Abstract
The question of whether conscious experience is best viewed as graded or dichotomous has received much scientific attention in recent years as the answer is relevant not only to models of consciousness, but also to the examination of neural markers of consciousness in patients and infants. Although some studies have found evidence of graded perception, it is unclear whether such perception is simply composed of individual stimulus features perceived in an all-or-none manner. Here, we examined whether the Kanizsa triangle (an illusory figure that is supposedly only perceived when all its parts are visible) has an impact on line length discrimination across four degrees of subjective visibility. We found that the presence of the Kanizsa triangle biases line length judgments (a phenomenon called the Ponzo illusion) when participants reported any experience (even a weak glimpse) of the stimulus. The results support the view that consciousness is a graded phenomenon. The strength of this support depends on the assumption that all parts of the illusory figure must be perceived for the illusion to work but this assumption is not resolved in the present literature. Currently, evidence can be found both for and against this notion.
Highlights
Conscious experience is often assumed to be an all-or-none phenomenon that is either present or absent on a single trial basis in experimental settings, and this assumption is critical to central theories of consciousness [2,3]
We present evidence supporting the notion that conscious experience is graded and that the gradedness is not caused by all-ornone perception of individual features, but instead that conscious experience is graded in a holistic sense
If perception can be degraded by complete perception of single fragments or features only, it can be hypothesised that the Kanizsa triangle induces a Ponzo illusion only when the threshold of awareness is crossed, i.e. we should expect the illusion to have a sudden impact on accuracy when the stimulus is reported as completely clear
Summary
Conscious experience is often assumed to be an all-or-none phenomenon that is either present or absent on a single trial basis in experimental settings (see for instance [1]), and this assumption is critical to central theories of consciousness [2,3]. “a feeling that something was shown, but not characterized by any content”) Whereas such arguments suggest alternative interpretations of dichotomous findings, they cannot rule out that the processing of single stimulus features explains intermediate levels of accuracy for weak perceptual experiences. If perception can be degraded by complete perception of single fragments or features only, it can be hypothesised that the Kanizsa triangle induces a Ponzo illusion only when the threshold of awareness is crossed, i.e. we should expect the illusion to have a sudden impact on accuracy when the stimulus is reported as completely clear (or possibly as almost clear). Participants reported which line they judged to be longer (upper vs lower) and how clear their experience was (on the Perceptual Awareness Scale, PAS [28]) This design allowed us to test for an effect of the presence of the illusion on the line comparison judgment for each PAS rating separately. Data for this rating allowed us examine whether the illusion was present when only a “weak glimpse” was perceived
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