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Research Article  |   December 2003
Differential effects of the Müller-Lyer illusion on reflexive and voluntary saccades
Author Affiliations
  • Jason S. McCarley
    Department of Psychology Mississippi State University, Mississippi State, MS, USAhttp://www.psychology.msstate.edu/jmccarley@psychology.msstate.edu
  • Arthur F. Kramer
    Beckman Institute and Department of Psychology, University of Illinois at Urbana-Champaign, Urbana, IL, USA
  • Gregory J. DiGirolamo
    Department of Experimental Psychology, University of Cambridge, Cambridge, England
Journal of Vision December 2003, Vol.3, 9. doi:10.1167/3.11.9
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      Jason S. McCarley, Arthur F. Kramer, Gregory J. DiGirolamo; Differential effects of the Müller-Lyer illusion on reflexive and voluntary saccades. Journal of Vision 2003;3(11):9. doi: 10.1167/3.11.9.

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Abstract

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.

Introduction
Perception and Action: Separate or Common Representations?
Despite much study, researchers have yet to reach consensus as to whether and how conscious perception and the control of action are linked. As described by Franz and colleagues (Franz, Fahle, Bülthoff, & Gegenfurtner, 2001), the relationship between perception and action has been hypothesized to take three different forms. The strong separate representation model proposes that distinct neural representations underlie motor behavior and conscious visual perception. The weak separate representation model likewise posits that separate spatial representations exist in the brain, but allows for crosstalk, modulated by task demands, to occur between them. The common representation model, finally, holds that conscious perception and visuomotor control proceed from the same mental representation.1 
Research in healthy observers has attempted to distinguish among these models by measuring the effects of perceptual illusions on visually guided behavior. Though counterintuitive, the strong separate representation model has received support from findings suggesting that visually guided behavior is uninfluenced by illusions that are evident in subjective reports. Aglioti, DeSouza, and Goodale (1995), for example, found large effects of the Titchener illusion on psychophysical judgments of object size, but found no effect on grip scaling. Bridgeman and colleagues, similarly, found that perceptual judgments but not pointing responses were biased by induced motion of a target object (Bridgeman, Kirch, & Sperling, 1981). Such findings are consistent with the hypothesis that perception and action are served by distinct spatial representations, with only the former representation being susceptible to illusion. Neurophysiological and neuropsychological evidence for distinct visual cortical streams, one primarily responsible for object recognition and the other for spatial representation and visuomotor control (Goodale & Milner, 1992), provides a plausible biological basis for this dissociation. 
Other data, however, cast doubt on the strong separate representation model. Consistent with a weak separate representation model, some findings suggest that illusion affects some aspects of visually guided movement (e.g., velocity of reach and grip force) but not others (e.g., grip scaling) (Brenner & Smeets, 1996; Jackson & Shaw, 2000), or that dissociations between perception and visually guided action are mediated by task demands (Bridgeman, Peery, & Anand, 1997; Gentilucci, Chieffi, Daprati, Saetti, & Toni, 1996; Wraga, Creem, & Proffitt, 2000). Other data appear to support a common representation model. Experiments by Franz and colleagues (Franz et al., 2001; Franz, Gegenfurtner, Bülthoff, & Fahle, 2000) found similar effects of the Titchener and Müller-Lyer (M-L) illusions on perception and grasping, while a study by Lopez-Moliner et al. (2003) demonstrated equal effects of a pictorial depth illusion on perception and manual tracking. A study by Vishton, Read, Cutting and Nunez (1999) found that subjective reports and grip scaling were both susceptible to the horizontal-vertical illusion, but only when observers were induced to encode the horizontal and vertical dimensions of the stimulus relative to each other. When observers were induced to encode the size of one dimension, ignoring the orthogonal dimension, no illusion was evident in either form of response. These findings are consistent with a common representation model in which the frame of reference induced by task demands determines what effect an illusion will have on a response, independent of the response mode. 
A complicating factor in the effort to distinguish between strong separate, weak separate, and common representation models using perception-action dissociations, it should be noted, is that different illusions may arise at different points within the visual processing stream. An illusion that arises early in processing and is propagated forward might therefore affect visually guided behavior even if perception and visuomotor behavior are ultimately based on separate representations. Visuomotor behavior will be resistant to an illusion only if the representations supporting perception and action are indeed separate, and only then if the illusion has its locus beyond the point at which those representations diverge (Dyde & Milner, 2002). 
Effects of Illusions on Oculomotor Behavior
Like the studies of reaching and grasping described above, studies of oculomotor behavior have produced apparently conflicting evidence for perception/action dissociations. A seminal study by Wong and Mack (1981) suggested that visually guided saccade programming is immune to illusion. Observers were asked to saccade toward a stepped target, then to saccade from memory back to the location of the original fixation (where no fixation marker remained). Motion of a surrounding frame was used to induce illusory changes in the target step. Data revealed that eye movements toward the target were impervious to illusory motion; saccade targeting was accurate despite changes in the perceived length or even direction of the target step. Only when observers made memory-guided return movements to the location of the original saccade launch point was any influence of the illusion evident in oculomotor behavior. 
The conclusion that saccades toward a visible stimulus are resistant to illusion, however, is challenged by data from an alternative line of research. Studies beginning as early as 1897 (Delabarre) have consistently demonstrated that saccadic eye movements are susceptible to misperceptions induced by the M-L illusion; when observers are asked to saccade toward the endpoint of an M-L figure, or to saccade from one endpoint of the figure to another, eye movement landing positions are reliably biased in the direction of illusory changes in the figure’s length (Binsted & Elliott, 1999; Delabarre, 1897; Festinger, White, & Allyn, 1968; Stratton, 1906; Yarbus, 1967). This effect obtains even when the stimulus figure remains visible throughout saccade execution and preparation, and as such is not the product of memory-guided targeting. 
Visually guided saccades thus appear to be susceptible to illusion under some circumstances and resistant under others. Given large differences between the methodology of Wong and Mack (1981) and that of Delabarre (1897), Yarbus (1967), Festinger (1968) and others, in addition to the inherent differences between the induced motion and M-L illusions, it is not immediately obvious what the relevant circumstances are. One mediating factor, however, may be the difference between reflexive/reactive/exogenously triggered and voluntary/volitional/endogenously triggered saccades (Deubel, 1995; Erkelens & Hulleman, 1993; Klein, Kingstone, & Pontefract, 1992;Klein & Shore, 2000). Reflexive saccades, although they can be inhibited or modified by top-down control (Machado & Rafal, 2000a, 2000b; Rafal, Machado, Ro, & Ingle, 2000), are programmed automatically, presumably by collicular mechanisms (Rafal, Smith, Krantz, Cohen, & Brennan, 1990), in response to a transient external signal at a saccade target location. In essence, they reflect a visual grasp reflex (Rafal et al., 2000). Voluntary saccades are programmed purposively, apparently by cortical mechanisms (Henik, Rafal, & Rhodes, 1994), in the absence of a transient signal to mark the target location. Thus, while exogenous and endogenous influences may interact to shape saccadic behavior in a particular task (Godjin & Theeuwes, 2002; Kopecz, 1995; Trappenberg, Dorris, Munoz, & Klein, 2001), the presence or absence of an external signal to directly specify a movement’s direction and amplitude appears to distinguish between qualitatively different subclasses of movement (Deubel, 1995; Erkelens & Hulleman, 1993). 
Past research into the effects of the M-L illusion on saccadic eye movements has generally asked subjects to saccade from end-to-end of an M-L figure, with no transient signal to mark saccade target locations. As such, movements were necessarily voluntary. In contrast, in the experiment described by Wong and Mack (1981), visually guided eye movements were toward a stepped target—a transient signal—and as such were likely to have been largely reflexive. This suggests that the effects of a visual illusion may in fact differ for voluntary and reflexive saccades, and more specifically that reflexive but not voluntary saccades may be resistant to illusion. The goal of the present research was to explore this possibility (see also DiGirolamo, McCarley, & Kramer, 2001). The observers’ task in the current experiments was to saccade from a central fixation point to one endpoint of a Brentano-style M-L configuration or a size-matched control stimulus (Figure 1). Reflexive saccades were produced by presentation of a transient go-signal at the saccade target location. Voluntary saccades were produced by use of a spoken go-signal. Experiment 1 revealed that both reflexive and voluntary saccades are susceptible to the M-L illusion, but to different degrees; while reflexive saccades are modestly affected, voluntary saccades show effects similar to those of subjective judgments. Experiment 2 demonstrated that the modest effects of the M-L illusion on reflexive movements were not the result of endogenous or anticipatory saccade planning, and were not modulated by the strength of the saccade go-signal. In total, results suggest that reflexive saccade programming occurs within a representation that is a weighted combination of retinotopic and higher level spatial frames, whereas voluntary saccade programming occurs within a reference frame more similar to that of visual consciousness. 
Figure 1
 
Illustration of the stimuli of Experiment 1. Stimulus dimensions were chosen such that wings-in and wings-out segments of the M-L stimulus were approximately equal in apparent length. Stimuli for Experiment 2 were identical to the M-L figures of Experiment 1, except that wings-in and wings-out segments were matched in physical length and differed in apparent length.
Figure 1
 
Illustration of the stimuli of Experiment 1. Stimulus dimensions were chosen such that wings-in and wings-out segments of the M-L stimulus were approximately equal in apparent length. Stimuli for Experiment 2 were identical to the M-L figures of Experiment 1, except that wings-in and wings-out segments were matched in physical length and differed in apparent length.
Experiment 1
The aim of Experiment 1 was to examine the effects of the M-L illusion on voluntary and reflexive saccades, and to compare the magnitude of these effects to that of the illusion on conscious perception. Toward that end, observers were asked to execute voluntary and reflexive saccades from the center to the endpoint of an M-L configuration or a size-matched control figure. Stimulus parameters were chosen so that saccade targets within M-L figures appeared to be equidistant from the movement launch point, when in reality one target was 1.3° farther than the other. By comparing saccades toward targets of different physical distances, therefore, it was possible to assess the degree to which movement amplitudes deviated from perceived target distances. 
Method
Observers
Observers were eight young adults recruited from the community of the University of Illinois at Urbana-Champaign. All observers had normal or corrected-to-normal vision. Observers were paid for participating. One observer, whose anticipatory movement rates were unacceptably high (53% in the reflexive saccade condition), was replaced. This did not change the pattern of results described below. 
Apparatus
Stimuli were presented on a 21″ monitor at a resolution of 800 × 600 pixels and a refresh rate of 85 Hz. Eye movements were recorded with an Eyelink eye tracker (SR Research Ltd.) with a temporal resolution of 250 Hz and a spatial resolution of 0.2°. An eye movement was classified as a saccade when its distance exceeded 0.2° and its velocity reached 30°/s, or when its length exceeded 0.2° and its acceleration had reached 9500°/s2. Subjects viewed displays from a distance of 91 cm, with viewing distance controlled by a chin rest. 
Stimuli
Stimuli were horizontally oriented Brentano-style M-L figures and size-matched control figures with vertical lines as wings. The middle wing of each stimulus divided the horizontal shaft into two segments of different lengths, one of 8.9° and the other of 7.6°. Within M-L configurations, the physically longer segment was always wings-in (illusively short), and the physically shorter segment always wings-out (illusively long). Stimulus dimensions were chosen on the basis of a psychophysical pilot experiment (n = 2, method of constant stimuli) to produce the perception that wings-in and wings-out segments of the illusion were of approximately equal extent (DiGirolamo, McCarley, & Kramer, 2001). Stimulus orientation (long segment on the left side vs. long segment on the right side) was counterbalanced within observers. The line segments that formed the wings of the stimuli were 2.1° in length. In the M-L figures, wings were angled at 30° relative to the shaft of the figure. In the control figures, they were at right angles relative to the shaft of the figure. Figures were drawn in gray (19.6 cd/m2) against a black (3.6 cd/m2) background. 
Procedure
Procedure is illustrated in Figure 2. The observer began each trial by pressing a key while fixating a filled gray circle (0.12° in diameter) in the center of an otherwise empty display. The stimulus figure for that trial appeared immediately thereafter, with the vertex of the middle wing centered on the fixation mark. A go-signal followed after a delay of 506 ms. On reflexive saccade trials, the go-signal was a filled white circle (90.7 cd/m2, 0.15° diameter), which appeared for 59 ms at one end of the stimulus figure. The observer’s task was to saccade to the end of the horizontal shaft at which the go-signal appeared. On voluntary saccade trials, the go-signal was a spoken word “left” or “right,” presented through the experimental computer’s speakers. The observer’s task was to saccade to the end of the horizontal shaft specified by the auditory signal. Reflexive and voluntary saccade trials were run in two separate blocks, with order of blocks counterbalanced across observers. Each block was composed of 20 randomly chosen practice trials, then 100 randomly ordered experimental trials for each combination of saccade target distance (8.9° vs. 7.6°) and stimulus configuration (M-L or control). 
Figure 2
 
Illustration of the procedure of Experiment 1. In reflexive saccade conditions, observers made eye movements toward a flashed go-signal. In voluntary saccade conditions, observers made eye movements toward the end of the stimulus figure specified by a spoken go-signal.
Figure 2
 
Illustration of the procedure of Experiment 1. In reflexive saccade conditions, observers made eye movements toward a flashed go-signal. In voluntary saccade conditions, observers made eye movements toward the end of the stimulus figure specified by a spoken go-signal.
Results
Data Loss, Error Rates, and Saccade Latencies
Trials on which gaze shifted prior to onset of the go-signal were excluded from analysis, as were trials on which saccade latency was less than 50 ms or greater than 750 ms. This resulted in the loss of 5% of all trials, approximately equivalent across reflexive saccade and voluntary saccade conditions. Also excluded from analysis were trials on which the saccade was not in the appropriate direction. As expected, saccade direction errors were less frequent for reflexive than for voluntary saccade trials (3% vs. 22%), and saccade latencies were shorter (182 ms vs. 365 ms). Neither error rates nor latencies varied as a function of saccade target distance or stimulus configuration. 
Saccade Amplitudes
Figure 3 presents mean horizontal amplitudes for reflexive (top) and voluntary (bottom) saccades. Note that within M-L figures, line segments of different physical lengths were of approximately the same apparent length. Thus, to the extent that saccade amplitudes were modulated by the M-L illusion, the difference between saccade amplitudes for near and far targets should have been smaller than in the control conditions. 
Figure 3
 
Reflexive (top) and voluntary (bottom) saccade amplitudes for Experiment 1.
Figure 3
 
Reflexive (top) and voluntary (bottom) saccade amplitudes for Experiment 1.
For omnibus analysis, amplitudes were submitted to a 2 × 2 × 2 ANOVA with saccade type (reflexive vs. voluntary), saccade target distance (7.6° vs. 8.9°), and stimulus configuration (M-L vs. control) as within-subjects factors. As can be seen in Figure 3, amplitudes for both reflexive and voluntary saccades were modulated by the M-L illusion; for both classes of movement, the difference in amplitude for near and far targets was smaller within the M-L configurations than within control figures. Effects of the illusion, however, were greater for voluntary than reflexive movements, with reflexive saccade amplitudes being biased by a mean of .29°, SE = .03, in the direction of the perceived changes in target distance (compared to saccade amplitudes within control figures) and voluntary saccade amplitudes being biased by a mean of .77°, SE = .08. Voluntary amplitudes were in fact slightly larger for targets that were 7.6° distant than for targets that were 8.9° distant. This finding may indicate either that perceived target distances within the M-L stimuli were not perfectly matched, or that the effects of the illusion were stronger on voluntary movements than on perception. In either case, it is clear that reflexive saccade programming was based on a weighted average of actual and perceived target distances, whereas voluntary saccade programming was dominated by perceived target distance. Statistical analysis confirmed these observations. A reliable main effect of saccade target distance [F(1, 7) = 120.157, p < .001, MSE = .057] indicated that on average saccade amplitudes were larger for more distant targets. This effect was qualified, however, by a two-way interaction of target distance by stimulus configuration [F(1, 7) = 130.846, p < .001, MSE = .033], confirming that saccade amplitudes were reliably modulated by perceived target distance, and by a three-way interaction of saccade type by target distance by stimulus configuration [F(1, 7) = 34.830, p = .001, MSE = .027], confirming that the effects of perceived target distance were larger for voluntary than for reflexive saccades. Nonetheless, two-way ANOVAs with stimulus configuration and target distance as factors produced reliable interactions for both voluntary and reflexive movements [F(1, 7) = 84.425, p < .001, MSE = .056 and F(1, 7) = 127.746, p < .001, MSE = .049], confirming that both classes of saccade were susceptible to the illusion. Post hoc t tests revealed that reflexive saccade amplitudes were shorter for near targets than for far targets within M-L figures [t(7) = −13.424, p < .001, SE = .048], whereas voluntary saccade amplitudes were shorter for far targets [t(7) = 2.674, p = .032, SE = .145]. 
Control Analyses
The results described above suggest that both reflexive and voluntary saccades are modulated by changes in perceived target distance produced by the M-L illusion, but that the effects of the illusion are larger for voluntary movements. An additional analysis was conducted to rule out alternative interpretations of these data. One possibility is that these results are not evidence of a functional difference between voluntary and reflexive saccades, but were produced by differences in saccade latency. As noted, voluntary latencies were considerably longer than reflexive latencies. Results might therefore be taken to indicate not that reflexive saccades are less susceptible to illusion than voluntary saccades, but that short latency movements are less susceptible than long latency movements. A related possibility is that the seeming effect of the M-L illusion on reflexive saccade amplitudes might actually have resulted from voluntary movements occasionally produced in response to a visual go-signal. The distribution of saccade amplitudes produced in response to a visual go-signals, in other words, might have comprised a mixture of voluntary saccade amplitudes showing a relatively large effect of the illusion and reflexive saccade amplitudes showing no effect. This hypothesis, like the differential-latency hypothesis noted above, suggests that the effects of stimulus configuration on reflexive saccade amplitude should be larger for long-latency than for short-latency saccades. 
An additional concern is that differential effects of wings-in and wings-out patterns on movement amplitudes might reflect a center-of-gravity tendency in saccade targeting (Coren & Hoenig, 1972; Findlay, 1982; He & Kowler, 1989), rather than the influence of an illusory percept, per se. Data would then indicate that reflexive saccades are more resistant to the center-of-gravity tendency than are voluntary saccades, but would not speak to the effects of illusion on oculomotor programming. Existing data cast doubt on this possibility; although center-of-gravity targeting appears to be the default strategy in eye movement programming, observers can easily target non-central positions within a shape when instructed to do so (He & Kowler, 1991). It thus seems unlikely that a center-of-gravity tendency would have strongly influenced performance in the current experiment, where observers were instructed to target a specific and well-defined target position (the end of the horizontal stimulus shaft). Nonetheless, it is useful to seek additional evidence against the center-of-gravity hypothesis. Evidence indicates, notably, that center-of-gravity effects are modulated by saccade latency. More specifically, center-of-gravity effects are larger for short-latency saccades than for long-latency movements (Coëffé & O’Regan, 1987; Deubel, 1996; Ottes, van Gisbergen, & Eggermont, 1985). The center-of-gravity account of the current data therefore predicts that the M-L illusion should affect short-latency movements more strongly than long-latency movements. 
To examine these various possibilities, saccade amplitude data within each cell of the design were subjected to a median split on the basis of movement latency, and were reanalyzed with a 2 × 2 × 2 × 2 ANOVA that included saccade latency (below median vs. above median), saccade type, stimulus configuration, and saccade target distance as within-subjects variables. Consistent with the possibility that the influence of the illusion might vary with saccade latency, the analysis revealed marginally reliable interactions of saccade latency by saccade type by target distance [F(1, 7) = 5.196, p = .057, MSE = .067] and of saccade latency by saccade type by stimulus figure by target distance [F(1, 7) = 5.207, p = .056, MSE = .105]. Upon examination, however, these effects did not contradict any of the conclusions drawn above. For closer analysis, voluntary and reflexive movements were submitted to separate three-way ANOVAs with saccade latency, stimulus configuration, and target distance as factors. A reliable three-way interaction indicated that the effect of the M-L illusion on voluntary saccades was stronger for long-latency than for short-latency movements [F(1,7) = 7.464, p = .029, MSE = .119] with mean magnitude of the bias produced by M-L wings increasing from .58°, SE = .12, for saccades of below-median latency, to 1.05°, SE = .10, for saccades of above-median latency. Conversely, a nonsignificant three-way interaction within the reflexive saccade data [F(1, 7) = .205, p = .665, MSE = .005] suggested that latency did little to modulate the influence of the M-L illusion on externally triggered movements, with mean effect magnitude being .33°, SE = .08, for saccades of below-median latency, and .24°, SE = .05, for saccades of above-median latency. Thus, contrary to a center-of-gravity explanation, the influence of the M-L illusion did not tend to decrease as saccade latencies increased. Contrary to the possibility that differences in mean latency might account for the differential susceptibility of reflexive and voluntary movements to the M-L illusion, or that a small number of voluntary movements might have contaminated the reflexive saccade data, latency did nothing to modulate the influence of the illusion on reflexive saccades. 
Discussion
The results of Experiment 1 indicate that voluntary saccades are as susceptible to the M-L illusion as perceptual judgments. Reflexive saccades show far smaller effects. These results do not appear to have been produced by a confound of saccade type with saccade latency, or by voluntary movements contaminating the reflexive saccade distributions, or by a center-of-gravity effect. 
In total, results are consistent with the hypothesis that voluntary and reflexive saccades are differentially susceptible to illusive changes in target distance. Contrary to expectations, however, reflexive movements in Experiment 1 were not fully resistant to illusion. A second experiment was conducted to further investigate the effects of the M-L illusion on reflexive saccade programming. 
Experiment 2
Experiment 1 revealed a modest but reliable influence of the M-L illusion on reflexive saccade programming. The first aim of Experiment 2 was to assess the role of top-down or endogenous factors in this effect. It is well known that observers can pre-plan saccades, anticipating and preparing for upcoming movements (e.g., Carpenter & Williams, 1995; Dorris & Munoz, 1998). Such endogenous saccade planning might be expected to have either of two contrary effects. The results of Experiment 1 indicate that the influence of the M-L illusion is larger for endogenously generated than for exogenously generated movements. This suggests that anticipatory movement planning might exacerbate the effects of the illusion on reflexive saccades. Alternatively, it is possible that endogenous saccade preparation might make saccade targeting more accurate, reducing the illusion’s influence much as foreknowledge of target position attenuates the center-of-gravity effect in saccade programming (Coëffé & O’Regan, 1987). Indeed, a failure to find that endogenous preparation reduced the M-L configurations’ effects would provide additional evidence against a center-of-gravity account of the above data. A third potential outcome, of course, is that the effect of the illusion on reflexive saccade programming is immune to top-down influences and thus unaffected by anticipatory saccade preparation. To test these possibilities, Experiment 2 employed a reflexive saccade task similar to that of Experiment 1, but manipulated target location probabilities to induce observers to prepare movements prior to onset of the go-signal. 
The second goal of Experiment 2 was to examine the role of stimulus-driven or exogenous factors in modulating the influence of the illusion on reflexive saccades. Past research has indicated that the strength or contrast of a flashed target has little effect on saccade accuracy (Deubel, 1996). This suggests that in the current reflexive saccade task, contrast of the go-signal should do little to modulate the effects of the M-L illusion. 
Method
Observers
Observers were 10 young adults recruited from the community of the University of Illinois at Urbana-Champaign. All observers had normal or corrected-to-normal vision. Observers were paid for participating. 
Apparatus
Apparatus were identical to those of Experiment 1. 
Stimuli
Stimuli were similar to those of Experiment 1, except for the following exceptions made to simplify data analysis. First, all stimuli were M-L configurations; vertical-wing control stimuli like those of Experiment 1 were not used. Second, wings-in and wings-out segments were now of the same physical length (8.25°), and thus differed in apparent length. 
Procedure
Procedure was similar to that of Experiment 1, with the following exceptions. First, all trials used visual go-signals. Thus, only reflexive saccades were studied. Second, the luminance transients used as go-signals were now of two different intensities. On weak signal trials, the go-signal was relatively dim (49.2 cd/m2). On strong signal trials, the go-signal was more intense (90.7 cd/m2). Third, the go-signal was no longer equiprobable on both sides of the display. Rather, the signal appeared on one side of the display on 80% of all trials, and on the opposite side on only the remaining 20% of trials. The assignment of left and right sides to high- and low-probability conditions was counterbalanced across observers. Each observer completed 10 randomly chosen practice trials, then 400 randomly ordered experimental trials, 200 with a high-contrast go-signal and 200 with a low-contrast go-signal. 
Results
Data Loss and Error Rates
Data from trials on which gaze shifted prior to onset of the go-signal were discarded, as were trials on which saccade latency was <50 ms or >750 ms. This resulted in the loss of 6% of all data. Saccade direction errors occurred on <1% of all trials. Data from these trials were also excluded from the analyses reported below. 
Saccade Latencies
To draw conclusions about the effects of cue contrast and location probability on saccade amplitudes, it is first necessary to examine saccade latency data for independent evidence that manipulations of these factors were effective. Mean saccade latencies are presented in Figure 4 (top). For statistical analysis, latency data were submitted to a within-subjects ANOVA with signal strength (weak vs. strong), target side (high probability vs. low probability), and perceived target distance (near vs. far) as factors. Of foremost importance, a main effect of signal contrast indicated that latencies were shorter for high-contrast than for low-contrast signals [F(1, 9) = 71.216, p < .001, MSE = 302.133], and a main effect of target side revealed that latencies were shorter for targets on the high-probability side than for targets on the low-probability side [F(1, 9) = 5.510, p = .043, MSE = 849.633]. Data thus confirmed that high-contrast signals served as more salient cues for reflexive saccade generation and that observers anticipated and prepared for saccades toward the high-probability side of the display. Manipulations of signal contrast and of location probability, in other words, were effective. Additional effects in the saccade latency data included an unexpected reliable main effect of perceived target distance [F(1, 9) = 9.309, p = .014, MSE = 124.089], indicating that saccade latencies were shorter for illusively far targets than for illusively near targets, a reliable interaction of signal strength by target side [F(1, 9) = 5.103, p = .050, MSE = 125.117], indicating that the benefits of endogenous cuing were larger for saccades toward low-contrast targets, and a marginally reliable interaction of exogenous signal strength by perceived target distance [F(1, 9) = 4.482, p = .063, MSE = 357.439], suggesting that effects of perceived distance on saccade latency were significant only with weak exogenous signals. 
Figure 4
 
Saccade latencies (top) and amplitudes (bottom) for Experiment 2.
Figure 4
 
Saccade latencies (top) and amplitudes (bottom) for Experiment 2.
Saccade Amplitudes
Figure 4 (bottom) presents mean horizontal saccade amplitudes. Note that because saccade target distances were physically equivalent in the current experiment, effects of the illusion on saccade programming are evident here as a difference in movement amplitude for wings-in and wings-out targets. Statistical analysis was identical to that for latency data. As in Experiment 1, saccade amplitudes were reliably biased by illusory changes in target distance, with saccade amplitudes being larger for targets which appeared to be farther from the saccade launch point [F(1, 9) = 34.486, p < .001, MSE = .112]. This effect, however, was independent of both exogenous and endogenous manipulations; no interactions approached significance (all Fs < 1). 
Discussion
Experiment 2 was designed to answer two questions. First, is the effect of the M-L illusion on reflexive saccades modulated by anticipatory or endogenous planning of eye movements? Second, is the influence of the M-L illusion on reflexive saccades modulated by the strength of the visual go-signal? In both cases, the answer is no. Effects of the illusion on reflexive saccade amplitudes were similar whether movements were made toward a likely or an unlikely target location. Similarly, the illusion was of the same magnitude for low-contrast target signals as for high-contrast signals. 
General Discussion
Earlier findings have suggested that reflexive and voluntary saccades might be differentially susceptible to visual illusions. The present results indicate that, at least in the case of the M-L illusion, this is so. Voluntary saccades show effects of the illusion similar in magnitude to those that are evident in subjective perceptual judgments. Reflexive saccades show effects of the illusion that, while reliable, are far smaller. The results of Experiment 2 indicate that the influence of the illusion on reflexive saccades is not modulated by endogenous saccade preparation or by strength of the transient signal marking the target location. 
How should these results be explained? As noted in the Introduction, past research has indicated that voluntary eye movements are programmed and initiated purposively by cortical mechanisms (Henik et al., 1994), whereas reflexive saccades are programmed automatically within the superior colliculus in response to transient visual signals (Rafal et al., 1990). The present data suggest that cortical planning of voluntary saccades occurs within a representation whose frame of reference is — in at least some aspects — similar or identical to that of conscious visual perception. This conclusion is compatible with a common representation model, or alternatively, with a separate representation model in which the M-L illusion arises prior to the bifurcation of the perception/action representations. Further research, employing different forms of visual illusion, will be necessary to distinguish these possibilities. 
In contrast, the current results support a weak separate representation model of reflexive saccade control. More specifically, findings suggest that reflexive saccade programming occurs within a representation that is a weighted average of a retinotopic map and a higher level spatial representation more closely matched to conscious perception. One possibility is that the integration of spatial frames is effected by the convergence of feedforward retinal signals and feedback cortical signals on the superior colliculus; the colliculus is known to receive input directly from the retina, and from the striate and extrastriate cortex (for a review, see Deubel, 1996). The results of Experiment 2 suggest that the weighting of the retinotopic and higher level information within the integrated representation is fixed, and not modulated either by top-down saccade preparation or by bottom-up strength of a retinotopically coded visual signal. Additional work, again using different forms of visual illusion, will be necessary to determine whether or not the higher level representation that biases reflexive saccade programming is identical to the representation that underlies conscious experience. 
Finally, it is useful to consider the apparent contradiction between the current results and those of Wong and Mack (1981), who found no effect of an induced motion illusion on reflexive saccade targeting. One potential explanation for this discrepancy lies in the different temporal characteristics of the M-L and induced motion illusions. The M-L illusion is extended in time, persisting for as long as the stimulus figure remains visible (ignoring gradual declines of the illusion that may occur with very long periods of extended viewing, e.g., Festinger et al., 1968). In the current experiments, the illusive percept, therefore, began each trial well before presentation of the saccade go-signal, and remained throughout saccade preparation and execution. In contrast, the induced motion illusion employed by Wong and Mack (1981) would have affected the perceived movement of the stepped target stimulus, but would not have existed either before that movement began or after it had been completed. The illusion may therefore have existed either too briefly or at the wrong moment to bias reflexive saccade programming. An alternative possibility, as noted in the preceding paragraph, is that the higher level spatial representation involved in reflexive saccade programming is not perfectly matched to a subjective frame of reference. The signals that bias reflexive saccade programming away from a perfectly retinotopic spatial frame, for example, might arise from a cortical representation that precedes the locus of the induced motion illusion. Once more, further research will be needed to test these possibilities. 
Acknowledgments
Portions of this work were completed while J.S.M. was a postdoctoral Fellow at the Beckman Institute, University of Illinois at Urbana-Champaign. Thanks to Eric Vidoni and Russell Smith for assistance with data collection. Commercial relationships: none. 
Footnote
Footnotes
1 This work is concerned primarily with visually guided movements. It is widely acknowledged that memory-guided movements share a mental representation with conscious perception (e.g., Bridgeman, 1999; Gentilucci et al., 1996; Wong & Mack, 1981).
References
Aglioti, S. DeSouza, J. F. X. Goodale, M. A. (1995). Size-contrast illusions deceive the eye but not the hand. Current Biology, 5, 679–685. [PubMed] [CrossRef] [PubMed]
Binsted, G. Elliott, D. (1999). The Müller-Lyer illusion as a perturbation to the saccadic system. Human Movement Science, 18.(1), 103–117. [CrossRef]
Brenner, E. Smeets, J. B. J. (1996). Size illusion influences how we lift but not how we grasp an object. Experimental Brain Research, 111, 473–476. [PubMed] [CrossRef] [PubMed]
Bridgeman, B. (1999). Separate representations of visual space for perception and visually guided behavior. In Aschersleben, G. Bachmann, T. Müsseler, J. (Eds.), Cognitive Contributions to the perception of spatial and temporal events (pp. 3–13). Amsterdam: Elsevier.
Bridgeman, B. Kirch, M. Sperling, A. (1981). Segregation of cognitive and motor aspects of visual function using induced motion. Perception & Psychophysics, 29, 336–342. [CrossRef] [PubMed]
Bridgeman, B. Peery, S. Anand, S. (1997). Interaction of cognitive and sensorimotor maps of visual space. Perception and Psychophysics, 59, 456–469. [PubMed] [CrossRef] [PubMed]
Carpenter, R. H. Williams, M. L. (1995). Neural computation of log likelihood in control of saccadic eye movements. Nature, 377, 59–62. [PubMed] [CrossRef] [PubMed]
Coëffé, C. O’Regan, J. K. (1987). Reducing the influence of non-target stimuli on saccade accuracy: Predictability and latency effects. Vision Research, 27, 227–240. [PubMed] [CrossRef] [PubMed]
Coren, S. Hoenig, P. (1972). Effect of non-target stimuli on the length of voluntary saccades. Perceptual and Motor Skills, 34. 499–508. [PubMed] [CrossRef] [PubMed]
Delabarre, E. B. (1897). A method of recording eye movements. American Journal of Psychology, 9, 572–574. [CrossRef]
Deubel, H. (1995). Separate adaptive mechanisms for the control of reactive and volitional saccadic eye movements. Vision Research, 35, 3529–3540. [PubMed] [CrossRef] [PubMed]
Deubel, H. Prinz, W. Bridgeman, B. (1996). Visual processing and cognitive factors in the generation of saccadic eye movements. Handbook of perception and action: Volume 1. Perception (Vol. 1, pp. 143–189). London: Academic Press.
DiGirolamo, G. J. McCarley, J. S. Kramer, A. F. (2001). Endogenously and exogenously driven eye movements to illusory locations. Poster presented at the 42nd Annual Meeting of the Psychonomic Society, Orlando, FL.
Dorris, M. C. Munoz, D. P. (1998). Saccadic probability influences motor preparation signals and time to saccadic initiation. Journal of Neuroscience, 18, 7015–7026. [PubMed] [PubMed]
Dyde, R. T. Milner, D. A. (2002). Two illusions of perceived orientation: One fools all of the people some of the time; the other fools all of the people all of the time. Experimental Brain Research, 144, 518–527. [PubMed] [CrossRef] [PubMed]
Erkelens, C. J. Hulleman, J. (1993). Selective adaptation of internally triggered saccades to visual targets. Experimental Brain Research, 93, 157–164. [PubMed] [CrossRef] [PubMed]
Festinger, L. White, C. W. Allyn, M. R. (1968). Eye movements and decrement in the Müller-Lyer illusion. Perception & Psychophysics, 3, 376–382.
Findlay, J. M. (1982). Global visual processing for saccadic eye movements. Vision Research, 22, 1033–1045. [PubMed] [CrossRef] [PubMed]
Franz, V. H. Fahle, M. Bülthoff, H. H. Gegenfurtner, K. R. (2001). Effects of visual illusions on grasping. Journal of Experimental Psychology: Human Perception & Performance, 27, 1124–1144. [PubMed] [CrossRef]
Franz, V. H. Gegenfurtner, K. R. Bülthoff, H. H. Fahle, M. (2000). Grasping visual illusions: No evidence for a dissociation between perception and action. Psychological Science, 11. [PubMed]
Gentilucci, M. Chieffi, S. Daprati, E. Saetti, M. C. Toni, I. (1996). Visual illusion and action. Neuropsychologia, 34, 369–376. [PubMed] [CrossRef] [PubMed]
Goodale, M. A. Milner, A. D. (1992). Separate visual pathways for perception and action. Trends in Neuroscience, 15, 20–25. [PubMed] [CrossRef]
Godjin, R. Theeuwes, J. (2002). Programming of endogenous and exogenous saccades: Evidence for a competitive integration model. Journal of Experimental Psychology: Human Perception and Performance, 28, 1039–1054. [[Pubmed] [CrossRef] [PubMed]
He, P. Kowler, E. (1989). The role of location probability in the programming of saccades: Implications for “center-of-gravity” tendencies. Vision Research, 9, 1165–1181. [PubMed] [CrossRef]
He, P. Kowler, E. (1991). Saccadic localization of eccentric forms. Journal of the Optical Society of America, 8, 440–449. [PubMed] [CrossRef] [PubMed]
Henik, A. Rafal, R. Rhodes, D. (1994). Endogenously generated and visually guided saccades after lesions of the human frontal eye fields. Journal of Cognitive Neuroscience, 6, 400–411. [CrossRef] [PubMed]
Jackson, S. R. Shaw, A. (2000). The Ponzo illusion affects grip-force but not grip-aperture scaling during prehension movements. Journal of Experimental Psychology: Human Perception & Performance, 26, 418–423. [PubMed] [CrossRef]
Klein, R. Kingstone, A. Pontefract, A. (1992). Orienting of visual attention. In Rayner, K. (Ed.), Eye movements and visual cognition: Scene perception and reading (pp. 46–65). New York: Springer-Verlag.
Klein, R. Shore, D. I. (2000). Relations among modes of visual orienting. In Monsell, S. Driver, J. (Eds.), Attention & performance XVIII (pp. 195–208). Cambridge, MA: MIT Press.
Kopecz, K. (1995). Saccadic reaction times in gap/overlap paradigms: A model based on integration of intentional and visual information on neural, dynamic fields. Vision Research, 35, 2911–2925. [PubMed] [CrossRef] [PubMed]
López-Moliner, J. Smeets, J. B. J. Brenner, E. (2003). Comparing the sensitivity of manual pursuit and perceptual judgments to pictorial depth effects. Psychological Science, 14, 232–236. [PubMed] [CrossRef] [PubMed]
Machado, L. Rafal, R. (2000a). Control of eye movement reflexes. Experimental Brain Research, 135, 73–80. [ [PubMed] [CrossRef]
Machado, L. Rafal, R. (2000b). Strategic control over saccadic eye movements: Studies of the fixation offset effect. Perception and Psychophysics, 62, 1236–1242. [PubMed] [CrossRef]
Ottes, F. P. van Gisbergen, J. A. M. Eggermont, J. J. (1985). Latency dependence of colour-based target versus non-target discrimination by the saccadic system. Vision Research, 24, 826–849. [PubMed]
Rafal, R. Machado, L. Ro, T. Ingle, H. (2000). Looking forward to looking: Saccade preparation and control of the visual grasp reflex. In Monsell, S. Driver, J. (Eds.), Attention & performance XVIII (pp. 155–174). Cambridge, MA: MIT Press.
Rafal, R. Smith, J. Krantz, J. Cohen, A. Brennan, C. W. (1990). Extrageniculate vision in hemianopic humans: Saccade inhibition by signals in the blind field. Science, 250, 118–121. [PubMed] [CrossRef] [PubMed]
Stratton, G. M. (1906). Symmetry, linear illusions, and the movements of the eye. Psychological Review, 13, 82–96. [CrossRef]
Trappenberg, T. P. Dorris, M. C. Munoz, D. P. Klein, R. M. (2001). A model of saccade inititiation based on the competitive integration of exogenous and endogenous signals in the superior colliculus. Journal of Cognitive Neuroscience, 13, 256–271. [PubMed] [CrossRef] [PubMed]
Vishton, P. M. Rea, J. G. Cutting, J. E. Nunez, L. N. (1999). Comparing effects of the horizontal-vertical illusion on grip scaling and judgment: Relative vs. absolute, not perception vs. action. Journal of Experimental Psychology: Human Perception & Performance, 25, 1659–1672. [PubMed] [CrossRef]
Wong, E. Mack, A. (1981). Saccadic programming and perceived location. Acta Psychologica, 48, 121–131. [PubMed] [CrossRef]
Wraga, M. Creem, S. H. Proffitt, D. R. (2000). Perception-action dissociations of a walkable Müller-Lyer configuration. Psychological Science, 11, 239–243. [PubMed] [CrossRef] [PubMed]
Yarbus, A. L. (1967). Eye movements and vision (B. Haigh, Trans.). New York: Plenum Press.
Figure 1
 
Illustration of the stimuli of Experiment 1. Stimulus dimensions were chosen such that wings-in and wings-out segments of the M-L stimulus were approximately equal in apparent length. Stimuli for Experiment 2 were identical to the M-L figures of Experiment 1, except that wings-in and wings-out segments were matched in physical length and differed in apparent length.
Figure 1
 
Illustration of the stimuli of Experiment 1. Stimulus dimensions were chosen such that wings-in and wings-out segments of the M-L stimulus were approximately equal in apparent length. Stimuli for Experiment 2 were identical to the M-L figures of Experiment 1, except that wings-in and wings-out segments were matched in physical length and differed in apparent length.
Figure 2
 
Illustration of the procedure of Experiment 1. In reflexive saccade conditions, observers made eye movements toward a flashed go-signal. In voluntary saccade conditions, observers made eye movements toward the end of the stimulus figure specified by a spoken go-signal.
Figure 2
 
Illustration of the procedure of Experiment 1. In reflexive saccade conditions, observers made eye movements toward a flashed go-signal. In voluntary saccade conditions, observers made eye movements toward the end of the stimulus figure specified by a spoken go-signal.
Figure 3
 
Reflexive (top) and voluntary (bottom) saccade amplitudes for Experiment 1.
Figure 3
 
Reflexive (top) and voluntary (bottom) saccade amplitudes for Experiment 1.
Figure 4
 
Saccade latencies (top) and amplitudes (bottom) for Experiment 2.
Figure 4
 
Saccade latencies (top) and amplitudes (bottom) for Experiment 2.
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