A specific subgroup of non-length tuned relay cells in the feline dorsal lateral geniculate nucleus.

Length tuning was first described for the "hypercomplex cell category" in the visual cortex. However it has subsequently become apparent that cells in the dorsal lateral geniculate nucleus (dLGN) also exhibit a high degree of length tuning and that for the majority of the population this matches or exceeds that associated with cortical hypercomplex cells (Cleland et al. 1983; Jones and Sillito 1987). In this paper we describe a distinct subpopulation of dLGN Y cells that lack length tuning. These cells were also characterised by poor centre-surround antagonism, and tended to be located close to laminar borders. They appeared to constitute 25% of the Y cell population. Following recent evidence showing relay cells to be powerfully excited by acetylcholine, and inhibitory interneurones to be inhibited, we have examined the responses of these non-length tuned cells to iontophoretic application of acetylcholine. Their brisk excitatory responses suggest that these cells are in fact relay cells. Their presence raises the possibility of a discrete non-length tuned component to the geniculate input to the cortex, and has potentially important implications for the way in which synaptic processes contributing to the length tuning profiles of visual cortical cells are modelled.