Saccade

Saccades are one of the fastest movements produced by the human eye (blinks may reach even higher peak velocities).

These saccades are generated by a neuronal mechanism that bypasses time-consuming circuits and activates the eye muscles more directly.

The burst neurons implement bang-bang control: they are either completely inhibited, or firing at its full rate of ~1000 Hz.

Each fixation involves binocularly coordinated movements of the eyes to acquire the new target in three dimensions: horizontal and vertical, but also in-depth.

[23] When making an upward saccade, the eyes diverged to be aligned with the most probable uncrossed disparity in that part of the visual field.

On the other way around, when making a downward saccade, the eyes converged to enable alignment with crossed disparity in that part of the field.

Pathological slow phases may be due to either an imbalance in the vestibular system or damage to the brainstem "neural integrator" that normally holds the eyes in place.

[citation needed] On the other hand, opsoclonus or ocular flutter are composed purely of fast-phase saccadic eye movements.

[24][25] Paroxysmal eye–head movements, termed aberrant gaze saccades, are an early symptom of GLUT1 deficiency syndrome in infancy.

[26][non-primary source needed] When the brain is led to believe that the saccades it is generating are too large or too small (by an experimental manipulation in which a saccade-target steps backward or forward contingent on the eye movement made to acquire it), saccade amplitude gradually decreases (or increases), an adaptation (also termed gain adaptation) widely seen as a simple form of motor learning, possibly driven by an effort to correct visual error.

[27] In these cases, it was noticed that the patients would make hypometric (small) saccades with the affected eye, and that they were able to correct these errors over time.

This led to the realization that visual or retinal error (the difference between the post-saccadic point of regard and the target position) played a role in the homeostatic regulation of saccade amplitude.

A person may observe the saccadic masking effect by standing in front of a mirror and looking from one eye to the next (and vice versa).

The subject will not experience any movement of the eyes or any evidence that the optic nerve has momentarily ceased transmitting.

It is thought that the brain does this by temporarily recording a copy of the command for the eye movement, and comparing this to the remembered image of the target.

[citation needed] It is also thought that perceptual memory is updated during saccades so that information gathered across fixations can be compared and synthesized.

[31] Therefore, while saccades serve in humans and other primates to increase the effective visual resolution of a scene, there must be additional reasons for the behavior.

It is thicker than the mammalian retina, has a higher metabolic activity, and has less vasculature obstruction, for greater visual acuity.

Experiments show that, during saccadic eye oscillations (which occupy up to 12% of avian viewing time), the pecten oculi acts as an agitator, propelling perfusate (natural lubricants) toward the retina.

Thus, in birds, saccadic eye movements appear to be important in retinal nutrition and cellular respiration.