The length-response properties of cells in the feline perigeniculate nucleus

Perigeniculate cells receive visual input from the dorsal lateral geniculate nucleus (dLGN) and from the visual cortex. In contrast to the extensive literature documenting dLGN and cortical cell responses, comparatively little quantitative data exists for perigeniculate nucleus cells, and very little is known about the role of the corticofugal input to the perigeniculate nucleus. We have previously shown that dLGN relay cells have sharply length-tuned receptive fields and that a significant component of this is dependent on the corticofugal system. In this report, we have explored the length-response properties of perigeniculate nucleus cells in the presence and absence of corticofugal feedback. The response profiles of most perigeniculate nucleus cells contrasted markedly with the sharply length-tuned fields of dLGN cells, but exhibited a notable resemblance to those exhibited by VI cells with short summation lengths, which have recently been shown to constitute a considerable proportion of the layer VI cell population. This might suggest that the responses of perigeniculate nucleus cells to long bars derive from their cortical input. However, our data failed to reveal a discernible change in their profiles after removal of the corticofugal drive. This surprising observation implies that their length-tuning profiles follow from subcortical circuitry. The ways in which this might occur are discussed.