The characteristics of eye movements made during visual search with multi-element stimuli.

Saccadic eye movements during visual search made to detect a target embedded in a set of randomly distributed, identical elements have been measured. Both the time T1/2 taken to press a button to denote target detection (the manual response time) and the time Tf taken to fixate the target were determined. It is established that these two response times are closely related to each other, and have similar dependence on the number of reference elements N, both in search tasks for which T1/2 is independent of N (parallel search) and in those for which T1/2 is proportional to N (serial search). The properties of the eye movements made by two subjects in response to a variety of stimuli have been subjected to detailed analysis. It is shown that the number of fixations made during a given search is proportional to the manual response time T1/2 and that the probability of locating the target with a single fixation falls steeply as T1/2 increases. The latency of the first saccade is approximately linearly dependent on T1/2, and latencies made in performing a given task are distributed in a Gaussian manner the spread of the latency values increasing as the mean latency increases. It is conclude that the measurement of eye movements provides a more detailed description of visual search than that given by measurement of manual response times, and that in terms of eye movements, only a subset of those tasks for which T1/2 is independent of N can be classified as parallel.