Inter-Individual Differences as Instrument to Investigate the Mechanisms in Metacontrast Masking

Abstract

In metacontrast masking the visibility of the first stimulus (the target) is reduced due to the appearance of the second stimulus (the mask). Two masking functions mainly occur: "Type A", when visibility is increasing with increasing SOA, and "Type B", where visibility is high in short and long SOAs and dropped to a minimum in between. In five studies it systematically investigated which influence different experimental parameters have on metacontrast masking functions. The longer the mask duration becomes in relation to the target duration, the more the minimum of the masking function shifts towards shorter SOAs, and the more Type-A-like (monotonically increasing) the function becomes. In the center and in the periphery masking is stronger in small stimuli than in large stimuli, and the SOA of the strongest masking occurred at the shortest SOA in both stimulus sizes. Masking functions are identical for predictable and unpredictable stimulus presentation locations. Apparent motion is better detected in long SOAs than in short SOAs. Furthermore, inter-individual differences were found, which were used to get insights in the mechanisms involved in metacontrast masking. Some participants show a Type A masking function and mostly report to perceive apparent movements in the sequence of both stimuli, mainly in long SOAs. Others show a Type B masking function and mostly report to perceive negative afterimages in the shape of the target inside the mask, mainly in short SOAs. Type A and Type B observers do differ in their top-down processing, but not in their bottom-up processing of metacontrast stimuli. Two processes could be revealed, which may be involved. According to the Integration-Segregation-Theory the perception of two stimuli being segregated, enabling apparent moving images, are associated to the segregation process. The percept of simultaneous, conjoint image of two physically succeeding stimuli are associated to the integration process. As Process 1 is strongly associated with a Type A masking function, which in turn goes along with a percept of motion, it may correspond to the segregation process. As Process 2 is strongly associated with a Type B masking function, which in turn goes along with a percept of negative afterimages inside of the mask stimulus, it may correspond to the integration process. It is assumed that these two processes are involved in the conscious perception of the target stimulus in metacontrast masking.