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Saccadic masking

A saccade is a fast eye motion. Because it's a motion which is optimized for speed, there is inevitable blurring of the image on the retina, as the retina is sweeping the visual field.

Blurred retinal images aren't very useful, and the eye has a mechanism that actually 'cuts off' the processing of retinal image when it's blurred. In the effect, humans become effectively blind during a saccade. This phenomenon is called saccadic masking. It doesn't mean that our vision becomes crippled due to saccadic masking -- it becomes crippled due to saccadic movement itself; the masking is there to shield our limited brain processing capacity from being bothered with an otherwise useless blurred image.

There's a beautiful experiment which demonstrates that saccadic masking is not fully related to the saccade itself. Saccadic masking starts with onset of the saccadic motion of the eye, and the onset of the associated blur. Yet, it finishes as soon as the image on the retina has stabilized -- whether due to finishing of the saccade itself, or not.

There are many ways in which the image on the retina during a saccade could be artificialy stabilized as to get rid of motion blur and thus finish the saccadic masking. A simple way which doesn't require a special apparatus can be realized when riding on a train, or on a lower deck of a bus. In this experiment, the situation is actually reversed: one starts with a blurred image, and uses the saccade itself to get rid of the blur.

Assume one is looking straight out of the train car's window at the adjacent track. If the train is moving fast enough, the track one is seeing will be just a blur -- the angular speed of the track's motion on the retina is too fast for the eye to compensate with optokinetic tracking[?]. Then, one starts looking to the left and right along the track -- like if one was were to catch something that was either speeding past on the track, or lagging behind.

Looking right and left along the adjacent track in fact means that one alternates the gaze between left and right portion of the track. Changing of the point of gaze is done as saccades. If, due to car's motion, the track is 'escaping' to one's left, a left-going saccade will try to 'catch up' with track's motion.

Saccadic velocity, plotted versus time, is a bell-shaped curve. If the peak velocity of the saccade (height of the peak of the curve) is at least as large as the angular velocity of the adjacent track, there will be at least one point in which the velocity of the eye is same as the velocity of the track. Imagine a bell shaped curve (velocity of the saccade) intersecting a horizontal line (constant velocity of the track).

For a very short period of time (about a thousandth of a second), the eye follows closely enough the track, with track's own velocity. Thus, the image on the retina gets stable for a fraction of a second. As soon as the image is stable, there is no more blur, and the saccadic suppression switches off. One should remember that this situation doesn't last long -- since a saccade doesn't have a constant velocity, very soon the eye is moving either faster or slower than the track, and the blur reappears in a course of a millisecond.

Yet, that millisecond (or so) is long enough for a snapshot of retinal image to be stored, and to enable its further processing. In another quarter of a second, after the image has been processed by the brain, one actually 'sees' the freeze-frame image of the adjacent track -- without any blur, and with a decent detail -- to to the extent that one easily notices details such as gravel, dirt in between the tracks, and so on.

A fragment of the possible timeline of the experiment follows. Note that it's not exactly known how long does a retinal image snapshot take -- it is assumed here that its less than 10 ms.

Initally: looking ahead on the adjacent track, the image on the retina moves with the tracks' angular velocity of 300 deg/s and is blurred
About T-0.1s: decision to switch gaze ('look') to behind on the adjacent track
T+0.000s: onset of the saccade: angular eye velocity starts rising from 0deg/s, relative motion of track on the retina starts falling
T+0.199s: angular eye velocity rises, and hits 300deg/s -- that matches the track velocity; the track is resting relative to the retina -- the blur ends
T+0.200s: there is no blur on the retina, the saccadic masking is off, snapshot of the image starts to be taken
T+0.202s: angular eye velocity is still rising, and exceeds the track velocity; the track starts moving on the retina -- blur resumes
T+0.205s: the snapshot starts being processed by the brain, retina 'sees' only the blur now
About T+.45s: the image has been processed and one becomes aware of it: one sees the image; at this time the saccade could have already finished.



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