Sound transmission and directional hearing in field crickets: Neurophysiological studies outdoors

Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, UK.
Journal of Comparative Physiology (Impact Factor: 2.04). 09/2010; 196(9):669-81. DOI: 10.1007/s00359-010-0557-x
Source: PubMed


Many studies provide detailed behavioural and neurophysiological information on the ability of crickets to localize a sound source under ideal acoustic conditions, but very little is known about how they perform in real habitats. We investigated directional hearing of crickets in the field using a neurophysiological approach, by recording the activity of the two prominent, bilaterally homologous AN1 neurons simultaneously in a cricket's habitat. The discharge and latency differences of the pair of neurons in response to conspecific chirps presented at different distances and directions were taken as a measure of directional information. The maximum hearing distance differed between individuals and weather conditions from 1 to 15 m (mean 9.2 m). Although the AN1 activity generally decreased with increasing distance, large fluctuations in the magnitude of responses occurred with distance, indicating that the intensity gradient over distance is often irregular. The directional information provided in the discharge differences of the two neurons also varied with distance. Again, there was no simple directional gradient on the transmission channel; rather, with decreasing distance to the source there were receiver locations providing suprathreshold responses, but no directional information. The consequences for the ability of field crickets to communicate acoustically close to the ground are discussed.

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Available from: Konstantinos Kostarakos
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    • "In a field study on acoustic preferences of female G. bimaculatus, Hirtenlehner and Römer (unpublished) found that differences in carrier frequency, intensity and chirp rate of the calling song needed to be larger for a significant preference compared to results under laboratory conditions. Several factors such as degradation of directional cues, species-specific amplitude modulations (AM) and non-linear intensity gradients over distance may have contributed to this difference in the observed behavioural performance (Römer 1998, 2001; Kostarakos and Römer 2010; Bradbury and Vehrencamp 2011). Furthermore, females are not completely free to walk towards a sound source in their natural environment, but have to circumvent obstacles in the transmission channel. "
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    ABSTRACT: A modified tracheal system is the anatomical basis for a pressure difference receiver in field crickets, where sound has access to the inner and outer side of the tympanum of the ear in the forelegs. A thin septum in the midline of a connecting trachea coupling both ears is regarded to be important in producing frequency-dependent interaural intensity differences (IIDs) for sound localization. However, the fundamental role of the septum in directional hearing has recently been challenged by the finding that the localization ability is ensured even with a perforated septum, at least under controlled laboratory conditions. Here, we investigated the influence of the medial septum on phonotaxis of female Gryllus bimaculatus under natural conditions. Surprisingly, even with a perforated septum, females reliably tracked a male calling song in the field. Although reduced by 5.2 dB, IIDs still averaged at 7.9 dB and provided a reliable proximate basis for the observed behavioural performance of operated females in the field. In contrast, in the closely related species Gryllus campestris the same septum perforation caused a dramatic decline in IIDs over all frequencies tested. We discuss this discrepancy with respect to a difference in the phenotype of their tracheal systems.
    Full-text · Article · Nov 2013 · Journal of Comparative Physiology
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    • "When females are free to orient and approach a sound source, such as in arena trials and outdoors, the changes in the female's position relative to the sound sources and some random deviations from a straight line to one source produce larger deviations in the amplitude of the two signals than those inherent to pure-tone or modulated signals. (2) Under natural conditions in the field, the properties of the transmission channel result in even larger random variations in the signal amplitude of alternative signals (Römer, 1998; Römer, 2001; Kostarakos and Römer, 2010), so that the minute differences between pure tone and modulated songs should play no role. For example, in an ongoing outdoor study, female G. bimaculatus needed a 5 dB difference in call amplitude for a significant approach to the louder signal (S.H. and H.R., unpublished), whereas on the trackball the minimum difference was 1–2 dB (Hedwig and Poulet, 2005). "
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    Full-text · Article · Mar 2013 · Journal of Experimental Biology
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    • "We did this by using two cricket species which communicate acoustically in the nocturnal tropical rainforest for which high masking noise levels have been reported [4], [39]. We take advantage of the fact that for acoustic insects experimental approaches are available to examine single neurons of the afferent auditory pathway in a portable preparation, which can be placed at any position outdoors, and its responses to conspecific stimuli under natural background be compared [40], [41], [42]. Our results show that three ‘bottom-up’ mechanisms exist in the afferent auditory pathway of tropical crickets, namely selective frequency filtering, spatial release from masking, and a gain control, which all contribute to the excellent performance of signal detection in high background noise levels. "
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    ABSTRACT: Insects often communicate by sound in mixed species choruses; like humans and many vertebrates in crowded social environments they thus have to solve cocktail-party-like problems in order to ensure successful communication with conspecifics. This is even more a problem in species-rich environments like tropical rainforests, where background noise levels of up to 60 dB SPL have been measured. Using neurophysiological methods we investigated the effect of natural background noise (masker) on signal detection thresholds in two tropical cricket species Paroecanthus podagrosus and Diatrypa sp., both in the laboratory and outdoors. We identified three 'bottom-up' mechanisms which contribute to an excellent neuronal representation of conspecific signals despite the masking background. First, the sharply tuned frequency selectivity of the receiver reduces the amount of masking energy around the species-specific calling song frequency. Laboratory experiments yielded an average signal-to-noise ratio (SNR) of -8 dB, when masker and signal were broadcast from the same side. Secondly, displacing the masker by 180° from the signal improved SNRs by further 6 to 9 dB, a phenomenon known as spatial release from masking. Surprisingly, experiments carried out directly in the nocturnal rainforest yielded SNRs of about -23 dB compared with those in the laboratory with the same masker, where SNRs reached only -14.5 and -16 dB in both species. Finally, a neuronal gain control mechanism enhances the contrast between the responses to signals and the masker, by inhibition of neuronal activity in interstimulus intervals. Thus, conventional speaker playbacks in the lab apparently do not properly reconstruct the masking noise situation in a spatially realistic manner, since under real world conditions multiple sound sources are spatially distributed in space. Our results also indicate that without knowledge of the receiver properties and the spatial release mechanisms the detrimental effect of noise may be strongly overestimated.
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