Jones, Helen E.; (1990) Thalamic contributions to mechanisms underlying genesis of sensitivity to contour length in the visual system. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Sensitivity to visual stimulus length was first described by Hubei and Wiesel for cells at the end of the proposed hierarchical sequence in the visual cortex, the so called "hypercomplex cells". However, it has subsequently become clear that length sensitivity is seen in a significant proportion of cells at the first level of processing in the visual cortex, and in lateral geniculate cells relaying retinal input to the visual cortex. This project is concerned with synaptic processes generating length preference at the level of the geniculate, and their significance for the cortical representation of stimulus length. Utilising single unit recording techniques, length response curves were obtained from cells recorded in the dorsal lateral geniculate nucleus (dLGN) and the perigeniculate nucleus (PGN). The majority of dLGN cells exhibited a degree of length tuning equivalent to that seen in tightly tuned cortical hypercomplex cells. Additionally, an apparently distinct sub-population of poorly tuned Y cells was identified. Most PGN cells also exhibited poorly tuned fields. These results are discussed in terms of possible synaptic circuitry generating length sensitivity at sub- cortical and cortical levels. It is suggested that the results have bearing on the modelling of synaptic processes contributing to cortical cell length tuning profiles. The corticofugal system provides the largest single input to the dLGN and has previously been shown to contribute to the generation of length tuning in the dLGN. Knowledge of the mode of synaptic action of the corticofugal system is therefore critical to an understanding of mechanisms generating length tuning. Koch suggested that corticofugal fibres mediate their effects by modulating NMDA receptor responses. While some studies suggest that the retinal neurotransmitter acts on NMDA and non- NMDA receptors, others suggest that it acts only on non-NMDA receptors. In the mammalian dLGN there is no clear evidence as to the identity of the optic nerve transmitter. As a preliminary attempt to address this issue, an iontophoretic technique was utilised to examine the effects and pharmacology of two putative optic nerve transmitters thought to act on NMDA receptors. The effects of selective NMDA and non-NMDA receptor antagonists were examined on the responses to these and to visual stimuli.

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