Spoken language processing in noisy environments, a hallmark of the human brain, is subject to agerelated decline, even when peripheral hearing might be intact. The present study examines the cortical cerebral hemodynamics (measured by fMRI) associated with such processing in the aging brain. Younger and older subjects identified single words in quiet and in two multi-talker babble noise conditions (SNR 20 and −5 dB). Behaviorally, older and younger subjects did not show significant differences in the first two conditions but older adults performed less accurately in the SNR -5 condition. The fMRI results showed reduced activation in the auditory cortex but an increase in working memory and attention-related cortical areas (prefrontal and precuneus regions) in older subjects, especially in the SNR -5 condition. Increased cortical activities in general cognitive regions were positively correlated with behavioral performance in older listeners, suggestive of a compensatory strategy. Furthermore, inter-regional correlation revealed that while younger subjects showed a more streamlined cortical network of auditory regions in response to spoken word processing in noise, older subjects showed a more diffused network involving frontal and ventral brain regions. These results are consistent with the decline-compensation hypothesis, suggestive of its applicability to the auditory domain.

High-resolution measurements of distortion product otoacoustic emissions (DPOAEs) from three different experimental paradigms are shown to be in agreement with the implications of a realistic "two-source" cochlear model of DPOAE fine structure. The measurements of DPOAE amplitude and phase imply an interference phenomenon involving one source in the region of strong nonlinear interaction of the primary waves (the strong "overlap" or generation region), and the other source region around the DPOAE tonotopic place. The component from the DPOAE place can be larger than the one from the generator region. These findings are supported by the analysis of the onset and offset of the DPOAE when the higher-frequency primary is pulsed on and off. The two-source hypothesis was further tested by adding a third tone closer in frequency to the DPOAE which modifies the amplitude of the component from the DPOAE place and leaves the one from the generator region unchanged. The results agree well with the model prediction that the variation with frequency, and implied latency, of the phase of the DPOAE tonotopic-place component are greater than the corresponding quantities for the component from the generation region.

Objectives-Previous research has attributed older adult's difficulty with perceiving speech in noise to peripheral hearing loss. Recent studies have suggested a more complex picture, however, and implicate the central nervous system in sensation and sensory deficits. This study examines the relationship between the neuroanatomical structure of cognitive regions and the ability to perceive speech in noise in older adults. In particular, the neuroanatomical characteristics of the left ventral and dorsal prefrontal cortex are considered relative to standard measures of hearing in noise.Design-The participants were fifteen older and fourteen younger right-handed native speakers of American English who had no neurological deficits and scored better than normal on standardized cognitive tests. We measured the participants' peripheral hearing ability as well as their ability to perceive speech in noise using standardized tests. Anatomical magnetic resonance images were taken and analyzed to extract regional volumes and thicknesses of several key neuroanatomical structures.Results-The results showed that younger adults had better hearing sensitivity and better speech perception in noise ability than older adults. For the older adults only, the volume of the left pars triangularis and the cortical thickness of the left superior frontal gyrus were significant predictors of performance on the speech-in-noise test.Discussion-These findings suggest that, in addition to peripheral structures, the central nervous system also contributes to the ability to perceive speech in noise. In older adults, a decline in the volume and cortical thickness of the prefrontal cortex (PFC) during aging can therefore be a factor in a declining ability to perceive speech in a naturalistic environment. Our study shows a link between anatomy of PFC and speech perception in older adults. These findings are consistent with the decline-compensation hypothesis, which states that a decline in sensory processing due to cognitive aging can be accompanied by an increase in the recruitment of more general cognitive areas as a means of compensation. We found that a larger PFC volume may compensate for declining peripheral hearing. Clinically, recognizing the contribution of the cerebral cortex expands treatment possibilities for hearing loss in older adults beyond peripheral hearing aids to include strategies for improving cognitive function. We conclude by considering several mechanisms by which the PFC may facilitate speech perception in noise including inhibitory control, attention, cross-modal compensation, and phonological working memory, though no definitive conclusion can be drawn.

We found cortical task-independent and noise-dependent effects concerning speech perception in noise involving bilateral mSTG and left pSTG. These results likely reflect demands in acoustic analysis, auditory-motor integration, and phonological memory, as well as auditory attention.

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