[Electrophysiologic characteristics of representations of the auditory and somatosensory systems in the turtle midbrain].

Experiments with unanesthetized tubocurarine-immobilized turtles show that the auditory representation is localized in the medio-dorsal part of tegmentum, torus semicircularis, which contains monomodal (auditory) and bimodal (auditory and somatosensory) units. The somatosensory system is wider represented, overlapping the auditory system in the medial parts of tegmentum. The focus of auditory representation is localized in lateral parts of the dorsal tegmentum (n. intercollicularis) where monomodal somatic units were recorded. The predominance of contralateral somatic projections is revealed. Frequency-threshold curves based on threshold of evoked potentials were V-shaped but flattened. Range of perceived frequencies was equal to 40-6000 Hz, range of optimal frequencies was 100-400 Hz. Auditory mesencephalic cells may be attributed to three types of discharge patterns: phasic, sustained and burst firing. Tuning to a single optimal frequency was typical of phasic cells but most of cells with sustained firing were sensitive to two or even three frequencies.