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Biomedical subjects

J D Hood

Publications and source records attributed to J D Hood.

63 records · Page 4Linked to original sources
PubMedDate not supplied

Vestibular and optokinetic evoked potentials.

Earlier studies of vestibular evoked potentials have adopted the expedient of rotating subjects with eyes closed and in consequences there exists a suspicion that the responses obtained might have been contaminated by corneo-retinal potentials. In order to minimise this artefact, subjects were required to fixate upon a small target light attached to and rotating with the chair while in total darkness. The rotational stimulus was a controlled angular rotation of the chair simulating a normal head movement. Averaged responses of ten such stimuli have revealed a consistent and well defined characteristic wave form not present in six patients with total vestibular loss. Similar evoked responses were obtained to a full field optokinetic stimulus approximating to that of the movement of the chair. Combined optokinetic and vestibular stimulation does not show algebraic summation. The evoked responses have features in common with nervous activity recorded in the monkey at vestibular nuclei and thalamic levels in comparable test situations.

Electronystagmography↗
PubMedDate not supplied

Oscillopsia of peripheral vestibular origin. Central and cervical compensatory mechanisms.

Eight patients with absent vestibular function categorized into four grades according to the disability they suffered from oscillopsia have been studied with a view to correlating its severity with the development of gaze stabilizing compensatory mechanisms. Eye movements were recorded while the following sinusoidal rotational stimuli were delivered: 1) trunk on head oscillation in the dark (COR); 2) head on trunk oscillation in the dark; 3) head on trunk and whole body (head and trunk) oscillation in the light in the presence of optic fixation. The COR was potentiated in all the patients regardless of their clinical status. Velocity gains (peak slow phase eye velocity/peak head velocity) during whole body rotation were significantly lower than head on trunk gains in the light in the better compensated patients. Since in the absence of vestibular function whole body rotation involves only the otokinetic system (OKN), this finding implies a depression of the OKN in these patients which can be corrected during head on trunk movements by virtue of a dynamic input from the neck. The results suggest that the processes of recovery from oscillopsia are dependent, in the main, upon the development of central mechanisms by means of which undesirable image movement across the retina is perceptually suppressed. Depression of OKN may be secondary to this perceptual rearrangement.

Adolescent↗
PubMedDate not supplied

Transfer of optokinetic activity to vestibular nystagmus.

Transfer of activity generated by prior optokinetic (OK) stimuli of one minute's duration to nystagmus induced in darkness by a subsequent vestibular stimulus consisting of step velocities to and from 40 degrees/s-1 was studied in 10 normal subjects. Four types of OK stimuli were used: (a) full field 'passive'; (b) full field 'active'; (c) full field in the presence of optic fixation, and (d) small OK drum stimulation. Transfer (T) was evident under all conditions and resulted in an enhancement of the vestibulo-ocular (VO) response when activity from the two stimuli were in the same direction (S) and a suppression when in the opposite (O). Expressed by the equation: Formula See Text. the respective transfer values obtained for the above conditions were (a) 66%, (b) 58%, (c) 22%, and (d) 54%. In all tests, rightward OK drum movement was more effective than leftward. In respect of passive OKN the resultant response can be well represented as the algebraic summation of the expected optokinetic after-nystagmus (OKAN) and the VO response, though opposing OKAN is more effective than enhancing. Passive OKN is more effective than active and this can be accounted for by the small contribution made by retinal slip in the former (the indirect path). Surprisingly, the small drum proved almost as effective as active OKN in terms of transfer. Fixation in the presence of full-field OK stimuli induces a non-directionally specific depression of the subsequent VO response, implying that retinal slip could contribute to the mechanism of VO response suppression.

Adolescent↗
PubMedDate not supplied

Evidence of direct thermal action upon the vestibular receptors in the caloric test. A re-interpretation of the data of Coats and Smith.

Although Bárány's convection theory has gained wide acceptance, there is strong evidence that the caloric response may include a component resulting from direct thermal stimulation of the vestibular end organs. It is argued on theoretical grounds that important new information could be obtained from a precise determination of the neutral head positions at which caloric irrigation fails to elicit a response. Although experimental studies using a bracketing procedure proved unsuccessful for this purpose, a re-examination of the extensive studies of Coats and Smith revealed that this information was already available. Reinterpreted in this way, their data clearly indicate that the hot and cold components resulting from direct thermal action equate to 42% and 34% respectively of the responses obtained in the planes of maximum reactivity. This is of sufficient magnitude to account for the anomalous findings in Spacelab I. Because of non-linearity in the relationship between neural discharge evoked by caloric irrigation and nystagmus magnitude, this component may be significantly less in the conventional caloric position. For this and other reasons there would seem to be no good reason to re-evaluate the clinical reliability of the caloric test.

Caloric Tests↗
PubMedDate not supplied

The influence of eye colour upon temporary threshold shift.

Controversy has arisen over the claim that melanin content of the iris as determined by eye colouration bears a direct relationship to susceptibility to temporary threshold shifts (TTS). A more comprehensive investigation of TTS has now been carried out which not only confirms the relationship but also indicates that earlier discordant results can be attributed to differences in methodology. The new evidence suggests that for stimulus intensities below 110 dB. (HTL re ISO, 1964) melanin content of the iris exerts its influence predominantly upon auditory adaptation while fatigue effects only become apparent at intensities above this level. The relevance of these findings to hearing conservation measures aimed at the prevention of noise-induced hearing loss is discussed.

Acoustic Stimulation↗
PubMedDate not supplied

An improved model for loudness coding during auditory adaptation.

A comparison of loudness adaptation measured at four baseline intensities and at test values 10 and 20 dB above the baseline intensities revealed an orderly decline. The magnitude of the decline in loudness adaptation with the increasing difference between the test values and the baseline was shown to agree with earlier theories. A method for closely estimating the change in loudness adaptation is presented in convenient graphic form and in algebraic equations.

Acoustic Stimulation↗
PubMedDate not supplied

Clinical implications in calibration requirements in bone conduction standardisation.

Although international agreement has long been reached on a standard threshold for air conduction audiometry, no similar standard exists for bone conduction autiometry. It is argued that the techniques applied to the determination of the air conduction thresholds are not applicable to those for bone conduction which should instead be brought into line with ISO 1964 and not established independently. A pilot study has been carried out which shows that by adopting the new approach now advocated international agreement could be attained with the expenditure of minimal time and effort.

Acoustic Stimulation↗
PubMedDate not supplied

Loudness balance procedures for the measurement of recruitment.

COLES and PRIEDE have recommended that in the alternate binaural loudness balance test use of the normal ear as the reference standard be now abandoned in favour of the worse ear. Their data are critically reviewed and shown to be subject to cumulative errors of methodology and interpretation.

Acoustic Stimulation↗