SENSORY BASIS FOR SOUND INTENSITY DISCRIMINATION IN THE BUSH-CRICKET REQUENA-VERTICALIS (TETTIGONIIDAE, ORTHOPTERA)

The ability of the female bushcricket, Requena verticalis, to discrimi nate between two conspecific sound signals that differed in sound pres sure level (SPL) was tested in a two-choice paradigm. Significant disc rimination was achieved with a 2-dB difference. The property of each p air of receptors to establish binaural discharge differences was inves tigated in electrophysiological experiments. The threshold to the cons pecific signal varies for each fibre from about 40 to 90 dB SPL, allow ing for a ran,se fractionation of the hearing organ. Each pair of rece ptors establishes significant binaural discharge differences only with in a restricted intensity range about 10 dB above threshold. Based on a model of the intensity response function of a receptor the total dis charge of the 22 receptors in both ears was calculated with monaural a nd binaural stimulation. The profile of receptors exhibiting significa nt discharge differences changes with increasing SPL, from the most se nsitive fibres with a characteristic frequency between 12 kHz and 35 k Hz at low SPLs to the least sensitive fibres at very low and high char acteristic frequencies at medium to high SPLs. The discharge differenc e with an intensity difference of 2 dB is rather small (4% of the tota l receptor activity) and limited only to a few pairs of receptors.