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Biomedical subjects

M R Leek

Publications and source records attributed to M R Leek.

14 recordsLinked to original sources
PubMed1992

The role of spectral and temporal cues in vowel identification by listeners with impaired hearing.

This study examined the use of duration and formant frequency in the labeling of synthetic CVC stimuli forming a beet-bit continuum. Durational and F2 frequency cues to vowel identity varied systematically across stimuli. Subjects with normal hearing tended to rely primarily on F2 frequency in vowel labeling, whereas subjects with impaired hearing relied less on F2 information. This group difference was observed even for stimuli with large F2 differences, which were easily discriminated by all subjects. The effect of vowel duration on labeling was similar for both groups, with long-duration stimuli receiving more "beet" responses than short-duration stimuli across the F2 range. Psychoacoustic measures of frequency resolution and temporal resolution were poor predictors of a subject's use of formant information and duration information in labeling.

Auditory Perception↗
PubMed1992

Improving the frequency specificity of the auditory brain stem response.

Several investigators have suggested that the use of tonal stimuli shaped with nonlinear windowing functions can improve the frequency specificity of the auditory brain stem response (ABR). This study investigated the effects of different windowing functions on the ABR for 30 normal-hearing adults and 30 adults with high-frequency hearing loss. These hearing-impaired patients often produce an abnormal click-evoked ABR because of the influence of the high-frequency loss. Each subject was evaluated using a click stimulus and a 500 Hz tone burst gated with one linear and four nonlinear windowing functions. There were no significant differences in wave V latency between the groups for any of the five windowed tone burst conditions. These results suggest that any of the windowing functions used would be effective for 500 Hz tonal ABRs with this population of hearing-impaired adults.

Acoustic Impedance Tests↗
PubMed1992

Estimation of psychometric functions from adaptive tracking procedures.

Because adaptive tracking procedures are designed to avoid stimulus levels far from a target threshold value, the psychometric function constructed from the trial-by-trial data in the track may be accurate near the target level but a poor reflection of performance at levels far removed from the target. A series of computer simulations was undertaken to assess the reliability and accuracy of psychometric functions generated from data collected in up-down adaptive tracking procedures. Estimates of psychometric function slopes were obtained from trial-by-trial data in simulated adaptive tracks and compared with the true characteristics of the functions used to generate the tracks. Simulations were carried out for three psychophysical procedures and two target performance levels, with tracks generated by psychometric functions with three different slopes. The functions reconstructed from the tracking data were, for the most part, accurate reflections of the true generating functions when at least 200 trials were included in the tracks. However, for 50- and 100-trial tracks, slope estimates were biased high for all simulated experimental conditions. Correction factors for slope estimates from these tracks are presented. There was no difference in the accuracy and reliability of slope estimation due to target level for the adaptive track, and only minor differences due to psychophysical procedure. It is recommended that, if both threshold and slope of psychometric functions are to be estimated from the trial-by-trial tracking data, at least 100 trials should be included in the tracks, and a three- or four-alternative forced-choice procedure should be used. However, good estimates can also be obtained using the two-alternative forced-choice procedure or less than 100 trials if appropriate corrections for bias are applied.

Attention↗
PubMed1991

An interleaved tracking procedure to monitor unstable psychometric functions.

In some experimental situations, the psychometric function underlying performance may not be stable, but instead may shift along the stimulus axis in response to changes in attention, learning, or task difficulty. When this occurs, the measured threshold may be influenced and the slope of the measured function will be inaccurately shallow. With commonly used experimental procedures, it is difficult to know whether a shallow psychometric function slope is a true reflection of the sensory process, or is a result of "averaging" a highly variable underlying function. Here, a new method is described of estimating psychometric function slope from the variability in two interleaved adaptive tracks, consulted on alternate trials, that is resistant to the effects of shifting performance levels. Further, a mechanism is described for assessing the likelihood that a threshold was, in fact, stable over the course of its measurement. Computer simulations are reported as well as verification of the method in measurements of human performance on a psychophysical task. Several conditions of externally imposed variability were simulated to establish the ability of these procedures to identify unstable functions and produce accurate slope estimates. The procedures worked well for thresholds shifting by as little as 4 dB if the variation did not occur too rapidly. The procedure and associated analyses are recommended as a relatively "free" means of calculating slope and quantifying threshold reliability with little extra experimental effort.

Attention↗
PubMed1991

Informational masking and auditory attention.

Informational masking is broadly defined as a degradation of auditory detection or discrimination of a signal embedded in a context of other similar sounds; it is not related to energetic masking caused by physical interactions between signal and masker. In this paper, we report a systematic release from informational masking of a target tone in a nine-tone rapid auditory sequence as the target is increasingly isolated in frequency or intensity from the remaining sequence components. Improved target-tone frequency difference limens as isolation increases are interpreted as a reflection of increasingly focused auditory attention. The change from diffuse to highly focused attention is gradual over the frequency and intensity ranges examined, with each 1-dB increment in target intensity relative to the remaining components producing performance improvements equivalent to those produced by a 2% increase in frequency isolation. The results are modeled as bands of attention in the frequency and intensity domains. For attention directed by frequency isolation, there is a strong correspondence with auditory filters predicted by the power spectrum model of masking. These data also support the existence of an attention band of intensity, with a bandwidth of about 5-7 dB at the moderate levels used in this experiment.

Adult↗
PubMed1990

Consideration of pigeon-holing and filtering as dysfunctional attention strategies in schizophrenia.

Difficulties in the use of both filtering and pigeon-holing strategies have been suggested as partial explanations for the information processing deficits typically noted with schizophrenic subjects. The study reported here examined the operation of both of these attentional strategies in a group of schizophrenic subjects and in a group of psychiatric control subjects. Subjects were asked to identify a target syllable in the presence of several levels of competing speech. A sensory filter could be profitably invoked when the target and the competing speech were presented to separate ears. Pigeon-holing was assumed to facilitate performance when both the target and the competition occurred in both ears. Results suggested that schizophrenic subjects are relatively efficient in using filtering strategies, but have more difficulty employing pigeon-holing strategies to allocate efficiently attentional resources.

Adult↗
PubMed1989

Perceptual asymmetry in schizophrenia and affective disorder: implications from a right hemisphere task.

The patterns of perceptual asymmetry exhibited by normal, schizophrenic, and affectively-disordered subjects on a dichotic tonal discrimination task, were compared. Affectively-disordered subjects' performances differed significantly from those of normal subjects, with normals demonstrating the expected left ear advantage, and affectively-disordered subjects showing no lateral advantage. The performance of the schizophrenic subjects fell between those of the normal and affective groups along a laterality continuum, with paranoid schizophrenic subjects tending to show a larger left ear advantage than non-paranoid schizophrenic subjects. The results do not support the hypothesis that schizophrenic subjects inappropriately transfer processing of right hemisphere stimuli to the left hemisphere, but do suggest that subgroup distinctions may be relevant to hypotheses of lateralized dysfunction in schizophrenia. Further, the performance of the affective group supports previous findings of right hemisphere abnormalities in affective disorder.

Adolescent↗
PubMed1987

Minimum spectral contrast for vowel identification by normal-hearing and hearing-impaired listeners.

To determine the minimum difference in amplitude between spectral peaks and troughs sufficient for vowel identification by normal-hearing and hearing-impaired listeners, four vowel-like complex sounds were created by summing the first 30 harmonics of a 100-Hz tone. The amplitudes of all harmonics were equal, except for two consecutive harmonics located at each of three "formant" locations. The amplitudes of these harmonics were equal and ranged from 1-8 dB more than the remaining components. Normal-hearing listeners achieved greater than 75% accuracy when peak-to-trough differences were 1-2 dB. Normal-hearing listeners who were tested in a noise background sufficient to raise their thresholds to the level of a flat, moderate hearing loss needed a 4-dB difference for identification. Listeners with a moderate, flat hearing loss required a 6- to 7-dB difference for identification. The results suggest, for normal-hearing listeners, that the peak-to-trough amplitude difference required for identification of this set of vowels is very near the threshold for detection of a change in the amplitude spectrum of a complex signal. Hearing-impaired listeners may have difficulty using closely spaced formants for vowel identification due to abnormal smoothing of the internal representation of the spectrum by broadened auditory filters.

Adult↗
PubMed1987

Vowel intelligibility in the absence of the acoustic reflex: performance-intensity characteristics.

Performance-intensity functions were defined for ten synthetic vowels whose durations (50 ms) were shorter than the time necessary for the acoustic reflex to alter the transmission characteristics of the middle ear. Recognition accuracy for ten listeners with normal auditory function was asymptotic from 72 to 90 dB and then fell linearly to 108 dB. Thus, at SPLs greater than approximately 90 dB, the auditory encoding of vowels is altered by the absence of the acoustic reflex.

Adult↗
PubMed1985

Auditory span of apprehension deficits in schizophrenia.

Previous research on the visual span of apprehension procedure has interpreted the performance deficits commonly demonstrated by schizophrenic subjects as indicative of a core central processing deficit. Little research has directly investigated the existence of similar information-processing deficits in an auditory modality in these patients. In the present study, groups of schizophrenic and nonschizophrenic patients and a comparison group of normal subjects reported the occurrence of a target syllable in the presence of background speech. Schizophrenic subjects performed as well as subjects from both comparison groups in the simpler background conditions, but were significantly less able to identify the target in the presence of auditory distracters when greater amounts of information were available to be processed. These data support the nonmodality specific nature of the deficit.

Adult↗
PubMed1984

Learning to detect auditory pattern components.

Listeners' abilities to learn to hear all the details of an initially unfamiliar sequence of ten 45-ms tones were studied by tracking detection thresholds for each tonal component over a prolonged period of training. After repeated listening to this sequence, the presence or absence of individual tones could be recognized, even though they were attenuated by 40-50 dB relative to the remainder of the pattern. Threshold-tracking histories suggest that listeners tend to employ two different learning strategies, one of which is considerably more efficient. Special training by reducing stimulus uncertainty and extending the duration of the target component was effective in increasing the rate of threshold improvement. Strategies acquired with the first pattern studied generalized to new sequences of tones. The possible implications of these results for the perceptual learning of speech or other auditory codes are discussed.

Auditory Perception↗
PubMed1983

Lateralization of rapid auditory sequences.

Subjects were trained to manually respond to the presence of a preassigned target order of a two-element sequence in a dichotic presentation. Reaction times to rapid target sequences presented to the right ear were significantly shorter than left ear reaction times. The speech or nonspeech character of the stimulus was not a significant factor. There were no lateral differences in accuracy of response. The lateral differences in reaction time are interpreted as a greater efficiency of processing by the right ear (and by implication, the left hemisphere) when temporal characteristics of the sequences were similar to temporal values found in speech.

Auditory Perception↗
PubMed1982

Frequency and intensity effects on ear dominance for tone bursts.

Normal-hearing Ss (N = 20) listened to dichotic pairs of 20-msec tones at 0.4, 1.5, or 3 kc/s differing by 76, 224, or 376 c/s between the two ears and also differing by 0, 15, or 35 db in SPL (the more intense tone always at 80 db SPL). S judged each pair "high-low" or "low-high" in pitch. Earlier investigations have shown that many normal-hearing Ss demonstrate a dominance for pitch perception of one ear over the other which is unaltered by large interaural intensity differences. In this study, ear dominance could be modified by a change in frequency location or interaural frequency difference of the dichotic stimuli. The effect of interaural level differences could be seen at all frequency conditions, but was strongest at low frequencies. It was suggested that ear dominance on this perceptual task is governed by several characteristics of the auditory system, including loudness perception, dichotic fusion, and frequency selectivity, as well as some as-yet ill-defined binaural processes.

Acoustic Stimulation↗