RAPID CHANGES IN THE FREQUENCY TUNING OF NEURONS IN CAT AUDITORY-CORTEX RESULTING FROM PURE-TONE-INDUCED TEMPORARY THRESHOLD SHIFT

The response areas (frequency by intensity) of single neurons in primary auditory cortex of anesthetized cats were studied before and after temporary threshold shifts in cochlear sensitivity induced by an intense pure tone. Cochlear temporary threshold shift was monitored through the threshold of the gross auditory nerve compound action potential and in most cases involved a notch-like loss centered at the characteristic frequency of the unit under study. Only two neurons showed changes in response area that mirrored the changes at the auditory periphery. Most neurons (14) showed more complex changes involving both expansion and contraction of response areas. Expansion of response areas was indicated by lower thresholds at some frequencies and by the emergence of sensitivity to previously ineffective frequencies. A change was classified as contraction when the response area after the intense-tone exposure was smaller than would be expected by applying the profile of the temporary threshold shift to the initial response area. Contraction of both upper (high intensity) and lower boundaries of response areas was found; in the most extreme cases, neurons were totally unresponsive after the intense-tone exposure. The complexity of effects of temporary threshold shifts on the response areas of cortical neurons is likely to be related to mechanisms that normally determine the frequency response limits of these neurons. The response areas of cortical neurons are more complex than those of auditory nerve fibers, and are thought to reflect the integration of excitatory and inhibitory inputs. The variety of effects observed in this study are consistent with the excitatory and inhibitory components of the response area of a given neuron being differentially affected by the temporary threshold shift.