SOUND LOCALIZATION IN CRICKETS .1. CONTRALATERAL INHIBITION OF AN ASCENDING AUDITORY INTERNEURON (AN1) IN THE CRICKET GRYLLUS-BIMACULATUS

Intracellular recordings were made in the brain of the cricket Gryllus bimaculatus from an ascending auditory interneuron (AN1). Acoustic st imuli with calling song temporal pattern were delivered via earphones in a preparation with the ''acoustic'' trachea cut (attenuation of cro ssing sound >30 dB). The input-output function of this cell was then d etermined by recording its responses to stimulation of the ipsilateral ear alone, of the contralateral ear alone and to stimulation of both ears simultaneously with the same or different carrier frequencies and intensities. This interneuron was excited by the ear ipsilateral to i ts axon and dendritic field and unresponsive to stimuli presented to t he axon-contralateral ear alone. However, in binaural stimulation expe riments, the response to a constant ipsilateral stimulus was progressi vely reduced as the intensity of a simultaneous contralateral stimulus was increased, above a threshold intensity. Tuning curves for thresho ld of this inhibition, determined in binaural stimulation experiments, indicated significant inhibition in the range 3-20 kHz with lowest th reshold at 4-5 kHz. The inhibition was unaffected by sectioning of the contralateral circumoesophageal or neck connective, indicating that t he inhibitory influence crosses the midline at the level of the protho racic ganglion. Intracellular recordings from AN1 in the prothoracic g anglion confirmed that it was indeed neurally inhibited by inputs from the contralateral ear. Tuning curves for excitation of an omega neuro n (ON1) by the ear ipsilateral to its soma and also the tuning of inhi bition of ON1 by its contralateral ON1 partner, closely match the tuni ng of inhibition of AN1 and to a lesser extent, of AN2. This was taken as evidence that each AN1 is inhibited by the 'contralateral' ON1. Th e significance of this interaction for directional hearing and phonota xis is discussed.