Does laminar position determine the receptive field properties of cortical neurons? A study of corticotectal cells in area 17 of the normal mouse and the reeler mutant.

The neocortex of the reeler (rl) mutant mouse develops abnormally; as a result, the orderly arrangement of cells in laminae containing neurons of similar size and configuration is severely disrupted. The reeler mutant thus offers an opportunity to study the role played by laminar position in establishing the interneuronal connections of the cortex. Since the receptive field properties of neurons in the primary visual cortex (area 17) are determined by these complex interconnections, a study of receptive field properties provides a useful way to test whether functionally important connections are altered by abnormal cell position. We chose the corticotectal (CT) cells of area 17 for this analysis because they have distinctive receptive field properties and can be identified positively by antidromic stimulation with electrodes in the superior colliculus. In addition, CT cells are located in a single lamina (layer V) in the visual cortex of normal mice, but are distributed from surface to white matter in the reeler visual cortex. We characterized the receptive fields of identified CT cells in area 17 of normal mice and reeler mutants and studied several properties of these cells quantitatively. Corticotectal cells in the visual cortex of the normal mouse have high rates of spontaneous activity and large receptive fields that are neither oriented nor directional; they respond to moving stimuli over a wide range of stimulus velocities, have high peak velocity sensitivities, and show very little spatial summation. Corticotectal cells in the visual cortex of the reeler mutant do not differ significantly in these characteristics, although CT cells with extremely large receptive fields are encountered somewhat more frequently in reeler cortex. These findings indicate that the detailed connections which determine the receptive field properties of CT cells are properly established in the reeler mutant mouse despite the markedly abnormal positions of CT cells in reeler visual cortex.