Unconscious and Conscious Processing of Color Rely on Activity in Early Visual Cortex: A TMS Study Henry Railo 1 , Niina Salminen-Vaparanta 1 , Linda Henriksson 2 , Antti Revonsuo 1,3 , and Mika Koivisto 1 Abstract ■ Chromatic information is processed by the visual system both at an unconscious level and at a level that results in con- scious perception of color. It remains unclear whether both conscious and unconscious processing of chromatic informa- tion depend on activity in the early visual cortex or whether un- conscious chromatic processing can also rely on other neural mechanisms. In this study, the contribution of early visual cor- tex activity to conscious and unconscious chromatic process- ing was studied using single-pulse TMS in three time windows 40–100 msec after stimulus onset in three conditions: con- scious color recognition, forced-choice discrimination of con- sciously invisible color, and unconscious color priming. We found that conscious perception and both measures of un- conscious processing of chromatic information depended on activity in early visual cortex 70–100 msec after stimulus presen- tation. Unconscious forced-choice discrimination was above chance only when participants reported perceiving some stimulus features (but not color). ■ INTRODUCTION Visual stimuli are processed by the brain and may influ- ence behavior even if they are not consciously perceived (Weiskrantz, 1997). A classical example of this is blind- sight, where patients who are clinically blind in certain visual field locations because of damaged visual cortex neverthe- less have a capacity to discriminate features of visual stimuli they do not consciously perceive ( Weiskrantz, Watrrington, Sanders, & Marshall, 1974; Pöppel, Held, & Frost, 1973; for a review, see Cowey, 2010). The neural mechanisms that underlie the conscious and unconscious modes of processing are not, however, well understood. Roughly speaking, they could rely on different anatomical and physiological systems, but they might also depend com- pletely or partly on overlapping neural mechanisms. In this study, we directly tested the contrasting accounts whether activity in the early visual areas (V1/ V2) is (Breitmeyer, Ro, & Singhal, 2004) or is not (Boyer, Harrison, & Ro, 2005) necessary for unconscious pro- cessing of chromatic information. Conscious visual perception of color is based on the processing of chromatic information by the visual system, most importantly, the discrimination of different wave- lengths (Gegenfurtner & Kiper, 2003). However, process- ing of chromatic information does not necessarily result in conscious perception of color. For instance, patients suffering from achromatopsia (Heywood & Cowey, 1999) or visual field defects caused by lesions to early visual areas (Stoerig & Cowey, 1989) may behaviorally respond to chromatic information in the absence of conscious color vision. Remarkably, in some of the patients with damaged V1, the spectral sensitivity curve of the blind visual field locations is similar to the sensitivity curve found in healthy controls (Stoerig & Cowey, 1989). One of the key questions in explaining unconscious chromatic processing concerns the pathways that relay chromatic information to cortex. Conscious color vision is mainly mediated by the LGN of thalamus and by ventral visual cortical areas (Claeys et al., 2004; Heywood & Cowey, 1999; Zeki et al., 1991; Meadows, 1974). In blindsight pa- tients with V1 lesions, the geniculate pathways that bypass V1 and project to extrastriate visual areas are possible me- diators of unconscious chromatic processing, but uncon- scious chromatic processing has also been proposed to rely on extrageniculate inputs to cortex (Weiskrantz, 1997; Cowey & Storeig, 1989; Stoerig & Cowey, 1989; Stoerig, 1987). The superior colliculus (SC) has been found to contribute to the residual visual abilities of blindsight pa- tients (e.g., Danckert & Rossetti, 2005), even for chromatic stimuli (Tamietto et al., 2009; Leh, Mullen, & Ptito, 2006). There is neural evidence from studies on monkeys that the SC responds to chromatic information, but the delayed onset latency of these color signals suggests a cortical rather than retinal origin (White, Boehnke, Marino, Itti, & Munoz, 2009). There is also some evidence that chromatic infor- mation is relayed to cortex via the pulvinar in monkeys (Felsten, Benevento, & Burman, 1983). 1 University of Turku, Finland, 2 Aalto University, Espoo, Finland, 3 University of Skövde, Sweden © 2012 Massachusetts Institute of Technology Journal of Cognitive Neuroscience 24:4, pp. 819–829