Visual Fixation

Fixation or visual fixation is the maintaining of the visual gaze on a single location. Humans (and other animals with a fovea) typically alternate saccades and visual fixations, the notable exception being in smooth pursuit, controlled by a different neural substrate that appear to have developed for hunting prey. 

Contrary to the old view, fixations are not simply the absence of eye movement; they are made up of much smaller movements. This is because should your eye be truly still, you would not actually be able to see as no neurons would fire. This has been demonstrated by retinal stabilization studies; a method of this is securing a contact lenses with a target image upon the eye; the visual stimulus moves in the same direction, speed and amplitude as the eye, so that the retinal image remains stable as eye movements are cancelled out.

After a few seconds, the image on the retina fades, thus showing that eye movements are needed to maintain visibility; there are three types of eye movements that support fixations.The photoreceptors need continuous stimulation to maintain visibility on a target, and there are three movements that make up fixations that achieve this. They are microsaccades, ocular drifts, and ocular microtremor. 

Microsaccades are miniature saccades, movement that corrects ocular drift and prevents fading. They are small, jerk-like, involuntary eye movements, similar to miniature versions of voluntary saccades. They typically occur during prolonged visual fixation (of at least several seconds), not only in humans, but also in animals with foveal vision (primates, cats, etc.). Microsaccade amplitudes vary from 2 to 120 arcminutes. 

It is notable that microsaccades have limited correctional accuracy, and also occur regardless of ocular drift. Furthermore, it was found that microsaccades are naturally suppressed by participants in complex attentional vision tasks such as threading a needle, suggesting that microsaccadic activity varies alongside the levels of focus, thus opening up new questions on how necessary microsaccades are. 

Ocular drifts are slow, random eye movements that drift away from the target of interest, and are said to be a consequence of neural noise.

Ocular microtremor is a constant, physiological, high frequency (peak 80Hz), low amplitude (estimated circa 150-2500nm) tremor of the eye, constant during the fixation period. Aside from stimulating the photoreceptors, the role of tremors in vision remains unclear. Although many aspects of these collective movements are under investigation to determine the purpose in vision, it is widely concluded that ocular drifts, micro saccades and tremors are at least necessary for maintaining visibility during fixations.