A reevaluation of the functional organization and cytoarchitecture of the feline lateral posterior complex, with observations on adjoining cell groups.

The organization of the cat's lateral posterior complex was reevaluated and its cytoarchitecture described. Analysis of visual representations within the complex confirmed that the pulvinar, lateral zone (LPL), and interjacent zone (LPi) correspond to separate representations of the visual field and established that the zones exhibit heterogeneous connections as a result of retinotopic interconnections with extrastriate areas which represent varying amounts of the visual field. A common system of visual representation extending through layers with differing anatomical connections was identified within zone LPL. The concept of an "isorepresentation cord" was introduced to describe these variable correspondences between sensory representations and connectional relationships. Isorepresentation cords are conceived as holding in common representation of a common locus on a sensory surface without functioning as a unit with respect to connections. Visual representations within the lateral posterior complex consist of many such cords arranged in orderly array. Zone LPm (medial) was also delineated more accurately on the basis of its connections with the ectosylvian visual area. Analysis of termination patterns which occupy boundary regions adjacent to and between the principal zones further established the existence of a collection of cell groups which form a thin, irregular shell investing the principal zones. The identification of these additional cell groups and the recognition of connectional heterogeneity within the principal zones of the complex made it possible to identify and describe the subtle cytoarchitectural differences which characterize the subdivisions of the lateral posterior complex and their boundaries with adjoining nuclear groups. The present findings are discussed with respect to the functions of the lateral posterior complex in interconnecting cortical visual and visuomotor areas, and with respect to the conceptual issues raised by variable correspondences between sensory representations and connectional relationships within thalamus.