Saccadic Eye Movements and Perceptual Judgments Reveal a Shared Visual Representation That Is Increasingly Accurate Over Time ⇑ Wieske Van Zoest A, , Amelia R

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Vision Research 51 (2011) 111–119

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Vision Research

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Saccadic eye movements and perceptual judgments reveal a shared visual representation that is increasingly accurate over time ⇑ Wieske van Zoest a, , Amelia R. Hunt b

a Cognitive Psychology, Vrije Universiteit Amsterdam, van der Boechorststraat 1, 1081 BT, Amsterdam, The Netherlands b School of Psychology, University of Aberdeen, Aberdeen AB24 2UB, UK

article info abstract

Article history: Although there is evidence to suggest visual illusions affect perceptual judgments more than actions, Received 7 July 2010 many studies have failed to detect task-dependant dissociations. In two experiments we attempt to Received in revised form 22 September 2010 resolve the contradiction by exploring the time-course of visual illusion effects on both saccadic eye movements and perceptual judgments, using the Judd illusion. The results showed that, regardless of whether a saccadic response or a perceptual judgement was made, the illusory bias was larger when Keywords: responses were based on less information, that is, when saccadic latencies were short, or display duration Perception and action was brief. The time-course of the effect was similar for both the saccadic responses and perceptual judge- Visual illusions ments, suggesting that both modes may be driven by a shared visual representation. Changes in the Saccadic eye movements Time-course of processing strength of the illusion over time also highlight the importance of controlling for the latency of different response systems when evaluating possible dissociations between them. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction driving behavior, it may be that behavior is controlled by one common visual representation that changes as a function of time Processes involved in recognition and identification of objects (Hunt, van Zoest, & Kingstone, 2010; van Zoest, Hunt, & Kingstone, rely on different parts of visual cortex than processes that assist 2010). When response decisions occur at different moments in localization and grasping of objects. This is uncontroversial. What time, they may be based on different time points in this emerging is controversial is the idea is that these different processes are dri- representation of objects, their identities, their task relevance, and ven by independent pathways and separate visual representations their spatial relationships. The present study aims to investigate (e.g., Goodale & Milner, 1992; Milner & Goodale, 1993, 1995, 2008). processing dynamics in a visual illusion, as measured by saccadic Neurophysiological and neuropsychological evidence supports the eye movements and perceptual judgments. idea that visually-guided actions can be dissociated from conscious report (e.g., Goodale & Milner, 1992). While this finding suggests that separate representations drive perception and action, several 1.1. Representations for perception and action studies have questioned the existence of a clear separation (Bruno & Franz, 2009; de Grave, Smeets, & Brenner, 2006; Franz, Support for separate processing streams for perception and ac- Fahle, Bulthoff, & Gegenfurtner, 2001; Franz, Gegenfurtner, tion is found in the primate visual system (e.g., Ungerleider & Bulthoff, & Fahle, 2000; Glover, 2002; Knox & Bruno, 2007; Vishton Mishkin, 1982). Under this model, the ventral stream runs from et al., 2007). Here we will focus on evidence that visual represen- occipital to temporal cortex and is responsible for the perception tation changes as a function of time, such that when in time visual and identification of objects. The dorsal stream runs from occipital information is used significantly influences human behavior (e.g., to parietal cortex and is responsible for motor control and actions Donk & van Zoest, 2009; Hunt, von Mühlenen, & Kingstone, towards objects. Further evidence in favor of the dual-route 2007; van Zoest, Donk, & Theeuwes, 2004). Little is known about hypothesis – the idea that these are indeed separate and indepen- how perception and action are affected by processing dynamics dent processing streams – was found in studies of neuropsycholog- in visual representations. Instead of two separate representations ical patients (Goodale & Milner, 1992; Goodale, Milner, Jakobson, & Carey, 1991; Milner & Goodale, 1995). Patients with damage to the ventral stream have difficulties identifying and recognizing objects, yet these patients are unimpaired in their motor abilities to- ⇑ Corresponding author. Address: Department of Cognitive Psychology, Vrije Universiteit Amsterdam, van der Boechorststraat 1, 1081 BT, Amsterdam, The wards the same objects. In contrast, patients with damage to the Netherlands. Fax: +31 20 598 8971. dorsal stream show no difficulties in object-identification, but E-mail address: [email protected] (W. van Zoest). show impaired abilities in grasping and reaching. These two

0042-6989/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.visres.2010.10.013 112 W. van Zoest, A.R. Hunt / Vision Research 51 (2011) 111–119 classes of patients together constitute a double dissociation be- 2003). Unlike reaching and grasping, eye movements can be tween the recognition of common objects and the ability to use elicited much earlier in time and providing a unique window in information about the size, shape and orientation of an object to early visual processing. Indeed, the eye movement system would control the hand and fingers during a grasping movement. These seem to provide conditions most likely to confirm the dual-route results suggest that distinct visual representations underlie per- hypothesis, given that the retinotectal visual pathway enables ception and action. eye movements to be executed in the absence of any cortical visual Supporting evidence for the dual-route hypothesis has also processing (Schiller, 1977). Nevertheless, similar to results with come from research showing that visual illusions affect perception manual movement responses described above, results from studies more than action (Aglioti, DeSouza, & Goodale, 1995; Bridgeman, on oculomotor behavior are inconsistent. Some have found that Lewis, Heit, & Nagle, 1979; Westwood & Goodale, 2003). For exam- oculomotor programming is resistant to visual illusions (Wong & ple, Aglioti et al. (1995) used a variation of the Titchener illusion to Mack, 1981) while others have found it is not (Bernardis, Knox, & investigate the effect of the illusion on perception and action. Bruno, 2005; de Grave, Franz et al., 2006; de Grave, Smeets et al., When participants were presented with two disks of equal size, 2006; Knox & Bruno, 2007; McCarley et al., 2003). they perceived a disk surrounded by smaller circles as larger than One way to explain the inconsistent effects of illusions on ocu- a disk surrounded by larger circles. However, when participants lomotor behavior was put forward by McCarley et al. (2003). They were asked to pick up one of the disks, grip size was similar for suggested that differences in saccadic control might account for the two disks. Aglioti et al. concluded that processing in the ventral differences in effect size of the illusion on saccadic performance stream was susceptible to the visual illusion while processing in (see also, DiGirolamo et al., 2008). Specifically, stimulus-driven the dorsal stream was not, or much less. Because contextual infor- reflexive eye movements may be differently influenced by a visual mation plays an important role in perceptual processes, they are illusion than goal-driven voluntary controlled eye movements. susceptible to context-based visual illusions. In contrast, the visu- McCarley et al. (2003) asked participants to saccade to the end- omotor system encodes information with respect to the appropri- point of a Müller-Lyer illusion stimulus. Saccades were controlled ate egocentric reference frame for a given action and does not either reflexively, by a transient go-signal that was presented at require a high-level perceptual representation that contains con- the endpoint of the stimuli, or voluntarily, by a spoken go-signal. textual information about the object. As a result, it was argued, vis- The results showed that the effect of the illusion was larger for vol- uomotor processing in the dorsal stream is better able to resist untary than for reflexive movements. In this study, voluntary sac- visual illusions. These findings not only support the idea that per- cades showed effects of the illusion similar in magnitude to those ception and action are served by distinct spatial representations, evident in perceptual judgments. The results are explained by the but also imply that the action representations exist outside of con- idea that reflexive and voluntary saccades are controlled by differ- scious visual experience. ent representations. Reflexive saccades rely on automatic collicular However, as briefly noted before, some have questioned the programs that are based on low-level retinotopic representations. idea of strong separate representations in the dual-route model These low-level representations are not susceptible to context (Clark, 2009; Franz & Gegenfurtner, 2008; Franz et al., 2000, and hence are not influenced by the illusion. Voluntary saccades, 2001; Glover, 2002; Glover & Dixon, 2001; Smeets & Brenner, it was argued, occur within a reference frame that is more similar 1995, 2006). This reservation is driven by a number of studies to conscious visual perception, and therefore do take into account showing that motor behavior is affected by visual illusions (e.g., high-level contextual information and are influenced by visual illu- de Grave, Franz, & Gegenfurtner, 2006; Glover & Dixon, 2001, sions. Given that an effect of the visual illusion was obtained for 2002); these results suggest that some aspects of motor control voluntary saccades, which can certainly be categorized as visu- such as velocity of reach and grip forces are not resistant to the ally-guided actions, the results of McCarley et al. (2003) do not effects of visual illusions (Jackson & Shaw, 2000; Smeets & Brenner, agree with the idea of strong independent representations. The re- 1995). Interactions between perception and action processes have sults of McCarley et al. (2003) may be reconciled with a weaker also posed challenges to the idea of separate streams. For instance, version of the dual-route hypothesis where the representations dorsal stream processing contributes to object categorization are not completely independent but allow for some interactions (Almeida, Mahon, & Caramazza, 2010), and manipulation of objects (Goodale & Westwood, 2004). can improve identification in agnosics (Schenk & Milner, 2006). Nevertheless, the discussion is complicated by a second study in Demonstrations of mutual interference and facilitation between which the effect of a visual illusion on reflexive and voluntary sac- perception and action tasks are the basis for the influential cades was investigated. In this work, Knox and Bruno (2007) re- common-coding theory of perception (Hommel, Musseler, ported the opposite pattern to McCarley et al. (2003). Knox and Aschersleben, & Prinz, 2001; Prinz, 1997) which has at its core Bruno found an effect of the illusion on reflexive saccades that the idea that visual and motor representations overlap. Represen- was comparable to that usually observed in verbal perceptual tations for perception and action may thus not be as independent tasks; a much smaller effect of the illusion was found for voluntary as initially assumed; some properties may be shared between the saccades. Methodological differences may explain some of the representations such that interactions are possible (Goodale & inconsistencies between this result and McCarley et al.’s finding. Westwood, 2004), or perhaps a single, shared representation drives For instance, instead of using a transient abrupt onset to trigger perception and action (e.g., Franz & Gegenfurtner, 2008; Franz, the reflexive saccade, Knox and Bruno (2007) used fixation offset Scharnowski, & Gegenfurtner, 2005; Franz et al., 2000, 2001; to facilitate the execution of fast reflexive saccades. Another differ- Mon-Williams & Bull, 2000; Smeets & Brenner, 1995; Smeets, ence is that McCarley et al. used a verbal spoken go-signal to elicit Brenner, de Grave, & Cuijpers, 2002). voluntary movements, while in Knox and Bruno’s experiment participants were required to saccade to the remembered endpoint of 1.2. Eye movements and visual illusions the illusion-inducing stimulus in the voluntary saccade condition. Although methodological details like those above may partly Additional insight into how actions are affected by visual illu- explain why effects of illusions are found on some behaviors but sions is provided by studies on eye movements and illusions not others (Bruno & Franz, 2009; Knox & Bruno, 2007), differences (Binsted & Elliott, 1999; de Grave, Franz et al., 2006; de Grave, in timing between different conditions and tasks makes further Smeets et al., 2006; DiGirolamo, McCarley, Kramer, & Griffin, comparisons between studies difficult. The moment in time at 2008; Knox & Bruno, 2007; McCarley, Kramer, & DiGirolamo, which a response is elicited can greatly influence performance W. van Zoest, A.R. Hunt / Vision Research 51 (2011) 111–119 113

(e.g., Hunt et al., 2007, 2010). Visual representations are not static 2. Experiment 1 but change as a function of time and the kind of information that guides behavior may depend on when in time the representation Participants were instructed to make an eye movement to the is accessed (Donk & van Zoest, 2008; Hunt et al., 2007; van Zoest middle of a line presented in the periphery. The line’s ends were et al., 2004). Therefore, short-latency responses are based on differ- capped by arrow heads pointing left, right, inward, or outward. A ent information than long-latency responses and this may affect bisection bias away from the direction of the arrows is predicted performance in visual illusions. For instance, the effects of saliency to occur when the two arrowheads point in the same direction of elements in a visual array have been found exclusively for short- (see Fig 1C). This misperception can be explained by a propensity latency responses (Donk & van Zoest, 2008; van Zoest & Donk, of the visual system to perceive the center of the whole object 2005; van Zoest et al., 2004). In contrast, responses elicited later rather than the center of a component within the image of the ob- in time are not affected by saliency, but are primarily controlled ject (e.g., Mon-Williams & Bull, 2000; Ro & Rafal, 1996). No bias is by task set and intentions (van Zoest & Donk, 2006, 2007). In the expected when the arrowheads both point inward or outward. case of visual illusions, perceptual judgments of illusions based Time-course analysis will reveal whether and how the bias on a rapid initial representation could include the gist of an image changes as a function of latency. and global context (e.g., Oliva & Torralba, 2006), while later processing may be guided by control mechanisms that are better able 3. Method to filter out irrelevant contextual information (e.g., Glover, 2002). This would lead to a diminished effect of the illusion over time. 3.1. Participants In contrast, if visually-guided actions are based on a separate representation, earlier responses may be driven by more purely motor Twelve students from the Vrije Universiteit Amsterdam partic- processing within the dorsal stream, while actions executed later ipated in Experiment 1 in exchange for course credit or money. in time would incorporate a broader representation that includes Participants ranged in age from 19 to 28 (mean age 21.2). All par- information about context and object identity. This would lead to ticipants had normal or corrected-to-normal vision. an increase in the effect of illusions on action over time. The current study will investigate the time-course of responses 3.2. Apparatus to a visual illusion. Responses that are based on less information are expected to be differentially influenced by the illusion than re- A Pentium IV computer with a processor speed of 2.3 GHz con- sponses that are based on more information. Time-course differ- trolled the timing of the events. Displays were presented on an Iiy- ences may help to clarify differences between the effect of ama 2100 SVGA monitor with a resolution of 1024 768 pixels and illusions for reflexive and voluntary eye movements (Knox & a 100-Hz refresh rate. Eye movements were recorded by means of Bruno, 2007; McCarley et al., 2003) and may help to explain dis- an Eyelink II tracker (SR Research Ltd.) with a 500 Hz temporal res- crepancies in results from other response systems as well (e.g., olution and a 0.2° of visual angle spatial resolution. The system Aglioti et al., 1995; de Grave, Smeets et al., 2006; Glover & Dixon, uses an infrared video-based tracking technology to compute the 2001). Additionally, comparison of perceptual and motor responses pupil center and pupil size of both eyes. Saccades were identified to visual illusions over time can shed light on the degree to which by means of a velocity threshold (35 deg/s) and an acceleration they are based on a shared representation. If perception and action threshold (9500 deg/s2). All subjects were tested in a sound-atten- share a dynamic representation, representational change will af- uated, dimly lit room with their heads resting on a chinrest. The fect performance in both modalities similarly, producing a similar monitor was located at eye level 75 cm from the chinrest. time-course of the effects across both tasks. To test this prediction, participants were presented with the Judd illusion (Judd, 1899), 3.3. Stimuli and the time-course of the effect of visual illusion on visual-guided eye movements (Experiment 1) and perceptual judgments (Exper- There were eight different types of line stimuli. Of these eight, iment 2) was investigated. the arrow heads pointed in the same direction in four stimuli

A. Illusion C. Bisection

actual middle perceived middle

B. Control

Fig. 1. An overview of the stimuli used in the two experimental conditions (A) and in the control conditions (B) and an illustration of the expected bias in the experimental condition (C). 114 W. van Zoest, A.R. Hunt / Vision Research 51 (2011) 111–119

(Fig. 1A). Of these, arrows positioned horizontally pointed either on initial saccade latency with condition (directional arrows and left or right, and arrows positioned vertically pointed either up control arrows) and SOA (200, 0, 200) as within-subject factors. or down. In the four remaining line types the arrow heads pointed The main effect of condition was significant F(1, 11) = 6.78, p = in opposite directions and these stimuli acted as control stimuli 0.0025. The mean saccade latency to directional arrows was a little (Fig. 1B). Two control stimuli were oriented horizontally and two bit slower (mean 221.9 ms) than the control arrows (219.9 ms). were oriented vertically. The shaft of the stimuli extended 8.12 de- Importantly the main effect of SOA was highly significant, gree of visual angle (DVA). The inclination of the wings with re- F(1, 11) = 245.49, p < 0.0001 and revealed that saccadic latencies spect to the shaft was 45°, and the length of the wings was 1.94 increased with an increasing overlap between fixation point and DVA. The horizontal stimuli appeared either above or below the display onset. Mean saccadic latency was 167.5 ms in the 200, fixation point, while the vertical stimuli appeared either the left 168.2 ms in the 0 ms condition and 327.1 ms in the 200 ms overlap or to the right of the fixation point. The stimuli were presented condition. The interaction between condition and SOA was not sig- at 9.09 DVA from fixation, and with a random jitter of plus or nificant, F(2, 22) < 1. minus 0, 0.3 or 0.6 DVA. To create the overall time-course of the effect, for each of the 12 The eight stimuli were grouped in two conditions based on the participants, trials were categorized per condition and sorted expected effect of the illusion. The bias was measured in distance based on saccade latency (collapsed over SOA manipulation) and from the actual middle of the line and was expressed in DVA. The divided in three equal bins (tertiles). Mean latencies for the three experimental condition consisted of four arrow types and these bins were 142.6 ms for the first bin, 187.3 ms for the second and either pointed left, right, up, or down (see Fig. 1A). It was predicted 339.7 ms for the third bin. The average distance to the center of that the eye movements directed to the middle of the line would the shaft (i.e., bias) was calculated for each condition (2) and tertile be biased to the mid-position of the figure. The control condition (3) of the sorted distribution of saccadic latencies. Mean saccadic was composed of the four symmetric stimuli (see Fig. 1B). Eye latency was calculated per condition per tertile. movements to the middle of the line in the control stimulus should From Fig. 2 it is clear the illusion is biasing the saccade bisection not significantly deviate from the actual middle of the stimulus. point, and, moreover, the bias in the illusion is greater earlier rather than later in time. These observations were tested in an AN- OVA with condition (directional arrows and control arrows) and 3.4. Design and procedure tertile (3) as within-subject factors and distance to the center in DVA as the dependent variable. The main effect of tertile was sig- Participants first viewed a calibration display consisting of nine nificant, F(2, 22) = 6.50, p = 0.006. The main effect of condition was points in a square array, which were fixated sequentially. The eye- significant, F(1, 11) = 19.20, p = 0.0011, as was the interaction be- tracking system was calibrated at the start of each block. In order tween condition and tertile, F(2, 22) = 9.40, p = 0.0011. Planned lin- to start each trial participants maintained fixation on a central ear contrasts revealed that the bias significantly decreased as a dot. Participants then pressed the spacebar in order to apply a drift function of tertile in the directional arrows condition, F(1, 11) = correction and to begin the trial with the presentation of a small 55.24, p < 0.0001, but not in the control arrows condition, fixation point for 500 ms followed by the bisection stimulus. To eli- F(1, 11) < 1. These tests confirm that saccadic eye movements were cit a large range of saccadic latencies, the fixation point was re- significantly influenced by the illusion and this bias decreased with moved at three different stimulus onset asynchronies (SOAs) of: an increase in saccade latency. 200, 0 and 200 relative to the target onset. Participants were in- To check whether it was the saccadic latencies per se that structed to make a saccade as quickly as possible following the off- decreased the illusion-induced bias or if it was instead the fixation set of the fixation point. It is known that saccadic latencies become manipulation that interacted with bias, we also ran the ANOVA on progressively faster when the fixation point is removed before the the bias measure with condition (directional arrows and control saccade target is presented and become slower when it is removed arrows) and SOA (3) as factors. There was a significant main effect following presentation of the saccadic target (Kingstone & Klein, of condition, F(1, 11) = 18.17, p = 0.0013, as well as a main effect of 1993; Saslow, 1967). This manipulation was expected to yield SOA, F (2, 22) = 11.56, p < 0.0001. However, in contrast with the short-latency as well high long-latency saccades. Participants were ANOVA with condition and tertile as factors, the interaction instructed to make an eye movement to the middle of the shaft of the stimulus. The stimulus was presented for 1000 ms after a saccade had ended, or for a total of 2000 ms. Participants were given written and oral instruction prior to the 1.2 beginning of the experiment. They completed 32 practice trials and Directional arrows Control arrows 504 experimental trials. Bisection stimuli (8) were randomly varied 1.0 within blocks of trials. Participants received feedback regarding 0.8 saccadic latency following every 25 trials and took a break after a block of 168 trials. 0.6

0.4 4. Results 0.2 Saccade latencies above 800 ms (3.39% of all trials) were counted as errors and were excluded from analysis. Saccade la- Distance from center (DVA) 0.0 tency shorter than 80 ms (0.22% of all trials) were regarded as 150 200 250 300 350 anticipatory responses and were not included in further analysis. -0.2 Eye movements that landed further than 3 DVA from the stimuli -0.4 (5.09% of all trials) where regarded as having missed the target Saccade latency (ms) and were not analyzed further. Fig. 2. The average distance from the center in DVA for the two experimental Only the first saccade was analyzed. To check whether the SOA conditions and the control condition as a function of saccade latency. Mean saccadic manipulation helped to generate short and long saccade latencies, latency was calculated per condition per tertile. Error bars represent the standard a repeated-measures analysis of variance (ANOVA) was performed error of the mean. W. van Zoest, A.R. Hunt / Vision Research 51 (2011) 111–119 115 between condition and SOA was not significant, F(2, 22) = 2.18, the illusion in perception, the duration of the arrow stimuli varied. p = 0.14. This pattern of results suggests that it was saccade Participants judged the position of a small line segment crossing latency, and not fixation offset time, that influences saccade bias. the shaft of the arrow relative to the perceived center of the arrow shaft. The line segment was initially placed at a large distance from the middle of the arrow shaft, either to the left or right for the hor- 5. Discussion izontally oriented stimuli, or above or below the center for the vertically oriented stimuli. During the experiment, the placement of The results of Experiment 1 revealed that saccades intended to the line segment on the bisection stimulus was contingent on the be directed to the center of the line were biased away from the response of the participant on the previous trial on the same stair- actual center by the arrowheads. Time-course analyses further case. For example, if a participant reported the line element as revealed that the bias decreased as a function of saccadic latency. positioned to the right of the center of the bisection stimulus in The later in time the eyes left the fixation point, the smaller the one staircase, the line element moved one step to the left on the effect of the illusion. next trial when the same staircase was presented. Consecutive Given that an effect of the visual illusion was obtained for visu- ‘‘right” responses will move the line element progressively closer ally-guided actions (saccades) the results of Experiment 1 do not to the center of the stimulus. When the line is perceived to cross agree with the idea of strong independent representations for the center, the response will reverse and the participant will re- perception and action. According to the idea that independent spond ‘‘left”. These responses reversals are therefore taken as an representations guide perception and action, saccadic eye move- estimate of the perceived center of the stimulus. ments were expected to be resistant to the effect of the visual illu- It is predicted that response reversals will be biased away from sion. These results are in line with other studies that have the center in the illusion stimuli compared to the control stimuli. demonstrated that eye movements are susceptible to visual illu- In order to investigate the time-course of the effect of the illusion, sions (e.g., de Grave, Franz et al., 2006; de Grave, Smeets et al., we manipulated the presentation duration. If the increase in sac- 2006). The existence of strong illusion effects that diminish over cade accuracy occurs because of changes to the representation of time also argues against the idea that faster, more reflexive the stimulus over time, the effect of the illusion on perceptual responses are refractory to illusions while slower, more voluntary judgments should also decrease as stimulus-presentation duration responses incorporate ventral stream processes and are therefore increases. In other words, observers are expected to be better able more likely to be susceptible to visual illusions (McCarley et al., to correct for initial misperception when responses are based on 2003). If that were the case, effects of illusions should grow, rather more information. than diminish, over time, as ventral stream processes would have more opportunity to influence responses executed later. The results of Experiment 1 may be best interpreted in line with 7. Method the planning-control model by Glover and colleagues (Glover, 2002, 2004; Glover & Dixon, 2001, 2002). According to this model, 7.1. Participants actions consist of a planning stage and a control stage where each stage uses its own representation. Whereas the visual representa- Twelve paid volunteers participated in Experiment 2. All partic- tion used in planning incorporates the visual context surrounding ipants had normal or corrected-to-normal vision. the target, the representation used in control operates relatively independent of context. As a consequence of the role of context 7.2. Apparatus in the two representations, planning is affected by visual illusions, whereas on-line control is not. Evidence for this model is provided The experiment was run on an HP Compaq with a 2.6-GHz Pen- by studies in which the effects of the illusion on grasping at a range tium 4 processor and 512 MB of RAM. The stimuli were presented of points throughout the reach were measured, rather than at the on a 19-in. Iiyama Vision Master Pro 454 CRT screen with a refresh point of the maximum grip aperture only (Glover & Dixon, 2001, rate of 120 Hz and a resolution of 1024 768 pixels. Viewing dis- 2002). The results showed that the effect of illusion was large early tance was at 68 cm. Participants responded using the keys, 2, 4, 8 in the reach and decreased as the hand approached the target. The and 6 on the number pad on the keyboard. results of Glover and Dixon suggest that the effects of a visual illusion on action are dynamic (however, see Franz et al., 2005; 7.3. Stimuli Meegan et al., 2004). The results of Experiment 1 are consistent with the idea of a The stimuli were identical to the stimuli used in Experiment 1, later control stage that corrects for early misperceptions that occur except that a small-superimposed line segment was presented at in a planning stage. However, the dynamic effects of illusions may varying locations along the shaft of the arrow stimuli (0.85 DVA). not be limited to motor responses. Effects of the illusion may In Experiment 2 the shaft of the stimuli extended 7.95 DVA and similarly be dynamic in perception. The idea here is that behavior the length of the wings was 1.85 DVA. The stimuli were presented – perception and action alike – is driven by one shared representa- at 8.78 DVA from fixation, with a random jitter of 0, 0.29 or 0.59 tion that develops over time. If the effects of visual illusion are DVA. Also a mask, extending across the entire display and com- ultimately guided by one representation, similar time-course dif- posed of crosshatched lines, immediately followed presentation ferences should become apparent if one looks at the time-course of the target stimulus. See Fig. 3. of perceptual responses. Similar to the saccadic responses in Exper- iment 1, responses driven by less information may be more biased 7.4. Procedure by the illusion than responses driven by more information. The directional arrows in the experimental condition were sep- 6. Experiment 2 arated based on the expected bias in unreferenced DVA relative to the middle of the line. For the horizontal arrows, left from the mid- A staircase procedure (e.g., Bertelson & Aschersleben, 1998) was dle was negative and right from the middle positive. For the verti- employed to investigate how perceptual responses are affected by cal arrows, up from the middle was negative and below from the the visual illusion. To investigate the time-course of the effect of middle positive. In turn, the predicted bias is a positive deviation 116 W. van Zoest, A.R. Hunt / Vision Research 51 (2011) 111–119

pressed ‘8’ or ‘2’ key, respectively. The position of the line element depended on the response given to that same staircase in a previous trial: the line was always shifted 2 pixels in the opposite direction of the response on the previous trial. A reversal is defined as a response different from the preceding one on the same staircase. A given staircase would be considered completed and would no long- er be presented to an observer when 10 reversals had been recorded. A trial started with the presentation of a fixation point for 500 ms and was followed by the presentation of the bisection stimulus. This stimulus was presented for 100, 200 or 400 ms and was 100, 200, 400 ms followed by a mask. Once a key response was recorded, the mask was presented for another 1000 ms. Completion of 10 reversals in each of the 18 staircases took on average 770 trials, taking participants around 45 min to finish. Prior to the experimental session, participants received written instructions and completed 48 prac- 500 ms tice trials.

Fig. 3. Illustration of the sequence of events in a trial in Experiment 2. 8. Results in DVA from the middle for both the left and up pointing arrows: Fig. 4 displays the results of the 18 staircases separately for each Observers are expected to perceive the middle as being to the right condition. The results show that the left and right staircases con- of the actual center in left pointing arrows and down from the ac- verge as the number of reversals increases. Critically, the mean tual center in up pointing arrows. The expected bias is a negative staircases of the three conditions converge at different locations deviation from the actual center for the right and down pointing relative to the actual center of the stimulus: The left and up arrow arrows: Observers are expected to perceive the middle as being left condition converge at a location biased positively from the actual from the actual center in right pointing arrows and up from the ac- center. Conversion in the control condition occurs close to the actual center in down pointing arrows. This distinction resulted in a tual center. The right and down arrow condition converge at a loca- total of three stimulus conditions, the control arrow condition, and tion biased negatively from the center of the stimulus. two experimental conditions (left and up pointing arrows and right The analysis of the data was based on the average over the last and down pointing arrows). five reversals, averaged over the left and the right staircase. Similar Eighteen staircases resulted from the combination of three to Experiment 1, the data was analyzed based on the ‘expected stimulus conditions (control, arrow left and up, arrow right and bias’ from the center of the line as dependent variable. The data down), three presentation durations (100, 200 and 400 ms), and was collapsed over the left and up and right and down arrows, two staircase start positions (left/above the center, right/below resulting in two main conditions involving the (1) directional ar- the center). Trials from each of the 18 staircases were randomly rows and (2) control arrows. The distance from center is positive intermingled. The start positions of the line segments were 40 pix- when the eyes are biased in the expected direction and negative els (1.21 DVA) out from the center of the bitmap of the arrow. when they are biased in the unexpected direction. These presentation durations were picked to resemble short, med- These averages are illustrated in Fig. 5, in which it is clear that ium and long-latency responses in the saccadic bisection task. The the bias decreases with an increase in presentation duration. In or- task of the participants in Experiment 2 was to indicate, using the der to statistically test these observations, a repeated measures number pad on the keyboard, whether the small line was posi- ANOVA was conducted with condition (directional arrows and con- tioned to the left or right of the center of the horizontal stimuli trol arrows) and presentation duration (100, 200 and 400 ms) as or above or below the center of the vertical stimuli. If they ob- within-subject factors. The main effect of presentation duration served the line as being positioned to the left or right of the center, was significant, F(2, 22) = 8.34, p = 0.0020; the main effect of condi- they pressed the ‘4’ or ‘6’ key, respectively. If the line was position was significant, F(1, 11) = 121.74, p < 0.0001, as well as the tioned above or below the center of the vertical stimuli, they interaction between condition and presentation duration,

Left & up arrows Control Right & down arrows right/ bottom stair, 100 ms 1.8 1.8 1.8 right/ bottom stair, 200 ms right/ bottom stair, 400 ms

1.2 1.2 1.2

0.6 0.6 0.6

0 0 0

-0.6 -0.6 -0.6 Distance from center (in DVA) center from Distance

-1.2 -1.2 -1.2 left/ top stair, 400 ms left/top stair, 200 ms left/top stair, 100 ms -1.8 -1.8 -1.8 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 reversal reversal reversal

Fig. 4. The average distance from the center of the stimulus (in DVA) of the 10 ﬁrst response reversals on each staircase. W. van Zoest, A.R. Hunt / Vision Research 51 (2011) 111–119 117

1.2 resistant to illusions (e.g., Aglioti et al., 1995). Second, the effect of Directional arrows Control arrows the illusion was seen to decrease as a function of processing time in 1.0 both Experiments 1 and 2. Finding a similar response pattern across different response modalities suggests that both perception 0.8 and action are based on a single visual representation. 0.6 The idea that perception and action may be driven by a single representation is not new (e.g., Franz & Gegenfurtner, 2008; Franz 0.4 et al., 2000, 2001, 2005; Mon-Williams & Bull, 2000; Smeets & Brenner, 1995; Smeets et al., 2002). While previous studies have 0.2 emphasized the importance of adequately matching the task- requirements if performance between two tasks is compared, the 0.0 Distance from center (DVA) present work stresses the importance of taking the passage of time 100 200 400 -0.2 into account. To the extent that different tasks and responses depend on different kinds of information, they will be influenced dif- -0.4 Presentation duration (ms) ferently by changes in the stimulus representation. The emerging representation of stimuli over time, therefore, can have different Fig. 5. The average distance from the control in DVA averaged for the left and right effects depending on both the type of response, and the time at staircases in each condition. Error bars represent the standard error of the mean. which the response is executed (Hunt et al., 2010; van Zoest et al., 2010). Previous results may have found differences between F(2, 22) = 11.77, p < 0.0001. Planned linear contrasts revealed that action and perception because they did not control for changes in the bias significantly decreased as a function of presentation dura- the illusion effects over time. tion in the experimental condition, F(1, 11) = 57.60, p < 0.0001, but The results of the current study are similar to results presented not in the control arrow condition, F(1, 11) < 1. These tests confirm by Glover and colleagues (Glover, 2004; Glover & Dixon, 2001, that the perceptual responses were significantly influenced by the 2002) in that they demonstrated that the effect of the visual illu- illusion and this bias significantly decreased as presentation dura- sion on action decreases over time. Based on their results, Glover tion increases. and colleagues proposed the planning-control model of action. In this model, action is composed of a planning and control stage and each stage is served by distinct visual representations. 9. Discussion Whereas the early planning stage is influenced by visual illusions, the later control stage is not. The data presented here, however, The results of Experiment 2 demonstrated that perceptual re- add to the planning-control model in one important way. In addi- sponses are subject to the dynamic influences of the visual illusion. tion to finding dynamic effects of the illusion in action, the present The bias in the perceptual judgment of center was smaller when data reveal the dynamic effect of the illusion in perception. Our display duration was long rather than short. In other words, per- data reveal that perceptual processes have, like action, an early ception seemed to partly overcome the effects of the arrowheads stage that is susceptible to context and a later control stage that when more information was available. is not, or much less. Finding a similar response pattern in both visuomotor and perceptual responses suggests that these two re- 10. General discussion sponse modalities may ultimately be driven by a single visual representation (Hunt et al., 2007, 2010; van Zoest et al., 2010). The purpose of the present study was to investigate the time- With this respect, our interpretation deviates from the planning- course of responses to a visual illusion. Perceptual representa- control model, to suggest that, rather than separate representations develop over time, and therefore short-latency responses tions guiding separate stages of action control, a single developing are based on different information than long-latency responses representation underlies changes in the influence of illusions over (Hunt et al., 2010; van Zoest et al., 2010). The results of Experi- time. ments 1 and 2 demonstrated that the effect of the arrow head A time-course approach may provide further insight when one direction is dynamic and decreases over time, regardless of considers differences in performance between reflexive and volun- whether a visually-guided saccade or a perceptual judgment tary control in visual illusions (Knox & Bruno, 2007; McCarley was made. Our results suggest that a shared representation that et al., 2003). Evidence suggest that reflexive control prevails during develops over time drives both perception and action. The early early selection, while voluntary control guides responses later in form of the representation appears to be driven by the global gist time (Theeuwes, Atchley, & Kramer, 2000; van Zoest et al., 2004). of the display, and it is influenced by misleading contextual infor- In line with the findings that illusions have a greater effect early mation. As time passes the representation changes and becomes in time rather than later, it is predicted that illusions should have more sophisticated as irrelevant information is discarded and a greater influence in reflexive responses rather than voluntary re- other information, such as prior knowledge and observer goals, sponses. The results of Knox and Bruno (2007) are in line with this is integrated. Thus, when observers are slower and more time reasoning. Knox and Bruno found an effect of the illusion on reflex- has been allowed for visual processing, performance is guided ive saccades. A much smaller effect of the illusion was found for by higher-order knowledge, allowing observers to filter out irrel- voluntary saccades. As with those results, our findings are difficult evant contextual information. to reconcile with those of McCarley et al. (2003), who found that There are at least two reasons why the present results do not reflexive (faster) saccades were less susceptible to the effects of agree with the idea of strong independent representations for per- arrowhead directions than voluntary ones. One important differ- ception and action as proposed by the dual-route hypothesis ence between our experiment and theirs is that here, saccades (Goodale & Milner, 1992). First, an effect of the visual illusion were directed towards the perceived center of a line, and so there was obtained for visually-guided saccades in Experiment 1. Sacc- was no point in space for the eye movement system to use as a tar- adic eye movements are a motor response and have been argued get along the line itself. It may be that a luminance transient at a to rely most strongly on a dorsal processing. According to the single point in space might provide special conditions where the dual-route hypothesis, motor programming should be more or less eye movement system can execute movements without taking 118 W. van Zoest, A.R. Hunt / Vision Research 51 (2011) 111–119 account of other visual information, particularly when successful References acquisition of a visible target depends on ignoring contextual information. Aglioti, S., DeSouza, J. F., & Goodale, M. A. (1995). Size-contrast illusions deceive the eye but not the hand. Current Biology, 5(6), 679–685. An alternative explanation for the observed differences be- Almeida, J., Mahon, B. Z., & Caramazza, A. (2010). The role of the dorsal visual tween fast responses and slow responses may be found in phys- processing stream in tool identification. Psychological Science, 21(6), 772–778. iological and functional differences in the way information is Bernardis, P., Knox, P., & Bruno, N. (2005). 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