Laminar connections of the cat's auditory cortex.

The retrograde and the anterograde transport of horseradish peroxidase were used to study the connections established by cells in different layers of the cat's primary auditory cortex (AI). Injections of peroxidase into the medial geniculate body show that pyramidal cells in layers V and VI of AI are the sources of the corticothalamic projections. Large pyramidal cells in the outer rim of layer V also send their axons to the inferior colliculus, and it is possible that some of these cells have axons that branch to innervate both the inferior colliculus and the medial geniculate body. Cells in AI that give rise to callosal axons lie principally in layers III and VI. The callosal neurons are found in irregular clusters as wide as 1100 microgram separated by spaces that contain relatively few callosal neurons. Experiments utilizing the anterograde transport of peroxidase show that callosal terminals are found in bands running from layers VI through I. These bands are about 500 microgram in width, and the terminals seem most densely packed in layers II and III. Since the dimensions of the cell clusters and bands of callosal terminals are not the same, it is likely that not all zones which give rise to callosal axons also receive them. The bands of callosal terminals labeled by orthograde transport may be seen in the same section along with the cell bodies labeled by retrograde transport, and the two zones of label are clearly not coextensive. Complete reciprocity, therefore, seems to be absent in the callosal auditory pathway.