Direct visualisation of gaze and hypometric saccades in cerebellar patients during visually guided stepping.

Four patients suffering from primary cerebellar degeneration and healthy matched controls undertook a test of functional mobility that demanded precise foot placement at each step. Vertical and horizontal eye movements were measured (using a head mounted eye tracking system) together with footfall patterns. Healthy subjects stepped accurately onto all targets and produced a clear pattern of saccadic eye movements, fixating each target in the sequence just prior to footlift. Still video frames, showing direction of gaze while walking, provide direct visual confirmation that these saccades serve to transfer gaze between successive targets in the walkway sequence. The planning of the saccade to the next target probably provides the locomotor control system with information useful for planning the corresponding (and shortly following) step. Cerebellar patients showed characteristic locomotor and oculomotor deficits. Dysmetric saccades to fixate footfall targets were seen in 39% of steps. Analysis confirms that these multi-saccadic eye movements include an initial hypometric saccade, which undershoots the target, followed by one or more additional saccades in the same direction. Direct visualisation of gaze at the end of a saccadic sequence confirms that these additional saccades are indeed corrective resulting in a foveal image of the footfall target.

Alcohol affects eye movements essential for visually guided stepping.

BACKGROUND: To understand how and why alcohol intoxication affects visually guided stepping, the eye movements and performance of 6 subjects (aged 22-35 years) were monitored as they progressed along a pathway of 18 irregularly placed stepping stones before and after consumption of an acute oral dose of alcohol. METHODS: Horizontal eye movements were measured with infrared reflectometry; footfall on or off target was monitored via copper fabric soles stuck to subjects' footwear. Breath alcohol concentration was monitored with an Alco-Sensor III breathalyzer. RESULTS: After alcohol loading, both locomotor and oculomotor deficits were evident. All subjects increased their step cycle duration-with prolonged stance, swing, and double support phases-and occasionally missed footfall targets. A large proportion of saccades made to fixate successive stepping stones were inaccurate and were accompanied by one or more corrective saccades. These problems with looking and stepping to footfall targets tended to occur together and were comparable to those seen previously in cerebellar patients undertaking the same task. CONCLUSIONS: The fact that healthy subjects acutely intoxicated by alcohol show symptoms of cerebellar dysfunction suggests that alcohol acutely and adversely affects the cerebellar contribution to performance of visually guided movements.

Rehearsal by eye movement improves visuomotor performance in cerebellar patients.

In order to assess the effect of rehearsal by eye movement alone on visuomotor performance, the eye movements and visually guided stepping of two cerebellar patients were monitored before and after a first and second batch of eye-movement rehearsals, in which patients made saccadic eye movements to the first 6 footfall targets (in a sequence of 18) whilst standing stationary at the start of the walkway. There was a marked improvement in oculomotor and locomotor performance following the second batch of eye-movement rehearsal. Both patients showed reduced occurrence of saccadic dysmetria, evident as a significant increase in the proportion of single to multi-saccadic eye movements (from 46 to 77% for DB and from 75 to 94% for TP). This was accompanied by increased regularity and accuracy of stepping in both patients, and decreased stance and double support phase durations (one patient only). Separate testing confirmed that these improvements in eye movements and stepping did not result from simple repetition of the task. This is the first demonstration of a technique--rehearsal by eye movement--that improves the visuomotor performance of cerebellar patients. It is compelling evidence for our proposal that during visually guided stepping the locomotor control system is dependent on assistance from the oculomotor control system.

Evidence for interactive locomotor and oculomotor deficits in cerebellar patients during visually guided stepping.

Eight patients suffering from primary cerebellar degenerative diseases undertook a walkway task, demanding precise foot placement at each step, and a visual fixation task, requiring only eye movements. Step cycle and horizontal eye movements were recorded throughout the tasks and compared to those of healthy adults (including age- and sex-matched controls). Cerebellar patients displayed both locomotor and oculomotor deficits. Increases in duration of the stance, swing and double support phases of the step cycle were all shown to contribute to ataxic gait. Dysmetric saccades to fixate the footfall targets were seen more frequently in patients than in controls. These hypometric saccades were followed by one or more corrective saccades (patients: >45% accompanied by one or more corrective saccades; controls: <10% accompanied by a single corrective saccade). Similarities between the oculomotor deficits displayed by patients during the visual fixation task and when walking indicate that the latter are not merely a consequence of ataxic gait. The existence of several links between these locomotor and oculomotor deficits provides evidence for considerable interaction between the two control systems in the production of patterned eye and stepping movements. These results also suggest that the cerebellum plays an active role in the co-ordination of visually guided eye and limb movements during visually guided stepping.