Some properties of extrastriate visual units in the cortex of the rabbit.

Receptive fields and trigger properties of 306 units in the visual cortex of the rabbit have been examined in detail. Reconstruction of the electrode tracks revealed that 276 of these recordings were made from cells located in the striate cortex, whilst the remaining 30 were found to lie in cortex lateral to the striate field. These latter units have been termed 'extrastriate'. Five penetrations involved both striate and extrastriate units enabling a direct comparison of their properties to be made. The most outstanding feature of the extrastriate units was their huge receptive field sizes (up to 80 degrees X 130 degrees) and high rates of spontaneous activity (usually between 10 and 20 imp./sec). None of these cells responded well to stationary stimuli and most preferred moving targets. Responses could be elicited regardless of the nature of stimulus-to-background contrast and were constant over a wide range of stimulus sizes. No cells were found which were orientation selective. Approximately half the extrastriate units were direction selective, the majority preferred anterior movement in the visual field. Movement in the counter-preferred direction generally resulted in suppression of the spontaneous discharge. In contrast to this the receptive fields of cells in the striate cortex were small, and were often sensitive to features such as stimulus size, orientation, and contrast, in addition to movement and direction of movement. The similarity between one class of extrastriate unit, and units recently observed in the pretectum of the rabbit is pointed out.