Perception, Action, and Roelofs Effect: A Mere Illusion of Dissociation

A prominent and influential hypothesis of vision suggests the existence of two separate visual systems within the brain, one creating our perception of the world and another guiding our actions within it. The induced Roelofs effect has been described as providing strong evidence for this perception/action dissociation: When a small visual target is surrounded by a large frame positioned so that the frame's center is offset from the observer's midline, the perceived location of the target is shifted in the direction opposite the frame's offset. In spite of this perceptual mislocalization, however, the observer can accurately guide movements to the target location. Thus, perception is prone to the illusion while actions seem immune. Here we demonstrate that the Roelofs illusion is caused by a frame-induced transient distortion of the observer's apparent midline. We further demonstrate that actions guided to targets within this same distorted egocentric reference frame are fully expected to be accurate, since the errors of target localization will exactly cancel the errors of motor guidance. These findings provide a mechanistic explanation for the various perceptual and motor effects of the induced Roelofs illusion without requiring the existence of separate neural systems for perception and action. Given this, the behavioral dissociation that accompanies the Roelofs effect cannot be considered evidence of a dissociation of perception and action. This indicates a general need to re-evaluate the broad class of evidence purported to support this hypothesized dissociation

Inverse Roelofs Effect for Remembered Space

<div><p>(A) An inverse Roelofs effect for immediate (solid line) and delayed (dashed line) sensorimotor responses toward remembered reference array locations, with a significant main effect of frame offset and a significant frame × delay interaction. When tested separately, the main effect of frame offset was significant for both immediate and delayed responses (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020364#pbio-0020364-t001" target="_blank">Table 1</a>; see also <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020364#sg003" target="_blank">Figure S3</a>).</p> <p>(B) The inverse Roelofs effect for remembered space can be used to predict the pattern of the Roelofs effect for targets presented within an offset frame. For example, a frame offset to the right would cause the remembered comparison array to be mislocalized as being shifted approximately 1° to the right (from <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020364#pbio-0020364-g003" target="_blank">Figure 3</a>A, solid line); a target presented at the center location of the comparison array (i.e., at the objective midline) would therefore be reported to lie approximately 1° to the left of the remembered center location. Computed in this way for all target and frame locations, the predicted Roelofs effect (gray lines and data points) closely matched the measured Roelofs effect for the perceptual judgment (black lines and data points, from <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020364#pbio-0020364-g002" target="_blank">Figure 2</a>A, solid line), with a predicted effect size (1.61°) that did not significantly differ from the measured effect (1.47° ± 0.32°; t[9] = 0.44, n.s.). Furthermore, the measured Roelofs effect did not differ from the predicted effect for any individual frame position (left frame: t[9] = 0.39, n.s.; center frame: t[9] = 0.01, n.s.; right frame: t[9] = 0.36, n.s.).</p></div

Perceptual and Sensorimotor Roelofs Effects

<div><p>(A) Effect of frame location on immediate (solid line) and delayed (dashed line) perceptual judgments of target location, with a significant main effect of frame offset but no frame × delay interaction (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020364#pbio-0020364-t001" target="_blank">Table 1</a>); error bars represent the standard error of the mean localization errors for each subject. (See also <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020364#sg001" target="_blank">Figure S1</a>A for a time line of the task events, <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020364#sg001" target="_blank">Figure S1</a>B for a plot of the Roelofs effect for each of the individual target locations, and <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020364#sg001" target="_blank">Figure S1</a>C for plots of the Roelofs effect within individual subjects.)</p> <p>(B) Effect of frame offset on immediate (solid line) and delayed (dashed line) saccadic eye movements, with a significant main effect of frame offset and a significant frame × delay interaction. When tested separately, the main effect of frame offset was not significant for immediate responses, but was significant for delayed responses (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020364#pbio-0020364-t001" target="_blank">Table 1</a>; see also <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020364#sg002" target="_blank">Figure S2</a>).</p></div

The Biased-Midline Hypothesis

<div><p>(A) A depiction of the manner in which a target (red circle), located directly in front of the subject, would be perceived as being a small distance to the right of the subject's apparent midline (dotted line), which has itself been biased to the left in the presence of the left-shifted frame.</p> <p>(B) An immediate open-loop sensorimotor response (pointing movement, as shown here, or saccade begun immediately after the target and frame are extinguished) would be accurate if the goal of the movement were encoded in the same distorted reference frame (that is, a small distance to the right of the distorted apparent midline).</p> <p>(C) With the frame and target extinguished during an imposed delay, the apparent midline would drift back to veridical (gray arrows), dragging the remembered location of the target (gray circle) with it. A subsequent sensorimotor response aimed at the remembered target (located a small distance to the right of the now-veridical apparent midline) would result in a delayed sensorimotor Roelofs effect.</p></div

Inverse Roelofs Effect for the Apparent Midline

<p>An inverse Roelofs effect for immediate (solid line) and delayed (dashed line) sensorimotor responses toward the apparent midline, with a significant main effect of frame offset and a significant frame × delay interaction. When tested separately, the main effect of frame offset was significant for both immediate and delayed responses (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020364#pbio-0020364-t001" target="_blank">Table 1</a>; see also <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020364#sg004" target="_blank">Figure S4</a>).</p

Effect of the Frame during the Delay Period

<p>Effect of frame offset on delayed saccadic eye movements, for trials in which the frame was either extinguished at the start of the delay period (brief frame, dashed line), or was present throughout the delay (extended frame, dotted line). There was a significant frame × delay interaction; when tested separately, the main effect of frame offset was not significant for the extended frame duration, but was significant for the brief duration, replicating the results shown in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020364#pbio-0020364-g002" target="_blank">Figure 2</a>B, dashed line (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020364#pbio-0020364-t001" target="_blank">Table 1</a>; see also <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020364#sg005" target="_blank">Figure S5</a>).</p

Schematic of the Induced Roelofs Effect

<div><p>(A) Example visual display (not drawn to scale) comprising a target (red circle) and a frame offset to the subject's left. Gray circles (unseen by subjects) represent the remembered positions of the items within the comparison array, centered on the subject's midline.</p> <p>(B) One possible mechanism for the inaccurate perceptual report of the target location, based on an illusory rightward shift of the perceived target location (green circle).</p> <p>(C) An alternative mechanism for the inaccurate perceptual report, based on a leftward shift of the memorized location of the comparison array (blue circles). Either mechanism (B or C) would result in the subject reporting the target to occupy the remembered location of item 4 in the comparison array.</p></div

A Roelofs Effect for Allocentrically Defined Targets

<div><p>(A) Visual display used to define allocentric targets; subjects were instructed to move the eyes to the missing corner of the partial rectangle (gray circle, not seen by subject). During experimental trials, this stimulus array was presented within a large rectangular frame that was either centered or offset left or right of the subject's midline.</p> <p>(B) Effect of frame offset on immediate (solid line) and delayed (dashed line) sensorimotor responses to targets defined allocentrically, with a significant main effect of frame offset and a significant frame × delay interaction. When tested separately, the main effect of frame offset was not significant for immediate responses, but was significant for delayed responses (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020364#pbio-0020364-t001" target="_blank">Table 1</a>; see also <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020364#sg006" target="_blank">Figure S6</a>)</p></div