Contrast masking effects change with practice

Contrast detection thresholds are known to increase with background contrast, a phenomenon called contrast masking. We found that, under some conditions, observers improved their masked detection performance by repetitive practice of a masking experiment. This learning effect resulted in a cancellation of suprathreshold contrast masking within the contrast range measured. A two-alternative forced-choice discrimination paradigm was used, with stimuli consisting of Gabor signals as maskers and target, presented at the same location and time. Untrained observers showed increased detection thresholds with increasing mask contrast for suprathreshold mask contrasts, but perceptual learning caused an elimination of this classical effect, with masked thresholds reaching the no-mask level and below. Learning did not decrease, but rather somewhat increased, discrimination thresholds when target and mask shared the same Gabor signal parameters. Performance improvement was found to be specific for orientation and mask configurations, though it did transfer between mirror symmetric mask configurations and between eyes. These results argue against a static transducer function-based account for contrast masking and are consistent with a theory assuming multiple feature-based interactive network capable of long-term gain modifications.