Effects of noise overexposure on tone detection in noise in nonhuman primates

Hierarchical differences in the encoding of amplitude modulation in the subcortical auditory system of awake nonhuman primates

Sinusoidal amplitude modulation (SAM) is a key feature of complex sounds. Although psychophysical studies have characterized SAM perception, and neurophysiological studies in anesthetized animals report a transformation from the cochlear nucleus' (CN; brainstem) temporal code to the inferior colliculus' (IC; midbrain's) rate code, none have used awake animals or nonhuman primates to compare CN and IC's coding strategies to modulation-frequency perception. To address this, we recorded single-unit responses and compared derived neurometric measures in the CN and IC to psychometric measures of modulation frequency (MF) discrimination in macaques. IC and CN neurons often exhibited tuned responses to SAM in rate and spike-timing measures of modulation coding. Neurometric thresholds spanned a large range (2-200 Hz ΔMF). The lowest 40% of IC thresholds were less than or equal to psychometric thresholds, regardless of which code was used, whereas CN thresholds were greater than psychometric thresholds. Discrimination at 10-20 Hz could be explained by indiscriminately pooling 30 units in either structure, whereas discrimination at higher MFs was best explained by more selective pooling. This suggests that pooled CN activity was sufficient for AM discrimination. Psychometric and neurometric thresholds decreased as stimulus duration increased, but IC and CN thresholds were higher and more variable than behavior at short durations. This slower subcortical temporal integration compared with behavior was consistent with a drift diffusion model that reproduced individual differences in performance and can constrain future neurophysiological studies of temporal integration. These measures provide an account of AM perception at the neurophysiological, computational, and behavioral levels.NEW & NOTEWORTHY In everyday environments, the brain is tasked with extracting information from sound envelopes, which involves both sensory encoding and perceptual decision-making. Different neural codes for envelope representation have been characterized in midbrain and cortex, but studies of brainstem nuclei such as the cochlear nucleus (CN) have usually been conducted under anesthesia in nonprimate species. Here, we found that subcortical activity in awake monkeys and a biologically plausible perceptual decision-making model accounted for sound envelope discrimination behavior.

An assessment of ambient noise and other environmental variables in a nonhuman primate housing facility

Acoustic noise and other environmental variables represent potential confounds for animal research. Of relevance to auditory research, sustained high levels of ambient noise may modify hearing sensitivity and decrease well-being among laboratory animals. The present study was conducted to assess environmental conditions in an animal facility that houses nonhuman primates used for auditory research at the Vanderbilt University Medical Center. Sound levels, vibration, temperature, humidity and luminance were recorded using an environmental monitoring device placed inside of an empty cage in a macaque housing room. Recordings lasted 1 week each, at three different locations within the room. Vibration, temperature, humidity and luminance all varied within recommended levels for nonhuman primates, with one exception of low luminance levels in the bottom cage location. Sound levels at each cage location were characterized by a low baseline of 58-62 dB sound pressure level, with transient peaks up to 109 dB sound pressure level. Sound levels differed significantly across locations, but only by about 1.5 dB. The transient peaks beyond recommended sound levels reflected a very low noise dose, but exceeded startle-inducing levels, which could elicit stress responses. Based on these findings, ambient noise levels in the housing rooms in this primate facility are within acceptable levels and unlikely to contribute to hearing deficits in the nonhuman primates. Our results establish normative values for environmental conditions in a primate facility, can be used to inform best practices for nonhuman primate research and care, and form a baseline for future studies of aging and chronic noise exposure.

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.

Correlations between cochlear pathophysiology and behavioral measures of temporal and spatial processing in noise exposed macaques

Noise-induced hearing loss (NIHL) is known to have significant consequences for temporal, spectral, and spatial resolution. However, much remains to be discovered about their underlying pathophysiology. This report extends the recent development of a nonhuman primate model of NIHL to explore its consequences for hearing in noisy environments, and its correlations with the underlying cochlear pathology. Ten macaques (seven with normal-hearing, three with NIHL) were used in studies of masked tone detection in which the temporal or spatial properties of the masker were varied to assess metrics of temporal and spatial processing. Normal-hearing (NH) macaques showed lower tone detection thresholds for sinusoidally amplitude modulated (SAM) broadband noise maskers relative to unmodulated maskers (modulation masking release, MMR). Tone detection thresholds were lowest at low noise modulation frequencies, and increased as modulation frequency increased, until they matched threshold in unmodulated noise. NH macaques also showed lower tone detection thresholds for spatially separated tone and noise relative to co-localized tone and noise (spatial release from masking, SRM). Noise exposure caused permanent threshold shifts that were verified behaviorally and audiologically. In hearing-impaired (HI) macaques, MMR was reduced at tone frequencies above that of the noise exposure. HI macaques also showed degraded SRM, with no SRM observed across all tested tone frequencies. Deficits in MMR correlated with audiometric threshold changes, outer hair cell loss, and synapse loss, while the differences in SRM did not correlate with audiometric changes, or any measure of cochlear pathophysiology. This difference in anatomical-behavioral correlations suggests that while many behavioral deficits may arise from cochlear pathology, only some are predictable from the frequency place of damage in the cochlea.

Changes in audiometric threshold and frequency selectivity correlate with cochlear histopathology in macaque monkeys with permanent noise-induced hearing loss

Exposure to loud noise causes damage to the inner ear, including but not limited to outer and inner hair cells (OHCs and IHCs) and IHC ribbon synapses. This cochlear damage impairs auditory processing and increases audiometric thresholds (noise-induced hearing loss, NIHL). However, the exact relationship between the perceptual consequences of NIHL and its underlying cochlear pathology are poorly understood. This study used a nonhuman primate model of NIHL to relate changes in frequency selectivity and audiometric thresholds to indices of cochlear histopathology. Three macaques (one Macaca mulatta and two Macaca radiata) were trained to detect tones in quiet and in noises that were spectrally notched around the tone frequency. Audiograms were derived from tone thresholds in quiet; perceptual auditory filters were derived from tone thresholds in notched-noise maskers using the rounded-exponential fit. Data were obtained before and after a four-hour exposure to a 50-Hz noise centered at 2 kHz at 141 or 146 dB SPL. Noise exposure caused permanent audiometric threshold shifts and broadening of auditory filters at and above 2 kHz, with greater changes observed for the 146-dB-exposed monkeys. The normalized bandwidth of the perceptual auditory filters was strongly correlated with audiometric threshold at each tone frequency. While changes in audiometric threshold and perceptual auditory filter widths were primarily determined by the extent of OHC survival, additional variability was explained by including interactions among OHC, IHC, and ribbon synapse survival. This is the first study to provide within-subject comparisons of auditory filter bandwidths in an animal model of NIHL and correlate these NIHL-related perceptual changes with cochlear histopathology. These results expand the foundations for ongoing investigations of the neural correlates of NIHL-related perceptual changes.

Noise-Induced Hearing Loss and its Prevention: Current Issues in Mammalian Hearing

Noise-induced hearing loss (NIHL) has been well investigated across diverse mammalian species and the potential for prevention of NIHL is of broad interest. To most efficiently develop novel therapeutic interventions, a good understanding of the current state of knowledge regarding mechanisms of injury is essential. The overarching goals of this review are to 1) concisely summarize the current state of knowledge, and 2) provide opinions on the most significant future trends and developments.

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.

Hematological and biochemical parameters for Chinese rhesus macaque

Rhesus macaque is an important animal model in biomedical research, especially human disease, developmental, translational, and pre-clinical research. Blood physiological and biochemical parameters are important markers for physiology, pathology, and toxicology research. However, these parameters have not been systematically reported for Chinese rhesus macaques. To characterize the reference for these parameters, this study collected 1805 Chinese rhesus macaques living in Southwestern China. A total of 24 blood physiological indexes and 27 biochemical parameters were determined. Sex and age were found to affect these parameters. In conclusion, a comprehensive and systematic reference of hematological and biochemical parameters for Chinese rhesus macaque was established in this work on the basis of a large cohort. Such reference will benefit biomedical research employing rhesus macaques as animal models.

No Reliable Association Between Recreational Noise Exposure and Threshold Sensitivity, Distortion Product Otoacoustic Emission Amplitude, or Word-in-Noise Performance in a College Student Population

OBJECTIVES

The purpose of this study was to evaluate the relationship between recreational sound exposure and potentially undiagnosed or subclinical hearing loss by assessing sound exposure history, threshold sensitivity, distortion product otoacoustic emission (DPOAE) amplitudes, and performance on the words-in-noise (WIN) test.

DESIGN

Survey data were collected from 74 adult participants (14 male and 60 female), 18 to 27 years of age, recruited via advertisements posted throughout the University of Florida campus. Of these participants, 70 completed both the survey and the additional functional test battery, and their preferred listening level was measured in a laboratory setting.

RESULTS

There were statistically significant relationships between hearing thresholds and DPOAE amplitude. In contrast, performance on the WIN was not reliably related to threshold sensitivity within this cohort with largely normal hearing. The two most common exposures included bars or dance clubs, followed by music player use. There were no statistically significant relationships between individual or composite measures of recreational sound exposure, including preferred listening level, years of music player use, number of reported sound exposures, previous impulse noise exposure, or previous noise-induced change in hearing, and functional measures including threshold, DPOAE amplitude, and WIN measures. Some subjects were highly consistent in listening level preferences, while others were more variable from song to song.

CONCLUSIONS

No reliable relationships between common recreational sound exposure or previous noise-induced changes in hearing were found during analysis of threshold sensitivity, DPOAE amplitude, or WIN performance in this cohort. However, the study sample was predominantly female and Caucasian, which limits generalizability of the results.

A Comparison of Auditory Oddball Responses in Dorsolateral Prefrontal Cortex, Basolateral Amygdala, and Auditory Cortex of Macaque

The mismatch negativity (MMN) is an ERP component seen in response to unexpected "novel" stimuli, such as in an auditory oddball task. The MMN is of wide interest and application, but the neural responses that generate it are poorly understood. This is in part due to differences in design and focus between animal and human oddball paradigms. For example, one of the main explanatory models, the "predictive error hypothesis", posits differences in timing and selectivity between signals carried in auditory and prefrontal cortex (PFC). However, these predictions have not been fully tested because (1) noninvasive techniques used in humans lack the combined spatial and temporal precision necessary for these comparisons and (2) single-neuron studies in animal models, which combine necessary spatial and temporal precision, have not focused on higher order contributions to novelty signals. In addition, accounts of the MMN traditionally do not address contributions from subcortical areas known to be involved in novelty detection, such as the amygdala. To better constrain hypotheses and to address methodological gaps between human and animal studies, we recorded single neuron activity from the auditory cortex, dorsolateral PFC, and basolateral amygdala of two macaque monkeys during an auditory oddball paradigm modeled after that used in humans. Consistent with predictions of the predictive error hypothesis, novelty signals in PFC were generally later than in auditory cortex and were abstracted from stimulus-specific effects seen in auditory cortex. However, we found signals in amygdala that were comparable in magnitude and timing to those in PFC, and both prefrontal and amygdala signals were generally much weaker than those in auditory cortex. These observations place useful quantitative constraints on putative generators of the auditory oddball-based MMN and additionally indicate that there are subcortical areas, such as the amygdala, that may be involved in novelty detection in an auditory oddball paradigm.