Biomedical subjects

T D Clack

Publications and source records attributed to T D Clack.

17 recordsLinked to original sources

PubMed1988

Labyrinthine functional tests and a case of midline cerebellar teratoma.

Electronystagmographic (ENG) examination may indicate clinically abnormal but nonspecific or even diagnostically deceptive results. Insights into the processes producing such observations sometimes can be suggested from correlations of findings to pathologic conditions in cases of demonstrated vestibular lesions. Such an opportunity occurred when unusual ENG results were obtained from a patient who had a history of equilibrium disturbances. The ENG revealed a direction-changing nystagmus in lateral head and body positions but not with the head similarly turned. Ocular counterroll measurements indicated hypofunctioning otolithic reactions despite normal vestibuloocular responses to calorizations. Radiologic follow-up showed a midline cerebellar tumor, later surgically confirmed, and found to be a rare teratoma. After partial excision and decompression, repeat tests revealed changes in both the positional nystagmus and ocular counterroll. These findings have implications for diagnosis, for mechanisms underlying ataxia, for inclusion of ocular counterroll testing in diagnostic evaluations, and for understanding the mechanisms underlying direction-changing positional nystagmus.

Cerebellar Neoplasms↗

PubMed1985

Ear-eye reflexes while riding in a car.

Persons with diminished or absent vestibulo-ocular reflexes (VORs) are thought to have difficulty with ocular stabilization during sufficiently rapid head movements. This suggests the hypothesis that labyrinthine defective individuals might have impaired visual abilities when in a moving motor vehicle. Ten licensed drivers likely to have abnormal VORs were recruited to assess this possibility. Their labyrinthine function was indexed by measurements of ocular counterroll responses to static head tilt and the nystagmus induced with caloric stimulations. Each person was evaluated on a visual task, i.e., answers to questions about alphanumeric information read from a stationary sign while sitting still and while riding in a car. Comparisons of vision scores tend to be lower in the moving than in the still condition. Furthermore, these decrements appear more closely related to the magnitudes of ocular counterroll than to the caloric reflexes. These results have implications concerning otolithic vs. semicircular canal function, vehicle operations by vestibular-impaired individuals, and evaluations of clinical treatments which employ vestibulotoxic drugs.

Automobiles↗

PubMed1984

Waardenburg's syndrome: a case report with CT scanning and cochleovestibular evaluation.

The Waardenburg's syndrome, a congenital ectodermal germ layer defect of autosomal dominant inheritance with variable phenotypic expressivity consists of 6 major characteristics: dystopia canthorum, broad nasal root, hypoplasia of the medial eyebrow, heterochromia irides, white forelock, and congenital deafness. Twelve additional characteristics have been reported including meningocele, atresia of the esophagus, and Hirshsprung's disease supporting an early hypothesis that the neural crest is the embryonic site linking defects of the cervical sympathic system with pigmentary abnormalities and inner ear anomalies. Congenital deafness reported in 20% of Waardenburg patients, and the most disabling characteristic, has been associated with ENG abnormalities in 30% of these deaf patients. A congenitally deaf young person with Waardenburg's syndrome is examined for basic cause of episodic vertiginous symptoms. Diagnostic assistance from previous investigations is lacking because they have not involved state-of-the-art functional evaluations or correlations of behavioral and radiographic findings. In this patient, high-resolution computerized tomography failed to demonstrate bony labyrinthine anomalies. Functional cochleovestibular analysis revealed a bilateral profound sensorineural hearing loss and a unilateral weakness in oculomotor responses to caloric stimulations. Repeated measurements of ocular counterroll show variations compatible with intermittent otolithic dysfunctioning. These findings are consistent with a diagnosis of an acquired active unilateral peripheral vestibular disorder. Some of the technical issues underlying the derivation of this conclusion are discussed.

Abnormalities, Multiple↗

PubMed1984

Detection of interaural phase shift between a subaudible and an audible tone.

Can a shift in interaural phase between a subthreshold signal and an audible contralateral probe tone affect perception of the probe? To obtain an answer, an 800-Hz tone was presented to both ears. The tone was presented continuously to one ear (-25 to + 10 dB SL) and in a sequence of four bursts per trial to the other ear (+ 10 dB SL). Interaural phase was reversed for either the second or the fourth burst in a 2 AFC task. Interaural phase-shift detection threshold (65% correct) varied with the intensity of the continuous signal; across subjects, this threshold varied from -21 to + 1 dB SL. When a 300-or 500-Hz masking tone was added to the ear with the continuous signal, phase-shift detection accuracy depended primarily upon the sensation level of the signal rather than its sound pressure level. These findings demonstrate temporal encoding at signal levels well below hearing threshold.

Adolescent↗

PubMed1984

Interaural octave phase-shift detection and aural harmonic distortion.

A continuous 400-Hz tone (60-75 dB SPL) and 250-msec bursts of an 800-Hz tone (10 dB SL) were delivered dichotically. Four out of nine listeners were able to detect a 180 degree interaural phase shift. When a subaudible continuous 800-Hz tone was added to the ear with the 400-Hz tone, interaural phase-shift detection depended on the phase relation of the added tone to the 400-Hz tone. These results are shown to be consistent with the hypothesis of aural harmonic distortion.

Attention↗

PubMed1981

Afterimage versus photographic ocular torsionometry.

Rotation of the eyeballs about the visual axis due to head tilt, counterocular torsion, can provide information relevant to the neuro-otologic status. Measurement of these reflexive movements, however, involves several difficulties. Two methods are examined: determinations by a behavioral afterimage technique are compared with those of a photographic method in the same subjects using a test-retest format. Results indicate that both procedures yield reliable measurements, and any differences tend to be inconsistent. Thus, the faster and less expensive afterimage technique appears suitable for many clinical purposes in assessing vestibulo-ocular function.

Adult↗

PubMed1978

Loudness and the power series transformation. Preliminary model and measurements.

A nonlinear-algebraic approach to monaural intensity processing is proposed: the purpose is to integrate the conscious loudness attribute evoked by a pure tone with the power-series description of auditory distortion. The preliminary model consists of six postulates leading to a mathematical definition for loudness. To evaluate this equation, two task-specific sets of loudness judgments from each of seven subjects are examined. When linked to loudness-interval responses via the equisection assumption, the equation describes the behaviors quite well. Extrapolations into other intensity ranges predict similar responses with relatively slight overestimates. By relaxing the assumption that subjects will adjust the loudness proportions exactly as instructed, their ratio productions also can be described and predicted with sometimes surprising accuracy. Particularly striking is the prediction of overall levels and the curvilinearities of "doublings" from the "halvings". In addition, the theory proposes absolute loudness measurement, an explanation for the growth of loudness including the principle underlying Steven's Power Law, and might prove useful in examining some exceptions to this relationship. Several aspects of this model differ from traditional approaches to intensity processing, but it appears to warrent further critical evaluations.

Adult↗

PubMed1978

Aural harmonics: estimation consistency from tone-on-tone masking measurements.

Amplitudes of inaudible "subjective" signals are inferred from tone-on-tone masking measurements. Several methodological issues are involved, e.g., the problems of how many and which measurements to include. To explore such sampling questions, sets of masked thresholds using a 250-Hz masker and a 500-Hz maskee are determined at various phase angles. These measurements are distributed into subsamples of (n=) 13, 9, 7, 5, or 4 each, and repeated estimates of the second aural harmonic inferred. For n greater than 4, these estimates averaged to within +/- 1 dB of one another,, although individual estimates vary considerably with the smaller n's (to 13 dB with n=4). Sets of five consecutive measurements also were analyzed when the masker : maskee phase angle was incremented in 90 degrees steps through two complete periods (of the maskee). Again, the reestimates for individuals can vary by at least 6 dB, but the averages over seven subjects remain within 2 dB of each other. Apparently, for n's of six or more, it matters little which masker: maskee phase angles are used to estimate the aural harmonic amplitudes.

Adult↗

PubMed1978