Frequency discrimination in the common marmoset (Callithrix jacchus)

Perceptual specializations for processing species-specific vocalizations in the common marmoset (Callithrix jacchus)

How humans and animals segregate sensory information into discrete, behaviorally meaningful categories is one of the hallmark questions in neuroscience. Much of the research around this topic in the auditory system has centered around human speech perception, in which categorical processes result in an enhanced sensitivity for acoustically meaningful differences and a reduced sensitivity for nonmeaningful distinctions. Much less is known about whether nonhuman primates process their species-specific vocalizations in a similar manner. We address this question in the common marmoset, a small arboreal New World primate with a rich vocal repertoire produced across a range of behavioral contexts. We first show that marmosets perceptually categorize their vocalizations in ways that correspond to previously defined call types for this species. Next, we show that marmosets are differentially sensitive to changes in particular acoustic features of their most common call types and that these sensitivity differences are matched to the population statistics of their vocalizations in ways that likely maximize category formation. Finally, we show that marmosets are less sensitive to changes in these acoustic features when within the natural range of variability of their calls, which possibly reflects perceptual specializations which maintain existing call categories. These findings suggest specializations for categorical vocal perception in a New World primate species and pave the way for future studies examining their underlying neural mechanisms.

Sound localization acuity of the common marmoset (Callithrix jacchus)

The common marmoset (Callithrix jacchus) is a small arboreal New World primate which has emerged as a promising model in auditory neuroscience. One potentially useful application of this model system is in the study of the neural mechanism underlying spatial hearing in primate species, as the marmosets need to localize sounds to orient their head to events of interest and identify their vocalizing conspecifics that are not visible. However, interpretation of neurophysiological data on sound localization requires an understanding of perceptual abilities, and the sound localization behavior of marmosets has not been well studied. The present experiment measured sound localization acuity using an operant conditioning procedure in which marmosets were trained to discriminate changes in sound location in the horizontal (azimuth) or vertical (elevation) dimension. Our results showed that the minimum audible angle (MAA) for horizontal and vertical discrimination was 13.17° and 12.53°, respectively, for 2 to 32 kHz Gaussian noise. Removing the monaural spectral cues tended to increase the horizontal localization acuity (11.31°). Marmosets have larger horizontal MAA (15.54°) in the rear than the front. Removing the high-frequency (> 26 kHz) region of the head-related transfer function (HRTF) affected vertical acuity mildly (15.76°), but removing the first notch (12-26 kHz) region of HRTF substantially reduced the vertical acuity (89.01°). In summary, our findings indicate that marmosets' spatial acuity is on par with other species of similar head size and field of best vision, and they do not appear to use monaural spectral cues for horizontal discrimination but rely heavily on first notch region of HRTF for vertical discrimination.

A silent two-photon imaging system for studying in vivo auditory neuronal functions

Two-photon laser-scanning microscopy has become an essential tool for imaging neuronal functions in vivo and has been applied to different parts of the neural system, including the auditory system. However, many components of a two-photon microscope, such as galvanometer-based laser scanners, generate mechanical vibrations and thus acoustic artifacts, making it difficult to interpret auditory responses from recorded neurons. Here, we report the development of a silent two-photon imaging system and its applications in the common marmoset (Callithrix Jacchus), a non-human primate species sharing a similar hearing range with humans. By utilizing an orthogonal pair of acousto-optical deflectors (AODs), full-frame raster scanning at video rate was achieved without introducing mechanical vibrations. Imaging depth can be optically controlled by adjusting the chirping speed on the AODs without any mechanical motion along the Z-axis. Furthermore, all other sound-generating components of the system were acoustically isolated, leaving the noise floor of the working system below the marmoset's hearing threshold. Imaging with the system in awake marmosets revealed many auditory cortex neurons that exhibited maximal responses at low sound levels, which were not possible to study using traditional two-photon imaging systems. This is the first demonstration of a silent two-photon imaging system that is capable of imaging auditory neuronal functions in vivo without acoustic artifacts. This capacity opens new opportunities for a better understanding of auditory functions in the brain and helps isolate animal behavior from microscope-generated acoustic interference.

Flexible auditory training, psychophysics, and enrichment of common marmosets with an automated, touchscreen-based system

Devising new and more efficient protocols to analyze the phenotypes of non-human primates, as well as their complex nervous systems, is rapidly becoming of paramount importance. This is because with genome-editing techniques, recently adopted to non-human primates, new animal models for fundamental and translational research have been established. One aspect in particular, namely cognitive hearing, has been difficult to assess compared to visual cognition. To address this, we devised autonomous, standardized, and unsupervised training and testing of auditory capabilities of common marmosets with a cage-based standalone, wireless system. All marmosets tested voluntarily operated the device on a daily basis and went from naïve to experienced at their own pace and with ease. Through a series of experiments, here we show, that animals autonomously learn to associate sounds with images; to flexibly discriminate sounds, and to detect sounds of varying loudness. The developed platform and training principles combine in-cage training of common marmosets for cognitive and psychoacoustic assessment with an enriched environment that does not rely on dietary restriction or social separation, in compliance with the 3Rs principle.

The authors present a cage-based stand-alone platform for autonomous, standardized, and unsupervised training and testing of visuo-auditory-cued behaviours of common marmosets. The experiments do not require dietary restriction or social separation.

Three psychophysical metrics of auditory temporal integration in macaques

The relationship between sound duration and detection threshold has long been thought to reflect temporal integration. Reports of species differences in this relationship are equivocal: some meta-analyses report no species differences, whereas others report substantial differences, particularly between humans and their close phylogenetic relatives, macaques. This renders translational work in macaques problematic. To reevaluate this difference, tone detection performance was measured in macaques using a go/no-go reaction time (RT) task at various tone durations and in the presence of broadband noise (BBN). Detection thresholds, RTs, and the dynamic range (DR) of the psychometric function decreased as the tone duration increased. The threshold by duration trends suggest macaques integrate at a similar rate to humans. The RT trends also resemble human data and are the first reported in animals. Whereas the BBN did not affect how the threshold or RT changed with the duration, it substantially reduced the DR at short durations. A probabilistic Poisson model replicated the effects of duration on threshold and DR and required integration from multiple simulated auditory nerve fibers to explain the performance at shorter durations. These data suggest that, contrary to previous studies, macaques are uniquely well-suited to model human temporal integration and form the baseline for future neurophysiological studies.

Cochlear shape distinguishes southern African early hominin taxa with unique auditory ecologies

Insights into potential differences among the bony labyrinths of Plio-Pleistocene hominins may inform their evolutionary histories and sensory ecologies. We use four recently-discovered bony labyrinths from the site of Kromdraai to significantly expand the sample for Paranthropus robustus. Diffeomorphometry, which provides detailed information about cochlear shape, reveals size-independent differences in cochlear shape between P. robustus and Australopithecus africanus that exceed those among modern humans and the African apes. The cochlea of P. robustus is distinctive and relatively invariant, whereas cochlear shape in A. africanus is more variable, resembles that of early Homo, and shows a degree of morphological polymorphism comparable to that evinced by modern species. The curvature of the P. robustus cochlea is uniquely derived and is consistent with enhanced sensitivity to low-frequency sounds. Combined with evidence for selection, our findings suggest that sound perception shaped distinct ecological adaptations among southern African early hominins.

Neural presbycusis at ultra-high frequency in aged common marmosets and rhesus monkeys

The aging of the population and environmental noise have contributed to high rates of presbycusis, also known as age-related hearing loss (ARHL). Because mice have a relatively short life span, murine models have not been suitable for determining the mechanism of presbycusis development and methods of diagnosis. Although the common marmoset, a non-human primate (NHP), is an ideal animal model for studying age-related diseases, its auditory spectrum has not been systematically studied. Auditory brainstem responses (ABRs) from 38 marmosets of different ages demonstrated that auditory function correlated with age. Hearing loss in geriatric common marmosets started at ultra-high frequency (>16 kHz), then extended to lower frequencies. Despite age-related deterioration of ABR threshold and amplitude in marmosets, outer hair cell (OHC) function remained stable at all ages. Spiral ganglion neurons (SGNs), which are the first auditory neurons in the auditory system, were found to degenerate distinctly in aged common marmosets, indicating that neural degeneration caused presbycusis in these animals. Similarly, age-associated ABR deterioration without loss of OHC function was observed in another NHP, rhesus monkeys. Audiometry results from these two species of NHP suggested that NHPs were ideal for studying ARHL and that neural presbycusis at high frequency may be prevalent in primates.

Pupillometry as a reliable metric of auditory detection and discrimination across diverse stimulus paradigms in animal models

Estimates of detection and discrimination thresholds are often used to explore broad perceptual similarities between human subjects and animal models. Pupillometry shows great promise as a non-invasive, easily-deployable method of comparing human and animal thresholds. Using pupillometry, previous studies in animal models have obtained threshold estimates to simple stimuli such as pure tones, but have not explored whether similar pupil responses can be evoked by complex stimuli, what other stimulus contingencies might affect stimulus-evoked pupil responses, and if pupil responses can be modulated by experience or short-term training. In this study, we used an auditory oddball paradigm to estimate detection and discrimination thresholds across a wide range of stimuli in guinea pigs. We demonstrate that pupillometry yields reliable detection and discrimination thresholds across a range of simple (tones) and complex (conspecific vocalizations) stimuli; that pupil responses can be robustly evoked using different stimulus contingencies (low-level acoustic changes, or higher level categorical changes); and that pupil responses are modulated by short-term training. These results lay the foundation for using pupillometry as a reliable method of estimating thresholds in large experimental cohorts, and unveil the full potential of using pupillometry to explore broad similarities between humans and animal models.

Nonadjacent dependency processing in monkeys, apes, and humans

The ability to track syntactic relationships between words, particularly over distances ("nonadjacent dependencies"), is a critical faculty underpinning human language, although its evolutionary origins remain poorly understood. While some monkey species are reported to process auditory nonadjacent dependencies, comparative data from apes are missing, complicating inferences regarding shared ancestry. Here, we examined nonadjacent dependency processing in common marmosets, chimpanzees, and humans using "artificial grammars": strings of arbitrary acoustic stimuli composed of adjacent (nonhumans) or nonadjacent (all species) dependencies. Individuals from each species (i) generalized the grammars to novel stimuli and (ii) detected grammatical violations, indicating that they processed the dependencies between constituent elements. Furthermore, there was no difference between marmosets and chimpanzees in their sensitivity to nonadjacent dependencies. These notable similarities between monkeys, apes, and humans indicate that nonadjacent dependency processing, a crucial cognitive facilitator of language, is an ancestral trait that evolved at least ~40 million years before language itself.

The use of nonhuman primates in studies of noise injury and treatment

Exposure to prolonged or high intensity noise increases the risk for permanent hearing impairment. Over several decades, researchers characterized the nature of harmful noise exposures and worked to establish guidelines for effective protection. Recent laboratory studies, primarily conducted in rodent models, indicate that the auditory system may be more vulnerable to noise-induced hearing loss (NIHL) than previously thought, driving renewed inquiries into the harmful effects of noise in humans. To bridge the translational gaps between rodents and humans, nonhuman primates (NHPs) may serve as key animal models. The phylogenetic proximity of NHPs to humans underlies tremendous similarity in many features of the auditory system (genomic, anatomical, physiological, behavioral), all of which are important considerations in the assessment and treatment of NIHL. This review summarizes the literature pertaining to NHPs as models of hearing and noise-induced hearing loss, discusses factors relevant to the translation of diagnostics and therapeutics from animals to humans, and concludes with some of the practical considerations involved in conducting NHP research.

High-resolution frequency tuning but not temporal coding in the human cochlea

Frequency tuning and phase-locking are two fundamental properties generated in the cochlea, enabling but also limiting the coding of sounds by the auditory nerve (AN). In humans, these limits are unknown, but high resolution has been postulated for both properties. Electrophysiological recordings from the AN of normal-hearing volunteers indicate that human frequency tuning, but not phase-locking, exceeds the resolution observed in animal models.

The coding of sounds by the cochlea depends on two primary properties: frequency selectivity, which refers to the ability to separate sounds into their different frequency components, and phase-locking, which refers to the neural coding of the temporal waveform of these components. These properties have been well characterized in animals using neurophysiological recordings from single neurons of the auditory nerve (AN), but this approach is not feasible in humans. As a result, there is considerable controversy as to how these two properties may differ between humans and the small animals typically used in neurophysiological studies. It has been proposed that humans excel both in frequency selectivity and in the range of frequencies over which they have phase-locking. We developed a technique to quantify these properties using mass potentials from the AN, recorded via the middle ear in human volunteers with normal hearing. We find that humans have unusually sharp frequency tuning but that the upper frequency limit of phase-locking is at best similar to—and more likely lower than—that of the nonhuman animals conventionally used in experiments.