Jingyi He¹, Maxwell J. Greene¹, William S. Tuten¹; ¹Herbert Wertheim School of Optometry and Vision Science, University of California, Berkeley, Berkeley, CA, USA

Adaptive optics platforms enable the delivery of small, diffraction-limited stimuli to known retinal locations. Attempts to relate the resultant cone activations to perception would benefit from a deeper understanding of how the intervening post-receptoral mechanism(s) integrate photoreceptor signals over space and time. The double-pulse technique has been widely used to estimate the temporal impulse response function (IRF) of the human visual system. In this study, we examined how the detectability of pulse pairs depended on stimulus-onset asynchrony (SOA). An adaptive optics scanning laser ophthalmoscope with two narrowband primaries (543 nm and 680 nm; “green” and “red”, respectively) was used to deliver increment flashes with higher-order and chromatic aberrations corrected. Stimulus pulses subtended either 6’×3’ (1 ms duration) or 24’×12’ (4 ms duration) and were presented in either isochromatic (“green-green”) or heterochromatic (“red-green”) configurations. Heterochromatic pulse pairs were presented with the intensities of the red and green primaries scaled according to single-pulse threshold measurements. A white background (2.2°×1.1°) was provided by an auxiliary display. Detection thresholds for SOAs ranging from 0 to 200 ms were obtained using a 2IFC task in three subjects. For the smaller stimulus size, threshold-versus-SOA curves were similar for both color conditions, exhibiting near-complete summation at the shortest SOA and rising monotonically to an intensity corresponding to ~67% of the single-flash threshold. By contrast, for the 24’×12’ isochromatic pulse pair, the highest thresholds were observed at an SOA of 50 ms, suggesting the emergence of an inhibitory lobe in the IRF as stimulus size increases. No such inhibition was observed for the red-green 24’×12’ pulse pair. Our results are broadly consistent with prior studies demonstrating that the form of the IRF depends on the spatio-chromatic properties of the stimulus, and can be incorporated into computational models of early visual processing in the human fovea.

Acknowledgements: This work was supported by the Air Force Office of Scientific Research under award numbers FA9550-20-1-0195 and FA9550-21-1-0230