Tonotopic representation and space map in the non-primary auditory cortex of the mustached bat.

As auditory system has no sensory epithelium into which auditory space are projected, we studied the physiological map of the auditory space in the non-primary auditory cortex of the mustached bat by using the echo of their orientation sound. Ten bats were used as experimental subjects. Tungsten wire electrodes were inserted obliquely in the dorsomedial (DM) and ventroposterior (VP) areas of the non-primary auditory cortex. When single neuron was isolated, best frequency (BF), best azymuth (BAZ) and best elevation (BEL) were measured and were plotted on a schematic figure. To mimic its biosonar, one loudspeaker, delivering synthesized orientation sounds, was placed in front of the animal, and another loudspeaker delivering synthesized echo was mounted on a movable hoop. Tonotopic representation was observed but complicated in both areas, and those areas could be divided into several subdivisions consisting of the neuron groups characterized by three frequency bands. The neurons were thought to be related to the processing of biosonar informations from the facts that their BFs agreed with the scope of the FM sweep of each echo harmonics. The magnitude of the response showed rapid increase at their BAZ or BEL, so that the neurons seemed to tune to a certain direction in the auditory space. Especially in the DM area, neurons assumed a systematic arrangement of their BAZs on the cerebral surface and showed some tendency of a systematic arrangement of their BELs. The DM area was thought to have a kind of neural map of the auditory space.