EMG responses of the vertical eye muscles to dynamic and static natural vestibular stimulation about different axes in alert rabbits

The EMG responses of the vertical eye muscles have been recorded in alert intact rabbits submitted to both dynamic and static natural vestibular stimulations about different axes. Following dynamic vestibular stimulation, the phase lead of the EMG response with respect to head position as well as its sensitivity increased with stimulus frequency in all the stimulation modalities. This indicates a progressive recruitment by the stimulus of the second-order vestibular neurons related to semicircular canals. The sensitivity of the response was consistently higher in the effective intermediate roll-pitch modality for all the 4 muscles. Following static stimulation, the EMG response showed an asymmetric modulation. The excitatory response was always higher than the inhibitory one and linearly related with the stimulus. Also for the static stimulation the amplitude of the response was significantly higher when the animal was tilted about the effective intermediate roll-pitch axis for all the 4 muscles. The presence of a maximal EMG response in the same modality for both static and dynamic stimulation indicates a similar spatial organization of those subgroups of ampullar and macular receptors projecting to the same eye muscle.     

Eye movements and vestibulo-ocular reflex in the blind

Summary To assess the effect of chronic deprivation of visual feedback, 21 blind patients underwent clinical and electro-nystagmographical examination. Patients with congenital blindness were characterized by spontaneous eye movements, inability to consciously move the eyes and absence of the vestibulo-ocular reflex (VOR), whereas eye movement abnormalities were practically absent in those with blindness acquired late in life. Active visual experience, at least in early life, seems to be crucial for the development of eye movements and VOR adjustment.     

Eye position and head velocity signals are conveyed to medial rectus motoneurons in the alert cat by the ascending tract of Deiters'

Eye position and head velocity signals are conveyed to medial rectus extraocular motoneurons in the alert cat by the ascending tract of Diiters'. Physiologically and behaviorally identified ascending Deiters' neurons have been injected intra-axonally and their morphology studied.     

Compensatory eye movements during head and body rotation in infants

Eye movements were measured in 2- and 4-month-old infants during rotation in the light and in the dark using a specially constructed infant seat that was rotated sinusoidally at 0.1, 0.25 and 0.5 Hz. As in adults, the phase of the vestibulo-ocular reflex (VOR) was nearly compensatory with respect to head position, and the gain values were generally in the range between 0.4 and 1.4. There were two types of phases for the cervico-ocular (COR) responses: compensatory and anticompensatory. The gain of the COR was 4-6 times larger than that reported for adults. In addition, it was both frequency- and age-dependent. These characteristics of the COR may reflect developmental processes in the infant nervous system. The interaction of the two reflexes (VOR + COR) gave responses similar to when the VOR was elicited alone. The subjects' use of a fixation light raised the gain of the VOR and VOR + COR to about 1.67 and 1.44, respectively. These values were higher than most gains reported for adults. However, they were close to what was predicted (1.53) for the experimental conditions wherein fixation was on a target 25 cm in front of the eyes.     

Influence of long-term optokinetic stimulation on eye movements of the rabbit

Two kinds of optokinetic afternystagmus (OKAN) have been studied in rabbits; positive and negative OKAN. Positive OKAN is the persistence of eye movements evoked by optokinetic stimulation following the termination of the stimulus, with the slow phase of the eye movements in the same direction as the inducing stimulus. Negative OKAN is evoked by long duration optokinetic stimulation, and has a slow phase of opposite direction to the inducing stimulus. The stimulus conditions which are optimal for inducing and maintaining negative OKAN were characterized. Rabbits were placed in an optokinetic drum for periods of 12-96 h (with appropriate intervening periods for food and water). Eye movements were recorded during and after the termination of optokinetic stimulation. The optimum optokinetic stimulus velocity for the induction of negative OKAN was 5 degrees/s. The minimum duration of stimulation for the induction of negative OKAN of maximum velocity was 48 h. Once induced, the slow phase of negative OKAN attained velocities of 50-100 degrees/s. Three conditions of restraint of the rabbits were studied after negative OKAN was induced during the intervening periods when eye movements were not being recorded. These conditions were: (1) unrestrained (full freedom of movement) without visual stimulation (in a dark enclosure); (2) restrained (horizontal head and body movement prevented) without visual stimulation; and (3) restrained with visual stimulation (in the stationary optokinetic drum). Conditions 1 and 2 caused negative OKAN to dissipate within 24 h. Condition 3 caused negative OKAN to be maintained for more than 70 h. The velocity imbalance of the horizontal vestibuloocular reflex (HVOR) was measured at different times following the induction of negative OKAN. It provided a more sensitive index of the central imbalance which caused negative OKAN, than did spontaneous nystagmus. One of the consequences of optokinetic stimulation measured over a 16 h period was a decrease in the gain of the optokinetic reflex. This reduction in gain could represent a central adaptation to maintained stimulation which in the absence of continued optokinetic stimulation is expressed as a nystagmus.     

Eye movements evoked by superior colliculus stimulation in the alert cat.

(1) The electrical stimulation of the superior colliculus (SC) in the cat evokes exclusively conjugate and contraversive eye saccades. (2) Their maximum velocity is markedly higher than that of spontaneous saccades. (3) The stimulus parameters (intensity, frequency, pulse width) have but little effect on the characteristics of the saccades. (4) In the anterior half of the SC, corresponding to the projection of the 12-15 central degrees of the retina, amplitude and direction of saccades depend exclusively on the position of the electrode. (5) In the posterior half, corresponding to the projection of the peripheral retina, saccades are 'goal directed' and the position of the goal is determined by the location of the electrode. (6) An increase in stimulus train length produces, in the anterior part, a succession of identical saccades and, in the posterior part, a goal fixation. (7) Taking all these data into consideration, a model of the foveation process in the cat is proposed.     

Painless head holder for unitary recordings from alert rats

The construction and implantation of a device is described that holds the head of an alert rat in a well-defined position. The main parts of the head holder consist of a plate with a wide opening to allow for vertical access to virtually all parts of the brain, a rectangular recording well, and two specially shaped elastic steel skull clamps. It is easy to fabricate, painless, long-lasting, and suitable for stable stereotaxic recordings of single units in small alert mammals.     

Vestibulo-ocular reflex (VOR), cervico-ocular reflex (COR) and its interaction in active head movements

Summary In normal adults the vestibulo-ocular reflex (VOR) and the cervico-ocular reflex (COR) were investigated during passive and active head or body movements, respectively. Sinusoidal rotations around the vertical axis of the body at frequencies of 0.05, 0.1, and 0.2 s^–1 and total amplitudes of 20°, 40°, 60°, or 80° were employed.The average eye deviations (Schlagfeld) during VOR were directed opposite to the direction of the head turning. During COR, however, slow eye deviations of higher amplitude were anticompensatory relative to the fixed head. During active head turnings the average eye deviations showed the same anticompensatory direction as in COR, but were still larger. They increased with stimulus amplitudes up to 60°.At least a weak cervical nystagmus was elicited in all subjects, with its fast phases beating in the direction of the relative head movement. Its gain reached marked values up to 0.5, but only for peak stimulus velocities below 25°/s. The nystagmus gain during active head turnings was only slightly higher than during VOR.With higher stimulus velocities, large anticompensatory saccades appeared just before the change of stimulus direction; these are typical for active head movements, but were also found during COR.Supported by Sonderforschungsbereich Hirnforschung und Sinnesphysiologie (SFB 70) der Deutschen Forschungsgemeinschaft (DFG)     

Eye movements during the Rorschach test in schizophrenia

In order to understand relationships between scanning behaviors, characteristics of visual stimuli and the clinical symptoms in schizophrenia, eye movements of 37 schizophrenic patients and 36 controls were recorded using an eye-mark recorder during a free-response period in a Rorschach test. Four cards (I, II, V and VIII) were used. Data were analyzed during 15 s from the presentation of each card. For all cards, the number of eye fixations and the number of eye fixation areas were fewer, and total scanning length and mean scanning length were shorter for schizophrenic patients than for controls. For card II, in the non-popular response group, eye fixation frequency upon area 5 + 6 (red) was higher for schizophrenic patients. For card VIII, in the popular response group, eye fixation frequency upon area 5 + 6 (pink) was lower for schizophrenic patients. For cards II and VIII, the number of eye fixations was inversely correlated with negative symptoms. For card II, total scanning length tended to be inversely correlated with negative symptoms, and mean eye fixation time was correlated with negative symptoms. The number of eye fixation areas was inversely correlated with positive symptoms. For card VIII, eye fixation frequency in a stimulative area tended to be correlated with positive symptoms. Scanning behaviors in schizophrenic patients are affected by characteristics of visual stimuli, and partially by clinical symptoms.     

Eye movements evoked by electrical stimulation in the superior colliculus of rats and hamsters

Although the organization of the sensory representations in the rodent superior colliculus is known to be similar to that of more advanced mammals, little is known about collicular motor organization in these animals. Since it has been suggested that rodents make head, rather than eye, movements to facilitate visual orientation, fundamental organizational dissimilarities could exist in the superior colliculi of different species to underlie these different orientation preferences. However, we observed that electrical stimulation of the rodent colliculus evoked the same pattern of movements observed in other mammals. Conjugate, contraversive saccadic eye movements were evoked and were found to be topographically organized. In addition, vibrissa and pinna movements could also be evoked. Although some species differences in evoked movements were evident, the same pattern of motor representations appears to exist in widely disparate mammalian species.     

Behavioral activities of axotomized abducens nucleus motoneurons in the alert cat

The activity of identified control and axotomized abducens motoneurons has been recorded during spontaneous eye movements in the alert cat. Axotomized motoneurons showed a quick fatigability during eye fixations not observed in controls. Discharge rate of axotomized motoneurons during on direction saccades was lower and shorter than in controls and started usually after the beginning of the saccade. Axotomized motoneurons could be antidromically invaded only in the presence of synaptic facilitation. Electrophysiological explanations for the behavior of axotomized motoneurons in the alert cat are discussed in the text.     

Identification of prepositus neurons projecting to the oculomotor nucleus in the alert cat

A population of prepositus hypoglossi nucleus neurons with discharges correlated to gaze parameters and antidromically activated by stimulation of the ipsilateral oculomotor nucleus has been recorded in the alert cat. The latency for antidromic invasion was of 0.6–0.8 msec. Neuronal firing rate encoded eye position and eye velocity in the horizontal plane, showing in all cases ipsilateral ‘on’ direction.     

Types of gaze movement: Variable interactions of eye and head movements

The patterns of coordinated eye-head movement in gaze shift are influenced by preconditions of the experimental protocol, such as the predictability and amplitude of the stimulus and the subject's endeavor to move as instructed, either naturally or as fast as possible. Gaze-shift movements fall into four distinct types with respect to eye-head latency, each one involving particular gaze-shift errors and head acceleration trajectories.     

Frontal myopia in the rabbit

The sharpness of a trans-scleral image was examined in the enucleated albino rabbit eye. Profound changes in the focus of the image were produced by changes in the relative position of the object in the visual field. These findings are discussed in relation to the view that the rabbit is frontally myopic and laterally hypermetropic.     

The cervico-ocular reflex in normal subjects and patients with absent vestibular function

The role of the cervico-ocular reflex (COR) has been studied in 12 patients with absent vestibular function and 13 normal subjects. Ramp and sinusoidal displacement stimuli were applied with trunk on head and head on trunk movements. In all patients, trunk on head movements evoked a marked slow-phase compensatory COR while in normal subjects it was weak and variable in direction. Fast components of the COR induced gaze shifts in the direction of the relative head movement ('anticompensatory' direction) which could be suppressed by imagining an earth fixed targed. No tonic component could be identified instead, in the case of ramp stimuli, a residual eye deviation was noted which was significantly enhanced in the patients and resulted from activity dynamically generated during the course of the trunk movement and not from its final angular displacement. Head on trunk ramp displacements in the dark evoked initial anticompensatory saccades followed by slow compensatory components, a pattern of eye movements remarkably similar to that seen during active head-eye target seeking. Thus, in the absence of labyrinthine function, the COR appears to take on the role of the vestibulo-ocular reflex in head-eye coordination in the initiation of the anticompensatory saccade which takes the eyes in the direction of the target, and the generation of the subsequent slow compensatory eye movements. Central pre-programming, as revealed by comparing the effect of different instructions and active versus passive neck-induced eye movements, has a profound influence on COR functioning.     

Method for effective rabbit head restraint during stereotaxic surgery

A method for rigid head restraint during stereotaxic surgical procedures is described. The method is based on modifications of a conventional rabbit stereotaxic head holder and provides reliable head fixation for accurate implantation of depth and surface electrodes, cannulae, microelectrode wells, etc.     

Oculomotor related interaction of vestibular and visual stimulation in vestibular nucleus cells in alert monkey

The activity of single cells in the vestibular nuclei in alert, behaving monkey was studied by extracellular recording. A majority of the neurons found in the superior and the rostral medial vestibular nuclei can be divided into two classes on the basis of their discharge relationship to eye movements evoked during head rotation, visual target pursuit, or visual suppression of the vestibulo-ocular reflex. The firing rate of the first unit type is proportional to head rotational velocity (and the resulting compensatory eye velocity) but is not modulated during slow eye movements of pure visual origin. During visual suppression of the vestibulo-ocular reflex, the relationship of this type of unit discharge to head velocity remains unchanged, although the eye velocity is now zero. The second type of unit more closely resembles oculomotor neurons in that its discharge pattern is proportional to eye position and velocity during eye movements of both visual and vestibular origin. However during suppression of the vestibuloocular reflex this type of unit continues to show a greatly reduced but consistent modulation of discharge rate proportional to head velocity. Thus the direct projection of vestibular neurons to oculomotor neurons cannot by itself account for the ability of the monkey to completely suppress its vestibulo-ocular reflex.