Abstract
Among the eye movements during fixation, the function of small saccades occuring quite commonly at fixation is still unclear. It has been reported that a substantial number of these microsaccades seem to occur in only one of the eyes. The aim of the present study is to investigate microsaccades in monocular stimulation conditions. Although this is an artificial test condition which does not occur in natural vision, this monocular presentation paradigm allows for a critical test of a presumptive monocular mechanism of saccade generation. Results in these conditions can be compared with the normal binocular stimulation mode. We checked the statistical properties of microsaccades under monocular stimulation conditions and found no indication for specific interactions for monocularly detected small saccades, which might be present if they were based on a monocular physiological activation mechanism.
Highlights
When the eyes are fixating a visual target, they are continuously moving in several ways which can be differentiated into three categories: (1) Tremor is a fast irregular movement with a high frequency spectrum above 50 Hz and an amplitude of about the size of one cone in the fovea, (2) drifts have a low frequency and an amplitude of a dozen cones, and (3) microsaccades are, by definition, those saccades which are not intended to bring a new target into the center of the fovea, but rather occur when maintaining the current fixation
(2) In the monocular condition only one of the eyes was presented with a fixation target while the fellow eye viewed a blank field without any fixation target
Though the microsaccade detection algorithm does not always detect them in both eyes we argue that it still seems more probable that this a weakness of the detection procedure than a real physiological phenomenon
Summary
When the eyes are fixating a visual target, they are continuously moving in several ways which can be differentiated into three categories: (1) Tremor is a fast irregular movement with a high frequency spectrum above 50 Hz and an amplitude of about the size of one cone in the fovea, (2) drifts have a low frequency and an amplitude of a dozen cones, and (3) microsaccades are, by definition, those saccades which are not intended to bring a new target into the center of the fovea, but rather occur when maintaining the current fixation. Microsaccades happen quite frequently, about two per second on average With respect to their kinematic traits, they resemble other saccades of the same size (Zuber, Stark, & Cook, 1965), e.g. they have the same relation of width and velocity (”main sequence”). As shown by the broad review of Martinez-Conde, Macknik, and Hubel (2004), possible physiological effects include the prevention of fading in the visual periphery, fine control of fixation position or enhancement of acuity. For some of these effects, experimental support has been found, e.g. MartinezConde et al (2006) showed that the fading of Troxler figures is correlated to lowered microsaccade activity before the loss of perception. How such integration could be accomplished in the real physiological system
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