PATTERNS OF ACOUSTICALLY EVOKED DISCHARGES OF NEURONS IN THE MESENCEPHALON OF THE BULLFROG

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Abstract
Exploration of the midbrain of the anesthetized bullfrog with microelectrodes indicated that responses evoked by auditory and vibratory stimuli were restricted to the torus semicurcularis. Units in the torus responded only to sound, only to vibration, or to both. Recordings from neurons responding to auditory stimuli indicate that most of them responded only with a short burst of spikes even when 10 second tones were used; a few, however, responded continually to such stimuli. The range of effective frequencies extended from below 100 cps to about 5000 cps. The range of the most sensitive thresholds was -3 to 10 decibels re 0.0002 dyne/cm2 at 200 to 1500 cps. Individual units were tuned more or less sharply to certain frequencies. A rise in stimulus intensity for a given frequency often caused a consistent increase in spike counts and decrease in initial latent period. For many units, however, at some or all effective frequencies, such a rise diminished or completely suppressed the activity and/or increased initial latent periods, presumably due to neural inhibition provoked by loud stimuli. A few units (some 10%) discharged most vigorously to a tone of specified length, and exhibited reduced activity with tones of either longer or shorter duration.