Timing of saccadic eye movements during visual search for multiple targets

Abstract

Visual search requires sequences of saccades. Many studies have focused on spatial aspects of saccadic decisions, while relatively few (e.g., Hooge & Erkelens, 1999) consider timing. We studied saccadic timing during search for targets (thin circles containing tilted lines) located among nontargets (thicker circles). Tasks required either (a) estimating the mean tilt of the lines, or (b) looking at targets without a concurrent psychophysical task. The visual similarity of targets and nontargets affected both the probability of hitting a target and the saccade rate in both tasks. Saccadic timing also depended on immediate conditions, specifically, (a) the type of currently fixated location (dwell time was longer on targets than nontargets), (b) the type of goal (dwell time was shorter prior to saccades that hit targets), and (c) the ordinal position of the saccade in the sequence. The results show that timing decisions take into account the difficulty of finding targets, as well as the cost of delays. Timing strategies may be a compromise between the attempt to find and locate targets, or other suitable landing locations, using eccentric vision (at the cost of increased dwell times) versus a strategy of exploring less selectively at a rapid rate.

Keywords: eye movements; saccades; saccadic planning; saccadic timing; visual search.

Figure 1

A sample trial of statistical estimation task illustrating the stimulus displays and the procedure. Displays for the look-only task were identical except that there were no response or feedback frames.

Figure 2

Preliminary psychophysical results. (a) Proportion of correct discriminations of line tilt as a function of target eccentricity. (b) d′ for distinguishing targets from distractors as a function of target eccentricity in the discrimination task. One “separation” equals the distance between any two adjacent circles = 150′.

Figure 3

Sample eye trace for a single trial shown as eye positions (left) and horizontal (top) and vertical (bottom) eye positions over time (right). Saccades are numbered in both graphs. The number on both graphs represents the order of saccades in the search sequence. The eye trace shows an example of primary and secondary saccades. The shaded area shows the dwell time between successive primary saccades.

Figure 4

Statistical estimation task: saccadic performance as a function of the width of distractors. Primary saccades only, excluding the infrequent revisits. (a) Hit rate: the ratio of the number of target circles fixated divided by the number of total circles fixated. (b) Saccade rate: number of circles fixated/s. (c) Number of target circles fixated/s. Three different lines represent three temporal epochs (blue: 1-s trial; black: first second of the 2-s trial; red: last second of the 2-s trial). Bars show ± SE; otherwise, SEs are smaller than the plotting symbols.

Figure 5

Look-only task: saccadic performance as a function of the width of distractors. Primary saccades only, excluding the infrequent revisits. (a) Hit rate: the ratio of the number of target circles fixated divided by the number of total circles fixated. (b) Saccade rate: number of circles fixated/s. (c) Number of target circles fixated/s. Three different lines represent three temporal epochs (blue: 1-s trial; black: first second of the 2-s trial; red: last second of the 2-s trial). Bars show ± SE; otherwise, SEs are smaller than the plotting symbols.

Figure 6

Dwell time as a function of saccade starting location, target or distractor, and landing position (HIT/MISS) in the estimation task (top row) and in the look-only task (bottom row). Data were only for longer trial duration (2 s). Blue lines represent the dwells of saccades that hit a target, and red lines represent the dwells of saccades that missed. The superimposed green lines represent the probability of hitting a target as a function of saccade starting location (green axis on the right hand side). Bars show ± SE; otherwise, SEs are smaller than the plotting symbols. The first saccade in the sequence was not included because the first saccade left from initial fixation.

Figure 7

Dwell time as a function of the ordinal position of primary saccades for three levels of selection difficulty in the estimation task (top) and the look-only task (bottom). Blue lines represent saccades going to a target (HIT) and red lines represent saccades going to a distractor (MISS). The superimposed individual circles represent the probability of hitting a target (axis on the right-hand side). Bars show ± SE; otherwise, SEs are smaller than the plotting symbols.

Figure 8

Eccentricity of the nearest target circle hit and the nearest target circle missed as a function of the ordinal position of primary saccades for three levels of selection difficulty in the estimation task (top) and the look-only task (bottom). Blue lines represent saccades going to a target (HIT) and red lines represent saccades going to a distractor (MISS). Bars show ± SE; otherwise, SEs are smaller than the plotting symbols.

Figure 9

Saccadic performance with variable contrast levels. Left: dwell times as a function of distractor width for three levels of stimulus contrast (low, medium, and high contrast). Right: selectivity as a function of distractor width for the three levels of stimulus contrast. Bars show ± SE; otherwise, SEs are smaller than the plotting symbols.

Figure 10

Dwell time as a function of saccade starting locations (target or distractor) and landing positions (HIT/MISS) in the estimation task of Experiment 2. Blue lines represent the dwells of saccades that hit a target, and red lines represent the dwells of saccades that missed. The superimposed green lines represent the probability of hitting a target as a function of saccade starting locations (green axis on the right-hand side). Bars show ± SE; otherwise, SEs are smaller than the plotting symbols. The first saccade in the sequence was not included because the first saccade left from initial fixation. Each row represents one level of stimulus contrast (low, medium, or high contrast).

Figure 11

Hit rate as a function of the number of remaining (not-yet-fixated) targets for three levels of selection difficulty. Blue lines represent observed hit rates in the estimation task, and black lines represent observed hit rates in the look-only task. The red lines are the predicted hit rate from acuity model described in the text and the Appendix.