Influence of vibration on cricket hearing: Interaction of low frequency vibration and acoustic stimuli in the omega neuron

Abstract

1. Low frequency substrate vibration affects the activity of the omega neuron (Wohlers and Huber 1978) in the cricket prothoracic ganglion (also called the large segmental auditory neuron; Popov et al. 1978). The details of this interaction have been studied inGryllus bimaculatus with extra- and intracellular recordings. 2. The threshold for excitation of the auditory omega interneuron by vibration is very high: 500 cm/ s2 acceleration (in the frequency range 100–800 Hz) or 300cm/s2 (1,000–2,000 Hz). This high threshold results in part from an active inhibitory process. 3. At accelerations of 3 cm/s2 or greater, vibrations in the range 100–2,000 Hz applied to the ipsilateral foreleg together with acoustic stimuli inhibit the response of the omega neuron to the acoustic stimulus. Vibration of the middle leg does not elicit this inhibition. 4. At a given acceleration the inhibitory effect of the various vibration frequencies tested is the mirror image of the threshold curve of the vibratory multi-unit response in the leg nerve as measured by Dambach (1972). This indicates that several vibration sensors in the foreleg (campaniform sensilla and subgenual organ) send inputs to the inhibitory circuit. 5. The amplitude of inhibition, expressed as the amount by which the response of the neuron to a 4.5-kHz, 70-dB sound pulse 70 ms long is reduced, is 10% for 3 cm/s2 at 400 Hz and averages 60% for 100 cm/s2 at 400 Hz. If vibratory and acoustic stimulus pulses 16 ms long are presented simultaneously, a 40% reduction in the response to a 60-dB sound is achieved with a vibration of only 25 cm/s2 at 400 Hz. 6. The peak of the inhibitory effect occurs 15–20 ms after the onset of vibration. The inhibition phase ends 40 ms after onset of the vibration pulse. 7. The NF neuron (Rheinlander et al. 1976) in the circumesophageal connective ofG. bimaculatus is also inhibited in its response to sound by vibration applied to the ipsilateral foreleg. 8. When synchronous acoustic and vibratory stimuli are presented in the temporal pattern of the conspecific song, the inhibitory circuitry can bring about an emphasis of the end of the syllable and of the pause in the song. A way in which this effect might occur even when the stimulus is entirely acoustic is discussed.