LAMINAR CONNECTIONS OF THE CAT'S PRIMARY AUDITORY CORTEX (AI)

Recent advances in neuroanatomical tracing methods that exploit the axonal transport of radioactive or histochemically indentifiable labels for the identification of fiber pathways have provided the basis for establishing the laminar connections of the primary visual and somatosensory cortices. These methods are more reliable than the older silver impregnation techniques for tracing degenerating fiber pathways. Furthermore, with the advent of a class of fluorescent dyes that have been demonstrated to be effective retrograde markers, it is now possible to use combinations of these tracers in double-labeling experiments to investigate collateral projections that may arise from single neurons. All of these techniques can be used to confirm the connections deduced from earlier degeneration studies, and to establish in detail the laminar connections of the primary auditory cortex (AI).; The peroxidase and radioautographic methods for tracing neural pathways were used to study the connections established by cells in different layers of AI. Injections of peroxidase into the medial geniculate body (MGB) show that pyramidal cells in layers V and VI of AI are the sources of the cortico-thalamic projection. Large pyramidal cells in the outer rim of layer V also project to the inferior colliculus. Injections of ('3)H-proline into AI show that these cells terminate bilaterally in the medial margin of the central nucleus of the inferior colliculus and in the dorsolateral margin of the caudal third of the central nucleus.; Callosal cells in AI arise mainly from layers III and VI and are found in irregular clusters as wide as 1100 um separated by spaces that contain relatively few callosal neurons. Experiments utilizing anterograde peroxidase transport show that callosal terminals are found in bands running from layers VI through I. These bands are about 500 um in width, and the terminals appeared most densely packed in layers II and III. Since the dimensions of the cell clusters and bands of callosal terminals differ, it is likely that not all zones which give rise to callosal axons also receive them. The bands of callosal terminals labeled by anterograde 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 auditory callosal pathway.; Finally, injections of ('3)H-proline into the ventral division of the MGB show that the majority of thalamo-cortical axons terminate within layers III-IV of AI.; To establish if dually projecting neurons are present in a single layer, we have devised a double-labeling technique, by using a novel combination of peroxidase and Nuclear Yellow (NY). Peroxidase labeling of the cell soma and NY-labeling of the cell nucleus can be visualized simultaneously in doubly labeled cells.; Injections of peroxidase into the ipsilateral MGB and NY into the contralateral AI show that there are two distinct populations of neurons in layer VI of AI that overlap topographically. Doubly labeled cells are not found. Cortico-geniculate cells are distributed throughout the thickness of layer VI, while callosal cells are generally situated superficial to them. Cortico-geniculate and callosal cells are also distributed homogeneously along the horizontal axis of layer VI.; Singly labeled neurons are found in the outer rim of layer V of AI after unilateral injections of NY into the MGB and of peroxidase into the inferior colliculus. No clustering of cortico-geniculate and cortico-collicular cells is seen along the horizontal axis of the layer. The ratio of these two cell types is about 1:1. Hence, in layer V or VI these dual projections arise from two separate populations of neurons that overlap topographically, but have distinctive patterns of connections.