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 prima ry auditory cortex of anesthetized cats were studied before and after temporary threshold shifts in cochlear sensitivity induced by an inten se pure tone. Cochlear temporary threshold shift was monitored through the threshold of the gross auditory nerve compound action potential a nd in most cases involved a notch-like loss centered at the characteri stic frequency of the unit under study. Only two neurons showed change s in response area that mirrored the changes at the auditory periphery . Most neurons (14) showed more complex changes involving both expansi on 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 cl assified 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. Contracti on of both upper (high intensity) and lower boundaries of response are as was found; in the most extreme cases, neurons were totally unrespon sive after the intense-tone exposure. The complexity of effects of tem porary threshold shifts on the response areas of cortical neurons is l ikely to be related to mechanisms that normally determine the frequenc y response limits of these neurons. The response areas of cortical neu rons are more complex than those of auditory nerve fibers, and are tho ught to reflect the integration of excitatory and inhibitory inputs. T he variety of effects observed in this study are consistent with the e xcitatory and inhibitory components of the response area of a given ne uron being differentially affected by the temporary threshold shift.