Marsha G. Clarkson - US grants

The proposed research seeks (1) to explore the development of auditory perceptual abilities, specifically pitch and timbre perception, during infancy, and (2) to delineate some of the acoustic cues infants can employ in those tasks. Both pitch and timbre perception appear to require the integration or synthesis of information gleaned from a peripheral analysis of the spectral characteristics of sound, whereas spectral analysis alone permits simple discrimination of sounds. On the basis of the animal literature, adult psychoacoustics work, and the available infancy research, it can be hypothesized that the synthesis of acoustic information will show dramatic developmental changes during early life. Furthermore, it appears that the ability to analyze sounds into their spectral components precedes the ability to integrate that information in the service of perception. The studies of pitch perception will attempt to establish the emergence of pitch for complex harmonic and inharmonic sounds, including the perception of the missing fundamental. Specifically, these studies will (1) evaluate pitch extraction from harmonic tonal complexes in 3 month olds, and (2) from inharmonic tonal complexes in 7 month olds; (3) determine whether 7 month olds can use spectral information to categorize tonal complexes, when they are unable to integrate that information; (4) determine whether 7 month old infants can use high frequency energy to extract pitch from tonal complexes; and (5) employ a psychophysical technique to assess the extent of the dominance region for pitch in 7 month olds, and (6) in adults. Since very little is known about timbre perception in infancy, an initial study will investigate infants' categorization of sounds on the basis of timbre, but in the presence of irrelevant variations in pitch. Infants from 3 to 8 months of age will be tested in an operant conditioning procedure. Since the proposed research will investigate both analytic and synthetic processing of sound, it is hoped that the general pattern of auditory development and the mechanisms underlying it can be uncovered. In doing so, the proposed research will provide a necessary preliminary step toward the formulation of a general model of auditory development. Furthermore, the brain-behavior relations uncovered will contribute to our understanding of the behavioral consequences of brain damage in human adults and children.

The proposed research seeks (1) to explore the development of auditory perceptual abilities, specifically pitch and timbre perception, during infancy, and (2) to delineate the acoustic cues infants use in those tasks as a means of determining the mechanisms underlying complex auditory processing. Both pitch and timbre perception appear to require the integration or synthesis of information gained from a peripheral analysis on the spectral characteristics of signals. Whereas it is known that infants can synthesize pitch and timbre from clear, unambiguous acoustic signals, and understanding of the mechanisms underlying those perceptions requires that the range of stimuli presented to infants be expanded and that the limits of perception be determined. The studies of pitch perception will assess the limits of infants' abilities to synthesize pitch from a variety of complex spectra that have been used with adults. Specifically, these studies will (1) assess the synthesis of pitch from harmonic tonal complexes containing different numbers of spectral components by 7-month-old infants and naive adults; (2) test infants' extraction of pitch from inharmonic tonal complexes with varying degrees of inharmonicity; (3) evaluate infants' perception of the missing fundamental from signals having high fundamental frequencies, which do not elicit a pitch for adults; and (4) measure the effects of different-frequency maskers on infants discrimination of pure tone and tonal complexes. Studies of timbre perception will move beyond the simple discrimination already demonstrated and attempt to determine the basis of infants' responding. Specifically, these studies, will (1) test infants' discriminate; and (2) assess infants' abilities to integrate spectral information across widely separate frequency regions in a profile analysis task. Seven-month-old infants will be tested in a conditioned head turning procedure where thy will be asked to discriminate complex acoustic signals and to categorize those signals on the basis of pitch or timbre. By delineating the nature of complex auditory perception for infants, the proposed research will provide an important first step in the formulation of a general model of auditory development. Comparisons of infants' performance with that of adults will indicate the degree to which adult theories of auditory perception can be generalized to infants and highlight the magnitude and quality of the developmental changes that must be accounted for by models of auditory development.

The proposed research seeks to understand the development of auditory spatial perception during infancy and to delineate the acoustic features of sounds that permit infants to localize them. Whereas young infants are sensitive to the location of sound sources, their precision of localization is considerably worse than that of adults. Do infants simply have problems detecting localization cues? Or do they have difficulty mapping those cues onto space? A two-pronged attack will address these questions. Eight and 24-week-old infants will be tested in an observer-based psychophysical procedure (OPP) . They will be asked to discriminate a shift in the location of a sound, and a minimal audible angle similar to that found for adults will be estimated. This sound field procedure provides maximal localization information from sounds, but the multiplicity of acoustic cues prohibits any attribution of their relative importance. Therefore, parallel studies will present sounds via earphones to isolate the relevant auditory cues in a lateralization task. Comparisons of performance across the two tasks will begin to tell how much localization acuity is limited by detection of binaural cues. One set of studies (1 and 2) will determine whether the span of potential sound sources influences infants' sound localization and lateralization. A second series of studies will provide successively more access to binaural cues to see how that information improves infants' localization and lateralization. Transient signals will provide interaural timing cues only in their onset (Studies 3 and 4), while repeating such transients will add timing cues from the resulting signal's envelope (Studies 5 and 6). Varying the number of transients presented will directly manipulate the amount of interaural information provided (Studies 7 and 8), and a final study (9) will determine how the rate of presentation influences infants' lateralization. Together localization and lateralization procedures can highlight the limitations of infants' auditory spatial perception and indicate the degree to which adult theories of auditory perception can be generalized to infants.

The applicant's long-term career goals are to characterize the development of complex auditory processing, to disseminate that knowledge to the lay and academic public, and to evaluate similar processing in infants at-risk for hearing impairments. In the short-term, the applicant intends to set- up the facilities needed for infant psychoacoustics procedures, to carry- out the proposed research, and to establish contact with local individuals involved in related research and service fields. The RCDA will provide the applicant time to develop contacts with the broader scientific community, a necessary condition for development of a productive and innovative research career. The award will also provide time for the infant laboratories to be set-up and necessary procedures to be developed. Some of these procedures will be difficult to implement and will require a major time commitment, which only the RCDA can provide. Finally, the RCDA will allow for the initiation of collaborative research. A wealth of scientists interested in areas related to the applicant's research reside in the institution. In addition, collaborative projects are being discussed both with individuals within the applicant organization and at neighboring facilities. The institution will release the applicant from all teaching, except for periodic courses related to the proposed research program, and administrative responsibilities will be reduced to the minimum required to maintain connections with her affiliated programs. The proposed research will investigate the development of infants' auditory spatial perception in localization and lateralization tasks and will delineate the acoustic features that underlie those abilities. Localization in the sound-field will be tested to assess performance in a natural environment, while earphone studies will permit isolation of the binaural cues likely to be used in the field. Across studies, the amount of interaural timing information provided by sounds will be varied to see how and in what way additional cues aid infants' spatial perception.