The Sander Parallelogram Illusion Dissociates Action and Perception Despite Control for the Litany of Past Confounds

cortex 98 (2018) 163e176

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Special issue: Research report The Sander parallelogram illusion dissociates action and perception despite control for the litany of past confounds

* Robert L. Whitwell a, , Melvyn A. Goodale b,c, Kate E. Merritt b and James T. Enns a a Department of Psychology, The University of British Columbia, Canada b The Brain and Mind Institute, The University of Western Ontario, Canada c Department of Psychology, The University of Western Ontario, Canada article info abstract

Article history: The two visual systems hypothesis proposes that human vision is supported by an Received 3 October 2016 occipito-temporal network for the conscious visual perception of the world and a fronto- Reviewed 09 November 2016 parietal network for visually-guided, object-directed actions. Two specific claims about Revised 7 February 2017 the fronto-parietal network's role in sensorimotor control have generated much data and Accepted 20 September 2017 controversy: (1) the network relies primarily on the absolute metrics of target objects, Published online 5 October 2017 which it rapidly transforms into effector-specific frames of reference to guide the fingers, hands, and limbs, and (2) the network is largely unaffected by scene-based information Keywords: extracted by the occipito-temporal network for those same targets. These two claims lead Visual illusion to the counter-intuitive prediction that in-flight anticipatory configuration of the fingers Sander Parallelogram during object-directed grasping will resist the influence of pictorial illusions. The research Action confirming this prediction has been criticized for confounding the difference between Perception grasping and explicit estimates of object size with differences in attention, sensory feed- Grasping back, obstacle avoidance, metric sensitivity, and priming. Here, we address and eliminate Two visual systems hypothesis each of these confounds. We asked participants to reach out and pick up 3D target bars resting on a picture of the Sander Parallelogram illusion and to make explicit estimates of the length of those bars. Participants performed their grasps without visual feedback, and were permitted to grasp the targets after making their size-estimates to afford them an opportunity to reduce illusory error with haptic feedback. The results show unequivocally that the effect of the illusion is stronger on perceptual judgments than on grasping. Our findings from the normally-sighted population provide strong support for the proposal that human vision is comprised of functionally and anatomically dissociable systems. © 2017 Elsevier Ltd. All rights reserved.

* Corresponding author. Department of Psychology, The University of British Columbia, Canada. https://doi.org/10.1016/j.cortex.2017.09.013 0010-9452/© 2017 Elsevier Ltd. All rights reserved. 164 cortex 98 (2018) 163e176

movements (Aglioti, DeSouza, & Goodale, 1995; Brenner & 1. Introduction Smeets, 1996; Daprati & Gentilucci, 1997; Haffenden & Goodale, 1998). Notably, the TVSH does not predict that all Object-directed actions and object recognition rely on aspects of an object-directed action should be influenced by different sources of information. The choice of which apple to the illusion. If the choice to pick up one target or the other is pick up at a fruit stand is influenced by a wide and diverse contingent on a relative judgment of their size, the illusion range of factors. We discriminate McIntosh apples from, say biases the decision (Aglioti et al., 1995; Haffenden & Goodale, neighboring Red Delicious apples, and amongst the McIntosh 1998). Similarly, the choice to use one hand or two hands to apples, which is freshest, largest, unmarked, and so on. In pick up a 3D shaft placed on the Muller-Lyer illusion is biased contrast, the muscle commands to reach out to pick up a by the illusion (van Doorn, van der Kamp, & Savelsbergh, chosen object must be accurate enough to deal with the ab- 2007). Grasping an object also entails programming the solute constraints imposed by geometry and, therefore, so too fingertip forces for lifting the object. These forces must be must the metrics that underlie those commands. Object- based on estimates of object weight. In contrast to object ge- directed actions require, at a minimum, the specification of ometry, which can be registered ‘directly’ (Goodale & Milner, the spatial relations between the object and the effectors, 2004), weight is a ‘hidden’ variable that is inferred through specification of the spatial relations between different com- other object properties (Wolpert, Diedrichsen, & Flanagan, ponents of the body, and the application of rules governing 2011). Some of those properties are perceived directly, such the relations between motor commands and the mechanics of as size, while others, such as material, are perceived indirectly the musculoskeletal system. The existence of these funda- through other direct properties such as texture. Thus, the mental differences in the computational requirements for programming of anticipatory grip forces will be biased by a ‘vision-for-action’ and ‘vision-for-perception’ is an axiom of pictorial illusion provided that 1) target size is influenced by Goodale and Milner's (1992) Two Visual Systems Hypothesis relative, scene-based metrics and 2) weight is inferred from (TVSH). This core idea has been accepted and elaborated in ventral-stream mediated target size. In line with this subsequent overarching theoretical frameworks for under- reasoning, the anticipatory kinematics of grasps but not the standing the anatomical and functional organization of vision fingertip forces, resist the Ponzo illusion (Brenner & Smeets, in the primate brain (e.g., Jeannerod & Jacobb, 2005; Kravitz, 1996; Jackson & Shaw, 2000). Notably, if the mass of the Saleem, Baker, & Mishkin, 2011; Milner & Goodale, 2006; target objects is identical and, therefore, independent of target Rizzolatti & Matelli, 2003). size, the Ponzo illusion no longer biases grip forces yet con- Common to these frameworks is the notion that the pos- tinues to influence perception (Westwood, Dubrowski, terior parietal cortex in the dorsal stream possesses sensori- Carnahan, & Roy, 2000), presumably because the underlying motor modules that code objects in terms of their effector- neural networks learn the real weights of the objects inde- specific spatial reference frames and form part of the fronto- pendently of awareness as happens, for example, in the size- parietal network for planning and executing object-directed weight illusion (Flanagan & Beltzner, 2000). actions (Buneo & Andersen, 2006; Cohen & Andersen, 2002). This coding format helps shield the programming of the ki- 1.2. The critical litany nematics of reaching and grasping movements from irrele- vant contextual information (Milner & Goodale, 2006). The In the wake of these early studies of the effects of pictorial TVSH further holds that the ventral stream in the occipito- illusions on reaching and grasping, critics pointed to possible temporal cortex codes objects in terms of their relative met- confounds that undermined the interpretation of a rics in scene-based spatial reference frames (Foley, Whitwell, perception-action dissociation. The list includes differences in & Goodale, 2015). Tethered to context, the ventral stream de- how attention is deployed during ‘perception’ and ‘action’ livers a conscious visual experience of a relatively stable tasks (e.g., Franz, Gegenfurtner, Bϋlthoff, & Fahle, 2000; environment and provides a perceptual foundation for Pavani, Boscagli, Benvenuti, Rabuffetti, & Farne, 1999); the cognitive operations, allowing us to recognize and think about hand's avoidance-like response to the surrounding pictorial objects and events, communicate their significance to others, elements when grasping the inner discs of the Ebbinghaus and make decisions about different courses of action display (Franz, Bulthoff, & Fahle, 2003; Haffenden & Goodale, (Kanwisher & Dilks, 2014; Kravitz, Kadharbatcha, Baker, 2000; Haffenden, Schiff, & Goodale, 2001; de Grave, Ungerleider, & Mishkin, 2013; Milner & Goodale, 2006). Biegstraatan, Smeets, & Brenner, 2005); differences in map- ping changes in stimulus-size to changes in the measured 1.1. Pictorial illusions response (Franz, Fahle, Bϋlthoff, & Gegenfurtner, 2001; Franz, Gegenfurtner, Bϋlthoff, Fahle, 2000; Franz, Scharnowski, & Against this backdrop, pictorial illusions provide an attractive Gegenfurtner, 2005); the availability of visual and/or haptic vehicle to test this core feature of the TVSH, because the TVSH feedback (Bruno & Franz, 2009; Franz, Hesse, & Kollath, 2009; predicts that the inflight, anticipatory kinematics of object- Haffenden & Goodale, 1998; Pavani et al., 1999; Vishton, Rea, interactive actions that require an analysis of object geometry Cutting, & Nunez,~ 1999); and the task used to probe partici- should resist the influence of the illusion-inducing back- pants' visual perception of target size (Franz, 2001, 2003). Ac- ground. Earlier seminal work supported this contention with cording to the critique, controlling these factors should target-directed reach-to-touch (Bridgeman, Kirch, & Sperling, eliminate the dissociation: grasps will follow the illusion just 1981; Gentilucci, Chieffi, Daprati, Saetti, & Toni, 1996; Mack, as the perceptual estimates do. Such a null finding supports a Heuer, Villardi, & Chambers, 1985) and reach-to-grasp monolithic account of visual processing, where there is no cortex 98 (2018) 163e176 165

fundamental distinction between perception and action. The stronger when participants make direct comparisons of the present experiment was designed to test the dissociability of sizes of the inner circles. If the discs are adjusted simulta- action and perception in a healthy population while control- neously to make them appear equal in size, superadditivity ling for each of these factors. But before describing the occurs: the effect of the illusion is significantly larger than the experiment, we examine each of these critiques in turn (see sum of the effects for each configuration measured using also Smeets & Brenner, 2006). single comparisons, regardless of whether single or dual displays are used (Franz et al., 2000; see also Foster & Franz, 2014). 1.3. Attentional deployment for grasping and perceptual If we also consider that the TVSH posits a larger effect of the judgment illusion on perception than on action, then one could conceivably reduce the effect of the illusion on perception Pavani et al. (1999) and others have argued that spatial until it matches that on action. attention is allocated differently in Aglioti et al.'s (1995) Here, using the Sander Parallelogram (e.g., DeLucia, Meyer, grasping and size discrimination task with the classic dual- & Tresilian, 2000; Runyon & Cooper, 1970) has an advantage configuration Ebbinghaus display (Fig. 1). According to this over the Ebbinghaus display, because it is noticeable more argument, spatial attention is deployed across both configu- powerful (see Fig. 1). As an added attentional safeguard, we rations for same/different judgments about the size of the two used the manual estimation technique, in addition to a more inner discs, whereas it is focused on only one of the configu- classic single comparison task, and avoided direct compari- rations when the participant picks up one of the discs. Thus, sons altogether. In manual estimation, participants simply the spread of attention across both configurations could have displace the tips of their thumb and index finger an amount increased the illusory influence of the background for the that matches the apparent size of only one of the targets. This judgment task only. Pavani et al.'s way of controlling attention technique has an advantage in that the hand acts as an iso- was to test participants with single configuration Ebbinghaus lated comparator stimulus. Thus, spatial attention is believed displays only. They found an equivalent effect of the display to be focused on the single target, albeit surrounded by the on grasps and explicit estimations of disc size. single configuration. Moreover, on each trial, we presented a Notably, the single configuration method introduces its single target 3D bar. For the perception tasks, the hand (or a own problem which is that it risks weakening the effect of the length-adjustable bar on the screen) was always isolated from illusion on perception by undercutting the very scene-based the Sander Parallelogram. processing that makes the illusion so compelling (Whitwell & Goodale, 2017). This is important since the TVSH attri- 1.4. Obstacle-avoidance and reach-to-grasp movements butes to the ventral perception system a scene-based spatial reference frame which is reasonably presumed to drive the Grasp kinematics are influenced by objects that are in close illusory effect of the background on conscious visual experi- proximity to either the target object or to the hand's path to ence in the first place. Indeed, there is some evidence to the target (Mon-Williams & McIntosh, 2000; Mon-Williams, suggest that scene-based processing of the Ebbinghaus Tresilian, Coppard, & Carson, 2001; Saling, Alberts, display boosts the effect of the illusion, because the effect is Stelmach, & Bloedel, 1998; Tresilian, 1998; Voudouris,

Fig. 1 e The Ebbinghaus and the Sander parallelogram illusions. The sizes of inner disks of the Ebbinghaus display (A) and the lengths of the vertical black lines in the Sander parallelogram (B) appear to differ but are in fact equal. In our judgment, the Sander parallelogram is a noticeably more powerful illusion. The two different orientations of the Sander Illusion conﬁgurations are shown along with the two conﬁgurations within the parallelogram which exaggerate (left pair, “long”)or understate (right pair, “short”) the apparent size of the vertical line. Note that the target is the only vertical line in the parallelogram. In the experiment, this line was replaced with a black 3D bar that participants reached out to pick up and made explicit estimates of its length. 166 cortex 98 (2018) 163e176

Smeets, & Brenner, 2012). Furthermore, neuropsychological grasps (e.g., Bruno & Franz, 2009; Deprati & Gentilucci, 1997; and neuroimaging work implicate a role for the sensorimotor Franz et al., 2009; Heath, Rival, & Neely, 2006; Heath, Rival, networks of the posterior parietal cortex, including the dorsal Westwood, & Neely, 2005). Nevertheless, exactly how visual stream, in mediating obstacle avoidance when reaching to feedback achieves this is poorly specified. There are at least targets or reaching out to pick them up (Chapman, Gallivan, two general ways in which visual feedback can mitigate the Culhama, & Goodale, 2011; Rice et al., 2008; Schindler et al., illusory effect on grasps. From a computational standpoint, 2004; Striemer, Chapman, & Goodale, 2009). Importantly, the first involves sensorimotor feedforward- and feedback- obstacle avoidance mechanisms operate even when the ob- coupled processes that minimize error and movement vari- stacles are pictorial (e.g., Haffenden & Goodale, 2000; Jax & ability and are already ascribed to the dorsal sensorimotor Rosenbaum, 2007). With respect to grasps directed at the system (Desmurget & Grafton, 2000; Flanagan & Beltzner, inner discs embedded in the Ebbinghaus illusion, wrist 2000; Whitwell, Ganel, Caitline, & Coodale, 2015; Whitwell, orientation is affected by the orientation of the surrounding Milner, Cavina-Pratesi, Barat, & Goodale, 2015; Whitwell, Mil- annulus of circles (de Grave et al., 2005). Critically, in the ner, Cavina-Pratesi, Byrne, & Goodale, 2014; Whitwell et al., Ebbinghaus illusion, the size of the gap between the inner 2016). According to this view, visual feedback would be edges of the surrounding circles and the edge of the inner disc exploited by coupled inverse and forward models to minimize influences grip aperture (Haffenden et al., 2001). Thus, the error, including error introduced by the illusion, over any kind chief concern here is that an obstacle avoidance effect could of iterative action (Gonzalez et al. 2008; Whitwell et al. 2016; be mistaken as an effect of the illusion (Haffenden et al., 2001; Wolpert & Kawato, 1998). Removing visual feedback would Whitwell, Buckingham, Enns, Chouinard, & Goodale, 2016; c.f. disrupt this process, reduce illusory error minimization, and Franz et al., 2003; Kopiske, Bruno, Hesse, Schenk, & Franz, inflate the effects of the illusion on the grasps. Removing vi- 2016). sual feedback might also invite cognitive supervision over the The Sander Parallelogram affords a solution to the grasp, introducing a perceptual influence of the illusion into obstacle avoidance problem. As Fig. 1B makes clear, the the response. Taking the theoretical and empirical work into strongest source for a putative effect of obstacle avoidance is consideration, it seems reasonable to assume that an absence the relationship between the parallelogram and the target 3D of visual feedback during the grasps should make the disso- bar, because the target is always presented in the same po- ciation more difficult to observe in pictorial illusions. Never- sition and only the position and orientation of the parallelo- theless, removing visual feedback from the grasp is a gram varies, i.e., the “short” versus “long” configuration. In necessary guard against the possibility that participants use a the latter configuration, the long-axis of the parallelogram is deliberate strategy to exploit visual feedback to minimize more or less equivalent on the two sides of the target. illusory error. Rotating the parallelogram 180 has little effect on this. In It is possible that the influence of the illusion on grasps is stark contrast, the “short” configuration introduces a notice- minimized because participants make deliberate visual ably large asymmetry in the extent of the parallelogram comparisons of the size of the visible target and the aperture either side of the target. Furthermore, rotating the parallelo- of the moving hand so that it can be shaped accordingly gram 180 results in a radical shift in the direction of this during the response (Haffenden & Goodale, 1998). However, asymmetrical extent and, therefore, the spatial relationship an on-the-fly version of this tactic is likely to fail, because a between the parallelogram and the hand. Thus, one way to conscious comparison would surely be corrupted by the in- test for an obstacle avoidance effect would be to determine if dividual's susceptibility to the illusion. Thus, the opportunity grip aperture depended on the orientation of the two “short” for an explicit comparison occurs at the end of the move- configurations. The advantage of this targeted comparison is ment during which the fingers are in full view and, impor- that it should be independent of any influence of the illusion; tantly, in contact with the target. At this point in the and we can test this assumption by making an analogous movement, participants have a clear view of both the finger comparison for the manual estimations. It is also possible, aperture and the target's size while these sources of infor- though we believe unlikely, that the putative obstacle mation are in spatial and temporal registration with the avoidance effects of the “short” configuration displays might target's felt (haptic) size. This final, end-of-the-reach com- cancel one another out, revealing no difference in grip aper- parison could serve as a kind of ‘knowledge of results’ ture. Whatever the case, a null finding would suggest that (Salmoni, Schmidt, & Walter, 1984; Schmidt, Young, either the parallelogram exerts no obstacle avoidance effect Swinnen, & Shapiro, 1989) and could be used to plan grip on grip aperture, or that on balance the effects are in fact aperture for the subsequent grasps performed on a (nor- independent of the illusion and, therefore, moot. Either pos- mally) small number of different target sizes. Thus, through sibility is an advantage for testing our principle hypothesis. A the use of this strategy over iterative responses, the grasps similar reasoning can be applied to the two “long” configu- could come to reflect the real sizes of the targets, introducing rations, although as mentioned above, the difference in an artefactual perception-action dissociation. Notably, a asymmetrical extent between the two orientations is much comparison tactic such as this should benefit the manual less noticeable. estimates of target size just as much if not more so. One conservative way to control for this is to remove visual 1.5. Visual feedback feedback throughout the entire reaching and grasping movement, forcing a stronger reliance on ‘ballistic’ visual Online visual feedback of the hand and the target plays a feedforward processing based on vision up to the moment significant role in reducing the effect of pictorial illusions on the hand moves. This is exactly what we did. cortex 98 (2018) 163e176 167

1.6. Haptic feedback Thus, any differences between grip aperture and size estimates that we found could not be biased by differences in the Removing visual feedback still leaves grasps with a putative slopes of their respective response functions or an underes- advantage, because the grasps are afforded haptic feedback timation of variance. We also tested the possibility that the from the hand making contact with the target. Haptic feed- manual mode of performing the explicit size estimation task back has been suggested as a possible source of target infor- yields an unusually large effect of the illusion (Franz, 2001)by mation that could minimize the effects of pictorial size- comparing it to a ‘classic’ method in which participants contrast illusions on grasps e even in the absence of visual adjusted the length of a comparator line stimulus on a display. feedback (e.g., Haffenden & Goodale, 1998; Vishton et al., Our findings did not support this contention. 1999). Notably, this could only happen over the course of Any one or more of these confounds in past studies may several trials, because the kinematics of the grasp are account for the dissociation between perception and action. measured before the hand makes contact with the target. Although some studies have addressed these concerns in part Nevertheless, size-estimation tasks typically offer no such (e.g., Dewar & Carey, 2006; Franz et al., 2000; Haffenden & opportunity for haptic feedback. Thus, to equate the grasping Goodale, 1998; Stottinger et al., 2012), none have controlled and estimation tasks for haptic feedback in the present study, all of them within the same experiment. Here we report the we alternated reach-to-grasp movements with explicit size- most stringent test to date of dissociated illusory effects on estimation as has been done in previous studies (e.g., Ganel, grasping and perceptual judgments in normally-sighted in- Tanzer, & Goodale, 2008; Haffenden & Goodale, 1998; Pavani dividuals using the noticeably powerful Sander Parallelogram et al., 1999). illusion. The results were clear: the effect of the illusion was significantly larger on perceptual judgments than it was on 1.7. Task priming grasps. This result was not modulated by task-priming or, more generally, by carry-over effects. We also found no evi- There is some evidence to suggest that mere exposure to a dence that obstacle avoidance was confounded with the visual perception task can sensitize subsequent actions to dissociation. illusory effects. For example, the Roelof's illusion biases target-directed pointing when the movements are performed after, but not before, judgments of target position (Bridgeman 2. Methods & Peery, 1997). Interestingly, visual feedback does not minimize the effect of the Muller-Lyer illusion on grasps if each 2.1. Participants grasp is immediately preceded by a manual estimate of the target bar (Heath & Rival, 2005; Heath, Rival, & Binsted, 2004). A total of 54 right handed subjects (14 males) aged 18e56 were Finally, the mere preview of a single distractor object may tested and paid for their participation. All participants have been enough to induce a size-contrast effect on grip verbally affirmed that they were right-handed, healthy, had aperture several seconds later when participants reached out no history of brain trauma, and had normal or corrected-to- to pick up a substituted target object of a different size (Hesse, normal vision. Participants did not have previous experience de Grave, Franz, Brenner, & Smeets, 2008). Taken together, with the task. All participants provided written and informed these carry-over effects suggest that object and scene-based consent, and all procedures were approved by the local ethics representations can influence action, perhaps reflecting in- committee. teractions between the dorsal and ventral streams through known connections (Borra et al., 2008; Distler, Boussaoud, 2.2. Apparatus and stimulus Desimone, & Ungerleider, 1993; Milner, 2017; Webster, Bachevalier, & Ungerleider, 1994). Here, we explored the pos- The 2D illusion background, consisting on a given trial of a sibility that manual estimates may sensitize grasps to the single Sander Parallelogram, was printed on a white back- illusion by testing to see if the effect of the illusion on grasps ground (Fig. 1B). The orientation of the background display and manual estimates depends on whether these tasks are could assume one of two possible orientations (‘up’ or ‘down’), performed in separate blocks of trials or are alternated one which were simply 180 rotations from one another. This trial after the other. If task priming is operating, then the ef- manipulation was counterbalanced between participants. As fect of the illusion on grasps and manual estimates should be Fig. 1B shows, the long axis of the parallelogram assumed the more similar when the tasks are alternated than when they same angle (~26 counter-clockwise from vertical). The targets are blocked separately. were 3D black rectangular bars made of aluminum placed on top of the display along the vertical black line within paral- 1.8. Measurement techniques lelogram (see Fig. 1B). The bars had a constant width and height (2 mm 4 mm) and differed only in length (3 cm vs. Another critical issue concerns the sensitivity of different re- 4 cm). A single target was placed within one of the parallelo- sponses to target-size, which would interact with any influ- grams and one parallelogram was presented per trial. Each ence of the illusion. Any comparison that does not take this parallelogram was sized to fit either the 3 cm or the 4 cm target into account will lead to biased estimates (Franz et al., 2001; bar and configured to make the target appear longer (“long” Glover & Dixon, 2002). Here, we tested for a between-task condition) or shorter (“short” condition). difference in the two response functions. We found evidence Participants stood facing a tabletop on which the illusory for a modest difference and adjusted the ‘raw’ illusory effects. display and target bars were always centered at the same 168 cortex 98 (2018) 163e176

position along the individual's sagittal plane. A pressure- participants with visual feedback for their manual estima- sensitive “start” button was embedded in the tabletop tions for a number of reasons. First, providing visual feedback directly in front of the participant, approximately 5 cm from permitted participants time to check their estimates. Second, the edge of the table. Computer-controlled PLATO goggles the availability of visual feedback in the manual estimation (Translucent Technologies, Toronto, Canada) were worn by task minimized the role that proprioceptive feedback might participants. The lenses of these goggles contain a liquid otherwise play in complicating a measurement that unfolds crystal substance which is translucent in its default state and over a much slower time scale than ballistic grasps do (Franz, suppresses the wearer's view of the workspace. Critically, the 2003). Third, it does not modulate the magnitude of the lenses can become transparent (~6 ms), providing the wearer Ebbinghaus illusion as measured using manual estimates with an unobstructed view of the workspace, which included (Kopiske et al., 2016). Fourth, performing either unimanual or their hand and the display. Before the experiment began, bimanual manual estimations beneath the table with the infrared-light emitting diodes (IRED) were attached to the hands entirely out of view failed to yield a significant effect of participant's right thumb, index finger, and wrist with adhe- the Ebbinghaus illusion. In contrast, if participants are pro- sive tape. The experimenter ensured that the pads of the vided a view of the hand during the manual estimation task, participant's thumb and index finger were unobstructed. The this illusion yields a significant effect (Vishton & Fabre, 2003). 3D positions of the IREDs were recorded with an optoelec- This latter set of findings is concerning for manual estimation tronic motion capture system, Optotrak Certus (Northern methods that do not permit at least some vision of the hand. Digital, Waterloo, Canada) at 200 Hz. 2.5. Reaching and grasping 2.3. Procedure and design At the start of the trial, the goggles cleared to permit the All participants performed a perceptual judgment task and a participant a full view of the workspace. Starting at the rest grasping task. These tasks were administered in separate position, the participants were instructed to reach out and blocks of trials and also in a single block of trials in which they pick up the target bar along its length using their right index were interleaved in an alternating fashion. Thus, each finger and thumb as soon as they saw the target. Critically, participant was administered three blocks of trials. The order when the participants released the start button at the onset of in which these three blocks were administered was counter- their reach, the lenses of the goggles immediately switched to balanced across the participants. Because there are six their translucent state to suppress the participant's vision for possible ways of ordering (i.e., permuting) three blocks of tri- the remainder of the trial. In short, the participants did not als, each participant was pseudo-randomly assigned to one of receive any visual feedback during their movement. After these six block orders such that there were nine participants picking up the target bar, participants were instructed to set it tested for each one of the six block orders. back down and return their hand to the rest position on the For the grasping and manual estimation tasks, each one of start button. four illusion backgrounds in the set of either the ‘up’ or ‘down’ parallelograms was pseudorandomly presented four times, 2.6. Testing the validity of the manual estimations for a total of 16 trials. Thus, 16 trials were administered for each separate block of perceptual judgments and grasps and To test for a possible influence of the manual estimation task in the block in which the two tasks were administered one in distorting the true magnitude of the illusion, we brought in after the other. Thus, 32 trials were administered for the a new group of 12 participants to measure the effect of the alternating block, and 64 trials were administered to each illusion using manual estimates and a more ‘classic’ method participant across the three blocks of trials. of adjustment. For both tasks, participants viewed the same displays that were used in the principal experiment, including 2.4. Perceptual estimates of target length the 3D bar. In this experiment, however, the participants viewed these displays on top of a height-adjustable horizon- Participants were instructed to keep their thumb and index tally-oriented touch screen rather than a table, so that a finger pinched together and depressing a start button as their comparator stimulus (a 2D black bar) could be presented on default rest position. The experimenter triggered the start of the screen. The participants adjusted the length of the the trial which cleared the lenses of the goggles to permit the comparator stimulus through keyboard presses. All aspects of wearer a full view of the workspace. The participants were the manual estimation task were identical to that used in the instructed to look at the target bar when the goggles cleared principal experiment. and then release the start button to displace their right thumb and index finger an amount that matched the perceived 2.7. Data processing and statistical analysis length of the target bar. They were also instructed to keep the base of their fist planted firmly on the surface of the table next The kinematics of each trial were analyzed offline. The prin- to the start button when making their estimate and to refrain cipal dependent measure for the grasping task was the peak from reaching towards the target. The participants were grip aperture (PGA) which was extracted from each trial instructed to keep their fingers as stable as possible when they separately. To isolate this anticipatory measure, a velocity- were satisfied with their estimate. When their vision was based search window was used which included only points occluded 3 sec later, they returned their hand to the start during which the hand was moving towards the target. We button and assumed the rest position. We provided the defined the beginning of this search window as the first cortex 98 (2018) 163e176 169

sample frame in which the indexefinger IRED exceeded a illusion. If haptic feedback from grasping the targets affords threshold velocity of 50 mm/sec for 200 ms. We defined the participants insight into the target's real size, we would expect end of the search window as the first sample frame, following the difference in the effect of the illusion between the grasps the start of the movement, in which the indexefinger IRED fell and manual estimates to be smaller when they are alternated below 75 mm/sec. The PGA was defined as the maximum than when they are blocked separately. vector distance between the thumb and index finger IREDs In order to test the obstacle avoidance hypothesis, we used that occurred during the movement. The principal dependent independent-samples t-tests to compare the mean PGA for the measure taken from the manual estimation trials was the ‘up’ and ‘down’ orientations of the 3-cm and 4 cm-‘short’ manual estimate aperture (MEA). In accordance with the ex- parallelograms based on the logic articulated in the Intro- perimenter's instruction, the MEA should correspond to a duction. With respect to the task priming hypothesis, grasps point in the participant's response, following adjustments, blocked separately were predicted to show a larger slope- during which the aperture between the thumb and index adjusted illusory effect if they were immediately preceded finger is highly stable. With rare exception, the manual esti- by a block of manual estimation trials, whereas the manual mations began with an the initial relatively fast opening of the estimations were predicted to show a smaller slope-adjusted thumb and indexefinger which was followed by a period of illusory effect if immediately preceded by a block of grasping adjustment during which the aperture velocity equilibrates, trials. All statistical tests used an alpha of .05. For tests fluctuating slightly above and below 0 mm/sec. This phase of involving more than one t-test, a was defined family-wise and the response corresponds to a plateau phase in the aperture was held to .05 using the Holm step-down procedure (Holm, profile. Thus, we used aperture velocity criteria to isolate the 1979). We denote Holm-adjusted alpha values in-text with a0. MEA towards the end of the plateau phase, accommodating participants' refinements to their estimate. The MEA was defined as the vector distance between the thumb and index 3. Results finger IREDs on the first sample frame in which the grip aperture velocity fell within ± 5 mm/sec for 25e250 ms. In a 3.1. Profiles for grip aperture, finger velocity, and grip limited number of cases, the 5 mm/sec threshold was aperture velocity increased or reduced to accommodate ‘shakier’ participants and/or sudden adjustments. Finally, we isolated an additional Fig. 2A shows the interpolated grip aperture after standard- estimation measure that occurred earlier in the response at a izing the profile to 100 time points beginning at the onset of point which was, on average, similar to the PGA for the grasps. the reach and ending at the point at which the peak grip We did this by extracting the maximum aperture achieved aperture occurred. The effects of the 1 cm-difference in target within a velocity-defined movement time window of 20 mm/ length and the (smaller) effect of the illusion each appear to sec than was used for the grasps, because the manual esti- evolve throughout this phase of the movement. Fig. 2C shows mates were not intended to be as ballistic as reaching and the interpolated velocity of the thumb and index finger grasping. markers across the same standardized time period as above. Condition means were computed for each participant and As these panels make clear, the fingers were still in-flight at for each measure: PGA and MEA. We calculated the unadjusted the point at which peak grip aperture was achieved. Thus, it effect of the illusion for each participant by subtracting their seems reasonable to conclude that the PGA is not a function of mean measure from responses directed at the illusory “short” the hand's contact with the table or the target on that trial. conditions from their mean measure from responses directed Fig. 2B shows the time-normalized aperture between the at the illusory “long” conditions. Thus, positive values are thumb and index finger for the manual estimates, beginning consistent with the direction of the illusion. For each partici- at the onset of the finger movements and ending at the point pant, we calculated the effect of the difference in real target at which the aperture velocity fell outside of 10 mm/sec to- length for the grasps and manual estimations by subtracting wards the end of the response. Similar to the grasps, the ef- the mean measure for responses to the 3-cm target bar from fects of the 1 cm-difference in target length and the (smaller) the mean measure for responses to the 4 cm target bar. We effect of the illusion each increased rapidly as the participants' calculated the slope for each task and for each participant by displaced their thumb and index fingers in the initial phase of dividing the difference in their mean response to the 3-cm and their manual estimates. Importantly, these estimates soon 4-cm target by the 1-cm difference in real target length. achieved a stable plateau phase which reflects their consid- Finally, the slope-adjusted illusory effect was calculated for ered judgment about the length of the target. Fig. 2D shows each participant following the Taylor-approximation proced- the velocity of the aperture for the manual estimates across ure articulated in Hesse, Franz, & Schenk, 2016. the same period as above. This figure makes it clear that not We tested the unadjusted effect of the illusion, the slopes, only is the 1-cm difference in target size visible early on in the and then the slope-adjusted effect of the illusion using sepa- response but that the effect of the illusion is as well. rate 2 (Task) 2 (Task Schedule) repeated measures Analysis Furthermore, these panels indicate that participants per- of Variance. The factor Task refers to whether participants formed the manual estimation task as instructed. were performing grasps or manual estimations. Thus, tests of this factor evaluate a core prediction of the TVSH. The factor 3.2. Unadjusted (Raw) effects of the illusion Task Schedule refers to whether or not the tasks were blocked separately or alternated. Thus, tests involving this factor The Sander Parallelogram illusion had a significant influence concern the role that haptic feedback plays in mitigating the on the grasps and manual estimations (see Table 1). Critically, 170 cortex 98 (2018) 163e176

Fig. 2 e Time-normalized grip aperture, velocity, and grip aperture velocity profiles. (A) Grasp grip aperture time-normalized up until the point at which the peak grip aperture occurred. The effects of the 1cm-difference in target length and the (smaller) effect of the illusion each increase as the reach unfolds. (B) Grip aperture for the manual estimates time-normalized up until the end of the movement. (C) The velocity of the thumb and index finger markers for the grasps normalized across the same time period as above. Following peak velocity, the smooth, downward trajectory of the curves that terminate well- above 100 mm/s indicate that the fingers are still in-flight at the point at which peak grip aperture is achieved. (D) Time- normalized grip aperture velocity profiles for the manual estimates. The influences of the illusion and target length are visible early on in the movement. Error bars reflect the ±SEM.

Table 1 e Unadjusted effects of the illusion (in mm).

Task N Task schedule Mean SEM Tests against zero Grasp 54 Blocked 2.3 .279 t* ¼ 8.31, p < 4 10 11 Grasp 54 Alternated 2.5 .283 t* ¼ 8.94, p < 4 10 12 Manual estimation 54 Blocked 4.0 .302 t* ¼ 13.08, p < 4 10 18 Manual estimation 54 Alternated 4.0 .299 t* ¼ 13.46, p < 1 10 18 Manual estimation 12 Blocked 4.2 .612 t* ¼ 6.92, p < 3 10 5 ‘Classic’ adjustment 12 Blocked 4.1 .489 t* ¼ 8.34, p < 5 10 6

Responses in all combinations of task (grasps and manual estimations) and task schedule (blocked and alternated) were affected by the illusion. The mean unadjusted effect of the illusion for the manual estimates and for the classic adjustments are remarkably similar to the mean unadjusted effect of the illusion from the main experiment. The asterisk (*) denotes signiﬁcant tests using the Holm (1979) multiple comparisons procedure. cortex 98 (2018) 163e176 171

Fig. 3 e The unadjusted (raw) and slope-adjusted effects of the illusion as a function of the task (grasp or manual estimations). Whether the tasks were administered in separate blocks of trials or alternated one after the other bore no inﬂuence on the effects (A) The raw effect of the illusion shows a clear dissociation between grasps and manual estimates (B) The slope-adjusted effect of the illusion does not affect the conclusions drawn from the analysis of the raw effects of the illusion. The grasps are clearly less affected by the illusion. (C) The difference in the effect of the illusion between the manual estimations and the grasps. All error bars reﬂect the ±SEM.

the grasps showed a significantly smaller effect of the illusion effect of Task Schedule was null, F(1,53) ¼ 1.46, p > .23; the than did the manual estimations, F(1,53) ¼ 36.33, p < 2 10 7, Task Task Schedule interaction was significant, however, 2 2 ƞp ¼ .41 (Fig. 3A and Fig. 3C). The main effect of Task Schedule F(1,53) ¼ 6.56, p < .02, ƞp ¼ .11. Follow-up tests showed that the [F(1,53) ¼ .30, p > .58] and the Task Task Schedule interaction slopes for the grasps were significantly smaller than the [F(1,53) ¼ .08, p > .77] were null. Thus, the fundamental factor slopes for the manual estimates when each task was blocked influencing the unadjusted effect of the illusion was the mode separately, t(53) ¼ 3.5, p < 3 10 7, a0 < .026. In contrast, the of response. slopes did not differ significantly when the tasks were alternated one after the other, t(53) ¼ .58, p > .56, a0 ¼ .05. Given the 3.3. Tests of the response functions (Slopes) influence of the interaction on the slopes, we adjusted each participant's raw illusion effect. Tests of the response function are tests of the slopes relating target length to the principal response measures. Table 2 lists 3.4. Slope-adjusted effects of the illusion the results of the tests for the condition slopes against zero and one. The tests of the mean slopes against one show two Overall, the results of this analysis reinforce those from the things: (1) the slopes for the grasps are comparable to the analysis of the unadjusted effects of the illusion. The Sander slope of .82 for grasps (Smeets & Brenner, 1999), and (2) the Parallelogram illusion had a significant influence on the manual estimations do not differ significantly from 1:1 cor- grasps and manual estimates (see Table 3). Critically, the respondence with the difference in target size. Thus, the grasps showed a significantly smaller effect of the illusion manual estimates reflect what we should expect to observe than did the manual estimates, F(1,53) ¼ 23.22, p < 2 10 5, ƞ2 ¼ from normally-sighted individuals and are not inflated as p .31 (Fig. 3B and C). The main effect of Task Schedule some studies have found (c.f., Haffenden et al. 2001; Kopiske [F(1,53) ¼ .05, p > .81] and the Task Task Schedule interaction et al., 2016). [F(1,53) ¼ .42, p > .51] were null. Thus, the fundamental factor The slopes for the manual estimations were greater than influencing the slope-adjusted effect of the illusion was the 2 those for the grasps, F(1,53) ¼ 6.51, p < .02, ƞp ¼ .11. The main mode of response.

Table 2 e The slopes relating the dependent measure to the 10-cm difference in target length.

Task N Task schedule Mean SEM Tests against zero Tests against one (unitary) Grasp 54 Blocked .81 .034 t* ¼ 23.62, p < 9 10 30 t* ¼5.67, p < 6 10 7 Grasp 54 Altanated .91 .033 t* ¼ 27.83, p < 3 10 33 t* ¼2.74, p < 9 10 3 Manual estimation 54 Blocked .98 .04 t* ¼ 24.18, p < 3 10 30 t ¼.59, p > .55 Manual estimation 54 Altanated .94 .034 t* ¼ 27.54, p < 5 10 33 t ¼1.85, p > .07 Manual estimation 12 Blocked .95 .071 t* ¼ 13.34, p < 4 10 8 t ¼.71, p > .49 ‘Classic’ adjustment 12 Blocked .97 .042 t* ¼ 22.92, p < 2 10 10 t ¼.67, p > .51

As the table shows, the mean slopes for all tasks and task schedules differed signiﬁcantly from zero. Neither the mean slopes for manual estimation nor the mean slope for classic adjustment differed from unitary (i.e. one). The asterisk (*) denotes signiﬁcant tests using the Holm (1979) multiple comparisons procedure. 172 cortex 98 (2018) 163e176

Table 3 e Slope-adjusted effects of the illusion (in mm).

Task N Task schedule Mean SEM Tests against zero Grasp 54 Blocked 2.9 .376 t* ¼ 7.65, p < 4 10 13 Grasp 54 Alternated 2.8 .305 t* ¼ 9.11, p < 5 10 10 Manual estimation 54 Blocked 4.0 .328 t* ¼ 12.35, p < 4 10 17 Manual estimation 54 Alternated 4.3 .341 t* ¼ 12.56, p < 2 10 17 Manual estimation 12 Blocked 4.5 .605 t* ¼ 7.36, p < 2 10 5 ‘Classic’ adjustment 12 Blocked 4.2 .562 t* ¼ 7.48, p < 2 10 5

Responses in all combinations of task (grasps and manual estimations) and task schedule (blocked and alternated) were affected by the illusion. The mean slope-adjusted effect of the illusion for the manual estimates and for the classic adjustments are similar to the mean slope-adjusted effect of the illusion from the main experiment. The asterisk (*) denotes signiﬁcant tests using the Holm (1979) multiple comparisons procedure.

3.5. Focused tests for an obstacle-avoidance effect on the manual estimation measurement technique provides peak grip aperture systematically-biased estimates of the magnitudeofthe illusion.

We used independent-samples t-tests to compare the mean 3.8. Testing an earlier point in the manual estimates PGA for the “up” and “down” orientations of the “short” configurations of the display. The test failed to yield a significant Although we argued that visual feedback for the manual es- effect of orientation for the 3-cm targets when the grasps were timates is important for a number of reasons, we isolated and either blocked or alternated (both ps > .65). Similarly, the test tested one additional measure, the peak MEA, defined analo- failed to yield a significant effect of orientation for the 4-cm gously to the PGA for the grasps. One advantage of this mea- targets when the grasps were either blocked or alternated sure is that it occurred 443 ms (SD ¼ ±180 ms) into the manual (both ps > .36). In other words, the display configurations that estimation response, a point that is comparable to the PGA, would be expected to bias PGA the most through obstacle which occurred 476 ms (SD ¼ ±112 ms) into the reach. Indeed, 0 avoidance showed no such influence. the difference in timing was null t(53) ¼ 1.46, p > .15, a ¼ .05. The mean slope-adjusted effect of the illusion on the peak 3.6. Focused tests for a modulatory influence of task MEA was about .3 mm larger than the one computed using the priming on the effect of the illusion plateau phase MEA, but the test of this mean difference was null, t(53) ¼ 1.21, p > .23, a0 < .026. Importantly, the slope- We used independent-samples t-tests to evaluate the influ- adjusted effect of the illusion on the peak MEA was signifi- ence of the order in which the blocked tasks were performed cantly greater than that observed on PGA when the tasks were on the mean effect of the illusion. The unadjusted effects of blocked separately [t(53) ¼ 3.17, p < 3 10 3, a0 < .013] and the illusion on the grasps did not depend significantly on when they were alternated with one another, t(53) ¼ 2.8, whether the manual estimations was performed immediately p < 8 10 3, a0 < .017. Thus, the grasps still show a smaller before or immediately after, t(34) ¼ .46, p > .65. An identical effect of the illusion than do the manual estimations despite test performed for the manual estimates was also null, comparing measures that occur at approximately the same t(34) ¼ 1.39, p > .17. Analogous tests performed on the mean point in time after the onset of the response. slope-adjusted effects of the illusion were consistent with the analysis of the unadjusted effects (both ps > .09). Thus, we found little evidence to support the notion that task priming 4. Discussion or carry-over effects were operating at time scales that would span whole blocks of trials. Two major points emerge from the present study. First, the Sander Parallelogram illusion influences both perception and 3.7. Testing the validity of manual estimations grasps performed without visual feedback (open loop). Sec- ond, compared to perception, open-loop grasps show more A comparison of the unadjusted effect of the illusion as resistance to the influence of the Sander Parallelogram. As measured using the classic adjustment and manual estimation such, these data support the more general claim of the TVSH techniques (see Table 1) yielded a null difference, t(11) ¼ .29, and expanded proposals (e.g., Kravitz et al., 2011) that vision- p > .77. The mean slopes for the classic adjustments and the for-action and vision-for-perception are mediated by disso- manual estimations each differed significantly from zero and ciable systems. Critically, these results were obtained while failed to differ significantly from unitary (see Table 2). Thus, simultaneously controlling for all of the methodological crit- like the manual estimations from the main experiment, icisms that have been leveled at previous versions of this test. the manual estimates and classic adjustments in this experi- As we pointed out in the Introduction, an effect of the ment yielded reasonable, essentially unitary, slopes. Neverthe- illusion on grasps performed in open loop is not novel. We less, we also computed the slope-adjusted effect of the illusion speculate that the unnatural nature of planning a movement for each task. A comparison of the adjusted illusion on classic using vision but executing it without vision invites uncer- adjustment and manual estimation was null t(11) ¼ .44, p > .67. tainty and, in turn, some cognitive supervision of the Thus, we found no evidence to support the notion that response. Concern about the use of targets that are cortex 98 (2018) 163e176 173

challenging to pick up have been expressed previously, and that, we would not be surprised to find that the effect of the the objects used in this experiment might warrant this Sander Parallelogram on grasping is weaker in closed loop concern (Carey, 2001). In addition, any automatic online or than in open loop as appears to be the case for the Muller-Lyer offline error-reducing adjustments in movement would have illusion in both grasping and pointing (for systematic reviews, been compromised under the open-loop condition used in this see Bruno, Bernardis, & Gentilucci, 2008; Bruno & Franz, 2009, study. Notably, these influences (and additional ones consid- respectively). Such a finding would be consistent with sys- ered below) only make the test for dissociation conservative. tematic review and with the notion that a ventral stream We devote the remainder of our discussion to issues that are contribution can influence a real-time response in open loop central to the wider debate concerning the susceptibility of conditions through a combination of iconic memory and grasps and perceptual judgments to pictorial illusions. cognitive supervision.

4.1. Spatial attention 4.3. Haptic feedback

The present findings are resistant to the criticism that atten- We did not find any evidence to support the notion that haptic tion is deployed differently in grasping and size estimation feedback from touching and lifting the target bars after each tasks, because identical stimuli were used to assess both target-size estimation mitigated the illusory effect on those tasks, and both tasks involved processing the length of a single perceptual judgments. According to the sensory calibration 3D target. This critique was originally applied to the case in hypothesis, the difference between grasping and perceptual which the effect of the illusion was measured while partici- report is not due to any fundamental difference in the func- pants compared the sizes of the inner discs in the classic dual tional requirements the task imposes and the different configuration Ebbinghaus illusion (Franz et al., 2000; Pavani anatomical systems that can fulfill those requirements, but et al., 1999). Direct comparisons induce super-additivity in instead arises from task differences in the availability of sen- the illusory effect if the targets are simultaneously adjusted, sory information in the two tasks, including haptic feedback but isolating the comparator circle from the display attenu- (Schenk, 2010, 2012a, 2012b; Schenk, Franz, & Bruno, 2011). If ates that possibility (Franz et al., 2000). In a manual size- this were true, however, then the manual estimates of target estimation task, the hand is considered the comparator and size should have benefitted at least as much as the grasps did is typically isolated from the target bar and the illusory display from touching the target. Manual estimates may even have an (as was the case here). We note that the hand is a natural advantage over grasps in that they do not require any planning comparator in this respect: we often use our thumb and or programming to manually locate the target. Thus, when indexefinger to indicate the sizes of objects (within an ac- given a chance to grasp the target, manual estimations are commodating range of course). Thus, even if we were to apply better poised to exploit haptic feedback to calibrate an esti- the logic of an attentional mismatch to the size estimation mate of target size and minimize illusory error. One might be task used here, super-additivity is predicted not to occur. tempted to respond to this point by positing a privileged grasp- specific access to spatial or size information about the target 4.2. Visual feedback and the mode of response, but this modification removes an appeal to parsimony and blurs the distinction between the We observed the dissociation between grasping and perceptual TVSH and the multimodal calibration proposal. In any case, as judgments of target length despite the absence of visual feed- we plainly observed, the manual estimations reaped no such back throughout the grasping movements and its availability benefit from haptic feedback, bolstering similar findings from throughout the estimation task. In this study, visual feedback other experiments that have incorporated identical manipu- could not be used either to modify the grasping movements as lations (e.g., Ganel et al., 2008; Haffenden & Goodale, 1998; they unfolded (i.e., ‘online’ updating) or to update movement Whitwell et al., 2016). In short, haptic feedback did not affect programming (i.e., ‘offline’ updating) of subsequent grasps. In the phenomenology of the illusion. previous studies, manual estimations have been performed with or without visual feedback (e.g., Franz, 2003; Haffenden & 4.4. Dissociable sensory feedback and updating for Goodale, 1998; Kopiske et al., 2016; Stottinger & Perner, 2009). perception and action Kopiske et al. found that the effect of the Ebbinghaus illusion on manual estimations did not depend on whether or not visual There are a number of additional findings that support the feedback was available after the response was initiated. Some hypothesis that visually-guided actions are served by different vision of the hand probably needs to be provided, however, sensory feedback, error-minimizing, and response-updating since removing it by asking participants to keep their hand processes from those serving visual perception (Pettypiece, below the work table fails to yield any effect of the Ebbinghaus Goodale, & Culham, 2010; Safstrom & Edin, 2004; Snow, illusion on manual estimates (Vishton & Fabre, 2003). For a Coodale, & Culham, 2015; Westwood & Goodale, 2003; number of additional reasons outlined in the Methods section, Whitwell, Ganel et al., 2015; Whitwell, Milner et al., 2015; we provided participants visual feedback throughout their es- Whitwell et al., 2014). This notion is compatible with a timates of target length. distinction between somatosensation-for-action and somat- Taking all of these considerations together, our results do sosensation-for-perception (Dijkerman & de Haan, 2007). not support the idea that visual feedback for grasps uniquely Nevertheless, we cannot rule out the possibility that visual explains the difference in the effect of illusion on actions and and haptic feedback influence a participant's response if such perceptual size estimations (Bruno & Franz, 2009). Having said feedback informs their belief about the length of the target. 174 cortex 98 (2018) 163e176

Just as one can discount an ongoing or recent conscious visual 5. Concluding remarks experience that conflicts with a contrary belief, participants could conceivably respond in accordance to their belief about These data show a dissociation between visually guided the length of the target despite their conscious visual experi- grasping and perceptual judgments in the context of the ence (i.e., phenomenology) of the target's distorted length. We Sander Parallelogram. The design of the experiment allowed speculate that measures of report would be more amenable us to conclude this, because it controlled for all the potential than ballistic reaching and grasping actions to expressing an confounds that critics have pointed to in previous compari- updated belief or updated declarative knowledge that was sons of grasping and perceptual judgments. These confounds derived from feedback. In any event, although haptic feedback included the deployment of spatial attention, visual and likely contributes towards mitigation of illusory effects on haptic feedback, obstacle avoidance, potentially different grasping, we assume that it is doing so independently of one's response sensitivity functions, and task priming. In addition, belief and of one's conscious visual experience. we tested perceptual judgments of target length using both a ’ 4.5. Obstacle avoidance manual estimation task and a ‘classic perceptual estimation task and found no difference in the magnitude of the illusion measured by these two methods. Given these considerations, Previous studies have shown that obstacle-avoidance can our results constitute strong empirical behavioral evidence potentially confound the interpretation of the effect of picto- that visually-guided grasping and visual perception dissociate rial background display on actions directed at 3D objects in healthy, normally-sighted individuals. placed on top of them (Haffenden & Goodale, 2000; Haffenden et al., 2001). In our experiment, the results of the comparisons of the orientation of the illusory display on grip aperture did not support obstacle avoidance as an explanation for our main Acknowledgements finding. The work was supported by NSERC PDF 487969 to R.L.W. and 4.6. Response sensitivity functions NSERC research grants RGPIN-2013-36796 to J.T.E. and RGPIN- 2017-04088 to M.A.G. Not surprisingly, both the grasps and manual estimations were sensitive to the 1-cm difference in the real lengths of the targets. Moreover, their sensitivities fell well within expected references ranges. We observed a difference in sensitivity between the tasks that depended on whether or not the tasks were blocked separately or alternated. Accordingly, we adjusted the raw Aglioti, S., DeSouza, J. F. X., & Goodale, M. A. (1995). Size-contrast illusory effect in each condition by indexing it to the response illusions deceive the eye but not the hand. Current Biology, 5, e function for that condition. Despite these adjustments, our 679 685. Borra, E., Belmalih, A., Calzavara, R., Gerbella, M., Murata, A., overall finding remained the same. Thus, task-differences in Rozzi, S., et al. (2008). Cortical connections of the macaque response-sensitivity to a difference in the length of the target anterior intraparietal (AIP) area. Cerebral Cortex, 18, 1094e1111. stimuli simply cannot account for our key finding and, logi- Brenner, E., & Smeets, J. B. J. (1996). Size illusion influences how cally, do not affect our conclusions. we lift but not how we grasp an object. Experimental Brain Research, 111, 473e476. 4.7. Differential task priming Bridgeman, B., Kirch, M., & Sperling, A. (1981). Segregation of cognitive and motor aspects of visual functioning in induced motion. Perception and Psychophysics, 29, 336e342. We also explored the possibility that illusion-biased perceptual Bridgeman, B., & Peery, S. (1997). Interaction of cognitive and judgments influence subsequent grasps by testing for task- sensorimotor maps of visual space. Perception and order effects. We found no evidence to support this conten- Psychophysics, 59(3), 456e469. tion. The slopes for the grasps, however, were affected by Bruno, N., Bernardis, P., & Gentilucci, M. (2008). Visually guided whether or not the grasps were administered in separate pointing, the Mu¨ ller-Lyer illusion, and the functional e blocks of trials or alternated with the manual estimations. interpretation of the dorsal ventral split: Conclusions from 33 independent studies. Neuroscience and Biobehavioral Reviews, Specifically, the slopes increased when alternated with 32(3), 423e437. manual estimates. It is tempting to suggest that this reflects Bruno, N., & Franz, V. H. (2009). When is grasping affected by the some sort of perception-based priming of action (e.g., Hesse Mu¨ ller-Lyer illusion? A quantitative review. Neuropsychologia, et al., 2008). If we assume that cross-talk between the ventral 47(6), 1421e1433. and dorsal streams occurred, then we still cannot say whether Buneo, C. A., & Andersen, R. A. (2006). The posterior parietal the estimate of target size was combined downstream at the cortex: Sensorimotor interface for the planning and online ‘proximal’ output stage or further upstream in ‘distal’ control of visually guided movements. Neuropsychologia, 44, 2594e2606. perceptual-cognitive stages. Whatever the case may be, the Carey, D. P. (2001). Do action systems resist visual illusions? effect was (1) limited to the sensitivity of the grasps to the 1-cm Trends in Cognitive Sciences, 5, 109e113. difference in target length, (2) did not result in a particularly Chapman, C. S., Gallivan, J. P., Culhama, J. C., & Goodale, M. A. unusual slope, and (3) did not influence the magnitude of the (2011). Mental blocks: fMRI reveals top-down modulation of illusion enough to produce a significant difference between the early visual cortex when obstacles interfere with grasp e unadjusted and slope-adjusted effect of the illusion. planning. Neuropsychologia, 49(7), 1703 1717. cortex 98 (2018) 163e176 175

Cohen, Y. E., & Andersen, R. A. (2002). A common reference frame right hand: Sensitivity to perceptual illusions with awkward grasps for movement plans in the posterior parietal cortex. Nature decreases with practice in the right but not the left hand Reviews Neuroscience, 3, 553e562. Neuropsychologia, 46 (pp. 624e631). Daprati, E., & Gentilucci, M. (1997). Grasping an illusion. Goodale, M. A., & Milner, A. D. (1992). Separate visual pathways Neuropsychologia, 35(12), 1577e1582. for perception and action. Trends in Neuroscience, 15,20e25. DeLucia, P. R., Meyer, L. E., & Tresilian, J. R. (2000). Geometrical Goodale, M. A., & Milner, A. D. (2004). Plans for action. Behavioural illusions can affect time-to-contact estimation and mimed Brain Sciences, 27,37e40. prehension. Journal of Experimental Psychology: Human Perception de Grave, D. D. J., Biegstraatan, M., Smeets, J. B. J., & Brenner, E. and Performance, 26(2), 552e567. (2005). Effects of the Ebbinghaus figure on grasping are not Deprati, M., & Gentilucci, E. (1997). Grasping an illusion. only due to misjudge of size. Experimental Brain Research, 163, Neuropsychologia, 35(12), 1577e1582. 58e64. Desmurget, M., & Grafton, S. (2000). Forward modeling allows Haffenden, A. M., & Goodale, M. A. (1998). The effect of pictorial feedback control for fast reaching movements. Trends in illusion on prehension and perception. Journal of Cognitive Cognitive Sciences, 4(11), 423e431. Neuroscience, 10(1), 122e136. Dewar, M. T., & Carey, D. P. (2006). Visuomotor ‘immunity’ to Haffenden, A. M., & Goodale, M. A. (2000). Independent effects of perceptual illusion: A mismatch of attentional demands pictorial displays on perception and action. Vision Research, 40, cannot explain the perception-action dissociation. 1597e1607. Neuropsychologia, 44, 1501e1508. Haffenden, A. M., Schiff, K. C., & Goodale, M. A. (2001). The Dijkerman, H. C., & de Haan, E. H. F. (2007). Somatosensory dissociation between perception and action in the Ebbinghaus processes subserving perception and action. Behavioral and illusion: Nonilluosry effects of pictorial cues on grasp. Current Brain Sciences, 30, 189e239. Biology, 11, 177e181. Distler, C., Boussaoud, D., Desimone, R., & Ungerleider, L. G. (1993). Heath, M., & Rival, C. (2005). Role of the visuomotor system in on- Cortical connections of inferior temporal area TEO in macaque line attenuation of a premovement illusory bias in grip monkeys. The Journal of Comparative Neurology, 334, 125e150. aperture. Brain and Cognition, 57(2), 111e114. van Doorn, H., van der Kamp, J., & Savelsbergh, G. J. P. (2007). Heath, M., Rival, C., & Binsted, G. (2004). Can the motor system Grasping the Muller-Lyer illusion: The contributions of vision resolve a premovement bias in grip aperture? Online analysis for perception in action. Neuropsychologia, 45, 1939e1947. of grasping the Mu¨ ller-Lyer illusion. Experimental Brain Flanagan, J. R., & Beltzner, M. A. (2000). Independent of perceptual Research, 158(3), 378e384. and sensorimotor predictions in the size-weight illusion. Heath, M., Rival, C., & Neely, K. (2006). Visual feedback schedules Nature, 3(7), 737e741. influence visuomotor resistance to the Mu¨ ller-Lyer figures. Foley, R. T., Whitwell, R. L., & Goodale, M. A. (2015). The two- Experimental Brain Research, 168(3), 348e356. visual-systems hypothesis and the perspectival features of Heath, M., Rival, C., Westwood, D. A., & Neely, K. (2005). Time visual experience. Consciousness and Cognition, 35, 225e233. course analysis of closed- and open-loop grasping of the Foster, R. M., & Franz, V. H. (2014). Superadditivity of the Mu¨ ller-Lyer illusion. Journal of Motor Behavior, 37(3), 179e185. Ebbinghaus and Muller-Lyer illusions depends on the method Hesse, C., de Grave, D. D. J., Franz, V. H., Brenner, E., & of comparison used. Perception, 42, 787e796. Smeets, J. B. J. (2008). Planning movements well in advance. Franz, V. H. (2001). Action does not resist visual illusions. Trends in Cognitive Neuropsychology, 25(7e8), 985e995. Cognitive Science, 5(11), 457e459. Hesse, C., Franz, V. H., & Schenk, T. (2016). Pointing and Franz, V. H. (2003). Manual size estimation: A antipointing in Mu¨ ller-Lyer figures: Why illusion effects need neurospyschological measure of perception? Experimental to be scaled. Journal of Experimental Psychology: Human Brain Research, 151, 471e477. Perception and Performance, 42(1), 90e102. Franz, V. H., Bulthoff, H. H., & Fahle, M. (2003). Grasp effects of the Holm, S. (1979). A simple sequentially rejective multiple test Ebbinghaus illusion: Obstacle avoidance is not the procedure. Scandinavian Journal of Statistics, 6,65e70. explanation. Experimental Brain Research, 149, 470e477. Jackson, S. R., & Shaw, A. (2000). The Ponzo illusion affects grip- Franz, V. H., Fahle, M., Bϋlthoff, H. H., & Gegenfurtner, K. R. (2001). force but not grip-aperture scaling during prehension Effects of visual illusions on grasping. Journal of Experimental movements. Journal of Experimental Psychology: Human Psychology: Human Perception and Performance, 27(5), 1224e1244. Perception and Performance, 26, 418e423. Franz, V. H., Gegenfurtner, K. R., Bϋlthoff, H. H., & Fahle, M. (2000). Jax, S. A., & Rosenbaum, D. A. (2007). Hand path priming in Grasping visual illusions: No evidence for a dissociation manual obstacle avoidance: Evidence that the dorsal stream between perception and action. Psychological Science, 11,20e25. does not only control visually guided actions in real time. Franz, V. H., Hesse, C., & Kollath, S. (2009). Visual illusions, Journal of Experimental Psychology: Human Perception and delayed grasping, and memory: No shift from dorsal to ventral Performance, 33(2), 425e441. control. Neuropsychologia, 47, 1518e1531. Jeannerod, M., & Jacobb, P. (2005). Visual cognition: A new look at Franz, V. H., Scharnowski, F., & Gegenfurtner, K. R. (2005). Illusion the two-visual systems model. Neuropsychologia, 43, 301e312. effects on grasping are temporally constant not dynamic. Kanwisher, N., & Dilks, D. (2014). The functional organization of Journal of Experimental Psychology: Human Perception and the ventral visual pathway in humans. In L. Chalupa, & Performance, 31(6), 1359e1378. J. Werner (Eds.), The new visual neurosciences. Ganel, T., Tanzer, M., & Goodale, M. A. (2008). A double Kopiske, K. K., Bruno, N., Hesse, C., Schenk, T., & Franz, V. H. dissociation between action and perception in the context of (2016). The functional subdivision of the visual brain: Is there a visual illusions: Opposite effects of real and illusory size. real illusion effect on action? A multi-lab replication study. Psychological Science, 19(3), 221e225. Cortex, 79, 130e152. Gentilucci, M., Chieffi, Daprati, E., Saetti, M. C., & Toni, I. (1996). Kravitz, D. J., Kadharbatcha, S. S., Baker, C. I., Ungerleider, L. G., & Visual illusion and action. Neuropsychologia, 34(5), 369e376. Mishkin, M. (2013). The ventral visual pathway: An expanded Glover, S., & Dixon, P. (2002). Dynamic effects of the Ebbinghaus neural framework for the processing of object identity. Trends illusion in grasping: Support for a planning/control model of in Cognitive Sciences, 17,26e49. action. Perception & Psychophysics, 64(2), 266e278. Kravitz, D. J., Saleem, K. S., Baker, C. I., & Mishkin, M. (2011). A Gonzalez, C. L. R., Ganel, T., Whitwell, R. L., Morrissey, B., & new neural framework for visuospatial processing. Nature Goodale, M. A. (2008). Practice makes perfect, but only with the Reviews Neuroscience, 12, 217e230. 176 cortex 98 (2018) 163e176

Mack, A., Heuer, F., Villardi, K., & Chambers, D. (1985). The Stottinger, E., & Perner, J. (2009). Consistency in exchange for dissociation of position and extent in Miiller-Lyer figures. inappropriately matched visual feedback? A comment on Perception & Psychophysics, 37(4), 335e344. Franz and Gegenfurtner (2008) “Grasping visual illusions: Milner, A. D. (2017). How do the two visual streams interact with Consistent data and no dissociation”. Cognitive each other? Experimental Brain Research, 235, 1297e1308. Neuropsychology, 26(4), 412e417. Milner, A. D., & Goodale, M. A. (2006). The visual brain in action (2nd Stottinger, E., Pfusterschmied, J., Wagner, H., Danckert, J., ed.). New York: Oxford University Press. Anderson, B., & Perner, J. (2012). Getting a grip on illusions: Mon-Williams, M., & McIntosh, R. D. (2000). A test between two Replicating Stottinger et al. [Exp Brain Res (2010) 202:79e88] hypotheses and a possible third way for the control of results with 3-D objects. Experimental Brain Research, 216(1), prehension. Experimental Brain Research, 134, 268e273. 155e157. Mon-Williams, M., Tresilian, J. R., Coppard, V. L., & Carson, R. G. Striemer, C. L., Chapman, C. S., & Goodale, M. A. (2009). “Real- (2001). The effect of obstacle position on reach-to-grasp time” obstacle avoidance in the absence of primary visual movements. Experimental Brain Research, 137, 497e501. cortex. Proceedings of the National Academy of Sciences of the Pavani, F., Boscagli, I., Benvenuti, F., Rabuffetti, M., & Farne, A. (1999). United States of America, 106(37), 15996e16001. Are perception and action affected differently by Titchener Tresilian, J. R. (1998). Attention in action or obstruction of circles illusion? Experimental Brain Research, 127,95e101. movement? A kinematic analysis of avoidance behavior in Pettypiece, C. E., Goodale, M. A., & Culham, J. C. (2010). Integration prehension. Experimental Brain Research, 120, 352e368. of haptic and visual size cues in perception and action Vishton, P. M., & Fabre, E. (2003). Effects of the Ebbinghaus illusion revealed through cross-modal conflict. Experimental Brain on different behaviors: One- and two-handed grasping; one- Research, 201, 863e873. and two-handed manual estimation; metric and comparative Rice, N. J., Edwards, M. G., Schindler, I., Punt, T. D., judgment. Spatial Vision, 16(3e4), 377e392. McIntosh, R. D., Humphreys, G. W., et al. (2008). Delay Vishton, P. M., Rea, G., Cutting, J. E., & Nunez,~ L. N. (1999). abolishes the obstacle avoidance deficit in unilateral optic Comparing effects of the horizontal-vertical illusion on grip ataxia. Neuropsychologia, 46, 1549e1557. scaling and judgment: Relative versus absolute, not Rizzolatti, G., & Matelli, M. (2003). Two different streams form the perception versus action. Journal of Experimental Psychology: dorsal visual system: Anatomy and functions. Experimental Human Perception and Performance, 25(6), 1659e1672. Brain Research, 153, 146e157. Voudouris, D., Smeets, J. B., & Brenner, E. (2012). Do obstacles Runyon, R. P., & Cooper, M. R. (1970). Enhancementof Sander illusion affect the selection of grasping points? Human Movement in minimal form. Perception & Psychophysics, 8(2), 110e111. Science, 31, 1090e1102. Safstrom, D., & Edin, B. B. (2004). Task requirements influence Webster, M. J., Bachevalier, J., & Ungerleider, L. G. (1994). sensory integration during grasping in humans. Learning & Connection of inferior temporal areas TEO and TE with Memory, 11, 356e363. parietal and frontal cortex in macaque monkeys. Cerebral Saling, M., Alberts, J., Stelmach, G. E., & Bloedel, J. R. (1998). Reach- Cortex, 5, 470e483. to-grasp movements during obstacle avoidance. Experimental Westwood, D. A., Dubrowski, A., Carnahan, H., & Roy, E. A. (2000). Brain Research, 118, 251e258. The effect of illusory size on force production when grasping Salmoni, A. W., Schmidt, R. A., & Walter, C. B. (1984). Knowledge objects. Experimental Brain Research, 135, 535e543. of results and motor learning: A review and critical Westwood, D. A., & Goodale, M. A. (2003). A haptic size-contrast reappraisal. Psychological Bulletin, 95(3), 335e386. illusion affects size perception but not grasping. Experimental Schenk, T. (2010). Visuomotor robustness is based on integration Brain Research, 153, 253e259. not segregation. Vision Research, 50(24), 2627e2632. Whitwell, R. L., Buckingham, G., Enns, J. T., Chouinard, P. A., & Schenk, T. (2012a). No dissociation between perception and Goodale, M. A. (2016). Rapid decrement in the effects of the action in patient DF when haptic feedback is withdrawn. The Ponzo display dissociates action and perception. Psychonomic Journal of Neuroscience, 3, 2013e2017. Bulletin Review, 23(4), 1157e1163. Schenk, T. (2012b). Response to Milner et al.: Grasping uses vision Whitwell, R. L., Ganel, T., Caitlin, B. M., & Goodale, M. A. (2015). and haptic feedback. Trends in Cognitive Sciences, 16, 258e259. Real-time vision, tactile cues, and visual form agnosia: Schenk, T., Franz, V., & Bruno, N. (2011). Vision-for-perception Removing haptic feedback from a “natural” grasping task and vision-foraction: Which model is compatible with the induces pantomime-like grasps. Frontiers in Neuroscience, 9, available psychophysical and neuropsychological data? Vision 1e18. Research, 51(8), 812e818. Whitwell, R. L., & Goodale, M. A. (2017). Real and illusory issues Schindler, I., Rice, N. J., McIntosh, R. D., Rossetti, Y., Vighetto, A., in the illusion debate (Why two things are sometimes better & Milner, A. D. (2004). Automatic avoidance of obstacles in a than one): Commentary on Kopiske et al. Cortex, 88, dorsal stream function: Evidence from optic ataxia. Nature 205e209. Neuroscience, 7(7), 779e784. Whitwell, R. L., Milner, A. D., Cavina-Pratesi, C., Barat, M., & Schmidt, R. A., Young, D. E., Swinnen, S., & Shapiro, D. C. (1989). Goodale, M. A. (2015). Patient DF's visual brain in action: Visual Summary knowledge of results for skill acquisition: Support feedforward control in visual form agnosia. Vision Research, for the guidance hypothesis. Journal of Experimental Psychology: 110, 265e276. Learning, Memory, and Cognition, 15(2), 352e359. Whitwell, R. L., Milner, A. D., Cavina-Pratesi, C., Byrne, C. M., & Smeets, J. B., & Brenner, E. (1999). A new view on grasping. Motor Goodale, M. A. (2014). DF's visual brain in action: The role of Control, 3, 237e271. tactile cues. Neuropsychologia, 55,41e50. Smeets, J. B. J., & Brenner, E. (2006). 10 years of illusions. Journal of Wolpert, D. M., Diedrichsen, J., & Flanagan, J. R. (2011). Principles Experimental Psychology: Human Perception and Performance, of sensorimotor learning. Nature Review Neuroscience, 12, 32(6), 1501e1504. 739e751. Snow, J. C., Goodale, M. A., & Culham, J. C. (2015). Preserved haptic Wolpert, D. M., & Kawato, M. (1998). Multiple paired forward and shape processing after bilateral LOC lesion. Journal of inverse models for motor control. Neural Networks, 11, Neuroscience, 35(40), 13745e13760. 1317e1329.