Cross-Species Mechanisms of Orientation Tuning in the Primary Visual Cortex
Simple cells in the primary visual cortex of all mammals are renown for having an ‘orientation preference’, meaning that they respond to a specific orientation of an elongated stimulus that lands on their spatial receptive field. Moreover, these cells respond to increments and decrements in luminance in slightly separate regions of their receptive fields, and the angle formed by the centers of these regions can predict their preferred orientation. Despite decades of research into the topic, the mechanism that gives rise to tuned simple cells and their receptive field structure remains an open question. Many models struggle to explain why the properties of simple cells are similar across mammals despite substantial inter-species differences in how simple cells are spatially organized across the cortex. For instance, in the primary visual cortex of cats and primates, vertical columns of cells have similar orientation preferences, and the preferences of these columns rotates smoothly, in a quasiperiodic fashion, as one moves horizontally across the cortex. In contrast, simple cells in mouse primary visual cortex with different tuning preferences are scattered randomly throughout the cortical tissue. Here I assess a mechanistic model of tuning asserting that a single mechanism can generate tuned cells that are either mapped out in an orderly manner across the cortex or arranged in a salt and pepper fashion. The two projects carried out in this thesis test a specific prediction made by the model and compare it to physiological data in mouse primary visual cortex and the thalamic afferents that innervate the cortex. Overall, the results corroborate the model and lend support to the notion that a universal, cross-species mechanism may be used to encode the orientation of a visual stimulus.