Analog simulation of prepyriform cortex in the cat

Abstract

The prepyriform cortex contains three types of intracortical feedback loops, negative, positive excitatory, and positive inhibitory, representing the possible modes of interaction in a mixed population of excitatory and inhibitory neurons. This system can be described by a linear differential equation with three variable coefficients corresponding to the three types of feedback gain. Solutions for the equation by analog computation for the appropriate initial conditions constitute the “single-shock evoked potential” for the model. Solutions are presented for four conditions, in each of which a single variable in the equation is modified. The changes in the output of the model are shown to replicate four patterns of variation previously described for the averaged evoked potential (AEP) of the prepyriform cortex. (1) An increase in AEP amplitude with food deprivation was replicated in the model by increasing forward gain of the system. (2) The increase in amplitude and decrease in frequency of AEP that occurs with increasing stimulus intensity was replicated by means of limiting diodes placed in the feedback limbs of the model. (3) A combined change in AEP amplitude, frequency, decay rate, and (inversely) phase, representing “spontaneous” variation in AEP, was replicated by decreasing network gain. (4) A decrease in phase and frequency of AEP associated with long-term learning was replicated by a relative increase in the gain of the positive excitatory feedback loop. The model is generally stable but can be rendered unstable in each of two ways (monotomic and oscillatory) previously observed in the prepyriform cortex. The role of the positive inhibitory feedback loop is shown to be particularly important in the maintenance of cortical stability.