Investigations of luminance- and contrast-modulated binocular rivalry

Binocular rivalry can occur when incompatible stimuli are presented separately to the eyes. Since the invention of the stereoscope by Wheatstone in 1838, binocular rivalry has been intensively investigated with visual stimuli, which are differentiated from the background by variations in luminance, so-called luminance-modulated stimuli. However, it is also possible to perceive stimuli for which luminance of the target does not differ from that of the background but instead varies in contrast: so-called contrast-modulated (CM) stimuli. The main aim of this thesis is to investigate CM and noisy luminance-modulated (LM) stimuli under binocular rivalry conditions as the gained knowledge would enhance our understanding of both CM processing, as well as binocular rivalry. Perceptual change rates, proportions of exclusive visibility, mixed percepts (i.e. piecemeal and superimposition), as well as changes of these proportions across time and distributions of perceptual phases were calculated and compared between various CM and LM stimulus conditions. To compare those stimulus types with each other, the detection threshold was measured in one experiment to determine the visibility of each stimulus type, i.e. multiples above threshold. LM stimuli engage in significantly more exclusive visibility and trigger more alternation even when CM stimuli are of comparable visibility. Lower proportions of exclusive visibility and numbers of perceptual alternation for CM stimuli were due to greater proportions of superimposition. When comparably visible LM and CM stimuli compete with each other under binocular rivalry conditions, CM exclusive visibility predominates over LM exclusive visibility. Even if LM visibility is many times above CM visibility, LM stimuli never reach perceptual predominance. This result suggests that CM stimuli are processed unlike LM stimuli by neurones that receive initial binocular input. The results obtained were integrated into models concerning alternation dynamics and underlying processing sites for LM and CM stimuli.