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Neural mechanisms of decision making when sensory evidence and priors conflict

Poster Session B - Sunday, March 8, 2026, 8:00 – 10:00 am PDT, Fairview/Kitsilano Ballroom

Stephanie L Wert¹ (stephaniewert@g.ucla.edu), Jesse Rissman^1,2, Barbara J Knowlton¹; ¹University of California, Los Angeles, Department of Psychology, ²University of California, Los Angeles, Department of Psychiatry & Biobehavioral Sciences

Humans form statistical priors through experience that bias perception, but accurate decisions require overriding these biases when evidence contradicts them. We examined brain mechanisms that reflect bias override in an fMRI study (N=38) using a perceptual decision-making task with implicitly learned priors. On each trial, participants judged the orientation of Glass pattern dot stimuli presented in red or green color with varying levels of coherence (0%, 13%, 35%, or 100%). Unbeknownst to participants, for stimuli in one color, one orientation occurred on 75% of the trials (positive prior) while stimuli in the other color occurred equally often in the two orientations (neutral prior). Most participants became sensitive to this base-rate prior across learning, reflected in a bias to choose the more frequently occurring direction on 0% coherence trials in which no diagnostic information was present. In the final block, we isolated moderately ambiguous trials (13/35% coherence) where the prior and sensory evidence conflicted. Participants either relied on the prior and erred or overrode it by using the weak sensory evidence to respond correctly. We found an interaction between prior condition (positive or neutral) and decision type (prior-based or sensory-based) in activation in the bilateral fusiform cortex and left ventral putamen. These results showed increased activity in these regions when participants overcame the acquired bias and based their decisions on the sensory evidence. These findings suggest that the putamen may help gate or suppress a biased response, while fusiform cortex enhances ambiguous sensory encoding, thereby supporting flexible perceptual decision-making.

Topic Area: THINKING: Decision making