Representation of waveform periodicity in the auditory midbrain of the bullfrog, Rana catesbeiana

Tadpole bioacoustics: Sound processing across metamorphosis

Many species of anuran amphibians (frogs and toads) undergo metamorphosis, a developmental process during which external and internal body morphologies transform dramatically as the animal transitions to a new ecosystem (from aquatic to terrestrial) and develops new behavior patterns (from filter-feeding to active pursuit of moving prey; from mostly mute to highly vocal). All sensory systems transform to some extent during metamorphosis, even in those "primitive" anuran species that remain fully aquatic in adult life. In this article, I review what is known about the development of the auditory system in anuran tadpoles. I identify crucial developmental windows for major maturational events in the ear and brainstem that showcase the structural and physiological reorganization of the substrates for hearing airborne sounds as the animal navigates the metamorphic transition. I argue that auditory development is dynamic and nonlinear, and I point out areas for future investigation. Understanding metamorphosis can shed light on how organisms adapt to major environmental challenges. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Auditory brainstem responses in Cope's gray treefrog (Hyla chrysoscelis): effects of frequency, level, sex and size

Our knowledge of the hearing abilities of frogs and toads is largely defined by work with a few well-studied species. One way to further advance comparative work on anuran hearing would be greater use of minimally invasive electrophysiological measures, such as the auditory brainstem response (ABR). This study used the ABR evoked by tones and clicks to investigate hearing in Cope's gray treefrog (Hyla chrysoscelis). The objectives were to characterize the effects of sound frequency, sound pressure level, and subject sex and body size on ABRs. The ABR in gray treefrogs bore striking resemblance to ABRs measured in other animals. As stimulus level increased, ABR amplitude increased and latency decreased, and for responses to tones, these effects depended on stimulus frequency. Frequency-dependent differences in ABRs were correlated with expected differences in the tuning of two sensory end organs in the anuran inner ear (the amphibian and basilar papillae). The ABR audiogram indicated two frequency regions of increased sensitivity corresponding to the expected tuning of the two papillae. Overall, there was no effect of subject size and only small effects related to subject sex. Together, these results indicate the ABR is an effective method to study audition in anurans.

"To ear is human, to frogive is divine": Bob Capranica's legacy to auditory neuroethology

Bob Capranica was a towering figure in the field of auditory neuroethology. Among his many contributions are the exploitation of the anuran auditory system as a general vertebrate model for studying communication, the introduction of a signal processing approach for quantifying sender-receiver dynamics, and the concept of the matched filter for efficient neural processing of complex vocal signals. In this paper, meant to honor Bob on his election to Fellow of the International Society for Neuroethology, I provide a description and analysis of some of his most important research, and I highlight how the concepts and data he contributed still inspire neuroethology today.

Cell's intrinsic biophysical properties play a role in the systematic decrease in time-locking ability of central auditory neurons

Studies in the vertebrates have shown that the time-locking ability of central auditory neurons decreases progressively along the ascending auditory pathway. This decrease is presumably attributed to a progressive reduction in the fidelity of synaptic transmission and an increase in the influence of synaptic inhibition along the cascade. The extent to which neurons' intrinsic biophysical properties contribute to the change in time-locking ability is unclear. We carried out whole-cell patch clamp recordings from the auditory thalamus of leopard frogs and compared their biophysical properties and time-locking abilities (determined by cell's responses to depolarizing pulse trains applied intracellularly) with those of lower auditory brainstem neurons. We found that frog thalamic neurons were homogeneous, exhibiting uniformly sustained, regular firing patterns, but not having low-threshold transient Ca2+ current which mammal thalamic neurons generally possess. Furthermore, intrinsic biophysical properties of the thalamic neurons are such that the time-locking ability of these neurons was very poor. The homogeneity of thalamic auditory neurons is in contrast to the heterogeneity of lower auditory brainstem neurons, with different phenotypes exhibiting different time-locking abilities and with sustained-regular phenotype consistently showing the worst time-locking ability among all biophysical phenotypes. Auditory nuclei along the ascending auditory pathway showed a progressive increase in the population of sustained-regular phenotype-this corresponded to a systematic decrease in the overall time-locking ability, with neurons in the dorsal medullary nucleus showing the best, and thalamic neurons exhibiting the poorest time-locking ability, whereas neurons in the torus semicircularis displayed intermediate time-locking ability. These results suggest that the biophysical characteristics of single neurons also likely play a role in the change in temporal coding ability along the ascending auditory pathway.

Subdivisions of the auditory midbrain (n. mesencephalicus lateralis, pars dorsalis) in zebra finches using calcium-binding protein immunocytochemistry

The midbrain nucleus mesencephalicus lateralis pars dorsalis (MLd) is thought to be the avian homologue of the central nucleus of the mammalian inferior colliculus. As such, it is a major relay in the ascending auditory pathway of all birds and in songbirds mediates the auditory feedback necessary for the learning and maintenance of song. To clarify the organization of MLd, we applied three calcium binding protein antibodies to tissue sections from the brains of adult male and female zebra finches. The staining patterns resulting from the application of parvalbumin, calbindin and calretinin antibodies differed from each other and in different parts of the nucleus. Parvalbumin-like immunoreactivity was distributed throughout the whole nucleus, as defined by the totality of the terminations of brainstem auditory afferents; in other words parvalbumin-like immunoreactivity defines the boundaries of MLd. Staining patterns of parvalbumin, calbindin and calretinin defined two regions of MLd: inner (MLd.I) and outer (MLd.O). MLd.O largely surrounds MLd.I and is distinct from the surrounding intercollicular nucleus. Unlike the case in some non-songbirds, however, the two MLd regions do not correspond to the terminal zones of the projections of the brainstem auditory nuclei angularis and laminaris, which have been found to overlap substantially throughout the nucleus in zebra finches.

Germinal sites and migrating routes of cells in the mesencephalic and diencephalic auditory areas in the African clawed frog (Xenopus laevis)

There is a clear core-shell organization in the auditory nuclei of amniotes. However, such organization only exists in the mesencephalic, but not in the diencephalic auditory regions of amphibians. To gain insights into how this core-shell organization developed and evolved, we injected a small dose of [(3)H]-thymidine into tadpoles of Xenopus laevis at peak stages of neurogenesis in the mesencephalic and diencephalic auditory areas. Following different survival times, the germinal sites and migrating routes of cells were examined in the shell (laminar nucleus, Tl; magnocellular nucleus, Tmc) and core (principal nucleus, Tp) regions of the mesencephalic auditory nucleus, torus semicircularis (Ts), as well as in the diencephalic auditory areas (posterior thalamic nucleus, P; central thalamic nucleus, C). Double labeling for [(3)H]-thymidine autoradiography and immunohistochemistry for vimentin was also performed to help determine the routes of cell migration. We found three major results. First, the germinal sites of Tp were intercalated between Tl and Tmc, arising from those of the shell regions. Second, although the germinal sites of Tl, Tmc, and Tp were located in the same brain levels (at rostromedial or caudomedial levels of Ts), neurogenesis in Tl or Tmc started earlier than that in Tp. Finally, the P and C were also generated in different ventricle sites. However, unlike Ts their neurogenesis showed no obvious temporal differences. These data demonstrate that a highly differentiated auditory region, such as Tp in Ts, is lacking in the diencephalon of amphibian. Our data are discussed from the view of the constitution and evolutionary origins of auditory nuclei in vertebrates.

Bats and frogs and animals in between: evidence for a common central timing mechanism to extract periodicity pitch

Widely divergent vertebrates share a common central temporal mechanism for representing periodicities of acoustic waveform events. In the auditory nerve, periodicities corresponding to frequencies or rates from about 10 Hz to over 1,000 Hz are extracted from pure tones, from low-frequency complex sounds (e.g., 1st harmonic in bullfrog calls), from mid-frequency sounds with low-frequency modulations (e.g., amplitude modulation rates in cat vocalizations), and from time intervals between high-frequency transients (e.g., pulse-echo delay in bat sonar). Time locking of neuronal responses to periodicities from about 50 ms down to 4 ms or less (about 20-300 Hz) is preserved in the auditory midbrain, where responses are dispersed across many neurons with different onset latencies from 4-5 to 20-50 ms. Midbrain latency distributions are wide enough to encompass two or more repetitions of successive acoustic events, so that responses to multiple, successive periods are ongoing simultaneously in different midbrain neurons. These latencies have a previously unnoticed periodic temporal pattern that determines the specific times for the dispersed on-responses.

Spatial location influences vocal interactions in bullfrog choruses

A multiple sensor array was employed to identify the spatial locations of all vocalizing male bullfrogs (Rana catesbeiana) in five natural choruses. Patterns of vocal activity collected with this array were compared with computer simulations of chorus activity. Bullfrogs were not randomly spaced within choruses, but tended to cluster into closely spaced groups of two to five vocalizing males. There were nonrandom, differing patterns of vocal interactions within clusters of closely spaced males and between different clusters. Bullfrogs located within the same cluster tended to overlap or alternate call notes with two or more other males in that cluster. These near-simultaneous calling bouts produced advertisement calls with more pronounced amplitude modulation than occurred in nonoverlapping notes or calls. Bullfrogs located in different clusters more often alternated entire calls or overlapped only small segments of their calls. They also tended to respond sequentially to calls of their farther neighbors compared to their nearer neighbors. Results of computational analyses showed that the observed patterns of vocal interactions were significantly different than expected based on random activity. The use of a multiple sensor array provides a richer view of the dynamics of choruses than available based on single microphone techniques.

Evolutionary significance of delayed neurogenesis in the core versus shell auditory areas of Mus musculus

Early comparative embryogenesis can reflect the organization and evolutionary origins of brain areas. Neurogenesis in the auditory areas of sauropsids displays a clear core-to-shell distinction, but it remains unclear in mammals. To address this issue, [3H]-thymidine was injected into pregnant mice on consecutive embryonic (E) days (E10-E19) to date neuronal birthdays. Immunohistochemistry for substance P, calbindin, and parvalbumin was conducted to distinguish the core and shell auditory regions. The results showed that: 1) cell generation began at E13 in the external or dorsal nucleus of the inferior colliculus (IC), but it did not start in the caudomedial portion of the central nucleus of IC, and significantly fewer cells were produced in the medial and rostromedial portions of the central nucleus of IC; 2) cells were generated at E11 in the dorsal and medial divisions of the medial geniculate complex (MGd and MGm, respectively), whereas cell generation was absent in the medial and rostromedial portions of the ventral medial geniculate complex (MGv), and fewer cells were produced in the caudomedial portion of MGv; 3) in the telencephalic auditory cortex, cells were produced at E11 or E12 in layer I and the subplate, which receive projections from the MGd and MGm. However, cell generation occurred at E13-E18 in layers II-VI, including the area receiving projections from the MGv. The core-to-shell distinction of neurogenesis is thus present in the mesencephalic to telencephalic auditory areas in the mouse. This distinction of neurogenesis is discussed from an evolutionary perspective.

Neurogenic development of the auditory areas of the midbrain and diencephalon in the Xenopus laevis and evolutionary implications

To study whether the core-versus-shell pattern of neurogenesis occurred in the mesencephalic and diencephalic auditory areas of amniotes also appears in the amphibian, [(3)H]-thymidine was injected into tadpoles at serial developmental stages of Xenopus laevis. Towards the end of metamorphism, [(3)H]-thymidine labeling was examined and led to two main observations: 1) neuron generation in the principal nucleus (Tp) started at stage 50, and peaked at stage 53, whereas it began at stage 48.5, and peaked around stage 49 in the other two mesencephalic auditory areas, the laminar nucleus (Tl) and the magnocellular nucleus (Tmc). 2) Neuron generation appeared at stage 40, and peaked around stage 52 in the posterior thalamic nucleus (P) and the central thalamic nucleus (C). Our study revealed that, like the cores of mesencephalic auditory nuclei in amniotes, Tp showed differences from Tl and Tmc in the onset and the peak of neurogenesis. However, such differences did not occur in the P and C. Our neurogenetic data were consistent with anatomical and physiological reports indicating a clear distinction between the mesencephalic, but not the diencephalic auditory areas of the amphibian. Our data are helpful to get insights into the organization of auditory nuclei and its evolution in vertebrates.

Information theory analysis of patterns of modulation in the advertisement call of the male bullfrog, Rana catesbeiana

Male bullfrogs often amplitude modulate the envelopes of the individual notes (croaks) in their multinote advertisement calls. These amplitude modulations change the envelope of the note from smooth and unmodulated to one with varying numbers of modulations. A Markov analysis shows the pattern of change in the envelope to be highly ordered, but not completely so (semi-Markovian). Three simple rules govern the presence or absence of modulations in individual notes. These rules are (1) all calls begin with an unmodulated note; (2) the first note to be modulated will contain only one modulation; and (3) when a change in modulation occurs from one note to the next, it does so with an increase or a decrease of one modulation only. The addition of modulations is correlated with an increase in note duration. Physiologically, the presence of modulations might increase the precision of temporal coding of note periodicities in the central auditory system.

Roles of the auditory midbrain and thalamus in selective phonotaxis in female gray treefrogs (Hyla versicolor)

Diencephalic and midbrain auditory nuclei are involved in the processing of auditory communication signals in anurans [Comparative Hearing: Fish and Amphibians, Springer-Verlag, New York, 1999, p. 218], but their exact roles in acoustically guided behavior, such as female phonotaxis, are unclear. To address this question, behavioral experiments were combined with lesions of dorsal thalamic nuclei and the midbrain torus semicircularis. Females were tested in two-alternative-forced-choice phonotactic experiments before and after a defined brain area was lesioned. During phonotactic tests, females had to choose between a "standard" synthetic call and one of three different variants, each of which had a single acoustic property (pulse rate, pulse rise-time, sound spectrum) that differed from the standard synthetic call. Results showed that dorsomedial thalamus lesions produced little or no effect on phonotaxis. In contrast, superficial and deep thalamus lesions, as well as lesions of the torus semicircularis, significantly decreased the number of phonotactic responses and increased the response time. Superficial thalamus lesions also abolished or reversed preferences for the standard call in the rise-time and sound spectrum tests. This effect is likely to have been caused by an imbalance in the stimulation of the thalamus by the low- and high-frequency pathways because these preferences were not affected in animals with more extensive lesions that included the superficial thalamus. Our data suggest that the torus semicircularis, but not the dorsal thalamus is crucial for phonotaxis in gravid, reproductively active females. Although dorsal thalamic nuclei seem to play a role in spectral sensitivity, they may additionally have motivational or attentional functions that contribute to achieving a state of phonotactic readiness.

Spectro-temporal response field characterization with dynamic ripples in ferret primary auditory cortex

To understand the neural representation of broadband, dynamic sounds in primary auditory cortex (AI), we characterize responses using the spectro-temporal response field (STRF). The STRF describes, predicts, and fully characterizes the linear dynamics of neurons in response to sounds with rich spectro-temporal envelopes. It is computed from the responses to elementary "ripples," a family of sounds with drifting sinusoidal spectral envelopes. The collection of responses to all elementary ripples is the spectro-temporal transfer function. The complex spectro-temporal envelope of any broadband, dynamic sound can expressed as the linear sum of individual ripples. Previous experiments using ripples with downward drifting spectra suggested that the transfer function is separable, i.e., it is reducible into a product of purely temporal and purely spectral functions. Here we measure the responses to upward and downward drifting ripples, assuming reparability within each direction, to determine if the total bidirectional transfer function is fully separable. In general, the combined transfer function for two directions is not symmetric, and hence units in AI are not, in general, fully separable. Consequently, many AI units have complex response properties such as sensitivity to direction of motion, though most inseparable units are not strongly directionally selective. We show that for most neurons, the lack of full separability stems from differences between the upward and downward spectral cross-sections but not from the temporal cross-sections; this places strong constraints on the neural inputs of these AI units.