ACOUSTIC INTERNEURONS IN BRAIN OF NOCTUID MOTHS

Ultrasonic stimuli were directed at one tympanic organ of various noctuid moth species. At low intensities these stimuli excite only one tympanic sense cell. Interneuron responses to this minimal input were detected in the suboesophageal and protocerebral ganglia. Typically, one phasic and one or two tonic spike sequences were detected on both sides of the brain. Commonly, these signals remained stable for hours in response to one or two 40 kHz pulses/sec delivered at low intensities. Sometimes intensities 20 to 30 dB higher partially or completely suppressed one of the tonic spike responses while leaving the other unaffected. In some preparations the protocerebral response did not remain steady but showed abrupt and marked lapses in acoustic excitability lasting from a few seconds to an hour. In a few visual stimulation caused temporary failure of the protocerebral response to low sound intensities. Return of the acoustic response might be delayed several minutes after with-drawal of the visual stimulus. Lapses in the acoustic response were frequently spontaneous and unpredictable, and on recovery the acoustic response was often more sensitive than before. Suppression by high sound intensities and lapses in responsiveness of the protocerebral acoustic response are discussed in relation to the acoustic avoidance behaviour shown by noctuid moths. © 1969.