How many faces can be processed during a single eye fixation?

Abstract

The purpose of our study was to estimate the perceptual span for facial information: how many faces can be processed during a single eye fixation. We used a visual-search task, in which the targets and distractors were facial photographs. The task of the observer was to search for and identify a target face in an array of faces. We measured the time needed for one search--threshold search time--by using a multiple-alternative staircase method. The threshold represents the duration of stimulus presentation at which the probability of correct responses was 79%. The array size was varied from 2 x 2 to 8 x 8 faces. Simultaneously with the performance measurements we measured eye movements with a video eye tracker. We found that threshold search time increased with increasing set size nearly linearly. The number of fixations also increased linearly from unity at the smallest set size to about fifteen at the largest set size. The result of 2 x 2 faces during a single fixation gave an estimate of 4 faces for the perceptual span. If, on average, only half of the elements had to be scanned for finding the target, 15 fixations at the largest set size (8 x 8) gave another estimate of 2.13 faces. The mean fixation duration was around 200 ms. Thus, the results suggest that 2-4 faces can be processed during one fixation of about 200 ms.