metacontrast

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met·a·con·trast

(met'ă-kon'trast),
Inhibition of the brightness of illumination when an adjacent visual field is illuminated.

metacontrast

This is an apparently paradoxical phenomenon because it consists of a reduction in subjective brightness of a flash of light which is caused by a second flash following shortly afterward in an adjacent region of the visual field. The effect depends upon the duration, intensity, surface areas of the two flashes, the retinal area stimulated, and particularly the interval of time between the two flashes. The phenomenon appears most clearly with an interval of about 0.1s and disappears when that interval reaches 0.3-0.4s. Syn. backward masking (this term is used to indicate when the test stimulus and the masking stimulus overlap spatially). A flash of light can also be made to appear slightly less bright when it is preceded by another flash in an adjacent region of the visual field and the interval of time is of the order of 0.05s. This second phenomenon is called paracontrast. Syn. forward masking (this term is used when the test stimulus and the masking stimulus overlap spatially). See masking.
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Moreover, it is plausible to believe that the intensity of the CSs may not be strong enough to trigger a conditioned response (CR) when subthreshold techniques and backward masking procedures are used.
To avoid contingency awareness of the S1-S2 relationship, S1 was presented below the perceptual threshold, using a backward masking procedure.
Backward masking works by presenting subjects with an irrelevant "mask" image that immediately follows an extremely brief exposure to a face, which is thought to terminate the brain's ability to further process the face and prevent it from reaching awareness.
A comparison of backward masking of faces in expression and gender identification
Indeed, in an experiment in which participants had to perform two different tasks on T1 and T2 (T1 was a diagonal line segment whose orientation was to be reported and T2 was a letter to be identified), Kawahara, Zuvic, Enns, and Di Lollo (2003) reported that such a task switching between T1 and T2 was sufficient to produce the AB without backward masking of T2.
Together with other visual, pattern, backward masking tasks, IT may tap these functions to different extents, depending on whether intellectual disability or childhood developmental or aging effects are involved.
It is of interest that an IT task using this dynamic backward masking principle ha recently been developed and found to correlate with IQ in both children and young adults (Knibb, 1992).
The first investigations of subliminal semantic priming using backward masking appeared in the early 1980s (e.
Egan (1986) suggested that apparent motion effects occur in all backward masking methodologies and that the influence of strategies has not been systematically documented, and argued that further research is required before it would be sensible to suggest that strategy use may confound IT as a measure of perceptual speed.
To date, a number of attempts have been made to develop a backward masking procedure that would eliminate the apparent movement effect (Chaiken & Young, 1993; Evans & Nettelbeck, 1993; Knibb, 1992).
Consideration of the backward masking procedure used to define IT suggests that the IT measure shares something of this characteristic.
VCD and VMD, however, require a broader attentional focus than IT and do not involve backward masking.

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