In addition, the human eye is in a constant state of vibration, oscillating back and forth at a rate of about 30-70 Hz. These microsaccades are tiny movements, roughly 20 arcseconds in excursion and are completely imperceptible under normal circumstances. They serve to refresh the image being cast onto the rod cells and cone cells at the back of the eye. Without microsaccades, staring fixedly at something would cause the vision to cease after a few seconds since rods and cones only respond to a change in luminance.
Saccades are the fastest movements produced by the human body. The peak angular speed of the eye during a saccade reaches up to 1000°/sec in monkeys (somewhat less in humans). Saccades to an unexpected stimulus normally take about 200 milliseconds to initiate and then last from about 20 to 200 milliseconds, depending on their amplitude. Under certain laboratory circumstances the latency of saccade production can be cut nearly in half (express saccades).
The amplitude of a saccade is the angular distance that the eye travels during the movement. For amplitudes up to about 60 degrees, the velocity of a saccade linearly depends on the amplitude (the so called "saccadic main sequence"). For instance, a 10° amplitude is associated with a velocity of 300°/sec, and 30° is associated with 500°/sec. In saccades larger than 60 degrees, the peak velocity starts to plateau (non-linearly) toward the maximum velocity attainable by the eye.
Saccades may rotate the eyes horizontally or vertically, or in any oblique direction to change gaze direction (the direction of sight that corresponds to the fovea), but normally saccades do not rotate the eyes torsionally. Torsion can be defined as clockwise or counterclockwise rotation around the line of sight when the eye is at its central primary position. Defined this way, Listing's law says that when the head is motionless, torsion is kept at zero.
Head-fixed saccades can have amplitudes of up to 90° (from one edge of the oculomotor range to the other), but in normal conditions saccades are far smaller, and any shift of gaze larger than about 20° is accompanied by a head movement. During such gaze saccades, first the eye produces a saccade to get gaze on target, whereas the head follows more slowly and the vestibulo-ocular reflex causes the eyes to roll back in the head to keep gaze on the target. During these head movements Listing's law is no longer obeyed.
Saccades are measured or investigated in four ways:
Without the use of objective recording techniques, it may be very difficult to distinguish between these conditions.
Eye movement measurements are also used to investigate psychiatric disorders. For example, ADHD is characterized by an increase of antisaccade errors and an increase in delays for visually guided saccade.
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 nor any evidence that the optic nerve has momentarily ceased transmitting. Due to saccadic masking, the eye/brain system not only hides the eye movements from the individual but also hides the evidence that anything has been hidden. Of course, a second observer watching the experiment will see the subject's eyes moving back and forth. The function's main purpose is to prevent smearing of the image.
When a visual stimulus is seen before a saccade, subjects are still able to make another saccade back to that image, even if it is no longer visible. This shows that the brain is somehow able to take into account the intervening eye movement. 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. This is called spatial updating. Neurophysiologists who have recorded from cortical areas for saccades during spatial updating have found that memory related signals get remapped during each saccade.
It is also thought that perceptual memory is updated during saccades so that information gathered across fixations can be compared and synthesized. However, the entire visual image is not updated during each saccade, only 3-4 features or objects if they are attended to. Some scientists believe that this is the same as visual working memory, but as in spatial updating the eye movement has to be accounted for. The process of retaining information across a saccade is called trans-saccadic memory and the process of integrating information from more than one fixation is called trans-saccadic integration.
In birds, saccadic eye movements serve a further function. The avian retina is highly developed. It is thicker than the mammalian retina and has a higher metabolic activity, but it lacks proper vasculature. Therefore, the retinal cells must obtain nutrients via diffusion through the choroid and from the vitreous humor. The pecten is a specialised structure in the avian retina. It is a highly vascular structure that projects into the vitreous humor. Experimentally, it has been shown that during saccadic eye oscillations (which occupy up to 12% of avian viewing time), the pecten acts as an agitator, propelling perfusate towards the retina. Thus, in birds, saccadic eye movements appear to be important in retinal nutrition and respiration.
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