Laminar organization and diversity of area MT receptive fields in the marmoset

The marmoset is a useful model for visual neuroscience due to its high acuity fovea, smooth cortex, and similar organization of visual processing areas with humans. One visual area of interest is cortical area MT, which contains highly motion-selective cells. Here we took advantage of the smooth cortical surface of the marmoset to examine spatial receptive fields across cortical layers and extracellular cell types, cataloging them by features including receptive field size, radial elongation, and motion selectivity. We mapped receptive fields in an awake marmoset free-viewing a full-field stimulus containing dot motion while recording across cortical depth using NeuroNexus linear arrays. The stimulus consisted of 4 to 32 dots, each plotted at a random position on the screen and moving in a random direction for a limited lifetime of 50ms with asynchronous updating. We estimated the receptive field by correlating neural responses to dot position and direction. Raw voltage tracers were preprocessed, and spikes sorted using Kilosort2. To perform current source density (CSD) analyses to estimate laminar depth, we measured local field potentials in response to a full-field contrast change. We find that the CSD – a method typically used in primary visual cortex – can provide an estimate of the MT input layer based on an early latency current sink. We also find that the distribution of extracellular waveform shapes is bi-modal based on peak-to-trough duration with separation of narrow and broad-spiking types (putative interneurons and principle cells). Properties such as size, elongation and motion selectivity varied as a function of cell type, depth, and eccentricity. Narrow spiking neurons exhibited lower direction selective index than broad-spiking neurons, consistent with a role as local interneurons. We also found that receptive fields scaled for eccentricity were proportionally larger and more elongated with reaching into the periphery when located near the fovea.