Academic journal article Perception and Psychophysics

Biases in the Perceived Timing of Perisaccadic Perceptual and Motor Events

Academic journal article Perception and Psychophysics

Biases in the Perceived Timing of Perisaccadic Perceptual and Motor Events

Article excerpt

Subjects typically experience the temporal interval immediately following a saccade as longer than a comparable control interval. One explanation of this effect is that the brain antedates the perceptual onset of a saccade target to around the time of saccade initiation. This could explain the apparent continuity of visual perception across eye movements. This antedating account was tested in three experiments in which subjects made saccades of differing extents and then judged either the duration or the temporal order of key events. Postsaccadic stimuli underwent subjective temporal lengthening and had early perceived onsets. A temporally advanced awareness of saccade completion was also found, independently of antedating effects. These results provide convergent evidence supporting antedating and differentiating it from other temporal biases.

In everyday life, people regularly make rapid saccadic movements of the eyes to bring objects of interest onto the high-acuity fovea. Saccades raise serious computational problems that the visual system must overcome (Bridgeman, van der Heijden, & Velichkovsky, 1994). Perisaccadic biases in spatial vision have provided insights into the way the brain solves these problems (Schlag & Schlag-Rey, 2002). Recent data suggest that temporal judgments are also biased when we make saccades to fixate new targets. Subjects consistently overestimate the duration of a postsaccadic stimulus, in comparison with the same stimulus seen at fixation, an illusion termed saccadic chronostasis (Yarrow, Haggard, Heal, Brown, & Rothwell, 2001; Yarrow, Haggard, & Rothwell, 2004; Yarrow, Johnson, Haggard, & Rothwell, 2004). In a typical experiment, subjects saccade to a target that changes form or color during the saccade. The subjects judge whether the new target stimulus was presented for a longer or a shorter time than were subsequently presented reference stimuli, and these judgments are used to determine a point of subjective equality (PSE; the point at which target and reference stimuli are perceived to have identical durations). The same task performed at fixation forms a control. Reduced PSEs in saccade conditions, in comparison with control conditions, imply temporal overestimation of the postsaccadic stimulus.

One finding from previous saccadic chronostasis experiments is particularly noteworthy. When saccades of different extents are made, the size of the illusion changes; the duration of the illusion reflects the duration of the preceding saccade (Yarrow et al., 2001). This suggests the following explanation. Both retinal blur and active suppression degrade visual input during saccades (Ross, Morrone, Goldberg, & Burr, 2001), leaving a gap in perception corresponding to the saccade duration. The brain may simply assume that the information in the postsaccadic image has remained constant across the saccade. Hence, the perceived onset of the saccade target is effectively antedated to a moment just prior to saccade initiation, helping to provide the visual continuity we experience. The illusion arises following many different kinds of saccades with partially separable neural substrates and has an effect size that typically exceeds the duration of the saccade by around 50 msec (Yarrow, Johnson, et al., 2004). These observations suggest that an efference copy signal originating in a subcortical region such as the superior colliculus could act as a trigger or time marker for the antedating process. The same signal may initiate receptive field shifts occurring in cells in the parietal cortex and other areas, which might, in turn, influence conscious visual perception (Duhamel, Colby, & Goldberg, 1992; Goldberg, Bisley, Powell, Gottlieb, & Kusunoki, 2002; Sommer & Wurtz, 2002; Umeno & Goldberg, 1997; Walker, Fitzgibbon, & Goldberg, 1995).

Two key objections have been made to this antedating account. These two problems are described below in some detail, because the experiments and analyses presented later were intended to provide new positive evidence for antedating that is not subject to these concerns. …

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