Interplay of excitation and inhibition elicited by tonal stimulation in pyramidal neurons of primary auditory cortex.

Tonal responses of neurons in the primary auditory cortex are a function of frequency, intensity and ear of stimulation. These responses occasionally display suppression. This review discusses how excitatory and inhibitory synaptic inputs interact to form suppressive responses and how changes in stimulus attributes affect the magnitude and timing of those responses. Stimulation at the characteristic frequency evokes a stereotyped sequence of depolarization (excitatory) and then hyperpolarization (inhibitory), as predicted from the canonical circuitry. Some neurons stimulated at higher sound intensities display a prominent increase in the magnitude of hyperpolarization or a decrease in its latency, both enabling counteraction with the preceding excitation. These interactions, in part, underlie the non-monotonic suppression. Furthermore, monaural non-dominant ear stimulation elicits such a powerful hyperpolarization as to cancel out the depolarization elicited at dominant ear stimulation, suggesting a linear mechanism for the binaural suppression. Alternatively, it elicits a depolarization almost equal in magnitude and time course to that elicited at binaural stimulation, suggesting a nonlinear interaction responsible for the suppression. Laminar differences are also noted for these inhibitory interactions.