Saccadic masking, also known as (visual) saccadic suppression, is the phenomenon in

visual perception Visual perception is the ability to detect light and use it to form an image of the surrounding Biophysical environment, environment. Photodetection without image formation is classified as ''light sensing''. In most vertebrates, visual percept ...

where the brain selectively blocks visual processing during

eye movements Eye movement includes the voluntary or involuntary movement of the eyes. Eye movements are used by a number of organisms (e.g. primates, rodents, flies, birds, fish, cats, crabs, octopus) to fixate, inspect and track visual objects of i ...

in such a way that neither the motion of the

eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

(and subsequent

motion blur Motion blur is the apparent streaking of moving objects in a photograph or a sequence of frames, such as a film or animation. It results when the image being recorded changes during the recording of a single exposure, due to rapid movement or l ...

of the image) nor the gap in visual perception is noticeable to the viewer. The phenomenon was first described by Erdmann and Dodge in 1898, when it was noticed during unrelated experiments that an observer could never see the motion of their own eyes. This can easily be duplicated by looking into a mirror, and looking from one eye to another. The eyes can never be observed in motion, yet an external observer clearly sees the motion of the eyes. The phenomenon is often used to help explain a

temporal illusion In psychology and neuroscience, time perception or chronoception is the subjective experience, or sense, of time, which is measured by someone's own perception of the duration of the indefinite and unfolding of events. The perceived time interval b ...

by the name of chronostasis, which momentarily occurs following a rapid eye-movement.

Mechanism

saccade In vision science, a saccade ( ; ; ) is a quick, simultaneous movement of both Eye movement (sensory), eyes between two or more phases of focal points in the same direction. In contrast, in Smooth pursuit, smooth-pursuit movements, the eyes mov ...

is a fast

motion, and because it is a motion that is optimised for speed, there is inevitable blurring of the image on the

retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...

, as the retina is sweeping the visual field. Blurred retinal images are not of much use, and the eye has a mechanism that "cuts off" the processing of retinal images when it becomes blurred. This phenomenon is called saccadic masking or saccadic suppression. There were two major types of saccadic masking claimed: flash suppression (the inability to see a flash of light during a saccade) and saccadic suppression of image displacement (characterized by the inability to perceive whether a target has moved or not during a saccade). Testing since then has revealed that these two theories may not be correct. Within-saccade movement detection was proven and detailed in a paper by Richard Schweitzer and Martin Rolfs at Humboldt University in Berlin. Because saccadic suppression starts before the actual onset of the saccade, it cannot be triggered by retinal motion and must be centrally activated by the brain. Supporting this idea, a significant reduction of the cortical signals retinotopically encoding stimuli briefly presented immediately before the execution of a saccade has been found as early as in primary visual cortex.

Intrasaccadic perception: relationship with 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. 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 A train (from Old French , from Latin">-4; we might wonder whether there's a point at which it's appropriate to talk of the beginnings of French, that is, when it wa ... , from Latin , "to pull, to draw") is a series of connected vehicles th ...

or on the lower deck of a

bus A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a motor vehicle that carries significantly more passengers than an average car or van, but fewer than the average rail transport. It is most commonly used ...

. 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.

References

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External links

Selected publications of David Burr
Visual system Vision 1890s neologisms

Mechanism

Intrasaccadic perception: relationship with saccadic movements and motion blur

See also

References

External links