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Differential Effects of the Müller-Lyer Illusion on Reflexive and Voluntary Saccades

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Differential Effects of the Müller-Lyer Illusion on Reflexive and Voluntary Saccades Journal of Vision (2003) 3, 751-760 http://journalofvision.org/3/11/9/ 751 Differential effects of the Müller-Lyer illusion on reflexive and voluntary saccades Department of Psychology Jason S. McCarley Mississippi State University, Mississippi State, MS, USA Beckman Institute and Department of Psychology, Arthur F. Kramer University of Illinois at Urbana-Champaign, Urbana, IL, USA Department of Experimental Psychology, Gregory J. DiGirolamo University of Cambridge, Cambridge, England Research has produced conflicting evidence as to whether saccade programming is or is not biased by perceptual illusions. However, previous studies have generally not distinguished between effects of illusory percepts on reflexive saccades, programmed automatically in response to an external visual signal, and voluntary saccades, programmed purposively to a location where no signal has occurred. Here we find that voluntary and reflexive saccades are differentially susceptible to the Müller-Lyer illusion; reflexive movements are reliably but modestly affected by the illusion, whereas voluntary movements show an effect similar to that of perceptual judgments. Results suggest that voluntary saccade programming occurs within a non-retinotopic spatial representation similar to that of visual consciousness, whereas reflexive saccade programming occurs within a representation integrating retinotopic and higher level spatial frames. The effects of the illusion on reflexive saccades are not subject to endogenous control, nor are they modulated by the strength of an exogenous target signal. Keywords: saccades, visually guided behavior, illusions DeSouza, and Goodale (1995), for example, found large Introduction effects of the Titchener illusion on psychophysical judgments of object size, but found no effect on grip Perception and Action: Separate or scaling. Bridgeman and colleagues, similarly, found that perceptual judgments but not pointing responses were Common Representations? biased by induced motion of a target object (Bridgeman, Despite much study, researchers have yet to reach Kirch, & Sperling, 1981). Such findings are consistent consensus as to whether and how conscious perception with the hypothesis that perception and action are served and the control of action are linked. As described by by distinct spatial representations, with only the former Franz and colleagues (Franz, Fahle, Bülthoff, & representation being susceptible to illusion. Gegenfurtner, 2001), the relationship between perception Neurophysiological and neuropsychological evidence for and action has been hypothesized to take three different distinct visual cortical streams, one primarily responsible forms. The strong separate representation model proposes for object recognition and the other for spatial that distinct neural representations underlie motor representation and visuomotor control (Goodale & behavior and conscious visual perception. The weak Milner, 1992), provides a plausible biological basis for separate representation model likewise posits that separate this dissociation. spatial representations exist in the brain, but allows for Other data, however, cast doubt on the strong crosstalk, modulated by task demands, to occur between separate representation model. Consistent with a weak them. The common representation model, finally, holds that separate representation model, some findings suggest that conscious perception and visuomotor control proceed illusion affects some aspects of visually guided movement from the same mental representation.1 (e.g., velocity of reach and grip force) but not others (e.g., Research in healthy observers has attempted to grip scaling) (Brenner & Smeets, 1996; Jackson & Shaw, distinguish among these models by measuring the effects 2000), or that dissociations between perception and of perceptual illusions on visually guided behavior. visually guided action are mediated by task demands Though counterintuitive, the strong separate (Bridgeman, Peery, & Anand, 1997; Gentilucci, Chieffi, representation model has received support from findings Daprati, Saetti, & Toni, 1996; Wraga, Creem, & Proffitt, suggesting that visually guided behavior is uninfluenced 2000). Other data appear to support a common by illusions that are evident in subjective reports. Aglioti, representation model. Experiments by Franz and doi:10.1167/3.11.9 Received April 4, 2003; published December 4, 2003 ISSN 1534-7362 © 2003 ARVO Downloaded from jov.arvojournals.org on 10/02/2021 McCarley, Kramer, & DiGirolamo 752 colleagues (Franz et al., 2001; Franz, Gegenfurtner, by data from an alternative line of research. Studies Bülthoff, & Fahle, 2000) found similar effects of the beginning as early as 1897 (Delabarre) have consistently Titchener and Müller-Lyer (M-L) illusions on perception demonstrated that saccadic eye movements are susceptible and grasping, while a study by Lopez-Moliner et al. (2003) to misperceptions induced by the M-L illusion; when demonstrated equal effects of a pictorial depth illusion on observers are asked to saccade toward the endpoint of an perception and manual tracking. A study by Vishton, Read, Cutting and Nunez (1999) found that subjective M-L figure, or to saccade from one endpoint of the figure reports and grip scaling were both susceptible to the to another, eye movement landing positions are reliably horizontal-vertical illusion, but only when observers were biased in the direction of illusory changes in the figure’s induced to encode the horizontal and vertical dimensions length (Binsted & Elliott, 1999; Delabarre, 1897; of the stimulus relative to each other. When observers Festinger, White, & Allyn, 1968; Stratton, 1906; Yarbus, were induced to encode the size of one dimension, 1967). This effect obtains even when the stimulus figure ignoring the orthogonal dimension, no illusion was remains visible throughout saccade execution and evident in either form of response. These findings are preparation, and as such is not the product of memory- consistent with a common representation model in which guided targeting. the frame of reference induced by task demands Visually guided saccades thus appear to be susceptible determines what effect an illusion will have on a response, independent of the response mode. to illusion under some circumstances and resistant under A complicating factor in the effort to distinguish others. Given large differences between the methodology between strong separate, weak separate, and common of Wong and Mack (1981) and that of Delabarre (1897), representation models using perception-action Yarbus (1967), Festinger (1968) and others, in addition to dissociations, it should be noted, is that different illusions the inherent differences between the induced motion and may arise at different points within the visual processing M-L illusions, it is not immediately obvious what the stream. An illusion that arises early in processing and is relevant circumstances are. One mediating factor, propagated forward might therefore affect visually guided however, may be the difference between behavior even if perception and visuomotor behavior are reflexive/reactive/exogenously triggered and ultimately based on separate representations. Visuomotor voluntary/volitional/endogenously triggered saccades (Deubel, behavior will be resistant to an illusion only if the representations supporting perception and action are 1995; Erkelens & Hulleman, 1993; Klein, Kingstone, & indeed separate, and only then if the illusion has its locus Pontefract, 1992; Klein & Shore, 2000). Reflexive beyond the point at which those representations diverge saccades, although they can be inhibited or modified by (Dyde & Milner, 2002). top-down control (Machado & Rafal, 2000a, 2000b; Rafal, Machado, Ro, & Ingle, 2000), are programmed Effects of Illusions on Oculomotor automatically, presumably by collicular mechanisms Behavior (Rafal, Smith, Krantz, Cohen, & Brennan, 1990), in response to a transient external signal at a saccade target Like the studies of reaching and grasping described location. In essence, they reflect a visual grasp reflex above, studies of oculomotor behavior have produced (Rafal et al., 2000). Voluntary saccades are programmed apparently conflicting evidence for perception/action purposively, apparently by cortical mechanisms (Henik, dissociations. A seminal study by Wong and Mack (1981) Rafal, & Rhodes, 1994), in the absence of a transient suggested that visually guided saccade programming is signal to mark the target location. Thus, while exogenous immune to illusion. Observers were asked to saccade and endogenous influences may interact to shape saccadic toward a stepped target, then to saccade from memory behavior in a particular task (Godjin & Theeuwes, 2002; back to the location of the original fixation (where no Kopecz, 1995; Trappenberg, Dorris, Munoz, & Klein, fixation marker remained). Motion of a surrounding 2001), the presence or absence of an external signal to frame was used to induce illusory changes in the target directly specify a movement’s direction and amplitude step. Data revealed that eye movements toward the target appears to distinguish between qualitatively different were impervious to illusory motion; saccade targeting was subclasses of movement (Deubel, 1995; Erkelens & accurate despite changes in the perceived length or even Hulleman, 1993). direction of the target step. Only when observers made Past research into the effects of the M-L illusion on memory-guided return movements to the location of the saccadic
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