Temporal Resolution Ability Research Articles

Temporal Resolution and Speech Recognition Ability of Patients with Retrocochlear Auditory Dysfunction

ObjectiveTo study the effect of temporal resolution measured by gap detection tasks on the speech recognition ability of patients with retrocochlear auditory dysfunction.Material and MethodsThe gap detection thresholds (GDTs) for dichotic stimuli were measured for 23 patients with retrocochlear auditory dysfunction confirmed by abnormal auditory brainstem responses the relationship between GDTs and maximum speech recognition scores (max. SRSs) in quiet was examined. In gap detection testing, narrow-band noises of the five center frequencies of 0.25, 0.5, 1.0, 2.0, 4.0 kHz were used as leading and trailing markers. Each of these two marker sounds was presented to the opposite ear at 20 dBSL with various gap durations between them.ResultsAbnormal GDTs were observed at one or more test frequencies in 17 of 23 patients. In 12 patients, abnormal GDTs were found at four or five test frequencies. These 12 patients included 4 patients with poor max. SRS less than 30%. However, no tendency was found for max. SRSs to be poorer as abnormal GDTs were observed for more test frequencies.ConclusionTemporal resolution measured by gap detection for dichotic stimuli was frequently reduced in patients with retrocochlear auditory dysfunction, and this reduction appeared to spread over a wide frequency range when speech recognition had markedly deteriorated. However, it also appeared that reduced temporal resolution for dichotic stimuli might not necessarily imply poor speech recognition in quiet.

Temporal resolution and temporal integration of short pulses at the auditory periphery of echolocating animals

To explain the temporal integration and temporal resolution abilities revealed in echolocating animals by behavioral and electrophysiological experiments, the peripheral coding of sounds in the high-frequency auditory system of these animals is modeled. The stimuli are paired pulses similar to the echolocating signals of the animals. Their duration is comparable with or smaller than the time constants of the following processes: formation of the firing rate of the basilar membrane, formation of the receptor potentials of internal hair cells, and recovery of the excitability of spiral ganglion neurons. The models of auditory nerve fibers differ in spontaneous firing rate, response thresholds, and abilities to reproduce small variations of the stimulus level. The formation of the response to the second pulse of a pair of pulses in the multitude of synchronously excited high-frequency auditory nerve fibers may occur in only two ways. The first way defined as the stochastic mechanism implies the formation of the response to the second pulse as a result of the responses of the fibers that did not respond to the first pulse. This mechanism is based on the stochastic nature of the responses of auditory nerve fibers associated with the spontaneous firing rate. The second way, defined as the repeatition mechanism, implies the appearance of repeated responses in fibers that already responded to the first pulse but suffered a decrease in their response threshold after the first spike generation. This mechanism is based on the deterministic nature of the responses of fibers associated with refractoriness. The temporal resolution of pairs of short pulses, which, according to the data of behavioral experiments, is about 0.1–0.2 ms, is explained by the formation of the response to the second pulse through the stochastic mechanism. A complete recovery of the response to the second pulse, which, according to the data of electrophysiological studies of short-latency evoked brainstem potentials in dolphins, occurs within 5 ms, is explained by the formation of the response to the second pulse through the repetition mechanism. The time constant of temporal integration, which, according to the behavioral experiments at threshold levels of pulses, is about 0.2–0.3 ms, is explained by the integrating properties of internal hair cells, etc. It is shown that, at the high-frequency auditory periphery, the temporal integration imposes no limitations on the temporal resolution, because both integration and resolution are different characteristics of the same multiple response of synchronously excited fibers.

Perceptual characterization of children with auditory neuropathy.

To characterize the perceptual abilities of a group of children with auditory neuropathy (AN)-type hearing loss, correlating results on a range of psychophysical tasks with open-set speech perception performance. Frequency resolution, temporal resolution and frequency discrimination ability was assessed in a group of 14 children with AN. Data also were obtained from a cohort of matched subjects with sensorineural hearing loss, and from a group of normally hearing children. Frequency resolution (notched noise masking) results for the AN subjects were equivalent to those of the normal-hearing subjects reflecting the "normal" outer hair cell function that characterizes the AN condition. Temporal resolution (TMTF) findings were, however, abnormal in many AN subjects and the degree of temporal disruption was correlated with speech discrimination (CNC) score. Frequency discrimination ability (for both fixed and frequency modulated stimuli) was also affected in those children with poor temporal resolution. The findings of this study indicate that the perceptual profiles of children with AN are quite different from those with sensorineural hearing loss. Where subjects in the latter group presented with impaired frequency resolution and normal temporal processing, the AN subjects typically showed normal frequency resolution and varying degrees of temporal disruption. The severity of this temporal abnormality, which appeared to affect both temporal resolution/amplitude modulation detection and the temporal aspects of frequency discrimination (such as phase locking), was strongly correlated to speech perception performance.

Use of temporal information in recognition of amplitude-compressed speech by older adults

Amplitude compression alters the speech envelope and under some conditions can reduce the availability of temporal cues for speech recognition. This may be a particular problem for older listeners, who demonstrated temporal resolution deficits in some studies. Older and younger hearing-impaired listeners were tested on their recognition of VCV syllables processed using the signal-correlated noise (SCN) method [Schroeder, J. Acoust. Soc. Am. 44, 1735–1736 (1968)]. This method results in a time-varying speech envelope modulating a noise carrier, and preserves temporal cues while minimizing spectral information. Recognition scores were obtained for uncompressed SCN syllables and for the same speech tokens after digital processing with a syllabic compression algorithm. In control conditions, recognition was tested for VCVs that included both temporal and spectral information. Younger and older listener groups were matched for mean hearing thresholds up to 4 kHz. A high-pass masker was included to eliminate contributions from the basal end of the cochlea. When compared to younger listeners with similar hearing losses, the older listeners demonstrated poorer performance on the compressed SCN signals. These results suggest that the poorer temporal resolution abilities of older listeners may play a role in their understanding of amplitude-compressed speech.

Auditory temporal resolution in specifically language-impaired and age-matched children.

Recently there has been renewed interest in the auditory processing capabilities of children with specific language impairment. In this study, eight children with specific language impairment and eight nonimpaired, age-matched peers completed a task to assess temporal resolution abilities. Children were asked to detect a tone in three masking conditions wherein the masker contained silent gaps of 0 msec., 40 msec., or 64 msec. in duration. Thresholds were measured in each masking condition at 500 Hz and 2000 Hz. Across the groups, thresholds decreased (improved) significantly as a function of increases in the duration of the gaps. Children in the two groups exhibited remarkably similar thresholds for the three masking conditions. However, children with specific language impairment required a significantly greater number of ascending trials to achieve the threshold criterion than did age-matched children. Results suggest that language-impaired children perceive temporal aspects of acoustic stimuli as well as their normally developing peers. Attentional mechanisms may play an important role in the difficulties they exhibit in auditory processing.

The effect of intensity on gap detection in hearing-impaired listeners

Previous investigations of gap detection by hearing-impaired listeners have yielded results that are inconsistent across listeners and studies. The full effect of stimulus intensity and audibility on hearing-impaired listeners’ performance has not been described. This study investigated the effect of intensity on gap detection abilities of normal-hearing listeners and listeners with similar mild to moderate high-frequency hearing impairments. Listeners were required to detect a gap in a high-frequency narrow-band noise over a wide range of intensities above threshold, and to judge the loudness of the signal. Results indicated that at low intensities all listeners showed reduced gap detection ability. As intensity increased, the listeners’ performance improved and stabilized at high sensation levels. At these higher intensities, the performance of hearing-impaired listeners was similar to, but still significantly different from that of normals (mean group difference=1 ms). This difference in listeners’ performance was found when results were compared in sensation level, sound-pressure level, and loudness. Consistent results across eight hearing-impaired listeners suggested that ensuring full audibility of the stimulus yields more consistent performance. These results suggest that temporal resolution ability is affected by both hearing impairment and by stimulus intensity.

Relationship between psychoacoustics and ‘‘say–stay’’ identification in hearing-impaired listeners.

The relationship between gap detection, glide discrimination, and identification of synthesized speech stimuli as more like ‘‘say’’ or more like ‘‘stay’’ was investigated in 18 listeners with varying degrees of hearing impairment. Speech stimuli were patterned after Best, Morrongiello, and Robson’s (1981) ‘‘say–stay’’ identification task, which involved the listeners’ use of silent gap (between signal-to-noise and vocalic portions of the signals) and first formant onset frequency, both indicating the presence or absence of stop consonants. A significant effect of hearing impairment was found on the gap detection and glide discrimination tasks, and on the identification functions of the ‘‘say–stay’’ stimuli. However, the speech identification functions were not correlated with the psychoacoustic results. In fact, the identification performance suggested listeners with mild to moderate hearing impairments were using weaker spectral cues than the normal-hearing listeners, which is contradictory to what is predicted by reduced spectral and temporal resolution abilities.

Central Auditory Skills in Blind and Sighted Subjects

Three different central auditory skills were compared and evaluated in 56 blind and 40 sighted subjects. The study consisted of three experiments conducted in three subgroups. Experiment A was performed in order to evaluate the localization function; experiment B for the temporal auditory resolution ability of the blind adult, and experiment C to test the ability of the blind person to discriminate speech material in noise. In all three experiments the blind subjects obtained significantly better results than the sighted subjects. From these results it was concluded that there is supporting evidence of a certain superiority of the blind individual with regard to central auditory function.

The resolution of brief interaural time-difference changes: recordings from primary auditory cortical projection field AI of the cat.

The ability of the auditory organ to resolve brief changes in an acoustic signal presented either monaurally or binaurally is not only of great importance in the processing of speech, it is also involved in the localization of sound stimuli and in selective listening. In the latter context, the electric activity of the primary auditory cortical projection field AI of the cat has been studied with the aim of evaluating specific response patterns evoked by brief changes in interaural time difference. The differences in response of the neuron populations sampled by two recording electrodes indicate that, within this area, there are significant differences in temporal resolution ability. Whereas click stimuli elicit distinct potential patterns at the two sites, with a brief change in interaural time difference, a marked response is recorded by only one of the electrodes. This response is characterized by a decrease in amplitude as the interaural time difference is reduced and as the duration of the time-shift stimulus decreases.

Neural processing mechanisms in echolocating bats, correlated with differences in emitted sounds.

Bats that emit pulses containing constant wavelength (CW) components preceding an FM sweep show neural adaptations that differ conspicuously from those of bats emitting purely FM pulses. In contrast to “FM bats,” which have sharply tuned units distributed broadly throughout the range of their emitted pulses, “CW bats” have auditory systems tuned predominantly to a much smaller range corresponding to the restricted range of emitted frequencies. There is an “off”-response sharply tuned to the emitted CW frequencies. “On” response sensitivity is greater at just lower frequencies (CW component of less than 8-msec duration) or on either side of the “off”-frequency, with a null at that point (CW component of 10–20 msec or longer). The “off”-response, which is present at the level of the auditory nerve, apparently represents rebound excitation following non-neural suppression peripheral to the primary afferent fibers. It reflects a mechanism for increasing frequency discrimination and has the effect as well of enhancing response to the FM portion of the emitted pulses. High sensitivity to frequencies above that of the emitted CW, in “long CW” bats, is apparently tied to a use of the Doppler-shifted echo CW to detect and determine the angle and/or relative velocity of echo sources. CW bats show much less dramatic temporal resolution ability than FM bats, although it seems probable that the same mechanism of distance determination—measurement of elapsed time between emitted and returning FM sweeps—is used by both. CW bats, on the other hand, appear to have more units that are sharply sensitive to signal angle, showing strong binaural interaction. The consistent and conspicuous differences in neurophysiology in different species indicates not only that central analytic mechanisms are highly adaptable, but that these adaptations are important and closely tied to the types of orientation sounds used.