Directional hearing in grasshoppers: Neurophysiological testing of a bioacoustic model

A recently proposed biophysical model for directional hearing in grasshoppe rs was tested using complex stimulus situations, with two loudspeakers, one on either side of the animal, synchronously emitting sinusoids with define d phase and amplitude relationships, Hearing responses were determined from whole nerve recordings and compared with the predictions of the model. In Schistocerca gregaria, there were only minor differences between the predic tions of the model and measurements and, by reducing the value of the gain of the internal sound path measured previously, a close agreement was achie ved between model and measured hearing responses. In Chorthippus biguttulus , larger discrepancies between model calculations using the values measured previously and neuronal response functions were found in both shape and am plitude. A better fit between measurements and model predictions was achiev ed by increasing the values of the internal delay over those measured previ ously. The measurements presented here indicate high inter-individual varia bility of the parameters of the internal pathway, with a range of 60 degree s for the internal phase delay. Calculating the directional characteristics using this range of values for the internal delay indicated that sufficien t directional information was available down to 5 kHz. Increasing the value of the internal delay over that measured in an earlier study therefore pro vides an explanation for the discrepancy between the poor directional infor mation attributed to C. biguttulus in that study and the excellent laterali zation ability of males of this species at 5 kHz.