High-yield methods for accurate two-alternative visual psychophysics in head-fixed mice

Research in neuroscience increasingly relies on the mouse, a mammalian species that affords unparalleled genetic tractability and brain atlases. Here, we introduce high-yield methods for probing mouse visual decisions. Mice are head-fixed, facilitating repeatable visual stimulation, eye tracking, and brain access. They turn a steering wheel to make two alternative choices, forced or unforced. Learning is rapid thanks to intuitive coupling of stimuli to wheel position. The mouse decisions deliver high-quality psychometric curves for detection and discrimination and conform to the predictions of a simple probabilistic observer model. The task is readily paired with two-photon imaging of cortical activity. Optogenetic inactivation reveals that the task requires mice to use their visual cortex. Mice are motivated to perform the task by fluid reward or optogenetic stimulation of dopamine neurons. This stimulation elicits a larger number of trials and faster learning. These methods provide a platform to accurately probe mouse vision and its neural basis

Mapping perceptual decisions to cortical regions

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

Distributed coding of choice, action and engagement across the mouse brain

Vision, choice, action and behavioural engagement arise from neuronal activity that may be distributed across brain regions. Here we delineate the spatial distribution of neurons underlying these processes. We used Neuropixels probes1,2 to record from approximately 30,000 neurons in 42 brain regions of mice performing a visual discrimination task3. Neurons in nearly all regions responded non-specifically when the mouse initiated an action. By contrast, neurons encoding visual stimuli and upcoming choices occupied restricted regions in the neocortex, basal ganglia and midbrain. Choice signals were rare and emerged with indistinguishable timing across regions. Midbrain neurons were activated before contralateral choices and were suppressed before ipsilateral choices, whereas forebrain neurons could prefer either side. Brain-wide pre-stimulus activity predicted engagement in individual trials and in the overall task, with enhanced subcortical but suppressed neocortical activity during engagement. These results reveal organizing principles for the distribution of neurons encoding behaviourally relevant variables across the mouse brain