Visual masking is a phenomenon of visual perception. It occurs when the visibility of one image, called a target, is reduced by the presence of another image, called a mask. The target might be invisible or appear to have reduced contrast or lightness. There are three different timing arrangements for masking: forward masking, backward masking, and simultaneous masking. In forward masking, the mask precedes the target. In backward masking the mask follows the target. In simultaneous masking, the mask and target are shown together. There are two different spatial arrangements for masking: pattern masking and metacontrast. Pattern masking occurs when the target and mask locations overlap. Metacontrast masking occurs when the mask does not overlap with the target location.
Factors affecting visual masking
Target-to-mask spatial separation Suppression can be seen in both forward and backward masking when there is pattern masking, but not when there is metacontrast. Simultaneous masking, however, will produce facilitation of target visibility during pattern masking. Facilitation also comes about when metacontrast is combined with either simultaneous or forward masking. This is because it takes time for the mask to reach the target's location through lateral propagation. As the target gets further from the mask, the time required for lateral propagation increases. Thus, the masking effect will increase as the mask gets closer to the target.
Target-to-mask temporal separation As the time difference between the target and the mask increases, the masking effect decreases. This is because the integration time of a target stimulus has an upper limit 200 ms, based on physiological experiments and as the separation approaches this limit, the mask is able to produce less of an effect on the target, as the target has had more time to form a full neural representation in the brain. Polat, Sterkin, and Yehezkel went into great detail in explaining the effect of temporal matching between target input and lateral propagation of the mask. Based on data from previous single-unit recordings, they concluded that the time window for any sort of efficient interaction with target processing is 210 to 310 ms after the target's appearance. Anything outside of this window would fail to cause any sort of masking effect. This explains why there is a masking effect when the mask is presented 50 ms after the target, but not when the inter-stimulus interval between mask and target is 150 ms. In the first case, mask response would propagate to the target location and be processed with a delay of 260 to 310 ms, whereas the ISI of 150 would result in a delay of 410 to 460 ms.
Monoptic vs. dichoptic visual masking In dichoptic visual masking, the target is presented to one eye and the mask to the other, whereas in monoptic visual masking, both eyes are presented with the target and the mask. It was found that the masking effect was just as strong in dichoptic as it was in monoptic masking, and that it showed the same timing characteristics.
Possible neural correlates There are multiple theories surrounding the neural correlates of masking, but most of them agree on a few key ideas. First, backward visual masking comes about from suppression of the target's "after-discharge", where the after-discharge can be thought of as the neural response to the target's termination. Impairments in backward masking have been consistently found in those with schizophrenia as well as in their unaffected siblings, thus suggesting that the impairments might be an endophenotype for schizophrenia. Forward masking, on the other hand, is correlated to the suppression of the target's "onset-response", which can be thought of as the neural response to the target's appearance.
Two-channel model Originally proposed by Breitmeyer and Ganz in 1976, the original version of this model stated that there were two different visual information channels- one being fast and transient, the other being slow and sustained. The theory asserts that each stimulus travels up each channel, and both channels are necessary for proper and full processing of any given stimulus. It explained backward masking by saying that the neural representation of the mask would travel up the transient channel and intercept the neural representation of the target as it travelled up the slower channel, suppressing the target's representation and decreasing its visibility. One problem with this model, as proposed by Macknik and Martinez-Conde, is that it predicts masking to occur as a function of how far apart, temporally, the stimulus onset is. However, Macknik and Martinez-Conde showed that backward masking is actually more dependent on how far apart stimulus termination is.
Retino-cortical dynamics model Breitmeyer and Ögmen modified the two-channel model in 2006, renaming it to the retino-cortical dynamics (RECOD) model in the process. Their main proposed modification was that the fast and slow channels were actually feed forward and feedback channels, instead of the magnocellular and parvocellular retino-geniculocortical pathways, which is what had previously been proposed. Thus, according to this new model, backward masking is caused when feed forward input from the mask interferes with the feedback coming from the higher visual areas' response to the target, thus reducing visibility.
Lamme's recurrent feedback hypothesis of visual awareness and masking This model proposes that backward masking is caused by an interference with feedback from higher visual areas. In this model, target duration is irrelevant because masking is supposed to occur as a function of feedback, which is generated when the target appears on screen. Lamme's group further supported their model when they described that the surgical removal of the extrastriate cortex in monkeys leads to a reduction of area V1 late responses.
Lateral inhibition circuits Proposed by Macknik and Martinez-Conde in 2008, this theory proposes that masking can be explained almost entirely by feed forward lateral inhibition circuits. The idea is that the edges of the mask, if positioned in close proximity to the target, may inhibit the responses caused by the edges of the target, inhibiting perception of the target.
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