Visual Masking

Visual masking refers to the reduced visibility of one stimulus, called the target, because of the presence of another stimulus, called the mask. As the generality of this definition suggests, visual masking is not a unitary phenomenon. Instead, a broad range of masking effects exists depending on the types of target and mask stimuli as well as their spatial and temporal relationship. This entry provides a classification of different masking types and illustrates their use in the study of vision and cognition.

Typology of Masking

Typically, the target and the mask are briefly presented (e.g., 10 milliseconds [ms]) and three types of masking occur according to their temporal order. Let us denote by stimulus onset asynchrony (SOA) the time delay between the onset of the target and the onset of the mask. When the target is presented before the mask (by convention negative SOAs), we have forward masking, and when the opposite holds, we have backward masking. The case when the target and mask are presented with SOA = 0 is called simultaneous masking.

The most basic form of visual masking, called masking by light, occurs when the mask stimulus is a spatially extended uniform field of light. Depending on the type of the target, masking by light can be divided into two subtypes: masking of light by light (the target is, like the mask, a spatially uniform stimulus but with much smaller spatial size) and masking of pattern by light (the target is a patterned stimulus such as an alphanumeric character). The more interesting types of visual masking occur in visual pattern masking, when a patterned target stimulus is masked by a patterned mask stimulus. From a methodological point of view, one can distinguish between three types of pattern masking: When the target and the mask do not overlap spatially, forward pattern masking is called paracontrast, and backward pattern masking is called metacontrast. When target and mask overlap spatially, if the mask does not contain any structural characteristics of the target, one has pattern masking by noise. On the other hand, when the mask has structural similarities to the target, one has pattern masking by structure.Figure 1 provides examples of different types of visual pattern masking.

Figure 1 Types of masking according to spatial and informational content of the stimuli. Left: paracontrast and metacontrast; middle: masking by noise; right: masking by structure.