Architecture and callosal connections of visual areas 17, 18, 19 and 21 in the ferret (Mustela putorius).

Visual areas 17, 18, 19 and 21 of the ferret can be distinguished on the grounds of cytoarchitecture, myeloarchitecture and cytochrome oxidase reactivity, and with transneuronal tract-tracing from the eye. Each visual area contains callosally connected, as well as acallosal, regions. The callosal connections originate mainly from layers 2 and 3 and, more widely, from layer 6. Callosally projecting neurons and callosal terminals are organized in three roughly medio-laterally oriented bands. The posterior and intermediate bands straddle the 17/18 and 19/21 border, respectively; the third band extends along the medial bank of the lateral suprasylvian sulcus. These bands are linked by a variable number of bridges of connections that demarcate acallosal islands. The distribution of callosal connections predicts the existence of vertical meridian representations corresponding to each of the bands and of non-isotropic representations of the visual field within the bridges and islands.