Biomedical subjects

S M Stufflebeam

Publications and source records attributed to S M Stufflebeam.

4 recordsLinked to original sources

PubMed1998

Peri-threshold encoding of stimulus frequency and intensity in the M100 latency.

Recent work has suggested that, in addition to spatial tonotopy, pitch and timbre information may be encoded in the temporal activity of the auditory cortex. Specifically, the post-stimulus latency of the maximal cortical evoked neuromagnetic field (M100 or N1m) is a function of stimulus frequency. We investigated the additional effect of varying the stimulus intensity on the M100 response. A 37-channel biomagnetometer recorded neuromagnetic fields over the temporal lobe of healthy volunteers in response to monaurally presented tones. The frequency dependence of the M100 latency remained remarkably invariant even at low stimulus intensity. Thus, for peri-threshold stimuli, frequency information appears encoded in the temporal form of the evoked response.

Acoustic Stimulation↗

PubMed1994

Multiple sclerosis lesions of the auditory pons are not silent.

To understand the relationship between brainstem lesions and auditory neurology in patients with multiple sclerosis, we compared behavioural, electrophysiological and imaging data in 38 patients with probable or definite multiple sclerosis and normal or near normal hearing. Behavioural measures included (i) general hearing tests (audiogram, speech discrimination) and (ii) hearing tests likely to be critically dependent upon brainstem processing (masking level difference, interaural time and level discrimination). Brainstem auditory evoked potentials provided the electrophysiological data. Multiplanar high-resolution MRI of the brainstem provided the anatomical data. Interaural time discrimination for high-frequency sounds was by far the most sensitive of all tests with abnormalities in 71% of all subjects. Whenever any other test was abnormal this test was always abnormal. Interaural time discrimination for low-frequency sounds and evoked potentials were closely related and next most sensitive with abnormalities in approximately 40% of all subjects. Interaural level discrimination and masking level difference were least sensitive with abnormalities in < 10% of subjects. Speech discrimination scores correlated significantly with the masking level differences, as well as with interaural time discrimination for high-frequency sounds. Pontine lesions were found in five of the 16 patients, in whom an objective method for detecting magnetic resonance lesions could be applied. All four with lesions involving the pontine auditory pathway had marked abnormalities in interaural time discrimination and evoked potentials. None of the other 12 had evoked potentials abnormalities. We conclude that neurological tests requiring precise neural timing can reveal behavioural deficits for multiple sclerosis lesions of the auditory pons that are otherwise 'silent'. Of all neurological systems the auditory system at the level of the pons is probably the most sensitive to multiple sclerosis lesions, because of its exceptional dependence upon neural timing in the microsecond range and the lack of redundancy in the encoding of high-frequency sounds. Precise neural timing may be critical for some aspects of speech processing.

Adult↗

PubMed1993

Binaural auditory processing in multiple sclerosis subjects.

In order to relate human auditory processing to physiological and anatomical experimental animal data, we have examined the interrelationships between behavioral, electrophysiological and anatomical data obtained from human subjects with focal brainstem lesions. Thirty-eight subjects with multiple sclerosis were studied with tests of interaural time and level discrimination (just noticeable differences or jnds), brainstem auditory evoked potentials and magnetic resonance (MR) imaging. Interaural testing used two types of stimuli, high-pass (> 4000 Hz) and low-pass (< 1000 Hz) noise bursts. Abnormal time jnds (Tjnd) were far more common than abnormal level jnds (70% vs 11%); especially for the high-pass (Hp) noise (70% abnormal vs 40% abnormal for low-pass (Lp) noise). The HpTjnd could be abnormal with no other abnormalities; however, whenever the BAEPs, LpTjnd and/or level jnds were abnormal HpTjnd was always abnormal. Abnormal wave III amplitude was associated with abnormalities in both time jnds, but abnormal wave III latency with only abnormal HpTjnds. Abnormal wave V amplitude, when unilateral, was associated with a major HpTjnd abnormality, and, when bilateral, with both HpTjnd and LpTjnd major abnormalities. Sixteen of the subjects had their MR scans obtained with a uniform protocol and could be analyzed with objective criteria. In all four subjects with lesions involving the pontine auditory pathway, the BAEPs and both time jnds were abnormal. Of the twelve subjects with no lesions involving the pontine auditory pathway, all had normal BAEPs and level jnds, ten had normal LpTjnds, but only five had normal HpTjnds. We conclude that interaural time discrimination is closely related to the BAEPs and is dependent upon the stimulus spectrum. Redundant encoding of low-frequency sounds in the discharge patterns of auditory neurons, may explain why the HpTjnd is a better indicator of neural desynchrony than the LpTjnd. Encroachment of MS lesions upon the pontine auditory pathway always is associated with abnormal BAEPs and abnormal interaural time discrimination but may have normal interaural level discrimination. Our data provide one of the most direct demonstrations in humans of relationships among auditory performance, evoked potentials and anatomy. We present a model showing that many of these interrelationships can be readily interpreted using ideas developed from work on animals, even though these relationships could not have been predicted with confidence beforehand. This work provides a clear advance in our understanding of human auditory processing and should serve as a basis for future studies.

Acoustic Stimulation↗

PubMed1993

Effects of multiple sclerosis brainstem lesions on sound lateralization and brainstem auditory evoked potentials.

Magnetic resonance (MR) imaging, brainstem auditory evoked potentials (BAEPs), and tests of interaural time and level discrimination were performed on sixteen subjects with multiple sclerosis (MS). Objective criteria were used to define MR lesions. Of the eleven subjects in whom no pontine lesions were detected and the one subject who had pontine lesions that did not encroach upon the auditory pathways, all had normal BAEPs and interaural level discrimination, although a few had abnormal interaural time discrimination. Of four subjects with lesions involving the pontine auditory pathway, all had both abnormal BAEPs and abnormal interaural time discrimination; one also had abnormal interaural level discrimination. Analysis of the data suggest the following: waves I and II are generated peripheral to the middle of the ventral acoustic stria (VAS); wave III is generated ipsilaterally in the region of the rostral VAS, caudal superior olivary complex (SOC) and trapezoid body (TB); and waves V and L are generated contralaterally, rostral to the SOC-TB. The region of the ipsilateral rostral SOC-TB is implicated as part of the pathway involved in the generation of waves V and L. Interaural time discrimination of both high and low frequency stimuli were affected by all brainstem lesions that encroached on auditory pathways. A unilateral lesion in the region of the LL affected interaural time discrimination for low-frequency stimuli less severely than bilateral lesions of the LL or a unilateral lesion of the VAS. The only interaural level discrimination abnormality occurred for a subject with a unilateral lesion involving the entire rostral VAS. It appears that detailed analysis of lesion locations coupled with electrophysiological and psychophysical data holds promise for testing hypotheses concerning the function of various human auditory brainstem structures.

Acoustic Stimulation↗