Studies looking specifically at filling-in illusions show varied results with regard to the subset of surface-responsive activity that correlates with
perceived brightness or color (but see Cornelissen, Wade, Vladusich, Dougherty, & Wandell,
2006; reviewed in Komatsu,
2006). Correlates have been found as early as V1 for certain illusions—simultaneous brightness contrast (Kinoshita & Komatsu,
2001; Rossi, Rittenhouse, & Paradiso,
1996), blind-spot filling-in (Komatsu, Kinoshita, & Murakami,
2002; Komatsu, Murakami, & Kinoshita,
1996; Tong & Engel,
2001), neon-color spreading (Sasaki & Watanabe,
2004), and the phantom effect (Meng, Remus, & Tong,
2005). Moreover, propagation of activities for neurons corresponding to a surface region following a boundary formation has been shown in electrophysiology in V1 (Lamme, Rodriguez-Rodriguez, & Spekreijse,
1999). On the other hand, correlates for other illusions—artificial scotoma (De Weerd, Gattass, Desimone, & Ungerleider,
1995) and Cornsweet effect (Perna, Tosetti, Montanaro, & Morrone,
2005; Roe, Fritsches, & Pettigrew,
2005)—were not found until V2 or even later. Finally, in a study using Troxler fading, the illusion most similar to discrete color filling, no isomorphic surface correlates were found at all (von der Heydt, Friedman, & Zhou,
2003). In this study, the neurons that were found to follow the monkey's perceptual experience were selective for the color spot's edge, not its interior surface.