The cortical visual areas of the sheep

1. A stereotaxic method for the sheep brain is described. 2. At its widest part the primary visual area (Visual I) of each hemisphere extends approximately 20 mm anteroposteriorly and, when unfolded, approximately 35 mm from side to side. It occupies both walls of the lateral sulcus, and extends medially to the medial wall of the hemisphere and to the depth of the ectolateral sulcus laterally. 3. The most lateral part of the primary visual area includes 10-15 degrees of the ipsilateral field; the contralateral field is represented to 135 degrees from the mid line. 4. Visual II also includes a strip of ipsilateral representation on its medial edge and extends to the supra-sylvian sulcus on the lateral surface of the brain. The furthest lateral representation recorded was 130 degrees lateral. 5. Most of both visual areas is concerned with the area centralis and the visual streak. The remainder of the retina has very little cortical representation. 6. Most cells in Visual I are simple with orientational and sometimes directional sensitivity. Some complex and hypercomplex cells have been seen in Visual I, and these predominate in Visual II. Receptive field sizes from 0-25 to 10 degree were found. Within 15 degrees of the vertical meridian, binocular cells are common in both Visual I and II.