Synaptic mechanisms of monaural and binaural processing in the locust

(1) Responses of auditory interneurones were recorded intracellularly within the metathoracic ganglion of the locust when stimulating each tympanic membrane with a piezoelectric transducer. Thus, in contrast to conventional sound stimulation, each of the two ears could be activated independently from the other at variable intensities, duration and stimulus onsets. By means of this ‘earphone-like’ stimulation technique the binaural integration properties of auditory interneurons could be analysed. (2) A minority of units (3 out of 43) was affected by input from one side only. Their synaptic input was purely excitatory and the intensity characteristics reflected those of auditory receptor fibres. (3) Most interneurones received input from both ears, each being excitatory or one excitatory or one excitatory and one inhibitory. In some units the unilateral synaptic response already included both an EPSP and an IPSP. As a result of varying temporal interactions between the EPSP and the IPSP within the unilaterally evoked complex response the intensity characteristics differed widely from unit to unit. (4) With binaural simultaneous stimulation the complexity of the postsynaptic responses of most interneurones increased as the synaptic input from both ears coincided at the level of the recorded interneurone. Although both ears were stimulated symmetrically (at the same time and intensity), units were recorded where the latencies of ipsilateral and contralateral synaptic input were different. Contralateral inhibition could either follow or precede ipsilateral excitation and in some cases both EPSP and IPSP had the same latency. On the basis of these findings the binaural synaptic mechanisms of directional coding are discussed and compared with corresponding results under free field stimulus conditions.