Connections of cat auditory cortex: II. Commissural system.

The commissural projections between 13 areas of cat auditory cortex (AC) were studied using retrograde tracers. Areal and laminar origins were characterized as part of a larger study of thalamic input and cortical origins of projections to each area. Cholera toxin beta subunit (CTbeta) and cholera toxin beta subunit gold-conjugate (CTbetaG) were injected separately within an area or in different areas in an experiment. The areas were identified independently with SMI-32, which revealed differences in neurofilament immunoreactivity in layers III, V, and VI. Each area received convergent AC input from 3 to 6 (mean, 5) contralateral areas. Most of the projections (>75%) were homotopic and from topographically organized loci in the corresponding area. Heterotopic projections (>1 mm beyond the main homotopic projection) constituted approximately 25% of the input. Layers III and V contained >95% of the commissural neurons. Commissural projection neurons were clustered in all areas. Commissural divergence, assessed by double labeling, was less than 3% in each area. This sparse axonal branching is consistent with the essentially homotopic connectivity of the commissural system. The many heterotopic origins represent unexpected commissural influences converging on an area. Areas more dorsal on the cortical convexity have commissural projections originating in layers III and V; more ventral areas favor layer III at the expense of layer V, to its near-total exclusion in some instances. Some areas have almost entirely layer III origins (temporal cortex and area AII), whereas others have a predominantly layer V input (anterior auditory field) or dual contributions from layers III and V (the dorsal auditory zone). A topographic distribution of commissural cells of origin is consistent with the order observed in thalamocortical and corticocortical projections, and which characterizes all extrinsic projection systems (commissural, corticocortical, and thalamocortical) in all AC areas. Thus, laminar as well as areal differences in projection origin distinguish the auditory cortical commissural system.