Temporal sensitivity for achromatic and chromatic flicker across the visual cortex

Abstract

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 degree) 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.

Cone-Driven, Geniculocortical Responses in Canine Models of Outer Retinal Disease

“… 35 , 36 Our work extends these techniques to fMRI measurements in normal and retinal disease models (as has been pursued in the human). 37 , 38 In WT dogs, we find reliable responses in the geniculocortical pathway to both luminance and chromatic stimuli. Measurements from inherited retinal disease models support both the selectivity of our stimuli for eliciting cone-driven responses and help to identify a promising disease model for the study of cone function restoration.…”

Cone-driven, geniculo-cortical responses in canine models of outer retinal disease

“…Manipulation of the spectral content of stimuli has been used previously in the dog to probe photoreceptor contributions in both ERG and pupil responses 35,36 . Our work extends these techniques to fMRI measurements in normal and retinal disease models (as has been pursued in the human 37,38 ). In WT dogs we find reliable responses in the geniculo-cortical pathway to both luminance and chromatic stimuli.…”