Receptive field properties of neurons in the squirrel striate cortex.

Functional organization of receptive fields of 192 neurons from the red squirrel striate cortex was studied. There were three functional types of units: non-selective, directionally selective, and orientationally selective. Non-selective neurons (16 percent) responded to stationary and even more vigorously to moving stimuli regardless of their orientation and direction of movement (with preferred velocities from 100 to 500°/s). The receptive fields were not separated into on- and off-areas. However, suppression of responses to stimuli of large size was observed. Directionally selective neurons (28 percent) retained preference for a specific direction of movement of a stimulus, irrespective of its size and contrast sign (from 100 to 500°/s). Orientationally selective neurons (56 percent) were categorized into simple, complex and intermediate. Selectivity of simple neurons was lacking when a single area on- or off- was stimulated. A number of simple neurons were also selective to contrast border sign, which depended on relative positions of the on- and off-areas. Complex neurons exhibited selectivity for orientation within either pure on- or on-off homogeneous areas. Intermediate receptive fields consisted of spatially separated on- and off-areas that retained self-dependent orientational selectivity. The preferred velocities for orientationally selective neurons varied from 2 to 500°/s. Neurons that yielded sustained responses displayed preference for lower speeds as compared to the cells with transient responses. Some neurons were orientationally selective at law speeds of moving stimuli and directionally selective at higher speeds. All types of neurons were responsive to blue and green stimuli but the color opponency between both inputs was not found.