The representation of priors and decisions in parietal cortex

Abstract

Animals actively sample their environment through orienting actions such as saccadic eye movements. Saccadic targets are selected based both on sensory evidence immediately preceding the saccade, and a salience map or prior built up over multiple saccades. In the primate cortex, the selection of each individual saccade depends on competition between target-selective cells that ramp up their firing rate to saccade release. However it is less clear how a cross-saccade prior might be represented, either in neural firing or through an activity-silent mechanism such as modification of synaptic weights on sensory inputs. Here we present evidence from magnetoencephalography for two distinct processes underlying the selection of the current saccade, and the representation of the prior, in human parietal cortex. While the classic ramping decision process for each saccade was reflected in neural firing rates (measured in the event related field), a prior built up over multiple saccades was represented via modulation of the gain on sensory inputs from the preferred target, as evidenced by rapid frequency tagging. A cascade of computations over time (initial representation of the prior, followed by evidence accumulation and then updating) provides a mechanism by which a salience map may be built up across saccades in parietal cortex. It also provides insight into why evidence accumulation signals are present in parietal cortex, when inactivation of the region has been shown not to affect performance on single-trial tasks.