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.

The relationship between sound duration and detection performance has long been thought to reflect temporal integration, and has been well studied in both humans and animal models. Reports of species differences are equivocal, with some metanalyses reporting no species differences, and others reporting substantial differences. This renders translational work in animals problematic. To re-evaluate this issue, tone detection performance was measured in rhesus macaques using a Go/No-Go reaction time detection task at various stimulus durations, and in the presence of broadband noise (BBN). Detection thresholds, reaction times (RT), and psychometric function slopes were calculated. All three measures were sensitive to tone duration, consistent with temporal integration. The rate of threshold change with duration was similar to human data, and was equally well fit by power law and exponential functions. The effect of tone duration on RT is the first reported in animals. BBN significantly affected how slopes changed with duration, and elevated thresholds, but did not affect how threshold or RT changed with duration. To begin investigating what processes may underlie these behavioral measures, the data were then compared to data generated using a probabilistic Poisson process model, previously compared to human data. The Poisson model recapitulated the effects of duration on threshold and psychometric slope. These behavioral data provide evidence that macaques are an exceptional model of human temporal integration, and the modeling results point the way forward for future neurophysiological studies.

Otitis externa (OE) is a condition that involves inflammation of the external ear canal. OE is a commonly reported condition in humans and some veterinary species (for example, dogs, cats), but has not been reported in the literature in macaques. Here, we present a case series of acute and chronic OE likely precipitated by abrasion of the ear canal with a tympanic membrane electrode in 7 adult male rhesus macaques (Macaca mulatta). All animals displayed purulent, mucinous discharge from 1 or both ears with 3 macaques also displaying signs of an upper respiratory tract (URT) infection during the same period. A variety of diagnostic and treatment options were pursued including consultation with an otolaryngologist necessitated by the differences in response to treatment in macaques as compared with other common veterinary species. Due to the nature of the studies in which these macaques were enrolled, standard audiological testing was performed before and after OE, including tympanometry, auditory brainstem responses (ABRs), and distortion product otoacoustic emissions (DPOAEs). After completion of study procedures, relevant tissues were collected for necropsy and histopathology. Impaired hearing was found in all macaques even after apparent resolution of OE signs. Necropsy findings included abnormalities in the tympanic membrane, ossicular chain, and middle ear cavity, suggesting that the hearing impairment was at least partly conductive in nature. We concluded that OE likely resulted from mechanical disruption of the epithelial lining of the ear canal by the ABR electrode, thereby allowing the development of opportunistic infections. OE, while uncommon in macaques, can affect them and should be included as a differential diagnosis of any macaque presenting with otic discharge and/or auricular discomfort.