Maintaining acoustic communication at a cocktail party: heterospecific masking noise improves signal detection through frequency separation

Abstract

SUMMARYWe examined acoustic masking in a chirping katydid species of the Mecopoda elongata complex due to interference with a sympatric Mecopoda species where males produce continuous trills at high amplitudes. Frequency spectra of both calling songs range from 1 to 80 kHz; the chirper species has more energy in a narrow frequency band at 2 kHz and above 40 kHz. Behaviourally, chirper males successfully phase-locked their chirps to playbacks of conspecific chirps under masking conditions at signal-tonoise ratios (SNRs) of −8 dB. After the 2 kHz band in the chirp had been equalised to the level in the masking trill, the breakdown of phase-locked synchrony occurred at a SNR of +7 dB. The remarkable receiver performance is partially mirrored in the selective response of a first-order auditory interneuron (TN1) to conspecific chirps under these masking conditions. However, the selective response is only maintained for a stimulus including the 2 kHz component, although this frequency band has no influence on the unmasked TN1 response. Remarkably, the addition of masking noise at 65 dB sound pressure level (SPL) to threshold response levels of TN1 for pure tones of 2 kHz enhanced the sensitivity of the response by 10 dB. Thus, the spectral dissimilarity between masker and signal at a rather low frequency appears to be of crucial importance for the ability of the chirping species to communicate under strong masking by the trilling species. We discuss the possible properties underlying the cellular/synaptic mechanisms of the 'novelty detector'. Supplementary material available online at

Novel underwater soundscape: acoustic repertoire of plainfin midshipman fish

“…Background noise can enhance the detection of auditory stimuli through a mechanism known as stochastic resonance (Jaramillo and Wiesenfeld, 1998). Recent studies of katydids show that the longlasting background trill of one species enhances the detection of conspecific chirps by a closely related, sympatric species (Siegert et al, 2013). In the case of the midshipman soundscape, the most prominent background noise is the hum of male conspecifics.…”

Processing of simple and complex acoustic signals in a tonotopically organized ear

“…A typical chirp of such a calling song consists of 10-14 syllables with increasing amplitude along the chirp structure [3,4]. The chirps, as well as each syllable, contain frequencies from about 2 up to at least 80 kHz [5,6]. Males have evolved a mechanism that leads to synchronous, alternating and phaselocked entrainment [4][5][6][7].…”

Insect Hearing

“…Because matched frequency fi ltering will typically make conspecifi c frequencies appear louder than heterospecifi c frequencies, this may help to fi lter out heterospecifi c "noise." That this mechanism can allow preferential representation and attention to conspecifi c signals in receiver auditory systems, even in the presence of spectral overlap in signals, has been shown in some elegant neurophysiological experiments on both crickets and katydids Siegert et al 2013 ).…”