Modulation of auditory responsiveness in the locust.

The auditory responsiveness of a number of neurones in the meso- and metathoracic ganglia of the locust, Locusta migratoria, was found to change systematically during concomitant wind stimulation. Changes in responsiveness were of three kinds: a suppression of the response to low frequency sound (5 kHz), but an unchanged or increased response to high frequency (12 kHz) sound; an increased response to all sound; a decrease in the excitatory, and an increase in the inhibitory, components of a response to sound. Suppression of the response to low frequency sound was mediated by wind, rather than by the flight motor. Wind stimulation caused an increase in membrane conductance and concomitant depolarization in recorded neurones. Wind stimulation potentiated the spike response to a given depolarizing current, and the spike response to a high frequency sound, by about the same amount. The strongest wind-related input to interneuron 714 was via the metathoracic N6, which carries the axons of auditory receptors from the ear. The EPSP evoked in central neurones by electrical stimulation of metathoracic N6 was suppressed by wind stimulation, and by low frequency (5 kHz), but not high frequency (10 kHz), sound. This suppression disappeared when N6 was cut distally to the stimulating electrodes. Responses to low frequency (5 kHz), rather than high frequency (12 kHz), sounds could be suppressed by a second low frequency tone with an intensity above 50-55 dB SPL for a 5 kHz suppressing tone. Suppression of the electrically-evoked EPSP in neurone 714 was greatest at those sound frequencies represented maximally in the spectrum of the locust's wingbeat. It is concluded that the acoustic components of a wind stimulus are able to mediate both inhibition and excitation in the auditory pathway. By suppressing the responses to low frequency sounds, wind stimulation would effectively shift the frequency-response characteristics of central auditory neurones during flight.