Concealed by a mask: The effects of mask duration, set size and target luminance on the object substitution masking effect

Deficits in visual attention are often demonstrated in visual masking experiments. A typical “masking effect” can be observed, in which the sheer presence of a mask causes noticeable impairment in the processing of a target stimulus. This is demonstrated using a four dot mask (e.g., : :) around a target in Object Substitution masking (OSM). Results of a typical OSM experiment demonstrate that impairment caused by a mask is greater with the dispersal of spatial attention amongst distracter items (a set size effect) and the duration that the mask remains on display after target offset (a mask duration effect). However, a novel contribution is to demonstrate that, although target accuracy is impaired, it is at least processed to some extent. This was demonstrated by manipulating target luminance (dull versus bright) in two experiments. The first experiment involved 3 trained observers, whilst the second experiment involved 30 untrained observers. Participants were required to complete a typical OSM task, in which mask duration, set size and luminance was varied. The major hypothesis was that, if luminance and OSM map onto the same component of visual processing (i.e., a low level of processing), a significant three-way interaction between set size, mask duration and luminance will be observed. Various results from analysis of variance (ANOVA) suggested that this hypothesis was supported; however, speculation regarding this issue was discussed. It was concluded that the results of this experiment indeed suggested that OSM effects target processing at a very low level in the visual system, which was theoretically consistent with ERP findings of previous studies