|dc.description.abstract||Uncertainty in various modalities (perceptual, motor, cognitive) is a fundamental problem that the brain must solve to ensure optimal responses to the demands of the environment. It has been shown theoretically that the optimal solution when faced with uncertainty is to represent and propagate full probability distributions over variables of interest and perform probabilistic inference according to Bayes’ rule. Previous studies in humans and other animals have shown that the brain does indeed implement a version of Bayesian Inference. Several computational theories have proposed different models for the underlying representation of probability distributions that this requires. The most prominent of these are Probabilistic Population Codes (PPC) and Sampling-Based Codes (SBC). However, existing studies have been unable to prove or disprove these theories conclusively, especially due to the lack of neural population-level recordings concurrent with single-trial measures of uncertainty.
In the present thesis, I investigated the representation of perceptual uncertainty in the brain, using the mouse primary visual cortex as a model system. I recorded the activity of large neuronal populations in vivo using two-photon calcium-imaging. I employed a variety of both passive (presentation of visual grating stimuli on a monitor) and active (visually-guided goal-directed behaviour) visual stimulation paradigms to manipulate the sensory uncertainty of the animals. Additionally, I derive a trial-by-trial behavioural uncertainty measure based on the licks that the animals make.
My results show that: 1) repeated exposure to behaviourally-relevant (but not neutral) stimuli increases the precision of their representation in the cortex. 2) In a passive viewing setting, manipulation of visual contrast modulates neural responses in a manner partially consistent with both PPC and SBC, with higher contrasts evoking, in general, larger mean responses and larger response variance, and leading to more discriminable stimulus representations. 3) During a two-alternative forced choice task, mouse behavioural uncertainty was modulated in an asymmetrical way by stimulus uncertainty (contrast/aperture) and orientation. Behavioural responses were explained by similarity of neural population responses to two stimulus “archetypes”.||en