Eye movement responses to combined linear and angular head movement

Lateral eye movements evoked by linear head motion were evaluated in human subjects by subtracting the eye movement responses to head-centred angular oscillation in the dark, about a vertical axis, from the responses evoked by similar oscillation with the head displaced 30 cm eccentrically from the axis. The centred oscillation gave a purely angular stimulus whereas the eccentric oscillation gave an additional tangential linear acceleration acting laterally to the head. The stimuli used were relatively unpredictable, enveloped sinewaves at 0.02 to 1.2 Hz, 60 degrees/s peak angular velocity, 0.004 to 0.24 g peak tangential acceleration, and subjects were either given no instructions or were told to imagine fixating on targets at 60 cm or 5 m distance. Eye movements of significantly higher velocity were evoked in the eccentric position, particularly at the higher frequencies and when subjects imagined near targets. The increase in velocity of eye movement was attributed to the linear stimulus and probably derives from stimulation of the otolith organs. The frequency response of the gain (degree/s/g) of these movements gave an approximate slope of -1, indicating that the eye velocity bears a constant proportionality to linear head velocity. The findings are in accord with the theoretical prediction that eye movements compensating for linear head motion should only be required for viewing near targets. These otolithic influences on eye movements could either the mediated by a direct "otolith-ocular reflex" which is subservient to viewing conditions, or, alternatively, the otolith signals may modify the activity of other oculomotor mechanisms.

Predictive eye saccades are different from visually triggered saccades

The metrics of eye saccades in response to random and predictable target motion were determined in 8 normal young subjects. The peak velocity of predictive saccades (PS) was significantly reduced when compared with randomly elicited saccades (RS) of equal amplitude. Saccadic gain (initial saccadic amplitude/target amplitude) was also decreased in PS. Saccadic inaccuracy was extremely frequent in PS and, in contrast to RS, it was independent of target amplitude displacement. These stimulus-dependent changes in saccadic characteristics may reflect activation of the saccadic pontine generators by different supranuclear pathways.

Suppression of visually evoked postural responses

Normal subjects standing on an earth-fixed force platform inside a movable room displaced at velocities comparable to those accompanying spontaneous body sway, exhibit a visually evoked postural response (VEPR) some 600 ms after the start of the room movement. It consists of a displacement of the centre of force of the body in the direction of the stimulus (primary component), followed shortly by a corrective displacement in the opposite (secondary component). On second presentation of the stimulus VEPR is markedly reduced, but only if full proprioceptive information from the lower limbs is available to the subjects. A patient deprived of this information showed much enhanced VEPR which he was unable to suppress, in contrast to a patient with absent vestibular function who presented normal VEPR. The results show that in the presence of conflict between different sensory clues, vision is initially dominant in sway control, although adaptive processes can quickly rearrange this hierarchy.

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.

Otolith stimulation evokes compensatory reflex eye movements of high velocity when linear motion of the head is combined with concurrent angular motion

Previous investigations have failed to find significant compensatory eye movements in response to linear motion of the head. However, the fact that visual acuity is essentially preserved during natural head movements, which combine both linear and angular components, is evidence that there must be compensation for linear translation. Hence we examined the lateral eye movements produced by angular oscillation, in the dark, in yaw at 0.5 and 1.5 Hz, 80 degrees/s peak, with the head both centred and positioned 30 cm eccentric from the axis of rotation in order to produce an additional linear acceleration acting tangentially. The combined stimuli produced high velocities of eye movement which were much greater than those produced by angular motion alone. The findings are evidence or a linear-compensatory reflex which is probably otolithic. The dependency of otolithic eye movements on concurrent stimulation of the semicircular canals is a possible explanation of positional nystagmus in neuro-otological disease in which it may be released by a pathological imbalance of canal function.

Vestibular involvement in spasmodic torticollis

Vestibular findings in a group of 35 patients with spasmodic torticollis without other otological or neurological symptoms were reviewed. The most consistent abnormality, present in more than 70% of cases, was a directional preponderance of vestibular nystagmus in the dark in a direction opposite to the head (chin) deviation. Rigidly clamping the head to a rotating chair did not abolish the directional preponderance. In the presence of optic fixation the directional preponderance was less frequent and its severity tended to diminish as a function of the duration of the disease. Smooth pursuit and optokinetic nystagmus were only occasionally affected. The results are indicative of primary involvement of the vestibular system in spasmodic torticollis and are discussed in terms of a break-down of the central mechanisms conveying sensory information responsible for head and eye orientation.

Predictive ocular motor control in Parkinson's disease

A comparison was made of predictive eye movements of both the saccadic and pursuit ocular motor systems in parkinsonian patients and aged-matched normal controls. In a predictive task our patients, who were mildly or moderately affected, showed a reduced tendency to make anticipatory saccades compared with normal controls. Although there was some impairment of pursuit during a nonpredictive task as shown by an increased phase lag, a normal amount of improvement took place with a predictive task. This difference between prediction in the saccadic and pursuit system is possibly explained by an increased reliance on a visual input by parkinsonian patients which prevents them making use of verbal instruction to generate anticipatory (eye in advance of target) saccades. Improvement of pursuit with a predictive target track is possible in such patients since normally in these circumstances the target is mainly followed rather than anticipated. The metrics of saccadic and smooth pursuit eye movements (saccadic velocity and pursuit turnover velocity) were normal.

Cervical nystagmus due to loss of cerebellar inhibition on the cervico-ocular reflex: a case report

Studies of the cervico-ocular reflex and the vestibulo-ocular reflex have been carried out separately and in combination on a patient with gait ataxia due to a cerebellar tumour. With the head fixed in space, body rotation to the right (left neck torsion) induced marked nystagmus to the left in darkness. Vestibulo-ocular responses to sinusoidal rotation were symmetrical while the neck was immobilised and asymmetric when it moved freely. It is suggested that the cervical nystagmus seen in this case was the result of removal of cerebellar inhibition upon the cervico-ocular reflex and that abnormal interaction of cervical and vestibular inputs could have played a role in the patient's unsteadiness.