Increases in auditory middle frequency discrimination thresholds after cortical ablations

Abstract

Monkeys were trained to respond to two tones (4000 and 5000 Hz) above selected criterion, using frequency discrimination conditioned avoidance procedures. The 4000-Hz tone was increased in frequency through a series of steps approaching 5000 Hz. When the animal fell below 75% correct response criterion and remained below this threshold in spite of several retraining attempts, the specific frequency was considered the monkey's threshold. Five of the seven animals reached relative thresholds of 30–50 Hz, one reached 60 Hz, another 125 Hz. Four of the seven monkeys having preoperative thresholds of 30–50 Hz showed remarkable increases in relative thresholds after cortical ablation (400–900 Hz). Cortical lesions in these monkeys were located in the middle third of the superior temporal gyrus, including damage to the opercular and insular cortices. Two other animals, in which superficial and minimal cortical removals were performed, showed no increase in threshold. One monkey with bilateral ablations performed in the superior third of the superior temporal gyrus demonstrated minimum increase in threshold of 100 Hz. Bilateral lesions involving not only the surface cortex of the middle portion of the superior temporal gyrus but also the opercular and insular cortices are necessary to produce maximum increases in relative threshold. Lesions in the superior third of the superior temporal gyrus produced only minimal increases in threshold.