Accumulation of Evidence in Perceptual Decisions

Recent research has accumulated insights into the neural processes underlying perceptual decision making by using mathematical models and neurophysiological experiments with monkeys. The studies performed on monkeys suggest that a decision is made by accumulating evidence in favor of the decision alternatives, until a decision threshold is reached. This process is closely linked to the planning of the motor response to a given choice, and the spike rate in the neurons performing this calculation show sustained activity until a response can be made. In order to investigate if an area in the human brain shows the same activations as accumulation neurons in the monkey brain, functional magnetic resonance imaging (fMRI) was used to record blood oxygenation level dependent (BOLD) responses while participants performed a forced two-choice perceptual decision making task with face and house stimuli containing a varying degree of visual noise. A 2 by 2 factorial design was used to investigate if there are brain regions that show both (a) greater activation for harder compared to easier trials in a reaction time task and (b) greater activation for easier compared to harder trials in a delayed response task. No such area was found, suggesting that humans rely on a different decision making mechanism than monkeys, where the accumulation of evidence is dissociated from the maintenance of the decision. The left intraparietal sulcus (IPS) and the dorsomedial prefrontal cortex (DMPFC) were identified as potential accumulator regions, showing more activity for hard than easy decisions across response conditions. A region in the left superior frontal gyrus was identified as a candidate for maintaining the decision until a response cue appears