Fixational eye movements are small displacements of the eyeballs which ensure that vision does not fade during fixation. There are three classes — tremor (the smallest), drifts and microsaccades (the largest). Traditionally, fixational eye movements have been studied with retinal stabilization techniques.
The functional role of fixational eye movements in perception is controversial. Microsaccades have received the most attention, and there is evidence for and against their role in perception. However, the existence of microsaccade-related activity in the visual pathway indicates that they might be involved in perception.
Several studies have focused on unravelling the neural code of the responses elicited by the different fixational eye movements. Microsaccades have been most extensively studied. The burst-like nature of the activity elicited by microsaccades has attracted particular attention, but its physiological meaning remains uncertain.
Despite the existence of fixational eye movements, perception is stable. This fact has been explained by a hypothetical microsaccadic suppression mechanism, and several models for such a mechanism have been proposed. The underlying phenomena have yet to be discovered.
Fixational eye movements can be modulated by environmental (illumination conditions) and cognitive (attention) factors. Other phenomena also seem to exert a modulatory influence on these movements, but such factors need to be investigated.
Among the outstanding questions in this field are: which circuits control fixational eye movements; what is the neural basis of microsaccadic suppression; what are the mechanisms that underlie the effect of cognitive factors during fixation; and what is the meaning of the burst firing that is correlated with microsaccades?
Our eyes continually move even while we fix our gaze on an object. Although these fixational eye movements have a magnitude that should make them visible to us, we are unaware of them. If fixational eye movements are counteracted, our visual perception fades completely as a result of neural adaptation. So, our visual system has a built-in paradox — we must fix our gaze to inspect the minute details of our world, but if we were to fixate perfectly, the entire world would fade from view. Owing to their role in counteracting adaptation, fixational eye movements have been studied to elucidate how the brain makes our environment visible. Moreover, because we are not aware of these eye movements, they have been studied to understand the underpinnings of visual awareness. Recent studies of fixational eye movements have focused on determining how visible perception is encoded by neurons in various visual areas of the brain.
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We thank Y. Duran for technical assistance and X. G. Troncoso for comments on the manuscript. This study was funded by the Barrow Neurological Foundation and the National Eye Institute.
The authors declare no competing financial interests.
- ENTOPTIC STRUCTURES
Structures within the eye. When these become visible they give rise to entoptic images.
Involuntary rhythmical oscillations of one or both eyes.
The retinal region with maximal concentration of photoreceptors, where visual acuity is highest.
- FLICKER FUSION THRESHOLD
The rate of flicker at which the flickering stimulus being viewed appears non-flickering (approximately 50–60 Hz in humans).
Coordinated in the two eyes.
- MAIN SEQUENCE
The linear correlation between saccadic speed and amplitude.
- RECEPTIVE FIELD
The area of the sensory space in which stimulus presentation leads to the response of a particular sensory neuron.
- BINOCULAR DISPARITY
The difference in gaze position of the two eyes that gives rise to stereovision.
- VISUAL MASKING
An illusion in which a normally visible target object is rendered invisible by a mask object.
- NEURAL CODE
The language, expressed as a pattern of neuronal impulses, that neurons use to send information to each other.
- EXTRARETINAL ACTIVATION
Responses in the visual system that occur in the absence of visual stimuli (such as one might see due to feedback from motor areas).
- SPATIAL SUMMATION
The way in which non-overlapping retinal stimulation is integrated within dendrites to produce a stronger neuronal response.
Clusters of action potentials.
- TEMPORAL SUMMATION
The way in which non-simultaneous synaptic events combine in time. One of the basic elements of synaptic integration.
- LONG, TIGHT BURST
A type of burst consisting of a large number of spikes that occur in rapid succession.
- SCOTOPIC CONDITIONS
Dim light conditions in which only the rods of the retina are active.
- PHOTOPIC CONDITIONS
Bright light conditions in which only the cones of the retina are active.
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Martinez-Conde, S., Macknik, S. & Hubel, D. The role of fixational eye movements in visual perception. Nat Rev Neurosci 5, 229–240 (2004). https://doi.org/10.1038/nrn1348
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