I. V. Orlov

Effects of artificial feedback to the vestibular input on postural instability induced by asymmetric proprioceptive stimulation.

The subjects in this study maintained a vertical posture standing on a rigid support. Postural stability was assessed in terms of the standard deviations (σ) from the mean amplitude of movements of the head relative to the null coordinate. Feedback at the vestibular input was created by transmastoid bipolar galvanic stimulation. Changes in the current in the feedback envelope were governed by a linear function based on the amplitude and rate of head movement. Variation in the coefficients of the feedback function could decrease the magnitude of σ for lateral movements which were increased (compared with values in calm standing in the dark) by unilateral vibrational stimulation of the gluteus medialis muscle. These results provide evidence that “rate” and “position” information have different values for maintaining the vertical posture in different subjects. They also demonstrate the ability of the central nervous system (CNS) to reevaluate the weightings of the different types of information arriving via a single channel. These results support the hypothesis that galvanic vestibular input can provide the CNS with sufficient information relating to the current orientation of the body. This information can be used for postural stabilization.

Artificial vestibular feedback in conditions of a modified body scheme

Subjects standing in the dark on a rigid immobile support maintained a vertical posture, which was destabilized by vibrostimulation of both Achilles tendons. Feedback was created via the vestibular pathway using transmastoid galvanic stimulation. Changes in the current in the feedback envelope were made using a linear function based on the amplitude and rate of head displacement. Changes in the body scheme were produced using rotation of the head relative to the trunk, rotation of the trunk with the relative head position fixed, and simultaneous rotation of the head and trunk. The result of these manipulations was that the head could be rotated through essentially 90° relative to the feet. In addition, rotation of one foot relative to the other through 90° was used. Artificial feedback damped head oscillations induced by vibration, but only those in the vertical plane passing through the interaural axis of the head. It is suggested that changes in the vectorial characteristics of vestibular responses and the results of applying artificial feedback on the background of modified orientation of the head relative to the feet may be associated with substitution of the ensembles of vestibular hair sets providing the dominant signals in the responses of vestibulospinal neurons.Subjects standing in the dark on a rigid immobile support maintained a vertical posture, which was destabilized by vibrostimulation of both Achilles tendons. Feedback was created via the vestibular pathway using transmastoid galvanic stimulation. Changes in the current in the feedback envelope were made using a linear function based on the amplitude and rate of head displacement. Changes in the body scheme were produced using rotation of the head relative to the trunk, rotation of the trunk with the relative head position fixed, and simultaneous rotation of the head and trunk. The result of these manipulations was that the head could be rotated through essentially 90 degrees relative to the feet. In addition, rotation of one foot relative to the other through 90 degrees was used. Artificial feedback damped head oscillations induced by vibration, but only those in the vertical plane passing through the interaural axis of the head. It is suggested that changes in the vectorial characteristics of vestibular responses and the results of applying artificial feedback on the background of modified orientation of the head relative to the feet may be associated with substitution of the ensembles of vestibular hair sets providing the dominant signals in the responses of vestibulospinal neurons.

Vestibular Influences on Postural Instability Induced by Movements of the Visual Environment and Support

Subjects were maintained in a vertical posture standing on a hard support with a limited degree of freedom in the frontal plane. The stability of the vertical posture was assessed on the basis of the standard deviations (σ) from the mean amplitude of head oscillations (in the frontal and sagittal planes) relative to the origin of the coordinate system. Sinusoidal rotations of the optokinetic cylinder in which subjects stood, sinusoidal rotations of the support, and combination of these rotations, with phase discordance between movements of the cylinder and the support, led to increases in σ in all subjects. Feedback via the vestibular input was created using transmastoid galvanic vestibular stimulation. Changes in the feedback current showed a linear function relating to the amplitude and speed of head movement. Introduction of variations in the feedback function could be used to decrease σ for lateral oscillations; increases (compared with values on calm standing in the dark) resulted from the use of any of the destabilizing treatments. Changes in σ for oscillations in the sagittal plane were not systematic.

Vestibular Prosthetics: Concepts, Approaches, Results

In contrast to hearing prosthetization, where the technology has been in development for more than 30 years, the challenge of vestibular prosthetization has been researched for no more than 15 years. However, the involvement of the vestibular system in supporting the normal functioning of the visual, motor, and other body systems defines its decisive contribution to spatial orientation in humans and animals. Damage to the vestibular apparatus (labyrinth) leads to serious impairment to posture control, gaze stabilization, spatial orientation, and psychological status, i.e., a person’s overall quality of life is sharply degraded. Animal studies have developed techniques for the prosthetization of the semicircular canals, which perceive angular acceleration and control eye movements in dynamic situations. New approaches based on replacement of the lost natural afferent spike activity in the vestibular nerve by electrical stimulation via a multichannel vestibular prosthesis have been successfully introduced into clinical practice.

Effects of Lateral Tilting on Optokinetic and Post-Optokinetic Nystagmus in Pigeons

Pigeons were exposed to optokinetic stimulation in the plane of the horizontal semicircular canals with an angular velocity of 10°/sec with normal head orientation in space during lateral tilting by 30° (dynamic tilting), after tilting (static tilting), and during rotation of the head to the initial position. No differences were seen in the effects of dynamic and static tilting on optokinetic and post-optokinetic nystagmus. Both types of tilting could induce both weakening and activation of oculomotor reflexes. There was no strong relationship between the direction of tilt and the sign of changes in the response. These data contradict the hypothesis of gravity-related changes in the time constant of the “velocity accumulator” as the sole cause of changes in optokinetic and post-optokinetic responses in conditions of dynamic and static tilting.

Habituation of horizontal nystagmus of the eyes in pigeons in conditions of alternating central and eccentric rotations.

Intact pigeons (n = 19) were rotated in the dark in the horizontal plane in different orientations relative to the axis of rotation. In central (evoking habituation) rotations, the animals head was located on the axis of rotation; in eccentric rotations, the animals head was 0.6 m from the axis of rotation. Pigeons were subjected to series of alternating central and eccentric rotations; rotation directions were also alternated. Series consisted of 2–5 rotation using a trapezoidal program. Each stimulus evoking habituation was used no more than 14 times during the experiment. Eccentric rotations were found not to prevent the gradual decrease in the peak rates of the slow components of primary nystagmus occurring on the transition from one series of central rotations to another in 17 individuals (group 1); these were increased in two individuals (group 2). Group 1 showed direct relationships between changes in this measure of primary nystagmus, changes in the duration of nystagmus, and changes in the peak rates of secondary nystagmus. Modifications of nystagmus within series varied. When two identical stimuli did not follow immediately one after the other, the second stimulus induced the same changes in nystagmus as observed in the individual in the first and next series of central rotations. If two identical stimuli followed one immediately after the other, the second stimulus in the pair often induced increases in the peak rates of primary and secondary nystagmus, along with increases in the time taken to reach the peak rate of primary nystagmus. These changes were non-random at a probability of >95%.

Fragmentary control of vestibuloocular responses

Intact pigeons (n=64) were rotated in the dark in the horizontal plane at different orientations relative to the axis of rotation. The overall patterns of changes in nystagmus of the eyes arising as a result of displacement of the otolith membranes in several directions were analyzed. In ten pigeons, all changes in nystagmus (type 1 general patterns) could be explained in terms of the dynamics of peripheral neuron activity and non-specific (identical for all combinations of interacting inputs) central influences. In the remaining pigeons, part of the changes in nystagmus (type 2 general patterns) was associated with central influences which were not identical for different combinations of interacting inputs. Repeated unusual combinations of vestibular stimuli and subsequent treatment with Nembutal transformed type 2 general patterns into type 1 general patterns. These data provide evidence for the fragmentary control of eye movements, whereby there is selective (fragmentary) modification of only some (individually specific) combinations of canal and otolith signals out of the whole set of vestibuloocular responses arising in response to stimulation of paired vestibular inputs; modification is mediated by changes in the sign of otolith influences on the canal components of these responses.

Mechanisms of the interaction of the angular and linear components of the horizontal vestibulo-ocular reflex in the pigeon

Intact pigeons (Columba livia,n=30) were rotated in a horizontal plane in the dark at different orientations relative to the axis of rotation. A total of 24 birds showed different directions of changes in the duration of contrarotatory nystamus (on transition from central rotation to eccentric), along with displacement of the otolith membranes in both the frontal and sagittal planes. These pigeons showed a direct relationship between changes in the duration of the primary phase of nystagmus and the peak rate of the slow component on the background of increasing centrifugal force, while no such relationship was seen in conditions of decreasing centrifugal force. Increases in the duration of the primary phase were accompanied by decreases in the duration of the secondary phase (i.e., the reversive phase) and vice versa. These data provide evidence that the otolith component is not decreased to zero by rotation at constant angular rates or immediately after this stopped; in conditions of negative angular acceleration, this component was biphasic. The results are in good agreement with a hypothesis [2] suggesting that the otolith component represents asymmetric (different in paired brain structures) neuronal activity modifying the canal component even when the level of asymmetry is itself insufficient to initiate eye movements.

Relationship Between Learning Characteristics and the Properties of Visual Objects in Rhesus Macaques with Bilateral Removal of Parietal Cortex Field 7

Behavioral experiments were used in rhesus macaques with bilateral excision of field 7 of the lower parietal cortex to study the relationship between visual differentiation learning processes and a variety of stimulus properties. All animals showed significant differences associated with stimulus properties, which produced different types of learning curves. For each monkey, visual stimuli were divided into compact groups in terms of the “similarity” of their learning characteristics. Removal of field 7 had no effect on the process of learning visual image discrimination when this was based on properties such as color and geometrical shape, but worsened the learning characteristics when visual differentiation was based on spatial information, when the learning process became unstable, with increases in the numbers of peaks and troughs on the learning curve and a significant increase in the duration of the learning period. The time to a stable motor response also became significantly greater than for visual images distinguished by shape and color. It is suggested that during the process of learning visual discrimination, processing and extraction of image signs by the visual system for objects characterized by spatial relationships is accompanied by the formation of spatial distinguishing signs, this process involving neuronal structures in field 7 of the lower parietal cortex, which appears to be the main area determining visual-vestibular interactions. Increases in oscillations and in the difficulty of the learning process for differentiation on the basis of spatial information aftr removal of field 7 might be due to a transfer from one strategy to another, resulting from disruption of the mechanisms which evaluate body image and egocentric orientation on the basis of visual-vestibular interactions.

Relationship between the characteristics of visual short-term memory in monkeys and the spatial properties of images.

Experiments were performed on Rhesus macaques to study the relationship between delayed visual differentiation processes and stimulus properties. These investigations showed that the processes of short-term storage of visual information in monkeys has significant features associated with differences in stimulus properties. These consisted of different durations of storage and motor response times. Because of these differences, stimuli (15 pairs) could be grouped into compact clusters on the bases of similarity between their delayed differentiation characteristics. These experiments characterized the processes of short-term information storage during the differentiation of stimuli differing in terms of spatial relationships between elements, as compared with stimuli differing in terms of other attributes (shape, color, etc.); spatial information was stored for shorter periods of time and motor response times were longer. It is suggested that visual short-term memory involves a set of mechanisms operating on attributes of different types and which, along with signs and working programs associated with the visual system, stores spatial discriminatory signs, in which the major role is played by visual-vestibular interactions.