Temporal sensitivity for achromatic and chromatic flicker across the visual cortex

The classes of retinal ganglion cells (RGCs) receive different combinations of L, M, and S cone inputs and give rise to one achromatic and two chromatic post-receptoral channels. Beyond the retina, RGC outputs are subject to filtering and normalization along the geniculo-striate pathway, ultimately producing the properties of human vision. The goal of the current study was to determine temporal sensitivity across the three post-receptoral channels in subcortical and cortical regions involved in vision, to better characterize post-retinal temporal processing. We measured functional magnetic resonance imaging (MRI) responses at 7 Tesla from participants viewing a high-contrast, flickering, spatially-uniform wide (~140°) field. Stimulus flicker frequency varied logarithmically between 2 and 64 Hz and targeted the L+M+S, L–M, and S–[L+M] cone combinations. These measurements were used to create temporal sensitivity functions (TSFs) of primary visual cortex (V1) across eccentricity, and spatially averaged responses from lateral geniculate nucleus (LGN), V2/V3, hV4, and MT. Functional MRI responses reflected known properties of the visual system, including higher peak temporal sensitivity to achromatic vs. chromatic stimuli, and low-pass filtering between the LGN and V1. V1 had the slowest peak temporal sensitivity across cortical regions, which increased at higher levels of the visual cortical hierarchy. Unexpectedly, peak temporal sensitivity decreased at greater eccentricities in area V1, especially for achromatic stimuli. Comparison of measured cortical responses to a model of integrated retinal output to our stimuli demonstrates that extensive filtering and amplification is applied to post-retinal signals.