Auditory stream segregation is the perceptual grouping of the acoustic mixture reaching the ear into coherent representations of sound sources. It has been described in a variety of vertebrates and underlies auditory scene analysis or auditory image formation. Here we describe a phenomenon in an invertebrate that bears an intriguing resemblance to auditory stream segregation observed in vertebrates: in Neoconocephalus retusus (Orthoptera, Tettigoniidae) an auditory interneuron segregates information about bat echolocation calls from background male advertisement songs. This process utilizes differences between the temporal and spectral characteristics of the two stimuli, a mechanism which is similar to those of auditory stream segregation in vertebrates. This similarity suggests that auditory stream segregation is a fundamental feature of auditory perception, widespread from invertebrates to humans.
"In the next step, spectral information is combined into very few, typically one or two semantic channels that represent conspecific signal or predators. The integration of spectral cues usually includes both lateral inhibition and summation across a specific spectral band conspecific signals from predators (e.g., categorical perception in crickets, Wyttenbach et al. 1996), (2) species discrimination in crickets and tettigoniids (Hill 1974; Hennig and Weber 1997; Schul et al. 1998), (3) sex discrimination in grasshoppers and tettigoniids (von Helversen and von Helversen 1997; Dobler et al. 1994), (4) stream segregation in katydids as evident in a single interneuron (Schul and Sheridan 2006) and (5) range fractionation for intensity discrimination (Römer et al. 1998). Even though a higher spectral resolution can often be observed in the hearing organs, most insects process sound in one or two channels with few exceptions (Phaneropteridae , Ensifera, retain the differential tuning to carrier frequencies in the auditory pathway, Stumpner 2002). "
[Show abstract][Hide abstract] ABSTRACT: Hearing in insects serves to gain information in the context of mate finding, predator avoidance or host localization. For these goals, the auditory pathways of insects represent the computational substrate for object recognition and localization. Before these higher level computations can be executed in more central parts of the nervous system, the signals need to be preprocessed in the auditory periphery. Here, we review peripheral preprocessing along four computational themes rather than discussing specific physiological mechanisms: (1) control of sensitivity by adaptation, (2) recoding of amplitude modulations of an acoustic signal into a labeled-line code (3) frequency processing and (4) conditioning for binaural processing. Along these lines, we review evidence for canonical computations carried out in the peripheral auditory pathway and show that despite the vast diversity of insect hearing, signal processing is governed by common computational motifs and principles.
"Stimulus-specific adaptation of the TN1 neuron may provide the answer. As noted above, TN1 quickly adapts to the rapid pulse trains (up to 200 pulses/s) that characterize many tettigoniid songs, but it faithfully follows the slower pulse rates (10 s of Hz) that bats produce during the initial stages of target acquisition (Schul and Sheridan 2006). The adaptation to high rates is carrier frequency specific, and as a result adaptation to rapid stimulation in one frequency band does not prevent responding to slower pulse rates in another, yet still high, frequency band. "
[Show abstract][Hide abstract] ABSTRACT: Ultrasound-driven avoidance responses have evolved repeatedly throughout the insecta as defenses against predation by echolocating bats. Although the auditory mechanics of ears and the properties of auditory receptor neurons have been studied in a number of groups, central neural processing of ultrasound stimuli has been examined in only a few cases. In this review, I summarize the neuronal basis for ultrasound detection and predator avoidance in crickets, tettigoniids, moths, and mantises, where central circuits have been studied most thoroughly. Several neuronal attributes, including steep intensity-response functions, high firing rates, and rapid spike conduction emerge as common themes of avoidance circuits. I discuss the functional consequences of these attributes, as well as the increasing complexity with which ultrasound stimuli are represented at successive levels of processing.
"At the single neuronal level, Schul and colleagues (Schul and Sheridan, 2006; Schul et al., 2012) described the highly selective encoding of batlike calls despite the simultaneous presence of a repetitive conspecific signal in the katydid Neoconocephalus retusus. Such a 'novelty detector' would allow this katydid to respond with evasive reactions to echolocation bat calls while listening to conspecifics (Schul and Sheridan, 2006). Novelty detection may also play a role in the context of species recognition. "
[Show abstract][Hide abstract] ABSTRACT: We 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-to-noise 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'.
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