The effect of reward expectation on the time course of perceptual decisions

Perceptual discriminations can be strongly biased by the expected reward for a correct decision but the neural mechanisms underlying this influence are still partially unclear. Using functional magnetic resonance imaging (fMRI) during a task requiring to arbitrarily associate a visual stimulus with a specific action, we have recently shown that perceptual decisions are encoded within the same sensory-motor regions responsible for planning and executing specific motor actions. Here we examined whether these regions additionally encode the amount of expected reward for a perceptual decision. Using a task requiring to associate a gradually unmasked female vs. male picture with a spatially-directed hand pointing or saccadic eye movement, we examined whether the fMRI time course of effector-selective regions was modulated by the amount of expected reward. In both the pointing-selective parietal reach region (PRR) and the saccade-selective posterior intraparietal region (pIPS), reward-related modulations were only observed after the onset of the stimulus, during decision formation. However, while in the PRR these modulations were specific for the preferred pointing response, the pIPS showed greater activity when either a saccadic or a pointing movement was associated with a greater reward relative to neutral conditions. Interestingly, the fusiform face area showed a similar reward-related but response-independent modulation, consistent with a general motivational signal rather than with a mechanism for biasing specific sensory or motor representations. Together, our results support an account of perception as a process of probabilistic inference in which top-down and bottom-up information are integrated at every level of the cortical hierarchy.