The organization of the visual hyperstriatum in the domestic chick. II. Receptive field properties of single units.

The functional properties of cells in the visual hyperstriatum have been studied in the anaesthetized, paralyzed, domestic chicken. Of 426 units 62% were activated by visual targets presented within a circumscribed region of the visual field of the contralateral eye. 13% were activated only by poor defined stimuli, such as a diffuse flash, and had no localized receptive fields. The remaining 25% were unresponsive. Of the cells with localized fields, 22% had circular, apparently uniform ON-OFF fields when explored with static spots of light. When stimulated with moving edges or bars a few showed a slight degree of orientation selectivity or directional asymmetry which only became apparent when responses were averaged. The remaining 78% responded optimally to moving bars or edges of a particular orientation. Less than 10% of these showed selectivity for stimulus length. Absolute directional selectivity was not seen, though a small proportion showed directional bias. Receptive field sizes were in the range of 20-45 degrees for all cells, with the exception of those few which were selective for stimulus length; these had field centres about 5-10 degrees across. Binocular cells were extremely rare, and cells driven exclusively from the ipsilateral eye were not found. The organization of the chick hyperstriatum is compared with that of the visual cortex in some lower mammals and the possible usefulness of the system for studies of the effects of visual deprivation is discussed.