How does masking work?

Question

Masking occurs when the delay between the target and the mask is less than a threshhold (say 50 milliseconds). If sensory data passes from lower to higher visual cortices/processing regions as in a linear one way street, how can the mask (which comes later) influence the perception of the target (which moved ahead of the mask)?

So does this mean masking works because the neuro-network consists of nested connections folding back and forth on itself? If so, at what scales do they operate? Do they occur both within higher regions and between higher and lower regions? Do we understand how they work in detail in the case of masking?

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I learned that masking is often used as a research tool for subconsciousness/unconsciousness. But maybe neuro-feedback disruted using masking is just one of the many ways subconscious perception can occur? If so, shouldn't we reserve some qualms about general conclusions about subconsciousness/unconsciousness drawn using the masking method?

Answer 1

Apparently your question is on backward masking, which means that the masker follows the stimulus (probe) in time. Backward masking generally occurs at higher levels, typically the cortex. In case of visual stimuli this can be the primary visual cortex, or V1 (Mace et al. 2005). Ongoing processing of the probe is then thought to be interfered with by the subsequent presentation of the masker, as long as the masker-probe interval is not too long. The PNAS article by Covaks et al., 1995 shows this nicely at the cortical level: neurons working on the stimulus change their response patterns when a masker is presented afterwards. So although there may be connections folding back and forth onto themselves, a serial 'one-way' route where the processing of a stimulus changes upon arrival of the masker is the most parsimonious, and experimentally supported explanation. Cortical processes are often affected by consciousness (alertness/vigilance) so the presence of backward masking effects can be interpreted as a marker of cortical higher-level processing.

Forward masking, and simultaneous masking, on the other hand, work at the neural level through adaptation processes. Although the effects of forward masking can be observed at the cortical level as shown by Brosch & Screiner 1997, forward masking is caused by processes occurring in the periphery (e.g., the cochlea or auditory nerve in hearing) and are unaffected by mental state. Hence, the presence of forward masking indicates that the neural process under investigation reflects a peripheral process that can also occur subconsciously, whereas the presence of backward masking generally point towards a higher-level neural process that is typically affected by mental state. Hope this helps.

Answer 2

There are different types of masking that may have different mechanisms. Even backward masking may mean:

• noise masking - such as when white noise is presented
• pattern masking - when target-resembling pattern of lines is shown
• metacontrast masking - when an object adjacent to target (but not occluding it) and highly different in contrast is used
• object substitution masking - when target itself is not occluded by mask (e.g., four dots are present near the corners of a target), but nevertheless is not perceived

Thus, different explanations exists for different types of masks and different conclusions can be made both about consciousness and about brain mechanisms. What I mean is that no clear answer on your question is possible without specifying further what kind of masking we are talking about.

See, for example, these papers:

Delord, S. (1998). Which Mask is the Most Efficient: A Pattern or a Noise? It Depends on the Task. Visual Cognition, 5, 313–338.

Enns, J. T., & Lollo, V. Di. (1997). OBJECT SUBSTITUTION:. A New Form of Masking in Unattended Visual Locations. Psychological Science, 8, 135–139.

Goodhew, S. C., Pratt, J., Dux, P. E., & Ferber, S. (2013). Substituting objects from consciousness: A review of object substitution masking. Psychonomic Bulletin & Review. doi:10.3758/s13423-013-0400-9