Gap Discrimination Research Articles

Reversing Pathological Neural Activity Using Targeted Plasticity

Reversing Pathological Neural Activity Using Targeted Plasticity

Inactivation of the left auditory cortex impairs temporal discrimination in the rat

The left auditory cortex (AC) in humans is involved in the processing of the temporal parameters of acoustical signals, specifically in speech perception, whereas the right AC plays the dominant role in pitch and melody perception. The hemispheric lateralization of acoustical signal processing in non-human mammals is less explored. The present study examined the ability of rats to detect or discriminate a series of gaps in continuous noise under conditions of unilateral or bilateral reversible inactivation of the AC. The results showed that muscimol-induced reversible inactivation of the left AC suppresses the ability of rats to discriminate between acoustical stimuli of different temporal parameters (duration or repetition rate), whereas inactivation of the right AC results in no change or only a mild decrease in discrimination ability. Hemispheric asymmetry was observed only in the case of gap discrimination tasks, but not in a gap detection task. Our findings demonstrate that, similarly as in humans, the left AC in the rat plays the dominant role in temporal discrimination. These data provide further evidence for the functional asymmetry of the mammalian brain, which appears in a relatively early phase of evolution.

Effect of age on silent gap discrimination in synthetic speech stimuli.

The difficulty that older listeners experience understanding conversational speech may be related to their limited ability to use information present in the silent intervals (i.e., temporal gaps) between dynamic speech sounds. When temporal gaps are present between nonspeech stimuli that are spectrally invariant (e.g., noise bands or sinusoids), older listeners are less able to resolve temporal gaps than are younger listeners. It has also been demonstrated that temporal gap perception deteriorates as the frequency difference between the sounds bordering the silent gap increases, and this effect becomes more pronounced with age (J. Lister, J. Besing, and J. Koehnke, 2002; J. Lister, J. Koehnke, and J. Besing, 2000). In this study, the effect of age on the ability to discriminate temporal gaps in dynamic stimuli (i.e., changing in frequency and duration over time) was measured in a gap duration discrimination (GDD) task. The participants were two groups of listeners with normal hearing sensitivity through at least 4000 Hz: (a) ages 21-38 years and (b) ages 50-72 years. Stimuli simulated the frequency characteristics of 1 consonant (/s/), a steady-state vowel (/a/), a weak diphthong (/eI/), and a voiced bilabial plosive (/burst/) in 6 combinations: (a) /s-a/, (b) /s-eI/, (c) /a-a/, (d) /eI-eI/, (e) /burst-a/, and (f) /burst-eI/. For each of the 6 phoneme combinations, 2 conditions of stimulus duration were used: (a) fixed, meaning that the durations of leading and trailing noises were fixed at values typical for the speech sounds being simulated, and (b) random, meaning that duration varied randomly within a range acceptable for accurate perception of the speech contrasts. Gap duration difference limens (GDDLs) were significantly larger for the older listeners than for younger listeners. For both groups, GDDLs were poorer for the spectrally dynamic marker pairs (e.g., /burst-eI/) than for the marker pairs that were relatively stable in frequency over time (/a-a/). GDD performance also was poorer for the random conditionthan for the fixed condition. Listener age and hearing sensitivity were significantly correlated, and age was not significantly correlated with GDD when controlling for hearing sensitivity. The authors conclude that temporal resolution (as measured by gap discrimination) is affected by age, stimulus complexity, and, perhaps, by hearing sensitivity in a speech context.

A Comparison of the Audiometric Performance of Bone Anchored Hearing Aids and Air Conduction Hearing Aids

To compare the function of bone anchored hearing aids (BAHA) with conventional air conduction hearing aids (ACHA) by means of objective audiometric tests focusing on temporal acuity and consonant discrimination in quiet and noise, as well as subjective quality-of-life questionnaires. Tertiary referral center. Prospective. Patients using BAHAs because of profuse drainage from chronic suppurative otitis media, and a comparison group of healthy volunteers. Objective measures: sound field audiograms, duration discrimination, gap discrimination, and final and initial consonant discrimination in quiet and in noise. Subjective measures: Sanders' Profiles, MOS SF-36 questionnaire. Normal-hearing subjects always performed better than hearing-impaired patients in all tests. When the BAHA was compared with the ACHA, there were no significant differences in any of the measures. The BAHA and the ACHA provided similar audiometric functioning in audiometric tests. The BAHA, although using a nonphysiologic sound conduction route, did not sacrifice temporal processing ability or speech perception in noise, and should be considered for patients with profuse ear drainage.

Effects of age and frequency disparity on gap discrimination.

Temporal discrimination was measured using a gap discrimination paradigm for three groups of listeners with normal hearing: (1) ages 18-30, (2) ages 40-52, and (3) ages 62-74 years. Normal hearing was defined as pure-tone thresholds < or = 25 dB HL from 250 to 6000 Hz and < or = 30 dB HL at 8000 Hz. Silent gaps were placed between 1/4-octave bands of noise centered at one of six frequencies. The noise band markers were paired so that the center frequency of the leading marker was fixed at 2000 Hz, and the center frequency of the trailing marker varied randomly across experimental runs. Gap duration discrimination was significantly poorer for older listeners than for young and middle-aged listeners, and the performance of the young and middle-aged listeners did not differ significantly. Age group differences were more apparent for the more frequency-disparate stimuli (2000-Hz leading marker followed by a 500-Hz trailing marker) than for the fixed-frequency stimuli (2000-Hz lead and 2000-Hz trail). The gap duration difference limens of the older listeners increased more rapidly with frequency disparity than those of the other listeners. Because age effects were more apparent for the more frequency-disparate conditions, and gap discrimination was not affected by differences in hearing sensitivity among listeners, it is suggested that gap discrimination depends upon temporal mechanisms that deteriorate with age and stimulus complexity but are unaffected by hearing loss.

Effects of age and hearing loss on discimination of silent gaps in dynamic stimuli

Temporal cues in speech occur between signals that are dynamic (changing in frequency and duration). Older listeners with and without hearing loss often experience difficulty discriminating these temporal cues important for speech understanding. This study was designed to measure silent gap discrimination between dynamic stimuli for three groups of listeners: (1) young with normal hearing; (2) older with normal hearing; and (3) older with sensorineural hearing loss. Synthetic stimuli were designed to simulate four speechlike sounds (/s/, /a/, /eI/, /b/) arranged in six pairs separated by a silent gap (/s-a/, /s-eI/, /a-a/, /eI-eI/, /b-a/, /b-eI/). Two conditions of duration were imposed upon the six speech sound pairs: (1) duration of each marker fixed at values typical for the speech sounds simulated and (2) duration of each marker varied randomly within a range appropriate for each speech sound simulated. The task was a 2I/2AFC procedure targeting 70.7% correct discrimination. Results indicate that gap discrimination for frequency dynamic markers deteriorates with age and is poorer for random duration conditions than for fixed duration conditions. Significant correlations were found between age and gap discrimination and between hearing sensitivity and gap discrimination. [Work supported by USF Research and Creative Scholarship Grant No. 1219-933-RO.]

Changes in Tactile Spatial Discrimination and Cutaneous Coding Properties by Skin Hydration in the Elderly

Neurosensory tactile functions were investigated in human subjects by two different and complementary experimental approaches. First, a conventional psychophysical method (two-point gap discrimination) was used to determine the tactile discrimination threshold by analyzing the subjects' ability to detect a gap of variable width between two contact points when a series of stimuli was applied to the skin. Using this method we confirmed the marked degradation of tactile spatial acuity with age and showed that skin discriminative function was partially restored after hydration of the skin with a moisturizer. The second approach consisted of a microneurographic recording of tactile afferent fibers in response to two types of mechanical stimuli applied reproducibly to the corresponding receptive fields. With this method, we found that the afferent messages were depressed following hydration of the skin surface. Thus, partial restoration of tactile spatial acuity after hydration appears to be due to both a softening of the stratum corneum permitting better localization of the stimulus and a weaker transfer of the stimulus toward the sensory receptors.

Beyond presbyacusis: Non-hearing loss-related temporal processing deficits in the elderly

A group of 56 elderly individuals were tested in a battery of tests including pure tone audiometry, various word- and sentence-level speech understanding tasks, and measurement of spectral, temporal, and spatial auditory abilities. Presbyacusic hearing loss, when present, was symmetrical in the two ears and at most mild to moderate, with no significant deficit below 1.5 kHz. Frequency resolution was estimated by calculating auditory filter width from notched-noise masking at 1 kHz; temporal resolution was estimated from gap discrimination and from auditory segregation of temporal patterns of harmonic complexes with components below 1.2 kHz (in the low envelope frequency range); spatial resolution in the horizontal plane was estimated from interaural time difference (ITD) thresholds for 500-Hz clicks. The largest differences between results of the elderly and those of normal-hearing young subjects were seen for speech understanding in spatially distributed babble, word recognition in reverberant rooms, ITD thresholds, and segregation of temporal patterns. The results portray the decline of temporal processing as an important component of auditory and speech understanding dysfunctions in aging and strongly suggest that a non-negligible part of these dysfunctions may be of central origin. [Supported by the National Institute on Aging and the V. A. Medical Research.]

Binaural gap duration discrimination in listeners with impaired hearing and normal hearing.

This study was designed to measure the ability of listeners with and without sensorineural hearing loss to discriminate silent gaps between noise band markers of different frequencies presented in an anechoic and a reverberant listening environment. A two-interval, two-alternative, forced-choice paradigm was used to measure gap discrimination ability for six listeners with normal-hearing and six listeners with sensorineural hearing impairment. Marker stimuli were narrow bands of noise centered at frequencies from 500 to 7000 Hz. The center frequency of the leading marker was held constant at 2000 Hz and the center frequency of the trailing marker was varied randomly across runs. Stimuli were presented in two virtual listening environments (anechoic and reverberant). The listeners' task was to indicate which interval contained the marker pair separated by the larger silent gap. Gap discrimination was measured as a function of the center frequency of the trailing marker and as a function of listening environment. Gap discrimination thresholds (msec) varied as a function of the center frequency of the trailing marker. As the trailing marker frequency increased above and decreased below the leading marker frequency (2000 Hz), gap thresholds increased significantly. Hearing loss and listening environment did not have a significant effect on gap discrimination thresholds. Analysis of the gap discrimination functions revealed significantly steeper slopes for trailing marker frequencies below 2000 Hz than for trailing marker frequencies above 2000 Hz. A possible age effect was observed in the data and significant correlations were found between age and function slopes for several conditions. Gap discrimination becomes more difficult as the frequency disparity between leading and trailing noise bands increases. This pattern of results occurs independent of hearing loss but may be influenced by listener age.

Gap discrimination and speech perception in noise

The relation between discrimination of silent gaps and speech-in-noise perception was measured in 20 normal-hearing listeners using speech-shaped noise as both the gap markers and the noise source for speech testing. In the gap discrimination experiment, subjects compared silent gaps marked by 60 dB SPL 250-ms noise bursts to standards of either 5, 10, 20, 50, 100, or 200 ms. The gap results were most similar to those reported by Abel [S. M. Abel, J. Acoust. Soc. Am. 52, 519–524 (1972)] as ΔT/T decreased non-monotonically with increased gap length. In a second experiment, the California Consonant Test (CCT) was administered at 50 dB HL via CD in three conditions: quiet, +10 S/N, and 0 S/N. Results from both experiments were correlated and the association between ΔT/T and CCT scores was generally negative. Listeners who discriminated the gaps with greater acuity typically had higher speech scores. The relation was strongest for the smaller gap standards at each S/N, or when performance for any gap duration was compared to the CCT results obtained in quiet.

Temporal resolution in birds and the perception of complex sounds

On many measures of temporal processing, including maximum and minimum temporal integration and duration discrimination, birds show sensitivities similar to those of humans and other mammals. More recent studies on difference limens for short duration tones, discrimination of gaps in sinusoidal markers, and detection of a mistuned harmonic, however, indicate that birds are much more sensitive than humans to certain temporal aspects of complex sounds. Further explorations of auditory temporal analysis by budgerigars (Melopsittacus undulatus) are described here, using sets of harmonic complexes with identical long-term spectra but with temporal waveforms differing either in envelope shape or fine structure. Behavioral discrimination and masking data using these temporal complexes show a high degree of temporal resolving power by the avian auditory system. These findings will be discussed in the context of previous studies on temporal modulation transfer functions and temporal windows in the budgerigar. In aggregate, these results strongly support the long-held belief that birds are more sensitive than humans and other mammals to temporal aspects of complex acoustic signals. [Work supported by NIH DC00198.]

Perceptual consequences of peripheral hearing loss: do edge effects exist for abrupt cochlear lesions?

There is a growing body of research that shows evidence of central neural reorganization in response to lesions in the auditory periphery, even if the lesions occur in maturity. This reorganization consists of an increased neural representation of frequencies corresponding to the edge frequency of the lesion. Data were collected to determine whether this over-representation might have consequences for human perception. The hypothesis was that increased central representation might increase acuity on some psychophysical tasks performed at the edge frequency. Tasks included frequency sweep detection (for tones), intensity discrimination (for 100-Hz-wide bands of noise and tones), gap detection and gap discrimination (both for 100-Hz-wide bands of noise). Results from observers with steeply sloping hearing losses were compared with results from normal-hearing observers performing these tasks with masking noise generated to simulate steeply sloping hearing loss. None of these data provide compelling evidence for the hypothesized edge effect. A 40-Hz following response to tone bursts was collected from a subset of the hearing-impaired observers in an attempt to confirm the animal physiology findings of neural over-representation of the edge frequency. No edge-frequency effect was noted in the results, though there was a non-significant tendency for one of the hearing-impaired observers to show shorter latency of response.

Discrimination of silent temporal gaps in sinusoidal markers by the budgerigar (Melopsittacus undulatus)

The tonal quality and temporal complexity of bird vocalizations have long suggested that birds may excel in auditory spectral or temporal processing. However, most psychophysical studies support the notion that birds have spectral and temporal resolving power roughly comparable to that measured in humans. Rarely have such psychophysical tasks involved both frequency and time cues. An intriguing aspect of gap-detection experiments with sinusoidal markers is the robust dependence on the frequency separation between the markers. In humans, silent gaps become more difficult to discriminate as the frequency difference between the markers increases. This frequency-dependent pattern of thresholds is well fit with a roex-filter model. Here, budgerigars were trained by operant conditioning to discriminate a change in the duration of a silent gap between two tonal markers. Frequency separation between the tonal markers had only a small effect on gap discrimination thresholds in budgerigars, suggesting an unusually broad peripheral auditory filter. This result stands in marked contrast to the results from humans and to estimates of the auditory filter in budgerigars using masking paradigms. Alternate explanations for the apparently broad tuning of the discrimination data will be considered. [Work supported by NIH grants DC-00198 and MH-00982 to RJD.]

Spatial acuity of the body surface over the life span.

Spatial acuity over 13 regions of the body was assessed cross-sectionally in 122 male and female subjects between 8 and 87 years of age. Of two measures, the primary one was a threshold for detecting a gap between two points (a refinement of the conventional two-point threshold). The secondary one was a threshold of point localization in 7 of these 13 body regions. The two measures yielded similar pictures of body acuity and age-related changes in acuity, and they agreed in essentials with an early acuity map dating back to Weber in 1835, as cited and confirmed experimentally by Weinstein (1968). To this acuity map, the present study added the dimension of age. The main finding was that aging is much harder on some body regions than on others. Declining acuity with age was found to characterize all regions to one degree or another, but the hands and feet turned out to be far more vulnerable than the more central regions, including the very acute lip and tongue. Deterioration of acuity in the great toe (averaging 400% between youth and advanced age) and fingertip (averaging 130%) may adversely affect such diverse activities as braille reading, grasping, and maintaining balance. The acuity map determined by gap discrimination was essentially the same for males and females; however, males gave significantly smaller localization thresholds than females. In two body regions tested (fingertip and upper lip), children significantly outperformed young adults at gap discrimination.

Human hearing without efferent input to the cochlea

The measurement of hearing without efferent input is possible in human patients whose vestibular nerve has been sectioned, to relieve severe vertigo usually caused by Ménière’s disease. The olivocochlear bundle, which runs in the inferior vestibular nerve, is also sectioned. Psychoacoustic measures have been made in over a dozen such patients, whose hearing levels range from normal to 40 or 50 dB. Most measurements were made only after the operation, but many were also made before. Thresholds measured before and after the operation revealed no change in the ability to detect tones either in the quiet or in noise, except when selective frequency focusing was involved. Comparisons between operated and normal ears showed a similar pattern. Other psychoacoustic tests, including measurements of intensity, frequency, and gap discrimination, of loudness functions and loudness adaptation, of lateralization, of the auditory filter (by notched noise), of overshoot, of TTS, showed either normal auditory functions or only the changes usually associated with sensorineural impairment. Besides the laboratory tests, most patients’ reports indicate no change in hearing subsequent to the vestibular neurectomy. Apparently, the one change—in selective listening—has little effect on such common auditory functions as speech perception. [Research supported by NIH.]

Stream segregation in normal‐hearing and cochlear‐impaired listeners

Cochlear‐impaired and normal‐hearing listeners were compared on two tasks intended to assess auditory stream segregation. One task measured gap discrimination between two tones that were presented either in isolation or were embedded into a sequential stream of tones. For tone pairs distally spaced in frequency, performance was poor when the tones were presented in isolation but improved when they were drawn into separate auditory streams, presumably because temporal judgments could now be made within a stream. Both groups of listeners gave similar patterns of results although some differences were apparent. The second task was a melody recognition procedure [J. A. P. M. de Laat and R. Plomp, J. Acoust. Soc. Am. 78, 1574–1577 (1985)] and involved identifying a 4‐note target melody embedded between two competing melodies as a function of the proximity and temporal synchrony of the competing melodies. In both groups, marked individual differences were observed. Results from both tasks will be discussed in terms of the effects of cochlear impairment on auditory grouping skills. [Work supported by the NIDCD R01‐DC01507.]

A tactile stimulation device for measuring two-point and gap discrimination thresholds in humans

A device to measure two-point-discrimination and gap-discrimination thresholds is described. The apparatus is capable of controlled and reliable delivery of forces up to 42.50 g while determining point of skin contact and subsequent depth of skin indentation during threshold determination. Stimulus delivery is controlled by computer software that deactivates a solenoid, allowing a counterbalanced beam with one of two stimulus wheels mounted on its end to rise and stimulate the skin. Depth of skin indentation is recorded by hardware that monitors the output of a photocell reflecting the position of the beam. The apparatus allows complete computer control of all data acquisition and recording.

Gap detection and discrimination in complex speechlike signals

Speechlike stimuli consisting of /s/‐like noise and F 1‐like frequency sweeps were used for gap detection and discrimination experiments. Five normal‐hearing subjects participated in the tasks: (1) detection of gaps in /s/‐like noise, (2) discrimination between gaps in noise, (3) detection of gaps between noise and glide, and (4) gap discrimination between noise and glide. Stimuli for the latter two experiments were patterned after synthesized “say” and “stay” speech signals. These stimuli consisted of 120 ms of high‐pass‐filter digital white noise (simulating the /s/ phoneme), followed by a silent gap varying from 0–120 ms in duration, and a 50‐ms linear frequency sweep from 230–611 Hz (simulating the F 1 transition). The same high‐pass‐filtered white noise was used for all experiments. An adaptive psychophysical procedure was used in the first two experiments; a method of constant stimuli procedure was used for the complex stimuli tasks because of the shallow‐sloping psychometric functions. Subjects required extensive training to reach asymptotic performance levels on the latter two experimental tasks. Results indicated significantly longer durations are required for the complex stimulus tasks.

Nonlinguistic auditory capabilities in aphasia

Nonlinguistic auditory capabilities were assessed through psychophysical tests in 11 left-CVA aphasic, four right-CVA nonaphasic, and eight normal male subjects selected from the same age group. The tests included frequency discrimination, gap detection, gap discrimination, frequency sweep discrimination, assessment of the magnitude of the frequency uncertainty effect in the detectability of tones in noise, and assessment of frequency selectivity through simultaneous masked thresholds. Results of these tests were compared to measures of auditory comprehension obtained from the Boston Diagnostic Aphasia Examination, the Porch Index of Communicative Ability, and the Token Test. Nonlinguistic auditory performance of the three subject groups differed significantly from each other. For the left-CVA subjects, frequency sweep discrimination, frequency discrimination, and the frequency uncertainty effect in tone-in-noise detection were the best predictors of verbal auditory comprehension. The right-CVA subjects displayed marked deficits with regard to all pitch-related tests. The findings stress the importance of considering the presence of nonlinguistic auditory dysfunctions when evaluating linguistic auditory capabilities in aphasia.

Visual gap and offset discrimination and its relation to categorical identification in brief line-element displays.

Visual gap and offset discrimination and its relation to categorical identification in brief line-element displays.