Receptive field properties of single neurons in rat primary visual cortex

The rat is used widely to study various aspects of vision including develop mental events and numerous pathologies, but surprisingly little is known ab out the functional properties of single neurons in the rat primary visual c ortex (VI). These were investigated in the anesthetized (Hypnorm-Hypnovel), paralyzed animal by presenting gratings of different orientations, spatial and temporal frequencies, dimensions, and contrasts. Stimulus presentation and data collection were automated. Most neurons (190/205) showed sharply tuned (less than or equal to 30 degrees bandwidth at half height) orientati on selectivity with a bias for horizontal stimuli (31%). Analysis of respon se modulation of oriented cells showed a bimodal distribution consistent wi th the: distinction between simple and complex cell types. Orientation spec ific interactions occurred between the center and the periphery of receptiv e fields, usually resulting in strong inhibition to center stimulation when both stimuli had the same orientation. There was no evidence for orientati on columns nor for orderly change in optimal orientation with tangential tr acks through V1. Responses were elicited by spatial frequencies ranging fro m zero (no grating) to 1.2 cycle/degree (c/degrees), peaking at 0.1 c/degre es, and with a modal cutoff of 0.6 c/degrees. Half of the neurons responded optimally to drifting gratings rather than flashing uniform field stimuli. Directional preference was seen for 59% of oriented units at all depths in the cortex. Optimal stimuli velocities varied from 10 to 250 degrees/s. So me units, mainly confined to layer 4, responded to velocities as high as 70 0 degrees/s. Response versus contrast curves (best fit with Naka-Rushton) v aried from nearly linear to extremely steep (mean contrast semisaturation 5 0% and threshold 6%). There was a trend for cells from superficial layers t o be more selective to different stimulus parameters than deeper layers cel ls. We conclude that neurons in rat V1 have complex and diverse visual prop erties, necessary for precise visual form perception with low spatial resol ution.