Abstract
We studied neural correlates accompanying the Fraser spiral illusion. The Fraser spiral illusion consists of twisted cords superimposed on a patchwork background arranged in concentric circles, which is typically perceived as a spiral. We tested four displays: the Fraser spiral illusion and three variants derived from it by orthogonally combining featural properties. In our stimuli, the shape of the cords comprised either concentric circles or a single spiral. The cords themselves consisted of black and white lines in parallel to the contour of the cords (i.e., parallel cords), or oblique line elements (i.e., twisted cords). The displays with twisted cords successfully induced illusory percepts, i.e., circles looked like spirals (the Fraser spiral illusion) and spirals looked like circles (i.e., a “reverse Fraser illusion”). We compared the event-related potentials in a Stimulus (Circle, Spiral) × Percept (Circle, Spiral) design. A significant main effect of Stimulus was found at the posterior scalp in an early component (P220-280) and a significant main effect of Percept was found over the anterior scalp in a later component (P350-450). Although the EEG data suggest stimulus-based processing in the posterior area in an early time window and percept-based processing in the later time window, an overall clear-cut stimulus-percept segregation was not found due to additional interaction effects. Instead, the data, especially in the later time window in the anterior area, point at differential processing for the condition comprising circle shapes but spiral percepts (i.e., the Fraser illusion).
Highlights
A typical aspect of visual illusions is that the actual percept differs from the presented stimulus
Kourtzi and Kanwisher (2001) showed that in a particular area in the ventral pathway, the neural activation did not depend on the particular stimulus properties per se but rather on the actual shape interpretation which remained constant despite stimulus change. All in all, such studies have shown dissociations between stimulus-related neural activation and percept-related neural activation. We focus on such dissociations with regard to one of the best known geometrical illusions, originally described by Fraser (1908)
Stimuli and Procedure The experiment included four kinds of stimuli: the original Fraser spiral illusion in which a set of concentric circles is made up of twisted cords; the reverse Fraser illusion with a single spiral made up of black and white elements; a display with a set of concentric circles made of black and white lines in parallel to the contour (Parallel Circles, see Figure 1C); and a display with a single spiral made of a cord parallel to the contour (Parallel Spiral, see Figure 1D)
Summary
A typical aspect of visual illusions is that the actual percept differs from the presented stimulus. Straight horizontal lines may appear skewed (e.g., the café wall illusion, Gregory and Heard, 1979) or colors may appear at positions that were not exposed to ‘‘colored’’ light (e.g., the neon color illusion, van Tuijl, 1975). We use neurophysiological measures to explore differences between illusory appearances and similar but non-illusory appearances. We aim to discern neural correlates for the veridical vs illusory perception of particular shapes. It is known that a single stimulus may yield very different shape interpretations. Rivalry displays and ambiguous stimuli have proven to be excellent materials to study neural correlates of perceptual interpretations. Using binocular rivalry in a functional magnetic resonance imaging set-up, differential percept-related cortical
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
