Human auditory cortex (HAC) is a complex area that contains multiple, functionally specialized auditory cortical fields (ACFs). Elucidating the organizational plan and functional properties of ACFs is necessary for understanding the cortical mechanisms of sound perception. We propose four experiments to elucidate the structural and functional organization of human ACFs and characterize changes in their anatomy and functional properties caused by aging and sensorineural hearing loss (SNHL). Three groups of subjects will be studied: (1) Young subjects with normal hearing;(2) Older subjects with normal hearing;and (3) Older subjects with mild to moderate bilateral sensorineural hearing loss (SNHL). Each subject group will participate in four experiments. During the previous granting period, we analyzed the tonotopic organization of HAC to reveal the organizational plan of human ACFs. HAC has an organization similar to that of other primate species: it contains 12 ACFs with three tonotopically organized core ACFs, surrounded by seven belt and two parabelt ACFs. In Exp. 1, we will replicate and extend these findings using a state-of-the-art MRI scanner. We will data from young subjects and evaluate age- and SNHL- related changes in ACF tonotopic organization, intensity functions, frequency-tuning bandwidths, spatial tuning, and attentional modulation. Speech processing is compromised by aging, particularly in association with SNHL. We previously found that lateral belt ACFs were specialized for processing speech sounds while medial belt ACFs were specialized for processing complex non-speech stimuli. In Exp. 2, we will map activations produced by spoken syllables and various control stimuli to elucidate speech-processing specialization in lateral ACFs. We will also measure changes in activation patterns to speech stimuli due to aging and SNHL. Previous studies and pilot data from our lab demonstrate age-related changes in HAC anatomy and shown that they correlate with auditory perceptual abilities. We have developed a number of innovative techniques to quantitatively analyze the anatomical structure of the cortex. In Exp. 3, we will use these techniques to characterize age- and SNHL-related alterations in HAC by measuring cortical thickness and tissue properties that reflect dendritic density, pericortical fiber organization and myelination. To evaluate the behavioral relevance of these changes we have developed new tests of speech comprehension in noise and auditory short- term verbal memory and found that these abilities are compromised by aging, particularly in patients with SNHL. In Exp. 4, we will use these and other tests to quantify auditory perceptual abilities in the three subject groups. The results will be correlated with the pattern of functional activations seen in Exps. 1 and 2 and with HAC anatomical structure measured in Exp. 3. Significance. Age-related impairments in speech comprehension are among the most common complaints of older veterans with and without hearing loss. This research program will elucidate the anatomical and functional basis of age- and SNHL-related impairments in speech perception. In addition, it will provide essential normative data that will clarify the basic anatomical and functional organization of HAC. The results will improve the precision of neuroimaging studies of HAC that has been implicated in a variety of audiological, neurological and psychiatric disorders including tinnitus, stroke, dementia, autism, schizophrenia, and traumatic brain injury (TBI).
The goal of this 4-yr research program is to use structural and functional MRI to clarify the organizational plan of auditory cortex in young Veterans, older Veterans with normal hearing, and older Veterans with hearing loss. We will clarify how auditory cortex processes both simple and complex sounds, particularly speech. Difficulties in speech comprehension affect Veterans increasingly as they age. Tests of speech comprehension with competing messages show abnormalities in 90% of Veterans aged 60-69 years. Our innovative neuroimaging and behavioral testing methods will elucidate the abnormalities in auditory cortex that underlie the impaired speech processing of older subjects with and without hearing loss. Moreover, they will permit more refined studies of the abnormalities of auditory cortex in other common disorders (e.g., TBI, schizophrenia, tinnitus, and dementia) in which auditory cortex plays a central role.