Selective habituation of vestibular nystagmus by visual stimulation in the monkey

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.

Unidirectional vestibular habituation in the squirrel monkey. The time course and its influence on optokinetic nystagmus

Twenty clockwise (CW) angular stimulations were given daily to squirrel monkeys over a 14-calendar day period. The gain of vestibulo-ocular reflex (VOR) to CW angular acceleration decreased after each daily session. Particularly, the maximum reduction of VOR was found after the first day's session. The reduction was retained until the time of the next day's session. But, the across-day course of the VOR reduction showed up-and-down sinusoidal patterns in all animals. With respect to the effect on optokinetic nystagmus (OKN), an increase of counter-clockwise (CCW) OK responses after repeated CW vestibular stimulations was found; however, there was no clear retention of this response modification across daily sessions.

Modulation of human velocity storage sampled during intermittently-illuminated optokinetic stimulation

When a stationary human subject is suddenly exposed to constant-velocity full-field optokinetic stimulation, slow-phase eye velocity rapidly approaches stimulus velocity without the gradual build-up ("velocity storage") readily seen in other species. Subsequent to extinguishing illumination, the presence of velocity storage is suggested by the persistence of a gradually-declining component of optokinetic after-nystagmus (OKAN). In the past, characterizing modulation of velocity storage has been tedious. It is now shown that such modulation can be characterized by periodic "sampling" of OKAN using "intermittently-illuminated" optokinetic stimuli (light-on: 9.7 s; darkness: 2 s). Six subjects viewed an intermittently-illuminated optokinetic drum turning with a square wave angular velocity profile of 60 deg/s peak amplitude and 0.01 Hz frequency. The resulting modulation of velocity storage was approximately exponential with time constant, T = 5-11 s, and asymptote, A = 10-17 deg/s. A significant negative correlation was observed between T and A values. In a given subject, T and A values agreed (generally within +/- 20%) with values obtained employing previously-used methods, suggesting that velocity storage behaved linearly during periodic optokinetic stimuli of less than 60 deg/s. The new method of sampling OKAN permits the use of arbitrary stimulus profiles required to observe, or to confidently predict, velocity storage response during natural behavioural movements, which was not feasible with older methods. Sampling also increases the ease and speed (roughly three-fold) of data acquisition.

Modification of the balance and gain of the vestibulo-ocular reflex in the cat

The characteristics of the vestibulo-ocular reflex (VOR) of a normal cat can be modified in response to visual demands. Two aspects of the VOR are modifiable independently by a normal cat: the gain and the balance. An imbalance results in a spontaneous nystagmus and an asymmetric VOR. Neither the gain nor the balance of a dark-reared cat's VOR is susceptible to visual modification. A cat whose crossed visual pathways are severed at the level of the optic chiasm is able to modify the gain of the VOR but not its balance. Both dark-reared and split-chiasm cats have only very short-lasting optokinetic after-nystagmus.

Unidirectional habituation of vestibulo-ocular responses by repeated rotational or optokinetic stimulations in the cat

1. Unilateral habituation of the vestibulo-ocular reflex was produced in adult cats stimulated by repeated unidirectional velocity steps (vestibular training) or by a continuously moving visual surround (optokinetic training). -- 2. Unidirectional vestibular training produced a strong asymmetry of vestibulo-ocular responses (VOR). Responses to velocity steps applied to the "trained" labyrinth were decreased both in gain and in time-constant. This effect generalized to responses to sinusoidal oscillations (0.03 Hz to 0.1 Hz), i.e. to a stimulus not used during training. -- No spontaneous nystagmus was ever observed in spite of the dynamic VOR asymmetry. -- 3. Unilateral vestibular habituation produced by vestibular training appeared to be a long-lasting phenomenon. It was still present 10 days after the end of training. -- 4. Optokinetic responses were not affected by vestibular training. -- 5. Unidirectional optokinetic training produced an increase in the slow phase velocity of optokinetic nystagmus (OKN) by about 25% in both directions. This effect did not persist for more than a few minutes. A marked spontaneous nystagmus was recorded in the dark after each session of optokinetic training, with a slow phase in the direction opposite to the previous OKN. -- 6. VOR in response to velocity steps and to sinusoidal oscillations were decreased unilaterally after optokinetic training. This effect was of short duration, however, and disappeared within the interval between training sessions. This lack of retention contrasted with the prolonged effect of vestibular training.

Changes in the horizontal vestibulo-ocular reflex of the rhesus monkey with behavioral and pharmacological alerting

It is well known that eye movements are influenced by an animal's state of arousal. Alterations in the dynamic characteristics of the horizontal vestibulo-ocular reflex of adolescent rhesus monkeys induced by changes in the animal's state of arousal were studied using linear system analysis employing both single frequency sinusoidal and white noise rotational stimulations. Arousal changes were induced by a behavioral task and/or the administration of amphetamines (0.5 mg/kg). Results indicate that highly alert animals display vestibulo-ocular reflex gains significantly different from less alert animals. Specifically, the gain of the vestibulo-ocular reflex is closer to unity over a wider range of frequencies in more alert animals. These changes were independent of the method used to maintain a high level of arousal.

Optokinetic suppression of aberrant vestibular reactions: an observation

It has long been known that optokinetic nystagmus may be modified by vestibular inputs. We have suppressed an aberrant vestibular response and associated gastrointestinal sensations by use of an optokinetic stimulus simultaneous to the positional stimulus in a patient with paroxysmal positional vertigo. This single subject study utilized several optokinetic conditions, and patterned its stimulus presentations to control for visual fixation and vestibular habituation. Objective recordings of eye movements (ENG) were made simultaneously with subjective evaluation of "dizziness" on a simple magnitude estimation scale.