Temporal and Spectral Processing of Complex Sounds

Projects under this focus area all involve the physiology and psychophysics of temporal or spectral processing of complex sounds in normal and damaged ears of either humans or animals. Several of these projects focus on the perceptual mechanisms underlying the temporal analysis of complex sounds, including human speech and animal vocalizations. Some of them are focused more on central processes and some on more peripheral processes. The ability to accurately analyze the temporal complexities of acoustic communication signals may be reduced in elderly human listeners and in humans suffering from cochlear impairments. In some animals, as in several bird species, temporal auditory abilities are exceptionally acute, perhaps supporting the primacy of the temporal code as a minimal cue system for communication. These differing temporal processing abilities across species may be related to anatomical and physiological differences in the auditory systems, as well as to specific alterations of structures at both peripheral and central processing levels in humans, and in birds with damaged and subsequently regenerated hair cells.

PEOPLE AND PROJECTS

Sandra Gordon-Salant, Hearing and Speech Sciences

Dr. Gordon-Salant's studies of speech perception and auditory psychophysics are aimed at examining the hypothesis that the predominant difficulties in speech understanding for elderly listeners are related to underlying problems in auditory temporal processing. Elderly listeners have particular problems recognizing speech that is degraded by alteration of speaking rate and/or reverberation, with rapid speech eliciting the strongest age-related processing deficits. Gordon-Salant has found a strong age-related deficit in processing brief changes in stimulus duration. The prominent age effect on the duration discrimination measures is even more robust when target stimuli are embedded in tonal sequences. There is no effect of hearing loss on measures of duration discrimination, indicating that the processing limitation is localized beyond the auditory periphery. Here the work of Poeppel, Shamma, and Simon examining more central processes with different techniques offer the possibility of new insights into these kinds of human auditory processing deficits.

Marjorie R. Leek, National Center for Rehabilitative Auditory Research

Leek's program of research explores the processing of complex sounds in hearing impaired listeners with a focus on peripheral stages of analysis. People with damaged cochleae experience a number of abnormalities in auditory analysis of sound in addition to the sensitivity loss that defines the deficit clinically. These include impaired frequency and temporal resolution which may be responsible in part for the difficulties listeners with hearing loss have in understanding speech. The perception of harmonic phase and temporal fine structure in harmonic complexes is related to the characteristics of speech and have led to important insights into hearing of both normal-hearing and hearing-impaired listeners. Leek has shown that the cochlear phase characteristic is altered with hearing loss, and with increases in stimulus level even for normal-hearing listeners, possibly reflecting the same increase in processing linearity that may be observed in the broadening of auditory filters in similar circumstances. Other studies of cochlear nonlinearities and the loss of nonlinearity with hearing impairment are also designed to provide insight into the mechanisms underlying the functional deficits accompanying cochlear damage. Phenomena similar to these have also been investigated in birds and species differences that emerge are providing new ways of thinking about phase perception.

Robert J. Dooling, Psychology (C-CEBH Director)

Dr. Dooling has been investigating the relation between hearing (predominantly measured psychophysically) and vocal learning and development in birds. These investigations are proceeding by comparing numerous temporal measures across avian species including temporal windows, gap detection in tonal markers, and now masking and discrimination of complex temporal waveforms in a series of collaborations with Leek and using moving spectral ripples with Shamma and his colleagues. Direct human/bird comparisons from this collaboration now show that birds are different from humans in the phase response of their auditory filters, and that birds have a highly developed capability for resolving temporal fine structure in harmonic complexes. In an effort to understand the peripheral basis for this temporal resolution, cochlear microphonic and compound action potentials have been recorded to these same harmonic complexes in budgerigars and also, for mammalian comparison, gerbils. The next step is to develop comparable electrophysiological measures in humans to gain greater insight into the relative contributions of peripheral auditory structures and auditory brainstem nuclei to temporal processing capabilities in humans. As these electrophysiological paradigms are developed, we will be able to track not only the effects of insult and senescence on the auditory system but also possible mechanisms of plasticity that may occur in the human ear with the use of amplification.