Saccade Amplitudes: A window on visual properties

Greene, Harold H, James M Brown, and Bryce A Paradis

The number of saccades executed to localize a target has been found reliably to increase with decreasing luminance contrast,  indicating a narrowing of the visual span.  In contrast,  saccade amplitude findings  have been inconsistent. We suspect that the inconsistency may be driven by the general (multi-parameter) saccade amplitude index utilized. We propose instead, the use of target-localizing saccade amplitudes (TLSA). Twenty-two participants searched (12 under monocular conditions)  to localize a target embedded in random grey dot displays.  The target was defined by a variation  in pattern that did not differ in average contrast from the rest of the display. Displays were presented at .04 and .54 Michelson contrasts. A chinrest was used to minimize head movement, and  eye positions were sampled at 500 Hz by an Eyelink II system.  Results indicated that across binocular and monocular viewing, TLSA decreased with decreasing luminance contrast. Hence we contend that TLSA, can be used to index visual span properties.