J. D. Hood

Observations Upon the Neurological Mechanism of Optokinetic Nystagmus with Especial Reference to the Contribution of Peripheral Vision

Electro-nystagmographic investigations of optokinetic nystagmus under a variety of test conditions have made possible the identification of two separate and distinct mechanisms, one involving peripheral vision and the other foveal vision. Certain striking differences are apparent in the characteristics of these mechanisms which throw new light upon the neurological mechanism of optokinetic nystagmus in man and at the same time resolve many of the conflicting opinions which appear in the literature upon the subject.

The significance of peripheral vision in the perception of movement.

Certain significant differences have been shown to occur in the character of the optokinetic response of a subject gazing actively and passively at a moving striped drum which suggest that different nervous mechanisms subserve each. Image motion across the retina appears to be the stimulus inducing the passive variety and is moreover largely responsible for the illusory sensations of self rotation. This same mechanism may be invoked by way of explanation of the phenomenon of reversed optokinetic nystagmus. The occurrence of this phenomenon in patients with non congenital as well as congenital nystagmus lends support to this explanation. Eye movements induced in the normal eye of subjects with unilateral ophthalmoplegia following stimulation of the paretic eye provide additional support for the contention that peripheral vision contributes to the control of normal ocular movements.

A Recent Clinico-Pathological Study of Cochlear Nerve Degeneration Resulting from Tumor pressure and Disseminated Sclerosis, with Particular Reference to the Finding of Normal Threshold Sensitivity for Pure Tones

Schuknecht and his co-workers have shown that partial lesions of the cochlear nerve in cats may be associated with normal hearing at threshold for pure tones. Comparable findings are described in two human subjects.

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.

Evidence of Direct Thermal Action upon the Vestibular Receptors in the Caloric Test: A Re-interpretation of the Data of Coats and Smith

Although Báránys 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.

Symmetrical Hearing Loss In Brain Stem Lesions

An examination of bilateral deafness resulting from proven lesions of the brain stem has shown that the pure tone audiograms at the two ears are invariably identical. This finding has interesting theoretical implications concerning the tonotopic organisation of the auditory pathways in the brain stem which will be discussed.

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.

Observations upon the nervous mechanism of vestibular habituation.

Rotational tests carried out upon a number of ballet dancers have shown their nystagmic threshold responses to angular acceleration to be normal when determined in darkness, but considerably raised in the presence of optic fixation. These results indicate that optic fixation plays an important role in vestibular habituation in man and this, in turn, calls for some reassessment of current views on its nervous mechanism.