Temporal Sensitivity Under Photopic and Scotopic Conditions Across the Cortical Visual Hierarchy

Behavioral and electrophysiological studies have shown that vision is slower under scotopic conditions (dark, activating only rods) than photopic conditions (light, activating only cones). However, slower scotopic processing cannot be solely explained by findings that rod signals are slower than cone signals, and it is unknown whether temporal processing differences persist in cortex. Flickering stimuli have previously been used in functional MRI (fMRI) studies to probe photopic cortical temporal sensitivity. This fMRI study investigates flicker sensitivity under photopic and scotopic conditions across the cortical visual hierarchy.

Fourteen participants viewed a stimulus flickering at six frequencies (2-10 Hz) under photopic and scotopic conditions during fMRI scanning. Retinotopic and high-level visual areas were delineated for each subject with population receptive field modeling (using a drifting bar) and a functional localizer (using images of objects).

In most areas, higher mean activation was observed under photopic than under scotopic conditions. However, peak activation was higher only in V1 and ventral retinotopic areas. The pattern of change over frequencies differed between lighting conditions in retinotopic areas, but not in most high-level areas. Under scotopic conditions, the largest BOLD response was observed at low frequencies throughout visual cortex. Under photopic conditions, BOLD responses appeared largely unchanging across frequencies, with a trend towards preferring higher frequencies in V1.

Selectivity for lower frequencies under scotopic conditions was observed throughout visual cortex, in contrast to limited selectivity under photopic conditions. This low-frequency preference could allow more time for extracting information from sparse scotopic stimuli.