Zatka Haas, Peter N.; (2018) Mapping perceptual decisions to cortical regions. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Perceptual decisions involve a complex interaction of several brain areas. The neocortex is thought to play a major role in this process, but it is unclear which cortical areas are causally involved, and what their individual roles are. To explore this problem, we trained head-fixed mice to perform a two-alternative unforced-choice visual discrimination task. Mice were rewarded with water for turning a wheel to indicate which of two stimuli had higher contrast, or for holding the wheel still if no stimuli were present. We developed a hierarchical Bayesian model of the choice behaviour and used this to quantify mouse behaviour in terms of perceptual states such as choice biases and stimulus sensitivities. We also used this model framework to quantify how these perceptual states vary across individual mice and across sessions. Using widefield calcium imaging, we found robust sequential activation in primary visual, secondary visual, secondary motor, primary motor and somatosensory cortices in response to stimulus presentation. Optogenetic inactivation revealed that only the first two regions: visual (VIS) and secondary motor (MOs) areas, were causally relevant. VIS inactivation was effective earlier than MOs inactivation, which suggests a sequential causal role for these regions. We observed a surprising effect of VIS inactivation which could only be explained by a downstream subtractive process which integrates information between the two hemispheres. We tested this idea by developing a mechanistic model which was fit to widefield fluorescence data, using the same Bayesian hierarchical framework used earlier. In this model, VIS activity enhances the decision variable associated with contraversive movements and suppresses the decision variable associated with ipsiversive movements. By contrast, activity in MOs enhances both. This model could predict average psychometric behaviour, trial-by-trial variation in choices within a stimulus condition, as well as simulate the effect of optogenetic inactivation. This thesis therefore shines light on the cortical contributions towards visual discrimination behaviour. This work has implications for the neural processes underlying perceptual decision making more broadly.

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