Abstract
The attempt to quietly fixate at a small visual object is continuously interrupted by a variety of fixational eye movements comprising, among others, a continuum of saccadic intrusions (SI) which range in size from microsaccades with amplitudes ≤0.25° to larger refixation saccades of up to about 2°. The size and frequency of SI varies considerably among individuals and is known to increase in neurodegenerative diseases such as progressive supranuclear palsy (PSP), and amyotrophic lateral sclerosis (ALS). However, studies of ALS disagree whether also the frequency of SI increases. We undertook an analysis of SI in 119 ALS patients and 47 age-matched healthy controls whose eye movements during fixation and tests of executive functions (e.g antisaccades) had been recorded by video-oculography according to standardised procedures. SI were categorised according to their spatio-temporal patterns as stair case, back-and-forth and square wave jerks (a subcategory of back-and-forth). The SI of patients and controls were qualitatively similar (same direction preferences, similar differences between patterns), but were enlarged in ALS. Notably however, no increase of SI frequency could be demonstrated. Yet, there were clear correlations with parameters such as eye blink rate or errors in a delayed saccade task that suggest an impairment of inhibitory mechanisms, in keeping with the notion of a frontal dysfunction in ALS. However, it remains unclear how the impairment of inhibitory mechanisms in ALS could selectively increase the amplitude of intrusions without changing their frequency of occurrence.
Highlights
The attempt to steadily fixate at a visual object is continuously interrupted by a variety of fixational eye movements comprising slow drifts and small saccades ranging in amplitude from less than 0.25° ('microsaccades') to about 2°
A more detailed view of the results is provided by Fig. 3 which shows polar representations of the sample averages of vectorial amplitude (VAMP) and INTV, and of displacement rate (DISR) and frequency of occurrence (FREQ), all sorted according to vector direction, intrusion pattern and group
This will lead us to the question of how the amyotrophic lateral sclerosis (ALS) patients' deficit of inhibitory control can lead to larger saccadic intrusions (SI) amplitudes without changing SI frequency
Summary
The attempt to steadily fixate at a visual object is continuously interrupted by a variety of fixational eye movements comprising slow drifts (typically ≤ 0.07°/s) and small saccades ranging in amplitude from less than 0.25° ('microsaccades') to about 2°. Observations of microsaccades were made by Barlow (1952), Ditchburn (1953), Ratliff (1950) and Yarbus (1967) who demonstrated saccades as small as 0.02°. Microsaccades may be to some degree spontaneous events but are thought to offset the effect of drift movements. They are not necessary for precise fixation because drift can be controlled in a smooth, non-saccadic way (for a review of this topic see Collewijn & Kowler, 2008). Other authors, while acknowledging that microsaccades counteract image fading, question whether they are essential for the prevention of fading. (Collewijn et al, 2008; Poletti & Rucci, 2010)
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