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. In the laboratory, this is typically studied by presenting a striped pattern that moves too fast to be seen, so that, when the eyes do not move, it appears as a homogeneous surface. But when the participant makes an eye movement in the same direction as the pattern movement, the velocity of the eye movement briefly matches that of the pattern movement. As a result, the pattern, which is normally invisible, briefly becomes stabilized on the retina, and consequently becomes visible. This phenomenon is known as
intrasaccadic perception. Outside of the laboratory, you can experience this as well, for example when riding on a
train or on the lower deck of a
bus. 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. ==See also==