The organization of the visual hyperstriatum in the domestic chick. I. Topology and topography of the visual projection.

The organization of the visual projection to the hyperstriatum, or Wulst, in the domestic chick has been investigated using extracellular recording with microelectrodes. The entire visual field of the contralateral eye projects retinotopically into the Wulst. The superior and inferior margins of the visual field are represented, respectively, in the posterior and anterior regions of the visual projection area. The projection of the nasal and temporal margins of the visual field is more complex, the superior and inferior parts of the temporal hemi-field being represented in the superficial and deep regions of the Wulst respectively, with an intervening projection of the nasal hemi-field at an intermediate depth. Visually responsive units appear to be restricted to the accessory hyperstriatum (HA) and possibly also to a narrow, medial region of the intercalated nucleus (HIS). Visual-evoked activity was never detected in the dorsal hyperstriatum (HD) or ventral hyperstriatum (HV). The organization found in the chick is discussed in relation to the previous physiological and anatomical findings of other workers in the visual Wulst of the pigeon and the owl.