Changes in auditory frequency discrimination thresholds after temporal cortex ablations

Abstract

Male monkeys (Macaca mulatta) were trained to discriminate two tones at 500 and 1000 Hz (cycle/sec) following normal conditioned avoidance procedures. At 500 and 1000 Hz a criterion of 90% correct response (CR) was required of each subject. The 1000-Hz tone was then reduced through a series of steps until 503 Hz was reached. All subjects were able to discriminate at 5-Hz separation of the two tones well above threshold (75% CR). At this point, bilateral ablation of portions of the primary and associated auditory cortices was performed. The animals were allowed to recover for 7 days, then were retested using the same discrimination procedure. Bilateral ablation of various portions of the postcentral gyrus, the superior temporal gyrus and the insular cortex caused varying increases in discrimination thresholds. One animal in which the ventral half of the superior temporal gyrus and the ventral limb of the insular cortex lying deep to the sylvian fissure were destroyed showed an increase in frequency discrimination threshold of only 7 Hz. On the other extreme, one animal in which the entire superior temporal gyrus and the extreme inferior portion of the postcentral gyrus were removed in depth showed an increase in threshold of between 25 and 30 Hz. This lesion included about 40% damage to the insular cortex. The remaining subjects exhibited increases in thresholds lying between 7 and 20 Hz depending upon the location and amount of damage to the superior temporal gyrus. Evoked auditory potentials were recorded from the viable cortex surrounding the lesions and typical surface response maps were constructed for each animal.