Our eyes are never at rest. Humans are not aware of continually performing small fixational eye movements in the period in which visual information is acquired and processed.
Investigation of the visual functions of fixational eye movements is an important component of research at the Active Perception Laboratory
The Active Perception Laboratory possesses advanced
tools to eliminate and manipulate the fixational motion of the retinal image. Many of these tools are custom made in the laboratory.
Microsaccades and foveal vision. Performance in high acuity tasks drops without precise microsaccade gaze shifts. Read more
Abstract: Humans rely on the fovea, the small region of the retina where receptors are most densely packed, for seeing fine spatial detail. Outside the fovea, it is well established that a variety of visual functions progressively decline with eccentricity. In contrast, little is known about how vision varies within the central fovea, as incessant microscopic eye movements prevent isolation of adjacent foveal locations. Using a new method for restricting visual stimulation to a selected retinal region, we examined the discrimination of fine patterns at different eccentricities within the foveola. We show that high-acuity judgments are impaired when stimuli are presented just a few arcminutes away from the preferred retinal locus of fixation. Furthermore, we show that this dependence on eccentricity is normally counterbalanced by the occurrence of precisely directed microsaccades, which bring the preferred fixation locus onto the stimulus. Thus, contrary to common assumptions, vision is not uniform within the foveola, but targeted microscopic eye movements compensate for this lack of homogeneity. Our results reveal that microsaccades, like larger saccades, enable examination of the stimulus at a finer level of detail and suggest that a reduced precision in oculomotor control may be responsible for the visual acuity impairments observed in various disorders.
The movie illustrates an example of how eye movements during the course of a trial in which the stimuli appeared at 15 arcmin eccentricity from the preferred locus of fixation. Blue traces indicate periods of drift and red segments indicate a microsaccedes.
M. Poletti, C. Listorti and M. Rucci (2013),
Microscopic eye movements compensate for nonhomogeneous vision within the fovea,
1691-1695. (See also Dispatch )
Spatial function of microsaccades. During natural fixation, microsaccades precisely relocate gaze to regions of interest. Read more
Abstract: The image on the retina is never stationary. Microscopic relocations of gaze, known as microsaccades, occur even during steady fixation. It has long been thought that microsaccades enable exploration of small regions in the scene in the same way saccades are normally used to scan larger regions. This hypothesis, however, has remained controversial, as it is believed that microsaccades are suppressed during fine spatial judgments. We examined the eye movements of human observers in a high-acuity visuomotor task, the threading of a needle in a computer-simulated virtual environment. Using a method for gaze-contingent display that enables accurate localization of the line of sight, we found that microsaccades precisely move the eye to nearby regions of interest and are dynamically modulated by the ongoing demands of the task. These results indicate that microsaccades are part of the oculomotor strategy by which the visual system acquires fine spatial detail.
The movie illustrates an example of how the gaze location (blue cross) is moved back and forth between the tip of the thread and the eye of the needle during the task.
H.-K. Ko, M. Poletti and M. Rucci (2010),
Microsaccades precisely relocate gaze in a high visual acuity task,
1549–1553. (See also News and Views )