Phenomenological considerations of metacontrast masking

Abstract

This thesis enriches the understanding of metacontrast masking as multidimensional phenomenon on the basis of phenomenological, behavioral as well as neurophysiological data. The phenomenological investigations (Experiment 1-2) depict the perception of seven qualitative different target aspects, regarding the perceived temporal distance of both stimuli (Target inside Mask, Target before Mask), the perceived contrast (Dark Target, Bright Target, No Target) and apparent motion perceptions (Rotation, Expansion). The unique relationships of these perceptions with SOA and Congruency showed that the experience of the target varies qualitatively in a metacontrast masking paradigm. This contradicts with the assumption that variations of the SOA influence the awareness of the target only quantitative, as indicated by one-dimensional objective and subjective measures. The underestimation of the target perception by objective measures is also indicated by the relationship between discrimination performance and the perception of the different target aspects (Experiment 2). In accordance to the phenomenological variety in the perception of the target, evidence for different processes was provided by the results of Experiment 3. Regarding these results, it seems to be inappropriate to conceptualize metacontrast masking as a unitary process underlying the whole range of SOAs. Instead, metacontrast can be defined as multidimensional phenomenon with three different processes underlying short and long SOAs. The process underling short SOAs was associated with two perceptions, which were found to depend on each other, the perceptions of a target integrated inside the mask and an expansion. At long SOAs, evidence for two independent processes was found, eliciting the perceptions of a rotation and a target temporally segregated from the mask. Finally, electrophysiological results supported the assumption that the perceptions at long SOAs were reflected by two distinct mechanisms, since unique neural signatures were found for both (Experiment 4).