Hearing in the frog: a neurophysiological study of the auditory response in the midbrain

The evoked response in the torus semicircularis of Rana temporaria and R. esculenta to acoustic stimulation has been investigated. The amplitude of the potential is maximal when the recording electrode is placed 1 mm below the tectal surface. We show that responses are evoked by tones over the frequency range 50 to 3500 Hz, but that the upper frequency limit can be extended beyond 5000 Hz by mechanical vibration of the middle ear. The projection from lower centres to the torus semicircularis is organized tonotopically; fibres responding to high frequencies project more rostrally than those responding to lower frequencies. Moreover, maximum response is evoked from each region in the torus semicircularis by incident sound from a particular direction. Best responses are obtained in the anterior region of the torus semicircularis for high frequencies presented rostrally and in the posterior region by lower frequencies presented at 90° to the rostrocaudal axis. In this way, a map of auditory space is represented in the midbrain.