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The frequency modulation (FM) of speech can convey linguistic information and also enhance speech-stream coherence and segmentation. The purpose of the present study was to use a clinically oriented approach to examine the effects of age and hearing loss on the ability to discriminate between stochastic patterns of low-rate FM and determine whether difficulties in speech perception experienced by older listeners relate to a deficit in this ability.

Stop-consonant discrimination was investigated in normal-hearing listeners and cochlear-implanted patients (CIP) by recording auditory evoked potentials (AEPs) to b and p syllables. This study demonstrates that (i) AEPs show time-locked components that mimic the temporal structure of the stimuli, indicating that both patients and control subjects encode those syllables according to the temporal cue (voice onset time) characterizing the voicedvoiceless contrast; (ii) the side of implantation does not affect the general structure of AEPs and b-p discrimination thresholds (measured separately with a psychophysical procedure); (iii) poor time-locking to the syllables' temporal structure is associated with poor discrimination. This suggests that EEG investigation of temporal-processing provides an objective index of speech perception in CIP and could be used in implanted children.

Speech intelligibility depends heavily on the accurate perception of auditory temporal envelope cues, that is the slower amplitude modulations present in the speech waveform. In a previous study, McAnally and Stein demonstrated that dyslexics may show impaired audibility (i.e. detectability) of these envelope cues. In the present psychophysical study, the ability to process temporal envelope cues was further investigated in dyslexic children by measuring detection thresholds of sinusoidal amplitude-modulation (SAM) and discrimination thresholds of SAM depth and SAM rate. Each threshold was measured at slow and fast SAM rates of 4 and 128 Hz, respectively. Overall, SAM thresholds were higher in dyslexics than in controls at both rates. The strongest deficit was observed at 4 Hz in the SAM detection task, but a deficit was also apparent at 128 Hz in the SAM discrimination tasks. Therefore, these results reveal that, in addition to reduced audibility of slow and fast envelope cues, some dyslexic children show poor encoding fidelity for these cues (as measured by the discrimination tasks). Overall, these findings are consistent with Tallal's hypothesis according to which the speech and reading deficits in some dyslexics may be caused by impaired temporal processes.

The cerebral representation of the temporal envelope of sounds was studied in five normal-hearing subjects using functional magnetic resonance imaging. The stimuli were white noise, sinusoidally amplitude-modulated at frequencies ranging from 4 to 256 Hz. This range includes low AM frequencies (up to 32 Hz) essential for the perception of the manner of articulation and syllabic rate, and high AM frequencies (above 64 Hz) essential for the perception of voicing and prosody. The right lower brainstem (superior olivary complex), the right inferior colliculus, the left medial geniculate body, Heschl's gyrus, the superior temporal gyrus, the superior temporal sulcus, and the inferior parietal lobule were specifically responsive to AM. Global tuning curves in these regions suggest that the human auditory system is organized as a hierarchical filter bank, each processing level responding preferentially to a given AM frequency, 256 Hz for the lower brainstem, 32–256 Hz for the inferior colliculus, 16 Hz for the medial geniculate body, 8 Hz for the primary auditory cortex, and 4–8 Hz for secondary regions. The time course of the hemodynamic responses showed sustained and transient components with reverse frequency dependent patterns: the lower the AM frequency the better the fit with a sustained response model, the higher the AM frequency the better the fit with a transient response model. Using cortical maps of best modulation frequency, we demonstrate that the spatial representation of AM frequencies varies according to the response type. Sustained responses yield maps of low frequencies organized in large clusters. Transient responses yield maps of high frequencies represented by a mosaic of small clusters. Very few voxels were tuned to intermediate frequencies (32–64 Hz). We did not find spatial gradients of AM frequencies associated with any response type. Our results suggest that two frequency ranges (up to 16 and 128 Hz and above) are represented in the cortex by different response types. However, the spatial segregation of these two ranges is not systematic. Most cortical regions were tuned to low frequencies and only a few to high frequencies. Yet, voxels that show a preference for low frequencies were also responsive to high frequencies. Overall, our study shows that the temporal envelope of sounds is processed by both distinct (hierarchically organized series of filters) and shared (high and low AM frequencies eliciting different responses at the same cortical locus) neural substrates. This layout suggests that the human auditory system is organized in a parallel fashion that allows a degree of separate routing for groups of AM frequencies conveying different information and preserves a possibility for integration of complementary features in cortical auditory regions.

AbstractThis paper presents a study in which five cochlear implantees were asked to discriminate the timbre of stimuli with temporally asymmetric envelopes. Stimuli were damped and ramped sinusoids presented acoustically. They were transformed by the speech processor of the implant and were presented through one electrode. All cochlear implantees could discriminate the damped and ramped sinusoids when the half-life was 4 ms, the carrier frequency was 400 Hz, and the period of the envelope was 50 ms. In a second experiment, timbre discrimination performance was measured as a function of half-life for two cochlear implantees. Both showed that timbre discrimination was possible over the range 1––24 ms. In normal-hearing listeners, the range is 1––64 ms and in cochlear implantees, stimulated directly without the speech processor, the range is 1––300 ms. At long half-lives, the decrease in discrimination performance observed with the speech processor appears to be due to the amplitude compression applied by the device. The present results suggest that it may be important to ensure that cochlear implants do not restrict temporal asymmetry unduly when applying compression to control level.

A physiological model of a post‐cochlear mechanism responsible for enhancing the phase‐locked activity of auditory neurons is proposed, when excited by sinusoi¨dally amplitude‐modulated (AM) tones. In the cochlear nuclei, chopper units respond to the summated low‐frequency components of their peripheral inputs. This feature is due to the time constant of the excitatory postsynaptic potential. It is also the result of the specific time course of the cellular membrane potential during the refractory period. The physiologically plausible model is derived from Hodgkin–Huxley equations. AM tones and white noises were applied into the chopper model. In both experiments, the responses of the model showed a strong phase‐locking for modulation frequencies between 100 and 300 Hz. The mean gain of the chopper model suggests that chopper units enhance the phase‐locked AM responses that they receive as inputs from auditory‐nerve fibers. These results agree with earlier electrophysiological studies.

A series of 2IFC experiments was performed with click trains to determine (a) the temporal jitter and (b) the amplitude jitter necessary to discriminate a regular click train from a jittered click train. The period of the regular click train was varied from 10 ms, where the sound is heard as a 100‐Hz buzzy tone, up to 120 ms, where it is heard as a stream of discreet clicks. A fixed percentage jitter was applied to the period (or the amplitude) of each click in the train with positive jitter, negative jitter, and no jitter being given equal probability. The results show that the auditory system is much more sensitive to time jitter (around 12%) than to amplitude jitter (around 50%), and there is a ‘‘hump’’ in the Weber fraction around periods of 40–50 ms between the ‘‘buzzy tone’’ and ‘‘distinct clicks’’ regions. The results are interpreted in terms of the auditory image model of auditory perception [Pattersonetal., J. Acoust. Soc. Am. 98, 1890–1894 (1995)] which preserves phase‐locking information in a buffer which is limited to time intervals less than about 40 ms.