Christopher Taylor¹ (), Yaffa Kizel; ¹New England College of Optometry

Spatial suppression and summation differ between young and old observers. We attempted to determine whether they could be altered suppression and summation in a simple task using Gabor stimuli. Duration thresholds were measured for discriminating motion (left/right) and/or orientation (±2° from vertical) in 14 younger (19-27 yrs) and 14 older (65-89 yrs) observers. We used Gabor stimuli (1 cycle/deg) with sizes (2σ = 0.7 or 5 degrees) and contrasts of 2% or 22.5%. The tasks were: (1) two baseline demand tasks, single-dimension stimuli and left-right motion (replication) or orientation (2) two stimulus check tasks with novel dual-dimension stimuli that varied in both motion direction and orientation, with certain responses to either motion or orientation within each task. (3) A dual-dimension stimulus with uncertain single-responses (motion/orientation) that were post-cued randomly on each trial. That is, the observer responded to either motion or orientation, depending on randomly selected post-stimulus cue (4) a dual-task where both motion and orientation responses were collected in randomized order. Seven duration thresholds were measured using a staircase procedure, with each task repeated twice and counterbalanced across two days. In the baseline and stimulus-check tasks for motion, younger observers showed reduced suppression. Orientation showed summation for both groups. When task demands were increased in the uncertain single-response tasks, dual-task summation was observed in both age groups. There was greater summation for older observers in the uncertain-response dual-task orientation tasks. Consistent with previous work, observers exhibited spatial suppression when discriminating motion direction with response certainty. However, when either response uncertainty (single response) or dual-task was used, both younger and older observers showed spatial summation. Task demands can alter whether spatial suppression and summation are observed even in simple spatial vision tasks with identical stimuli. While we used aged observers, this effect could extend to other special populations.

Acknowledgements: Support: Meta Reality Labs, NECO internal funding, NIH T-35-007149