Central Neural Processing of Sound Signals in Insects

An auditory-responsive interneuron descending from the cricket brain: a new element in the auditory pathway

Crickets receive auditory information from their environment via ears located on the front legs. Ascending interneurons forward auditory activity to the brain, which houses a pattern recognition network for phonotaxis to conspecific calling songs and which controls negative phonotaxis to high-frequency sound pulses. Descending brain neurons, however, which are clearly involved in controlling these behaviors, have not yet been identified. We describe a descending auditory-responsive brain neuron with an arborization pattern that coincides with the ring-like auditory neuropil in the brain formed by the axonal arborizations of ascending and local interneurons, indicating its close link to auditory processing. Spiking activity of this interneuron occurs with a short latency to calling song patterns and the neuron copies the sound pulse pattern. The neuron preferentially responds to short sound pulses, but its activity appears to be independent of the calling song pattern recognition process. It also receives a weaker synaptic input in response to high-frequency pulses, which may contribute to its short latency spiking responses. This interneuron could be a crucial part in the auditory-to-motor transformation of the nervous system and contribute to the motor control of cricket auditory behavior.

Stink Bug Communication and Signal Detection in a Plant Environment

Plants influenced the evolution of plant-dwelling stink bugs' systems underlying communication with chemical and substrate-borne vibratory signals. Plant volatiles provides cues that increase attractiveness or interfere with the probability of finding a mate in the field. Mechanical properties of herbaceous hosts and associated plants alter the frequency, amplitude, and temporal characteristics of stink bug species and sex-specific vibratory signals. The specificity of pheromone odor tuning has evolved through highly specific odorant receptors located within the receptor membrane. The narrow-band low-frequency characteristics of the signals produced by abdomen vibration and the frequency tuning of the highly sensitive subgenual organ vibration receptors match with filtering properties of the plants enabling optimized communication. A range of less sensitive mechanoreceptors, tuned to lower vibration frequencies, detect signals produced by other mechanisms used at less species-specific levels of communication in a plant environment. Whereas the encoding of frequency-intensity and temporal parameters of stink bug vibratory signals is relatively well investigated at low levels of processing in the ventral nerve cord, processing of this information and its integration with other modalities at higher neuronal levels still needs research attention.

Impaired Binaural Hearing in Adults: A Selected Review of the Literature

Despite over 100 years of study, there are still many fundamental questions about binaural hearing that remain unanswered, including how impairments of binaural function are related to the mechanisms of binaural hearing. This review focuses on a number of studies that are fundamental to understanding what is known about the effects of peripheral hearing loss, aging, traumatic brain injury, strokes, brain tumors, and multiple sclerosis (MS) on binaural function. The literature reviewed makes clear that while each of these conditions has the potential to impair the binaural system, the specific abilities of a given patient cannot be known without performing multiple behavioral and/or neurophysiological measurements of binaural sensitivity. Future work in this area has the potential to bring awareness of binaural dysfunction to patients and clinicians as well as a deeper understanding of the mechanisms of binaural hearing, but it will require the integration of clinical research with animal and computational modeling approaches.

Internal state transition to switch behavioral strategies in cricket phonotaxis

Animals employ multiple behavioral strategies for exploring food and mating partners based on both their internal state and external environment. Here, we examined how cricket phonotaxis, which was considered an innate reactive behavior of females to approach the calling song of conspecific males, depended on these internal and external conditions. Our observation revealed that the phonotaxis process consisted of two distinctive phases: wandering and approaching. In the latter phase, crickets moved directly towards the sound source. The transition into this phase, referred to as the 'approach phase', was based on changes in the animal's internal state. Moreover, retention of the approach phase required recognition of the calling song, while song loss downregulated cricket mobility and induced frequent stopping. This is a typical movement in local search behaviors. Our results indicate that phonotaxis is not only a reactive response but a complicated process including multiple behavioral strategies.

Directional hearing in insects: biophysical, physiological and ecological challenges

Sound localisation is a fundamental attribute of the way that animals perceive their external world. It enables them to locate mates or prey, determine the direction from which a predator is approaching and initiate adaptive behaviours. Evidence from different biological disciplines that has accumulated over the last two decades indicates how small insects with body sizes much smaller than the wavelength of the sound of interest achieve a localisation performance that is similar to that of mammals. This Review starts by describing the distinction between tympanal ears (as in grasshoppers, crickets, cicadas, moths or mantids) and flagellar ears (specifically antennae in mosquitoes and fruit flies). The challenges faced by insects when receiving directional cues differ depending on whether they have tympanal or flagellar years, because the latter respond to the particle velocity component (a vector quantity) of the sound field, whereas the former respond to the pressure component (a scalar quantity). Insects have evolved sophisticated biophysical solutions to meet these challenges, which provide binaural cues for directional hearing. The physiological challenge is to reliably encode these cues in the neuronal activity of the afferent auditory system, a non-trivial problem in particular for those insect systems composed of only few nerve cells which exhibit a considerable amount of intrinsic and extrinsic response variability. To provide an integrative view of directional hearing, I complement the description of these biophysical and physiological solutions by presenting findings on localisation in real-world situations, including evidence for localisation in the vertical plane.

Wiring patterns from auditory sensory neurons to the escape and song-relay pathways in fruit flies

Many animals rely on acoustic cues to decide what action to take next. Unraveling the wiring patterns of the auditory neural pathways is prerequisite for understanding such information processing. Here, we reconstructed the first step of the auditory neural pathway in the fruit fly brain, from primary to secondary auditory neurons, at the resolution of transmission electron microscopy. By tracing axons of two major subgroups of auditory sensory neurons in fruit flies, low-frequency tuned Johnston's organ (JO)-B neurons and high-frequency tuned JO-A neurons, we observed extensive connections from JO-B neurons to the main second-order neurons in both the song-relay and escape pathways. In contrast, JO-A neurons connected strongly to a neuron in the escape pathway. Our findings suggest that heterogeneous JO neuronal populations could be recruited to modify escape behavior whereas only specific JO neurons contribute to courtship behavior. We also found that all JO neurons have postsynaptic sites at their axons. Presynaptic modulation at the output sites of JO neurons could affect information processing of the auditory neural pathway in flies.

Phonotactic steering and representation of directional information in the ascending auditory pathway of a cricket

Directional hearing is crucial for animals depending on acoustic signals to locate a mate. We focused on crickets to explore the reliability of directional information forwarded to the brain by the ascending auditory interneuron AN1, which is crucial for phonotactic behavior. We presented calling song from -45° to +45° in steps of 3° and compared the phonotactic steering of females walking on a trackball with the directional responses of AN1. Forty percent of females showed good steering behavior and changed their walking direction when the speaker passed the body's longitudinal axis. The bilateral latency difference between right and left AN1 responses was small and may not be reliable for auditory steering. In respect to spike count, all AN1 recordings presented significant bilateral differences for angles larger than ±18°, yet 35% showed a mean significant difference of 1-3 action potentials per chirp when the frontal stimulus deviated by 3° from their length axis. For small angles, some females had a very similar AN1 activity forwarded to the brain, but the accuracy of their steering behavior was substantially different. Our results indicate a correlation between directional steering and the response strength of AN1, especially for large angles. The reliable steering of animals at small angles would have to be based on small bilateral differences of AN1 activity, if AN1 is the only source providing directional information. We discuss whether such bilateral response difference at small angles can provide a reliable measure to generate auditory steering commands descending from the brain, as pattern recognition is intensity independent.NEW & NOTEWORTHY The ascending auditory interneuron AN1 has been implicated in cricket auditory steering, but at small acoustic stimulation angles, it does not provide reliable directional information. We conclude that either the small bilateral auditory activity differences of the AN1 neurons are enhanced to generate reliable descending steering commands or, more likely, directional auditory steering is mediated via a thoracic pathway, as indicated by the reactive steering hypothesis.

Evolutionary diversification of the auditory organ sensilla in Neoconocephalus katydids (Orthoptera: Tettigoniidae) correlates with acoustic signal diversification over phylogenetic relatedness and life history

Neoconocephalus Tettigoniidae are a model for the evolution of acoustic signals as male calls have diversified in temporal structure during the radiation of the genus. The call divergence and phylogeny in Neoconocephalus are established, but in tettigoniids in general, accompanying evolutionary changes in hearing organs are not studied. We investigated anatomical changes of the tympanal hearing organs during the evolutionary radiation and divergence of intraspecific acoustic signals. We compared the neuroanatomy of auditory sensilla (crista acustica) from nine Neoconocephalus species for the number of auditory sensilla and the crista acustica length. These parameters were correlated with differences in temporal call features, body size, life histories and different phylogenetic positions. By this, adaptive responses to shifting frequencies of male calls and changes in their temporal patterns can be evaluated against phylogenetic constraints and allometry. All species showed well-developed auditory sensilla, on average 32-35 between species. Crista acustica length and sensillum numbers correlated with body size, but not with phylogenetic position or life history. Statistically significant correlations existed also with specific call patterns: a higher number of auditory sensilla occurred in species with continuous calls or slow pulse rates, and a longer crista acustica occurred in species with double pulses or slow pulse rates. The auditory sensilla show significant differences between species despite their recent radiation, and morphological and ecological similarities. This indicates the responses to natural and sexual selection, including divergence of temporal and spectral signal properties. Phylogenetic constraints are unlikely to limit these changes of the auditory systems.