Sound-by-sound thalamic stimulation modulates midbrain auditory excitability and relative binaural sensitivity in frogs

Neuroethology of sound localization in anurans

Albert Feng pioneered the study of neuroethology of sound localization in anurans by combining behavioral experiments on phonotaxis with detailed investigations of neural processing of sound direction from the periphery to the central nervous system. The main advantage of these studies is that many species of female frogs readily perform phonotaxis towards loudspeakers emitting the species-specific advertisement call. Behavioral studies using synthetic calls can identify which parameters are important for phonotaxis and also quantify localization accuracy. Feng was the first to investigate binaural processing using single-unit recordings in the first two auditory nuclei in the central auditory pathway and later investigated the directional properties of auditory nerve fibers with free-field stimulation. These studies showed not only that the frog ear is inherently directional by virtue of acoustical coupling or crosstalk between the two eardrums, but also confirmed that there are extratympanic pathways that affect directionality in the low-frequency region of the frog's hearing range. Feng's recordings in the midbrain also showed that directional information is enhanced by cross-midline inhibition. An important contribution toward the end of his career involved his participation in neuroethological research with a team of scientists working with frogs that produce ultrasonic calls.

Descending projections to the auditory midbrain: evolutionary considerations

The mammalian inferior colliculus (IC) is massively innervated by multiple descending projection systems. In addition to a large projection from the auditory cortex (AC) primarily targeting the non-lemniscal portions of the IC, there are less well-characterized projections from non-auditory regions of the cortex, amygdala, posterior thalamus and the brachium of the IC. By comparison, the frog auditory midbrain, known as the torus semicircularis, is a large auditory integration center that also receives descending input, but primarily from the posterior thalamus and without a projection from a putative cortical homolog: the dorsal pallium. Although descending projections have been implicated in many types of behaviors, a unified understanding of their function has not yet emerged. Here, we take a comparative approach to understanding the various top-down modulators of the IC to gain insights into their functions. One key question that we identify is whether thalamotectal projections in mammals and amphibians are homologous and whether they interact with evolutionarily more newly derived projections from the cerebral cortex. We also consider the behavioral significance of these descending pathways, given anurans' ability to navigate complex acoustic landscapes without the benefit of a corticocollicular projection. Finally, we suggest experimental approaches to answer these questions.

Tuned in to communication sounds: Neuronal sensitivity in the túngara frog midbrain to frequency modulated signals

For complex communication signals, it is often difficult to identify the information-bearing elements and their parameters necessary to elicit functional behavior. Consequently, it may be difficult to design stimuli that test how neurons contribute to communicative processing. For túngara frogs (Physalaemus pustulosus), however, previous behavioral testing with numerous stimuli showed that a particular frequency modulated (FM) transition in the male call is required to elicit phonotaxis and vocal responses. Modeled on such behavioral experiments, we used awake in vivo recordings of single units in the midbrain to determine if their excitation was biased to behaviorally important FM parameters. Comparisons of stimulus driven action potentials revealed greatest excitation to the behaviorally important FM transition: a downward FM sweep or step that crosses ~600 Hz. Previous studies using long-duration acoustic exposure found immediate early gene expression in many midbrain neurons to be most sensitive to similar FM. However, those data could not determine if FM coding was accomplished by the population and/or individual neurons. Our data suggest both coding schemes could operate, as 1) individual neurons are more sensitive to the behaviorally significant FM transition and 2) when single unit recordings are analytically combined across cells, the combined code can produce high stimulus discrimination (FM vs. noise driven excitation), approaching that found in behavioral discrimination of call vs. noise.

Background firing in the auditory midbrain of the frog

Statistical characteristics of background firing in the midbrain auditory units of grass frog (Rana t. temporaria) located in torus semicircular (TS) were investigated. Only about 5% of the cells demonstrated prominent spontaneous firing. For such units the following characteristics were obtained: the distribution of interpulse intervals, the autocorrelation functions (ACF) for the real firing process and for the process with shuffled intervals, the hazard function (HF) and the joint distribution of adjacent interpulse intervals. The burstiness of firing was also estimated. In the absolute majority of the cells, the background firing demonstrated considerable deviation from the renewal process. There was weak but significant positive correlation between adjacent interpulse intervals. The burstiness of firing in the midbrain auditory units was moderate but higher than reported for medullary auditory neurons. The value of burstiness did not decrease after interval shuffling. Along with the reduction in excitability (generalized refractoriness) in many neurons observed post-spike facilitation effects were observed. Comparing background activity in medullary and midbrain nucleus suggests that there is an increase in complexity of the information processing along the auditory pathway.

Anatomical characterization of subcortical descending projections to the inferior colliculus in mouse

Descending projections from the thalamus and related structures to the midbrain are evolutionarily highly conserved. However, the basic organization of this auditory thalamotectal pathway has not yet been characterized. The purpose of this study was to obtain a better understanding of the anatomical and neurochemical features of this pathway. Analysis of the distributions of retrogradely labeled cells after focal injections of retrograde tracer into the inferior colliculus (IC) of the mouse revealed that most of the subcortical descending projections originated in the brachium of the IC and the paralaminar portions of the auditory thalamus. In addition, the vast majority of thalamotectal cells were found to be negative for the calcium-binding proteins calbindin, parvalbumin, or calretinin. Using two different strains of GAD-GFP mice, as well as immunostaining for GABA, we found that a subset of neurons in the brachium of the IC is GABAergic, suggesting that part of this descending pathway is inhibitory. Finally, dual retrograde injections into the IC and amygdala plus corpus striatum as well into the IC and auditory cortex did not reveal any double labeling. These data suggest that the thalamocollicular pathway comprises a unique population of thalamic neurons that do not contain typical calcium-binding proteins and do not project to other paralaminar thalamic forebrain targets, and that a previously undescribed descending GABAergic pathway emanates from the brachium of the IC. J. Comp. Neurol. 525:885-900, 2017. © 2016 Wiley Periodicals, Inc.

Sound source localization and segregation with internally coupled ears: the treefrog model

Acoustic signaling plays key roles in mediating many of the reproductive and social behaviors of anurans (frogs and toads). Moreover, acoustic signaling often occurs at night, in structurally complex habitats, such as densely vegetated ponds, and in dense breeding choruses characterized by high levels of background noise and acoustic clutter. Fundamental to anuran behavior is the ability of the auditory system to determine accurately the location from where sounds originate in space (sound source localization) and to assign specific sounds in the complex acoustic milieu of a chorus to their correct sources (sound source segregation). Here, we review anatomical, biophysical, neurophysiological, and behavioral studies aimed at identifying how the internally coupled ears of frogs contribute to sound source localization and segregation. Our review focuses on treefrogs in the genus Hyla, as they are the most thoroughly studied frogs in terms of sound source localization and segregation. They also represent promising model systems for future work aimed at understanding better how internally coupled ears contribute to sound source localization and segregation. We conclude our review by enumerating directions for future research on these animals that will require the collaborative efforts of biologists, physicists, and roboticists.

Female túngara frogs do not experience the continuity illusion

In humans and some nonhuman vertebrates, a sound containing brief silent gaps can be rendered perceptually continuous by inserting noise into the gaps. This so-called "continuity illusion" arises from a phenomenon known as "auditory induction" and results in the perception of complete auditory objects despite fragmentary or incomplete acoustic information. Previous studies of auditory induction in gray treefrogs (Hyla versicolor and H. chrysoscelis) have demonstrated an absence of this phenomenon. These treefrog species produce pulsatile (noncontinuous) vocalizations, whereas studies of auditory induction in other taxa, including humans, often present continuous sounds (e.g., frequency-modulated sweeps). This study investigated the continuity illusion in a frog (Physalaemus pustulosus) with an advertisement vocalization that is naturally continuous and thus similar to the tonal sweeps used in human psychophysical studies of auditory induction. In a series of playback experiments, female subjects were presented with sets of stimuli that included complete calls, calls with silent gaps, and calls with silent gaps filled with noise. The results failed to provide evidence of auditory induction. Current evidence, therefore, suggests that mammals and birds experience auditory induction, but frogs may not. This emerging pattern of taxonomic differences is considered in light of potential methodological, neurophysiological, and functional explanations.