Saccadic eye movements

A saccade (sakad′ik – French, twitch, jerk) is a quick, simultaneous movement of both eyes between two or more phases of fixation in the same direction. Saccadic eye movements are extremely fast voluntary movements of the eyes, allowing them to accurately “refix” on an object in the visual field, and change retinal foci from one point to another. Some Saccadic eye movements can be involuntary.

Saccades are one of the fastest movements produced by the human body with peak angular speeds of the up to 900°/s. An unexpected stimulus can commence a saccade in about 200 milliseconds (ms), and last from about 20–200 ms, depending on their amplitude. 20–30 ms is typical movement for language reading.

We do not look at the world with fixed steadiness, although our brain tells us otherwise. Our eyes move around, locating interesting parts of the scene and building a mental map in three-dimensions. Our eyes saccade, or jerk/twitch quickly, stop, scan, and then move again. The fovea (the high-resolution portion of vision, 1-2 degrees of vision) is one of the main reasons for Saccadic eye movements – we must move our eyes to resolve objects in our minds.

Saccadic masking
One of the most interesting points about Saccadic eye movements involves what we don’t perceive as our eyes move. One would think that no information is passed through the optic nerve to the brain while the eyes move in saccade, that is at least our perception experience, but that’s not correct. Saccadic masking or saccadic suppression begins just before your eyes move and keeps us from experience a blurred or smeared image. You can experience the saccadic masking effect with a very simple experiment: look in a mirror, look at your left eye, then change your gaze to look at your right eye – you won’t perceive any movement of your eyes, which is evidence that the optic nerve has momentarily ceased transmitting or that the brain just refuses to process the transmission.

Spatial updating and Trans-saccadic perception
One of the continually amazing things about perception is that our brain often perceives information that isn’t there. Spatial updating occurs when you see an object just before a saccade, and allows you to “make another saccade back to that image, even if it is no longer visible.” The brain somehow “takes into account the intervening eye movement by temporarily recording a copy of the command for the eye movement” and compares it to the remembered target image.

Trans-saccadic memory is the process of retaining information across a saccade. Neurologist think that perceptual memory is updated during saccades so information gathered across fixations can be compared and produced, creating what researches believe is a type of visual working memory.

Saccadic Dysfunction
There are a series of disorders that can produce abnormal eye movements. One is Nystagmus (also known as “dancing eyes”) a condition of involuntary eye movement (side to side, up and down, and other) that may reduce or limit vision.

Web Development Application
The understanding of saccadic eye movements has had a remarkable impact on Web usability in the form of eye tracking studies. By employing technology that monitors eye movements that can pinpoint precisely where a user is looking on a page, usability testers can study and better understand how people interact with text or online documents.

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