Receptive fields of directionally selective units in the optic nerve of the ground squirrel.
These units responded vigorously to stimuli moving entirely across their receptive field centers in one direction (preferred) and not at all when the direction of motion was reversed (null). The directional selectivity was the result of an inhibitory mechanism which prevented responses to null movements. Surrounding each field center was a concentric antagonistic region produced by a second inhibitory mechanism.
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