Masking that disrupts late phases of visual processing eliminates location specificity of visual perceptual learning

Visual perceptual learning (VPL) is characterized by location specificity, in which learning is specific to the trained retinal location. Some studies have suggested that the location specificity of VPL is a manifestation of the involvement of VPL in early visual areas that have smaller receptive fields than higher areas. Other studies have suggested that location specificity results from higher-level involvement. To test which possibility is more likely, a psychophysical experiment was conducted using the following two types of masking. Previous neurophysiological studies have suggested that forward masking disrupts early phases of visual processing, while backward masking disrupts later processing phases. Since feedback occurs at later phases, disruption of location specificity only by backward masking would support the hypothesis that feedback is involved in location specificity. On the other hand, no disruption of location specificity by backward masking would support the hypothesis that location specificity occurs in early visual areas. During six training days, participants performed an orientation discrimination task at one location with either forward or backward masking. In the forward masking group, a noise mask was presented before a grating with a constant orientation. In contrast, in the backward masking group, a ring-shaped mask was presented after the grating, termed metacontrast masking. The strength of the masking effect was equated across participants from the two masking groups by adjusting the luminance contrast of the mask stimuli before training. In the pretest and posttest, participants performed the orientation discrimination task at the trained and untrained locations without masking. We found that while performance was improved only at the trained location in the forward masking group, performance improved equally at the trained and untrained locations in the backward masking group. These results suggest that temporally late processing, including feedback processing, contributes to the location specificity of VPL.