Melissa L. Caras, Ph.D. - US grants

DESCRIPTION (provided by applicant): Gambel's white-crowned sparrows are songbirds that breed seasonally. Like many other songbirds, they exhibit dramatic seasonal differences in song behavior, and in the morphology and physiology of the neural circuit underlying song production. Androgenic and estrogenic steroid sex hormones mediate these seasonal changes. While much work has focused on the hormonal mechanisms underlying seasonal plasticity in songbird vocal production, comparatively little work has investigated seasonal and hormonal effects on songbird auditory processing. Preliminary auditory brainstem response (ABR) data demonstrate that breeding condition white-crowned sparrows have significantly increased thresholds and latencies than non-breeding birds;however, a detailed assessment of these effects remains to be accomplished. The focus of my research is to use white-crowned sparrows as a model for further characterizing seasonal and hormonal influences on auditory function. Using extracellular single-unit recording methods, Specific Aim 1 will determine where in the auditory system seasonal differences originate. Specific Aim 2 will identify which hormone receptors mediate these changes by utilizing a variety of hormone treatments, immunohistochemistry and ABR recording. Finally, Specific Aim 3 will determine if seasonal differences observed in the peripheral/brainstem auditory system are conserved in forebrain processing regions. PUBLIC HEALTH RELEVANCE: A growing body of literature links human hearing and steroid hormones, but the nature and mechanism of these interactions remain unknown. To better understand the extrinsic and intrinsic conditions that regulate the sensitivity and selectivity of hearing as related to communication signals, I have chosen to study the peripheral and central auditory system of songbirds. The research I propose here will use white-crowned sparrows as a model to better understand the hormonal mechanisms that influence and underlie normal hearing function, and is thus directly relevant to the mission of NIDCD.

? DESCRIPTION (provided by applicant): Childhood hearing loss disrupts central processing of spectral and temporal cues, and may contribute to perceptual deficits, including impaired speech and language processing. Though difficulties can persist even after normal auditory input is restored, earlier restoration is correlated with superior recovery. These findings suggest that there is a critical period of development during which the neural mechanisms subserving auditory processing are particularly vulnerable to sensory perturbations. The key goal of this proposal is to determine whether a brief period of developmental hearing loss disrupts the neural and perceptual detection of amplitude modulation (AM), a temporal cue necessary for speech comprehension and non-human animal communication. The core hypothesis is that transient hearing loss during a critical period disrupts the neural encoding of AM stimuli, thereby producing a perceptual impairment that persists after normal peripheral function is restored. This hypothesis will be explored in detail, with two experimental aims. Specific Aim 1 will determine whether there is a critical period during which temporary hearing loss impairs subsequent AM detection. Sound attenuation will be induced by the insertion of bilateral earplugs at various postnatal ages. After a period of restored auditory input (via earplug removal), animals will be trained and tested on an aversive conditioning paradigm designed to assess amplitude modulation detection capabilities. Behavioral performance will be quantified by calculating correct responses and false alarms. Detection thresholds will be estimated from psychometric functions and compared between earplug-reared animals and normally-reared littermates. These measures will reveal whether there is a critical period during which the maturation of AM detection is sensitive to hearing loss. Specific Aim 2 will examine the impact of temporary, developmental hearing loss on central sensory encoding in auditory cortex of awake-behaving animals performing the AM detection task described in Aim 1. Multi- and single-unit responses will be recorded wirelessly from chronically implanted electrode arrays in left auditory cortex. AM-evoked firing rates and power will be calculated and used to calculate neurometric curves that can be directly compared to behavioral psychometric functions. Additional analyses will assess the relationship between behavior and neural activity on trial-by-trial basis. Results will be compared between earplug-reared and normally-reared animals. The results of this experiment will elucidate whether behavioral deficits arising from developmental hearing loss can be explained by disrupted sensory coding, as is generally assumed, or whether non-sensory (e.g. cognitive) factors contribute to perceptual dysfunction. Together, these findings will reveal whether there is a discrete developmental critical period for the neural representation and perceptual detection of AM. Ultimately, these results may further our understanding of speech and language delays associated with childhood hearing loss, a goal directly relevant to the mission of NIDCD.

PROJECT SUMMARY/ABSTRACT Candidate: The candidate's long-term goal is to establish an independent research program focusing on the neural mechanisms of long-term perceptual plasticity in the auditory system. Her previous training has provided her with a strong foundation in auditory physiology, sensory plasticity, and behavioral neuroscience. Here, she proposes to expand her skill set with additional training in awake-behaving physiology, adeno- associated virus (AAV)-mediated, cell-type specific opsin expression, and wireless optogenetic manipulation of neural activity in awake-behaving animals. During the K99 phase, she will prepare for the transition to independence by attending workshops on chalk talks, teaching practices, job interviews, negotiation, and lab management. By the end of the mentored phase, the candidate will have the academic and practical skills needed to transition to establish her own laboratory. By the completion of the R00 period of this award, she will have the publication record and preliminary data needed to generate a highly competitive R01 application. Environment: K99 phase training will take place at New York University's (NYU) Center for Neural Science, an outstanding environment for postdoctoral level training in systems-level neuroscience. Dr. Dan Sanes, the primary mentor for this application, has an established auditory neuroscience research program that uses a range of approaches, including in vitro slice physiology, in vivo awake-behaving physiology, calcium imaging, psychophysics, and more recently, optogenetics. A collaboration with Dr. Gordon Fishell, located at NYU's School of Medicine, has provided the Sanes Lab with AAV vectors, allowing for targeted, cell-type specific opsin expression. Dr. Fishell has provided a letter of support, indicating his willingness to continue this collaboration, both with Dr. Sanes during the K99 phase of this award, and with the candidate directly, once she achieves independence. Additional mentoring will be provided by Dr. Daniel Polley (Harvard), and Dr. Jonathan Fritz (University of Maryland), both leaders in the auditory neuroscience community. Research: Long-term improvement in sound detection, a process known as perceptual learning, is critical to language acquisition and musical training. Despite its importance, our understanding of the neural mechanisms underlying perceptual learning remains limited. Furthermore, evidence suggests that top-down modulations of cortical activity related to active listening are involved in perceptual learning, but it is unknown whether a causal relationship exists between these processes. The proposed research will address these issues. Wireless recordings will be made from the auditory cortex of animals as they are trained on a sound detection task, revealing the temporal relationship between neural and behavioral improvement as animals learn (K99). Similar recordings from frontal cortex (R00) will establish the dynamics of top-down activity during perceptual training. Wireless optogenetic activation of local inhibitory circuits within auditory cortex (K99) and frontal cortex (R00) will reveal the causal roles of these brain regions in perceptual learning.

Summary The ability to improve our perceptual skills- to get better with practice- is critical for the acquisition of many complex behaviors, including speech and language. Despite its importance, our understanding of the neural mechanisms underlying this process, known as perceptual learning, remains limited. Recent evidence suggests that training-based plasticity occurs in the top-down networks that modulate auditory cortical activity during task engagement, likely playing an important role in generating long-term improvements in sound encoding and perception. The goal of this project is to explore this hypothesis, specifically focusing on one candidate brain region- the orbitofrontal cortex (OFC). First, to establish the anatomical substrate of this modulation, direct and indirect pathways from OFC to ACx will be characterized in Mongolian gerbils (Meriones unguiculatus) using anatomical tract tracing. Second, to determine the temporal dynamics of top- down activity during perceptual learning, multichannel extracellular recordings will be acquired wirelessly from OFC as animals train and improve on an auditory detection task. Third, to determine whether OFC-mediated modulation of auditory cortical activity plays a causal role in perceptual learning, optogenetics will be used to manipulate OFC activity while simultaneously monitoring auditory cortical response properties and behavioral output. This line of inquiry is well positioned to answer fundamental questions in the fields of auditory learning and memory, and holds the potential to inform the development of clinical treatments to improve perceptual skills in patients with hearing, speech, or language deficits.

Summary The ability to improve our perceptual skills- to get better with practice- is critical for the acquisition of many complex behaviors, including speech and language. Despite its importance, our understanding of the neural mechanisms underlying this process, known as perceptual learning, remains limited. Recent evidence suggests that training-based plasticity occurs in the top-down networks that modulate auditory cortical activity during task engagement, likely playing an important role in generating long-term improvements in sound encoding and perception. The goal of this project is to explore this hypothesis, specifically focusing on one candidate brain region- the orbitofrontal cortex (OFC). First, to establish the anatomical substrate of this modulation, direct and indirect pathways from OFC to ACx will be characterized in Mongolian gerbils (Meriones unguiculatus) using anatomical tract tracing. Second, to determine the temporal dynamics of top- down activity during perceptual learning, multichannel extracellular recordings will be acquired wirelessly from OFC as animals train and improve on an auditory detection task. Third, to determine whether OFC-mediated modulation of auditory cortical activity plays a causal role in perceptual learning, optogenetics will be used to manipulate OFC activity while simultaneously monitoring auditory cortical response properties and behavioral output. This line of inquiry is well positioned to answer fundamental questions in the fields of auditory learning and memory, and holds the potential to inform the development of clinical treatments to improve perceptual skills in patients with hearing, speech, or language deficits.