Figure 7.

Structure of feedback MSPRT models. (A) Feedback implementation of MSPRT following Bogacz (2009). The basal ganglia are part of a cortico-cortical feedback loop that provides an excitatory feedback signal representing the logarithm of the posterior probability. Note that in this implementation the integration of the sensory evidence is mediated by the cortico-cortical feedback loop involving the basal ganglia. The yellow boxes with “Δt” in them indicate some time delay in the feedback loop. (B) Alternative feedback implementation of MSPRT. The feedback signals in (A) have been separated into an excitatory component responsible for integrating the sensory evidence and an inhibitory component responsible for normalizing the posterior probabilities. This implementation no longer requires the basal ganglia to participate in the temporal accumulation process. Furthermore, an excitatory offset has been added to the inputs of all integrators to move the operating point of the circuit into the positive range (recall that the logarithm of a probability is a negative value). As long as the same offset is added to the input of all integrators, this does not affect the result of calculating the posterior probabilities. Thus, each integrator now carries the sum of the logarithm of the posterior probability and the offset. The integrators now also have the property that they cannot represent negative values (like the integrators in the earlier models).