Origins of feature selectivity in the visual system of two mammalian species
AffiliationOptometry and Vision Sciences
Document TypePhD thesis
Access StatusThis item is embargoed and will be available on 2020-12-19.
© 2018 Dr. Yamni Mohan
Visual information is transmitted from the retina to the primary visual cortex (V1) through the lateral geniculate nucleus (LGN). At each stage along this visual pathway, the receptive field properties of neurons are transformed, with sharp feature selectivity originating for the first time in the primary visual cortex. In this thesis, I studied the mechanism underlying the generation of the full range of, as well as the sharpening of, two such feature selectivities - the orientation and spatial frequency tuning of neurons- along the visual pathways of tree shrews and macaques. Undertaking this study in two species helps us examine the mechanisms that may be conserved during evolution. In experiment 1 (chapter 4), I used the differences in spatial scale between the geniculate inputs and the V1 spiking outputs in the optical imaging of intrinsic signals to examine the differences in the preferred orientations of the inputs and outputs in V1 of anaesthetized macaques. I determined that the majority of inputs were tuned to the radial orientation (the orientation of the line joining a point on the visual field to the centre of gaze, or fovea in the macaque). A bias for the radial orientation is already evident in the retina. I suggest that the full range of orientation preferences observed in the outputs are generated from a limited number of broadly tuned channels. In experiment 2 (chapter 5), I explored the mechanism underlying the sharpening of orientation tuning from layer 4 to layer 2/3 in tree shrew V1. I found that the orientation selectivity of layer 2/3 is generated from sharpening the broad biases observed in layer 4 of the cortex. It is likely that intracortical inhibitory connections play a bigger role in sharpening feature selectivity in the tree shrews (and by extension in macaques) compared to cats, where most such studies are undertaken. In experiment 3 (chapter 6), I found that neurons in the visual layers of the superior colliculus (SC), which form part of an alternate pathway to the visual cortices, and those in the LGN and layer 4 of V1 show similar orientation and spatial frequency tuning. Hence, it is likely that neurons in these two pathways inherit their feature selectivity from biases established in the retina. In experiment 4 (chapter 7), I found that unlike the macaques, simple cells in the tree shrew V1 act more as Fourier analysers; i.e., they deconstruct the visual scene into their spatial frequency components. I conclude that the tree shrew has several spatial frequency tuning channels in V1 in comparison. Together, my results suggest that sharp feature selectivity observed in the primary visual cortex may be generated from broad biases that are present sub-cortically. Further, in tree shrews and macaques, where sharpening of feature selectivity occurs from layer 4 to layer 2/3, intracortical mechanisms, such as cross-orientation inhibition also play an important role in elaborating feature selectivity. The full range of orientation and spatial frequency preferences in the cortex may be generated from a limited number of broadly tuned channels in both the macaques and tree shrews. These results indicate that sub-cortical biases play an important role in elaborating feature selectivity within the primary visual cortex.
Keywordsprimary visual cortex; orientation selectivity; spatial frequency tuning; superior colliculus; optical imaging of intrinsic signals; tree shrews; macaques; sub-cortical biases
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