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1. Echo-location of blinded Yuma bats (Myotis yumanensis) was studied after ablation of the auditory cortex (A.C.). A task of obstacle-avoidance was given to the bats. Hits and misses of strands were counted, and orientation sounds emitted by the bats during flight were recorded.
2. After bilateral ablation of A.C., two bats out of six failed to avoid even large obstacles such as 3·7 mm strands and wall. These bats emitted orientation sounds at a rate of 10-15/sec during flight, but did not change that rate before crossing the obstacles and crashed into them. In these bats, other cortical areas in addition to A.C. were probably ablated.
3. In three bats out of six, obstacle-avoidance performance was quite normal. These bats avoided even 0·2 mm strands with orientation sounds, the repetition rate of which was systematically increased before crossing the obstacles. In two of them, the dorsal half of the inferior colliculus (I.C.) was bilaterally ablated in addition to A.C. But ability to avoid the obstacles was not impaired at all. Their cerebral cortices did not show the normal positive—negative diphasic potential change in response to tonal stimuli, although the normal diphasic potential change was retained in A.C. of bats which could not echo-locate as a result of bilateral ablation of the main nucleus of I.C. A.C. appeared to be not essential for echo-location.
4. Unilateral ablation of A.C. and the internal capsule had no effect on echo-location, but bilateral ablation of them usually resulted in death from operational trauma.
5. It was suggested that A.C. was less important for sound localization in bats than in cats.
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