Speech scientists have long proposed that formant exaggeration in infant-directed speech plays an important role in language acquisition. This event-related potential (ERP) study investigated neural coding of formant-exaggerated speech in 6-12-monthold infants. Two synthetic ⁄ i ⁄ vowels were presented in alternating blocks to test the effects of formant exaggeration. ERP waveform analysis showed significantly enhanced N250 for formant exaggeration, which was more prominent in the right hemisphere than the left. Time-frequency analysis indicated increased neural synchronization for processing formantexaggerated speech in the delta band at frontal-central-parietal electrode sites as well as in the theta band at frontal-central sites. Minimum norm estimates further revealed a bilateral temporal-parietal-frontal neural network in the infant brain sensitive to formant exaggeration. Collectively, these results provide the first evidence that formant expansion in infant-directed speech enhances neural activities for phonetic encoding and language learning.

Gaussian noise simultaneous maskers yield higher masked thresholds for pure tones than lowfluctuation noise simultaneous maskers for listeners with normal hearing. This increased masking effectiveness is thought to be due to inherent fluctuations in the temporal envelope of Gaussian noise, but effects of fluctuating forward maskers are unknown. Because differences in forward masking due to age and hearing loss are known, the current study assessed effects of masker envelope fluctuations for forward maskers in younger and older adults with normal hearing and older adults with hearing loss. Detection thresholds were measured in these three participant groups for a pure-tone probe in quiet and in Gaussian and low-fluctuation noise forward maskers with either 1 or 1/3 equivalent rectangular bandwidths. Higher masked thresholds were obtained for forward maskers with greater inherent envelope fluctuations for younger adults with normal hearing. This increased effectiveness of highly fluctuating forward maskers was similar for older adults with normal and impaired hearing. Because differences in recovery from forward masking between listeners with normal and impaired hearing may relate to differences in cochlear nonlinearities, these results suggest that mechanisms other than cochlear nonlinearities may be responsible for recovery from rapid masker envelope fluctuations.

Forward-masked thresholds increase as the magnitude of inherent masker envelope fluctuations increase for both normal-hearing (NH) and hearing-impaired (HI) adults for a short masker-probe delay (25 ms). The slope of the recovery from forward masking is shallower for HI than for NH listeners due to reduced cochlear nonlinearities. However, effects of hearing loss on additional masking due to inherent envelope fluctuations across masker-probe delays remain unknown. The current study assessed effects of hearing loss on the slope and amount of recovery from forward maskers that varied in inherent envelope fluctuations. Forward-masked thresholds were measured at 2000 and 4000 Hz, for masker-probe delays of 25, 50, and 75 ms, for NH and HI adults. Four maskers at each center frequency varied in inherent envelope fluctuations: Gaussian noise (GN) or low-fluctuation noise (LFN), with 1 or 1/3 equivalent rectangular bandwidths (ERBs). Results suggested that slopes of recovery from forward masking were shallower for HI than for NH listeners regardless of masker fluctuations. Additional masking due to inherent envelope fluctuations was greater for HI than for NH listeners at longer masker-probe delays, suggesting that inherent envelope fluctuations are more disruptive for HI than for NH listeners for a longer time course.

Forward masking is generally greater for Gaussian noise (GN) than for low-fluctuation noise (LFN) maskers for listeners with and without sensorineural hearing loss (SNHL). Because older age and SNHL may affect recovery from masker envelope fluctuations differently, the current study explored which of these two factors contributed more substantially to the persistence of GN disruption. GN disruption was measured using three masker-signal delays (25, 75, and 150 ms) for three adult participant groups: younger listeners with normal hearing (YNH), older participants with normal, or near-normal, hearing (ONH), and older participants with sensorineural hearing loss (OSNHL). The role of underlying mechanisms was tested using a computational model for midbrain neurons. The primary result suggests that older listeners with normal or near normal hearing may be more susceptible to the deleterious effects of masker-envelope fluctuations than younger listeners with normal hearing. Results from the computational model propose that there may be a larger influence of efferent feedback and saturation of inner hair cells on GN disruption than previously expected.

The effect of masking release is still the source of numerous active investigations. However, differences in findings are sometimes noted among studies, especially related to potential gate frequency effects. The present study investigated the effect of masking release on listeners using a variety of speech materials . The main goal of this study was to investigate the effect of speech material on measures of masking release for listeners with normal hearing and hearing loss. The test stimuli were IEEE sentences and modified spondee words. To eliminate confounds of audibility and duration, the test stimuli were equated for duration and audibility. Listeners were tested across a wide range of audibility and gate frequencies. Performance and masking release results will be presented for these speech stimuli. [Work supported by NIDCD 008306.]