Cerebral Mechanisms in Auditory Localization. ∗

Abstract

Bikov 1 has reported that following complete transection of the corpus callosum it was possible to condition a dog to salivate to the sound of a whistle placed at the level of, and some distance from, the left ear. He was unable to subsequently develop a discrimination so that the animal did not salivate to the same stimulus sounded from its right side. Pavlov 2 concluded from this study that “a differentiation of the direction of a sound required a united activity of both hemispheres.”

Bikov's data and Pavlov's interpretation are consistent with the view that hearing is crossed. That is, impulses produced in the left cochlea end in the right hemisphere, while those impulses originating in the right cochlea terminate in the left hemisphere. Since both ears are essential for the correct localization of sounds in space (L-R habit), destruction of the corpus callosum would prevent the integration of the impulses produced in the peripheral mechanisms. From this logic, it would also follow that extirpation of the auditory cortex of one hemisphere (temporal lobe) would have the same effect as does the destruction of one cochlea.

Recent studies, however, indicate that this view is incorrect. Results procured in the dog 3 and the cat 4 support the contention that hearing involves uncrossed as well as crossed components. Impulses from each cochlea reach both cortices. There is also some question as to the reliability of the salivary conditioning technic when it is employed in studies involving cerebral extirpation. Zeliony, 5 using this method failed to establish conditioned reflexes in a totally decorticate dog, wherleas positive results have been reported both by Culler 6 and Girden 7 who made use of the motor conditioning technic. In the light of these facts, a retest of the question was made with the latter method, The L-R habit⁸ was established by training the dogs to flex a forepaw upon the presentation of an auditory stimulus (buzzer or pure tone) from its right side, but to inhibit this response if the stimulus was sounded from the left side. All surgical operations were made under aseptic conditions and general, injection, anesthesia.