Dr. James T. Enns
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Object substitution is a type of visual masking that occurs when a briefly presented display is followed by several small dots that surround the location of a target image, but do not touch it. An example of a display sequence from a typical object substitution masking experiment is shown in the figure. The visual display consists of a number of different shapes, with one shape (the circle) singled out by the four small dots that surround it. The task of the observer (also called the study participant) is to identify this shape. If the display terminates after a single brief flash of these shapes, identification accuracy of the shape surrounded by the four dots is nearly perfect. If, however, the four dots continue without interruption to a second display in which only the four dots remain on view, then identification accuracy is reduced, sometimes to the levels that are only possible by pure guessing. It is as if the square shape that is merely implied by the outline of the four dots has replaced the original target shape in the consciousness of the observer. | Object substitution is a type of visual masking that occurs when a briefly presented display is followed by several small dots that surround the location of a target image, but do not touch it. An example of a display sequence from a typical object substitution masking experiment is shown in the figure. The visual display consists of a number of different shapes, with one shape (the circle) singled out by the four small dots that surround it. The task of the observer (also called the study participant) is to identify this shape. If the display terminates after a single brief flash of these shapes, identification accuracy of the shape surrounded by the four dots is nearly perfect. If, however, the four dots continue without interruption to a second display in which only the four dots remain on view, then identification accuracy is reduced, sometimes to the levels that are only possible by pure guessing. It is as if the square shape that is merely implied by the outline of the four dots has replaced the original target shape in the consciousness of the observer. | ||
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Masking by four surrounding dots was first reported by Enns & Di Lollo (1997), who noted that it differed in several important ways from metacontrast masking, which is a reduction in visibility of a briefly presented target image that is followed by a second image that fits snugly around the contours of the target image, but does not touch it. A first point of difference was that the four dots did not act as a mask when the target location was known in advance. That is, unlike metacontrast masking, which is very effective when the target appears at the center of gaze, the four dots act as effective masks only when the observer is uncertain about the location of the target in advance of its appearance. This means that for the four dots to be an effective mask, spatial attention must be distributed over a larger area rather than focused on a single location. | Masking by four surrounding dots was first reported by Enns & Di Lollo (1997), who noted that it differed in several important ways from metacontrast masking, which is a reduction in visibility of a briefly presented target image that is followed by a second image that fits snugly around the contours of the target image, but does not touch it. A first point of difference was that the four dots did not act as a mask when the target location was known in advance. That is, unlike metacontrast masking, which is very effective when the target appears at the center of gaze, the four dots act as effective masks only when the observer is uncertain about the location of the target in advance of its appearance. This means that for the four dots to be an effective mask, spatial attention must be distributed over a larger area rather than focused on a single location. | ||
Revision as of 16:58, 8 January 2008
Object substitution is a type of visual masking that occurs when a briefly presented display is followed by several small dots that surround the location of a target image, but do not touch it. An example of a display sequence from a typical object substitution masking experiment is shown in the figure. The visual display consists of a number of different shapes, with one shape (the circle) singled out by the four small dots that surround it. The task of the observer (also called the study participant) is to identify this shape. If the display terminates after a single brief flash of these shapes, identification accuracy of the shape surrounded by the four dots is nearly perfect. If, however, the four dots continue without interruption to a second display in which only the four dots remain on view, then identification accuracy is reduced, sometimes to the levels that are only possible by pure guessing. It is as if the square shape that is merely implied by the outline of the four dots has replaced the original target shape in the consciousness of the observer.
Masking by four surrounding dots was first reported by Enns & Di Lollo (1997), who noted that it differed in several important ways from metacontrast masking, which is a reduction in visibility of a briefly presented target image that is followed by a second image that fits snugly around the contours of the target image, but does not touch it. A first point of difference was that the four dots did not act as a mask when the target location was known in advance. That is, unlike metacontrast masking, which is very effective when the target appears at the center of gaze, the four dots act as effective masks only when the observer is uncertain about the location of the target in advance of its appearance. This means that for the four dots to be an effective mask, spatial attention must be distributed over a larger area rather than focused on a single location.
