Effects of signal duration on the recognition of masked communication signals by the grasshopper Chorthippus biguttulus.

The detection and recognition of acoustic communication signals masked by noise was investigated in a grasshopper (Chorthippus biguttulus) whose auditory system exhibits only poor spectral resolution and therefore has to operate in the time domain. The signals of this species consist of numerous identical subunits that enable the receiver, in principle, to make repetitive measurements. We aimed at determining the maximum integration time in this species by using stimuli of different durations under increasing noise levels. As a criterion for recognition the typical phonotactic turning response of the males was evaluated, which is reliably triggered by a female song, and thus is a sensitive indicator for recognition of conspecific signals. When confronted with a long signal (1000 ms) males tolerated a 2.4 dB higher noise level as compared to a short signal (250 ms). Noise tolerance improved with increasing signal duration from 250 ms to 450 ms. Beyond this signal duration, however, no further improvement was observed, indicating an upper limit for temporal integration that corresponds to only five song subunits. The gain in noise tolerance had a slope of 2.7 dB per doubling duration, which corresponds to the expectation derived from an energy detector model (3 dB per doubling duration) rather than to the value expected from signal detection theory (1.5 dB per doubling duration).