Patterns of sensory intermodality relationships in the cerebral cortex of the rat

Patterns of connections underlying cross-modality integration were studied by injecting distinguishable, retrograde tracers (Fluoro-Gold and diamidino yellow) in pairwise manner into different sensory representations (visual, somatosensory, and auditory) in the cerebral cortex of the rat. In agreement with previous single tracer studies, our results indicate that the central core of sensory areas receives projections mainly from a set of association areas located in a ringlike fashion along the margin of the cortical mantle. The visual cortex received projections from areas 48/49, area 29d, posterior agranular medial cortex (AGm), area 11, area 13, and area 35. All these areas were also connected to the auditory cortex with the exception of areas 29d and AGm. However, lateral to area 29d and posterior AGm, a band of neurons projecting to the auditory cortex was present. Somatosensory cortex was connected mainly with the more anterior aspect of the hemisphere, which included primary motor area, area 11, and area 13. The patterns of intermodality relationships revealed in the present study were of two main categories. In the anterior and lateral areas, an intermingling of cells projecting to different sensory modalities was observed. In contrast, in areas located along the medial aspect of the hemisphere, cells connected to different sensory modality representations tended to be segregated from each other. Postsubicular cortex (areas 48/49) contained both intermingled and segregated groups of cells. The incidence of clearly identified double-labeled cells concurrently projecting to two different sensory representations was extremely rare. These patterns may form a substrate for different levels of cross-modal sensory integration in the rat cortex.