Relationship Between Saccadic Masking, Saccadic Movements and Motion Blur
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 artificially stabilised to get rid of motion blur and thus finish the saccadic masking. A simple method which does not require special apparatus can be observed when riding on a train or on the lower deck of a bus. In this experiment, the situation is actually reversed, as 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 - just as if one was 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 the left and right portions of the track. Changing the point of gaze is done as saccades. If, due to the car's motion, the track is 'escaping' to one's left, a left-going saccade will try to 'catch up' with the track's motion.
Saccadic velocity, plotted against 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 the 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 the track closely enough. 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. This situation does not 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 the 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 -- 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. Although it is not known exactly how long a retinal image snapshot takes, it is assumed here that it is less than 10 ms.
| Initially: | 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 0 deg/s, relative motion of track on the retina starts falling |
| T+0.199s: | angular eye velocity rises, and hits 300 deg/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. |
Read more about this topic: Saccadic Masking
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