Spatial position information accumulates steadily over time

Abstract

One of the more enduring mysteries of neuroscience is how the visual system constructs robust maps of the world that remain stable in the face of frequent eye movements. Here we show that encoding the position of objects in external space is a relatively slow process, building up over hundreds of milliseconds. We display targets to which human subjects saccade after a variable preview duration. As they saccade, the target is displaced leftwards or rightwards, and subjects report the displacement direction. When subjects saccade to targets without delay, sensitivity is poor; but if the target is viewed for 300-500 ms before saccading, sensitivity is similar to that during fixation with a strong visual mask to dampen transients. These results suggest that the poor displacement thresholds usually observed in the "saccadic suppression of displacement" paradigm are a result of the fact that the target has had insufficient time to be encoded in memory, and not a result of the action of special mechanisms conferring saccadic stability. Under more natural conditions, trans-saccadic displacement detection is as good as in fixation, when the displacement transients are masked.

Figure 1.

Experimental setup for saccade and fixation trials. The black circles indicates eye position. The red squares indicate targets for fixation or saccades. A, Saccade trials. A trial started with subjects directing gaze to a fixation point. After 100 ms, the fixation point was turned off and subjects continued to maintain fixation on the blank screen. The saccade target T1 appeared 1000 ms later. Subjects saccaded to it on auditory cue 0–500 ms after saccade target onset. As soon as the saccade was detected, the saccade target was displaced either leftwards or rightwards (T2). At the end of the trial, the subject indicated the direction of the target displacement by key press. B, Fixation trials. The sequence was similar for the saccade trials, except subjects maintained fixation at the position of the fixation point for the entire trial. T1 was displayed for 0–900 ms, and followed by 60 ms of high-contrast mask (simulating the masking effect of the saccade). T2 was then displayed leftwards or rightwards of its original position and subjects reported the direction of the shift by key press.

Figure 2.

A, Example psychometric functions for the saccade condition, for three representative subjects. Filled triangles refer to trials where the saccade was cued as soon as the target was presented (the standard condition for this type of study). Open circles refer to trials with the longest preview duration (731 ms; SD, 92 ms). The data are fitted with a Gaussian error function whose SD (σ) is taken as an estimate of threshold (just-noticeable difference). Thresholds are lower (steeper curves) for all subjects with the longest preview duration. B, Psychometric functions for the fixation condition. Same conventions as in A. The longest preview duration in fixation trials was 900 ms.

Figure 3.

A, Displacement thresholds (geometric mean of thresholds, calculated separately for all subjects) for the saccade condition as a function of presentation duration. Saccade latencies were measured for all trials, and target presentation time binned into 100 ms bins. Error bars represent standard error across subjects. The continuous curves show exponential fits to the data (Eq. 1). B, Displacement thresholds in the fixation condition.

Figure 4.

A, Psychometric functions for a typical subject, calculated either as a function of the physical displacement of the target (gray open circles), or the difference between the target position (after displacement) and eye landing position (black triangles). The slope of the functions is similar for both conditions. B, Average displacement thresholds, calculated from individual psychometric functions considering saccadic landing (ordinate) against those considering only the physical position of the target (abscissa). The continuous line is a linear regression (−0.09 + 1.23x, R²⁼ 0.98). The dashed line is the equality line. C, Average displacement bias, calculated from individual psychometric functions considering saccadic landing (ordinate) against those considering only the physical position of the target (abscissa). The continuous line is a linear regression (0.07 + 0.67x, R²⁼ 0.74). The dashed line is the equality line.

Figure 5.

Thresholds as a function of target exposure duration for two different contrasts, 97% (open circles) and 10% (filled triangles), averaged over two subjects. Error bars refer to ±1 standard error across subjects.