Homocysteine, B vitamins, and cardiovascular disease.
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Atherosclerosis↗
Biomedical subjects
Richard F Lewis
Publications and source records attributed to Richard F Lewis.
Acclimation to chronic constant-rate peripheral stimulation provided by a vestibular prosthesis.
We are developing two types of vestibular prosthetics that electrically stimulate afferent neurons. One type replaces absent sensory function by providing stimulation that modulates above and below a baseline established with the head stationary. The other type provides constant stimulation and is turned on only when necessary, for example, to override unnatural variations like those experienced by patients suffering from Ménère's syndrome; this prosthesis does not provide motion information. Both prostheses require neural plasticity, which we investigated by providing chronic constant-rate stimulation to semicircular canal neurons in three guinea pigs. The stimulation was alternately switched on or off for eight consecutive weeks before being switched daily. A brisk horizontal nystagmus was measured when the stimulation was first turned on and then dissipated over the course of a day. The nystagmus demonstrated an after-effect in the opposite direction when the stimulation was turned off. The nystagmus that we measured after just a few (2 to 5) off-to-on transitions returned to baseline more rapidly than when first turned on. In fact, after many such off-to-on or on-to-off transitions, little nystagmus was evoked by turning the stimulation on or off. These findings show that the brain acclimates to constant-rate stimulation.
Acclimatization↗
Tilt psychophysics measured in nonhuman primates.
A method to measure tilt psychophysics in nonhuman primates has been developed and tested. Two rhesus monkeys were trained to orient a light bar parallel to the direction of gravity by rotating a small steering wheel attached to the primate chair. After adequate training, they performed this task during motion paradigms that rotated the gravitoinertial force (GIF) in the roll plane. The animals accurately aligned the light bar with the direction of gravity during and following roll tilts. Subtle aspects of perceived roll tilt previously recorded in human subjects were also observed in the monkeys. During centrifugation and linear translation, an interaural inertial force rotates the GIF in the roll plane. Similar to humans, the animals' perceived direction of gravity shifted toward alignment with the GIF during these paradigms, and the dynamics of these shifts demonstrated an interaction between the otolith afferents that transduce the GIF and rotational cues from the semicircular canals. Finally, optokinetic roll stimulation induced an illusion of roll tilt that was dependent on head orientation, similar to results described in human subjects. Taken together, these findings validate the methodology employed to measure tilt psychophysics; the monkeys' responses were qualitatively similar to those described in human subjects and also recapitulated subtle aspects of the perceptual response observed in humans. This new methodology sets the stage for innovative experimental approaches that will help to elucidate the neural mechanisms underlying the perception of spatial orientation.
Animals↗
Frequency-specific mal de debarquement.
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Automobile Driving↗
Vergence-dependent adaptation of the vestibulo-ocular reflex.
The gain of the vestibulo-ocular reflex (VOR) normally depends on the distance between the subject and the visual target, but it remains uncertain whether vergence angle can be linked to changes in VOR gain through a process of context-dependent adaptation. In this study, we examined this question with an adaptation paradigm that modified the normal relationship between vergence angle and retinal image motion. Subjects were rotated sinusoidally while they viewed an optokinetic (OKN) stimulus through either diverging or converging prisms. In three subjects the diverging prisms were worn while the OKN stimulus moved out of phase with the head, and the converging prisms were worn when the OKN stimulus moved in-phase with the head. The relationship between the vergence angle and OKN stimulus was reversed in the fourth subject. After 2 h of training, the VOR gain at the two vergence angles changed significantly in all of the subjects, evidenced by the two different VOR gains that could be immediately accessed by switching between the diverged and converged conditions. The results demonstrate that subjects can learn to use vergence angle as the contextual cue that retrieves adaptive changes in the angular VOR.
Adaptation, Physiological↗
Context-dependent adaptation of visually-guided arm movements and vestibular eye movements: role of the cerebellum.
Accurate motor control requires adaptive processes that correct for gradual and rapid perturbations in the properties of the controlled object. The ability to quickly switch between different movement synergies using sensory cues, referred to as context-dependent adaptation, is a subject of considerable interest at present. The potential function of the cerebellum in context-dependent adaptation remains uncertain, but the data reviewed below suggest that it may play a fundamental role in this process.
Animals↗
Slow target-directed eye movements in ataxia-telangiectasia.
PURPOSE: To analyze the slow eye movements that shift the direction of gaze in patients with ataxia-telangiectasia (A-T). METHODS: Eye and head movements were recorded with search coils in three patients with A-T during attempted gaze shifts, both with the head immobilized and free to move. RESULTS: Gaze shifts frequently included both saccadic and slow components. The slow movements were recorded after 42% of saccades and had an average peak velocity of 6.1 deg/sec and a mean amplitude of 2.0. They occurred with the head stationary and moving, could be directed centripetally or centrifugally, had velocity waveforms that were relatively linear or exponential, and always moved the eyes toward the visual target. CONCLUSIONS: The slow movements appear to differ from pursuit and vestibular eye movements and are not fully explained by the various types of abnormal eye movements that can follow saccades, such as gaze-evoked nystagmus or postsaccadic drift. Their origin is uncertain, but they could represent very slow saccades, due to aberrant inhibition of burst cell activity during the saccade.
Adolescent↗
Vestibular adaptation studied with a prosthetic semicircular canal.
We have developed and tested a prosthetic semicircular canal that senses angular head velocity and uses this information to modulate the rate of current pulses applied to the vestibular nerve via a stimulating electrode. In one squirrel monkey, the lateral canals were plugged bilaterally and the prosthesis was secured to the animal's head with the angular velocity sensor parallel to the axis of the lateral canals. In the first experiment, the stimulating electrode was placed near the ampullary nerve of one lateral canal. Over a period of two weeks, the gain of the horizontal VOR during yaw axis rotation gradually increased, although the response magnitude remained relatively small. In the second experiment, the stimulating electrode was placed near the ampullary nerve of the posterior canal, but the orientation of the velocity sensor remained parallel to the axis of the lateral canals. Over a one-week period, the axis of the VOR response gradually shifted towards alignment with the (yaw) axis of head rotation. Chronic patterned stimulation of the eighth nerve can therefore provide adequate information to the brain to generate a measurable VOR response, and this can occur even if the prosthetic yaw rotation cue is provided via a branch of the VIIIth nerve that doesn't normally carry yaw rotational cues. The results provided by this pilot study suggest that it may be feasible to study central adaptation by chronically modifying the afferent vestibular cue with a prosthetic semicircular canal.
Adaptation, Physiological↗