Contributed Session II: The relationship between temporal summation at detection threshold and fixational eye movements.

We studied the relationship between the threshold temporal summation of increment pulses and fixational eye-movements. Six participants completed a 2AFC increment detection task. Stimuli were 0.16 x 2.2 arcmin increments of 543 nm light presented via an AOSLO with a 60 Hz frame rate. Stimuli for temporal integration were two single frame presentations with a 16 ms (consecutive frames), 33 ms, 100 ms, or 300 ms inter-stimulus interval (ISI). Data were also collected for increments presented on a single frame. Stimuli were presented in either world-fixed coordinates (natural retinal image motion) or were stabilised on the retina. There were large differences in overall sensitivity across individuals, but the time-course of performance change with ISI was similar across participants. Thresholds for ISI=33 ms were close to performance with two consecutive frames, suggesting complete summation of light energy; whereas thresholds for ISI=300 ms were closer to the single-frame case, suggesting limited summation; and thresholds for ISI=100 ms were intermediate, suggesting residual summation. The effect of ISI on threshold was similar for stabilised stimuli and natural viewing, but there was a small trend towards lower thresholds for stabilised stimuli at short ISI and vice-versa at long ISI. We plan to present our results in the context of an ideal observer calculation that may clarify how the initial visual encoding, including temporal summation within cones, shapes performance.