Previous studies have found a significant correlation between spectral-ripple discrimination and speech and music perception in cochlear implant (CI) users. This relationship could be of use to clinicians and scientists who are interested in using spectral-ripple stimuli in the assessment and habilitation of CI users. However, previous psychoacoustic tasks used to assess spectral discrimination are not suitable for all populations, and it would be beneficial to develop methods that could be used to test all age ranges, including pediatric implant users. Additionally, it is important to understand how ripple stimuli are processed in the central auditory system and how their neural representation contributes to behavioral performance. For this reason, we developed a singleinterval, yes/no paradigm that could potentially be used both behaviorally and electrophysiologically to estimate spectral-ripple threshold. In experiment 1, behavioral thresholds obtained using the single-interval method were compared to thresholds obtained using a previously established three-alternative forcedchoice method. A significant correlation was found (r=0.84, p=0.0002) in 14 adult CI users. The spectralripple threshold obtained using the new method also correlated with speech perception in quiet and noise. In experiment 2, the effect of the number of vocoderprocessing channels on the behavioral and physiological threshold in normal-hearing listeners was determined. Behavioral thresholds, using the new singleinterval method, as well as cortical P1-N1-P2 responses changed as a function of the number of channels. Better behavioral and physiological performance (i.e., better discrimination ability at higher ripple densities) was observed as more channels added. In experiment 3, the relationship between behavioral and physiological data was examined. Amplitudes of the P1-N1-P2 "change" responses were significantly correlated with d′ values from the single-interval behavioral procedure. Results suggest that the single-interval procedure with spectral-ripple phase inversion in ongoing stimuli is a valid approach for measuring behavioral or physiological spectral resolution.

This study evaluated the role of temporal fine structure in the lateralization and understanding of speech in six normal-hearing listeners. Interaural time differences (ITDs) were introduced to invoke lateralization. Speech reception thresholds (SRTs) were evaluated in backgrounds of two-talker babble and speech-shaped noise. Two-syllable words with ITDs of 0 and 700 ms were used as targets. A vocoder technique, which systematically randomized fine structure, was used to evaluate the effects of fine structure on these tasks. Randomization of temporal fine structure was found to significantly reduce the ability of normal-hearing listeners to lateralize words, although for many listeners, good lateralization performance was achieved with as much as 80% fine-structure randomization. Most listeners demonstrated some rudimentary ability to lateralize with 100% fine-structure randomization. When ITDs were 0 ms, randomization of fine structure had a much greater effect on SRT in two-talker babble than in speech-shaped noise. Binaural advantages were also observed. In steady noise, the difference in SRT between words with 0-vs 700-ms ITDs was, on average, 6 dB with no fine-structure randomization and 2 dB with 100% fine-structure randomization. In two-talker babble this difference was 1.9 dB and, for most listeners, showed little effect of the degree of finestructure randomization. These results suggest that (1) improved delivery of temporal fine structure would improve speech understanding in noise for implant recipients, (2) bilateral implant recipients might benefit from temporal envelope ITDs, and (3) improved delivery of temporal information could improve binaural benefits.

. Effects of inhibitory feedback in a network model of avian brain stem. J Neurophysiol 94: 400 -414, 2005. First published March 2, 2005; doi:10.1152/jn.01065.2004. The avian auditory brain stem consists of a network of specialized nuclei, including nucleus laminaris (NL) and superior olivary nucleus (SON). NL cells show sensitivity to interaural time difference (ITD), a critical cue that underlies spatial hearing. SON cells provide inhibitory feedback to the rest of the network. Empirical data suggest that feedback inhibition from SON could increase the ITD sensitivity of NL across sound level. Using a bilateral network model, we assess the effects of SON feedback inhibition. Individual cells are specified as modified leakyintegrate-and-fire neurons with time constants and thresholds that vary with inhibitory input. Acoustic sound level is reflected in the discharge rates of the model auditory-nerve fibers, which innervate the network. Simulations show that with SON inhibitory feedback, ITD sensitivity is maintained in model NL cells over a threefold range in auditory-nerve discharge rate. In contrast, without SON feedback inhibition, ITD sensitivity is significantly reduced as input rates are increased. Feedback inhibition is most beneficial in maintaining ITD sensitivity at high-input rates (simulating high sound levels). With SON inhibition, ITD sensitivity is maintained for both interaurally balanced inputs (simulating an on-center sound source) and interaurally imbalanced inputs (simulating a lateralized source). Further, the empirically observed temporal build-up of SON inhibition and the presence of reciprocal inhibitory connections between the ipsi-and contralateral SON both improve ITD sensitivity. In sum, our network model shows that inhibitory feedback can substantially increase the sensitivity and dynamic range of ITD coding in the avian auditory brain stem. I N T R O D U C T I O NThe submillisecond difference in the time of arrival of sound to the two ears, interaural time difference (ITD), is a critical cue in the localization and processing of sound sources (cf. Blauert 1997). Nucleus laminaris (NL), the avian homologue of the mammalian medial superior olive (MSO), is the first neural center to receive input from the two ears. Fibers from nucleus magnocellularis (NM) project bilaterally and provide phase-locked, glutamatergic, excitatory input to NL. Single neurons in NL show sensitivity to ITD by performing coincidence detection (Jeffress 1948) on the input from left and right NM (Carr and Konishi 1990;Sullivan and Konishi 1984). Empirical data indicate that ITD sensitivity is maintained in NL over a 50-dB range in sound level (Peña et al. 1996). However, simple coincidence models (Peña et al. 1996; Reed and Durbeck 1995) lose ITD sensitivity as the discharge rate of NM fibers monotonically increases threefold or more with sound level (Warchol and Dallos 1990).Two mechanisms have been demonstrated to maintain ITD sensitivity in models of NL despite variations in sound level. First, synaptic d...