Visual strategies used for time-to-arrival judgments in driving.

To investigate the sources of visual information that are involved in the anticipation of collisions we recorded eye movements while participants made relative timing judgments about approaching vehicles at a junction. The avoidance of collisions is a critical aspect in driving, particularly where cars enter a line of traffic from a side road, and the present study required judgments about animations in a virtual driving environment. In two experiments we investigated the effects of (i) the angle of approach of the vehicle and the type of path (straight or curved) of the observer, and (ii) the speed of both the observer and the approaching car. Relative timing judgments depend on the angle of approach of the other vehicle (judgments are more accurate for perpendicular than for obtuse angles). Eye-movement analysis shows that visual strategies in relative timing judgments are characterised by saccadic eye movements back and forth between the approaching car and the road ahead, particularly the side line which may serve as a spatial reference point. Results suggest that observers use the distance of the car from this reference point for their timing judgments.

Saccadic latency as a function of target duration in a spatial localization task.

This study examined a potential confound in the inverse relationship between target duration and saccadic latency reported by Adam, Ketelaars, Kingma, and Hoek in 1993. Eight participants located a briefly flashed target by moving the eyes and the cursor toward its position in a (backward) mask condition and in a no-mask condition. Analysis showed similar saccadic latencies in both conditions, thereby refuting the potentially confounding role of the backward masking procedure. It is tenatively suggested that the longer saccadic latencies noted for shorter target durations may be associated with delayed accumulation of evidence for the detection of the target.

Manipulating saccadic decision-rate distributions in visual search.

The Gaussian shape of reciprocal latency distributions typically found in single saccade tasks supports the idea of a race-to-threshold process underlying the decision when to saccade (R. H. Carpenter & M. L. Williams, 1995). However, second and later saccades in a visual search task revealed decision-rate (=reciprocal latency) distributions that were skewed Gamma-like (E. M. Van Loon, I. T. Hooge, & A. V. Van den Berg, 2002). Here we consider a related family of Beta-prime distributions that follows from strong competition with a signal to stop the sequence, and is described by two parameters: a fixate and saccade threshold. In three saccadic search experiments, we tried to manipulate the two thresholds independently, thereby expecting change in shape and mean of the reciprocal latency distribution. Interestingly, rate distributions for later saccades were significantly better fit by Beta-prime than by Gamma functions. Increases in the distribution's skew were found with higher display density, but only for second and later saccades. First saccade rate distributions were not altered by the expected target location or by visual information presented prior to the search, but making pre-search saccades did influence both thresholds. The mean rate remained a stereotyped function of ordinal position in the saccade sequence. Our results support strong competition between two decision signals underlying the timing of saccades.