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Naotsugu Tsuchiya, Christof Koch; Continuous flash suppression. Journal of Vision 2004;4(8):61. doi: 10.1167/4.8.61.
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Visual illusions that produce perceptual suppression despite constant retinal input, such as binocular rivalry (BR) or flash suppression (FS), are potent tools to study the neural correlates of consciousness. However, the duration and timing of perceptual suppression in BR are difficult to control, and FS attains only short duration of suppression, too short to produce aftereffect. Furthermore, FS requires pre-adapting period, which makes it impossible to use in studies that require complete perceptual unawareness. Here, we introduce Continuous Flash Suppression (CFS). With CFS, we can control the timing of suppression and suppress stimuli for long duration (> several minutes) without subjects noticing the suppressed stimuli at all. We used several categories of figures (Gabor patches and angry faces) as suppressed stimuli and other categories as continuously changing dominant stimuli (Gabor patches of different orientations, random textures consisted of small color rectangles, gray or color neutral faces, scrambled faces). Both stimuli were presented at the fovea (10×10 deg, with a stereoscope, 50 cm from the display). The dominance duration of suppressed stimuli during a 60s trial was the observation variable. As the contrast and saliency of suppressed stimuli decreases, dominance duration of the suppressed figure decreases. As suppressed stimuli are low-pass filtered, the dominance of the suppressed figure decreases. ISI between dominant stimuli is crucial; in general, short ISI (<1sec) produces strong and prolonged suppression. Under a wide range of parameters, a stimulus that usually dominates about 30s in a 60s BR trial can be suppressed completely (i.e. 0s dominance in a 60s trial). We report a classical human fear conditioning study that utilized CFS. Subjects could report their on-going subjective visibility of figures during 2s stimuli presentation, while they tried to associate electrical shocks with angry face stimuli that were completely masked by CFS.
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