Luminance Contrast and Eye Movements During Visual Search

Polisuk, Brittany M., Ebony N. Fails, Perrier T. Greene, and Harold H. Greene

The Processing Span is the area around an eye fixation from which useful information may be obtained. Whereas the number of fixations made in search of a target and saccade amplitudes are reasonable indices of Processing Span properties, saccade amplitude findings have a priori, been inconsistent. We contend that when all saccades are considered, the amplitude distribution is noisy. Hence, a better Processing Span index is the amplitude of the saccade that detected the target. Assuming that the Processing Span is narrower for lower luminance contrast environments, we expected a decrease in target-detecting saccade amplitudes with decreasing contrast. From previous studies we also expected, longer fixations, and increased numbers of fixations with decreasing contrast. Twelve participants searched with one eye to localize a target embedded in random grey dot displays.  Displays were presented at .04 and .54 Michelson contrasts. The target was defined by a variation in pattern that did not differ in average contrast from the rest of the display.  With decreasing contrast, fixation count and fixation duration increased. Also, as predicted, target-detecting saccade amplitudes decreased.  We conclude that visual search in low contrast environments (e.g. driving in the fog) is made difficult by (i) a narrower Processing Span, and (ii) longer fixation durations. Our findings add to a growing body of evidence that eye movement indices give sufficient access to the Processing Span.