stream appears to be deeply suppressed under interocular suppres- sion, as shown by psychophysics (Zimba and Blake, 1983; Alais and Melcher, 2007), neurophysiological data in monkeys (Logothetis, 1998), and in single-cell recordings in humans (Kreiman et al., 2005). However, it has recently been proposed that there is a dif- ference between the ventral and dorsal stream for the processing of invisible pictures of animals and tools (Fang and He, 2005; Almeida et al., 2008, 2010). It was suggested that while dorsal stream neurons responded to invisible tools that carried the characteristic of being “graspable” the categorization of invisible animals was widely sup- pressed in the ventral stream (Fang and He, 2005). Even though the human and non-human primate brain can achieve visual categorization very fast, theories of visual awareness propose that a rapid feed-forward mechanism might not be sufficient for visual awareness, which might also require horizontal connections between different brain areas (Lamme, 2000) and/or late feedback projections from prefrontal areas (Sergent et al., 2005). The study of the neural correlates of invisible stimuli is valuable to disentangle the minimal set of processes that are necessary and sufficient for visual awareness to occur (Koch, 2003). Further investigation is needed to understand the relationship between the first, rapid feed-forward pass of information and the emergence of visual awareness. We studied the timing of categorization of seen and unseen images of animals, tools, and scrambled control images employing continu- ous flash suppression (CFS; Tsuchiya and Koch, 2005), EEG record- ings and single trial analysis. Based on the EEG signal, our classifiers INTRODUCTION The human brain continuously performs visual categorization of stimuli in everyday life. Studies of rapid visual categorization sug- gest that the first 100–200 ms are crucial to this process, consist- ent with categorization during the first pass of visual processing (Potter and Faulconer, 1975; VanRullen and Thorpe, 2001b; Liu et al., 2009). For go/no go tasks, for example, early event-related potentials (ERPs) at approximately 150 ms reflect the decision that there was a target present in a natural scene (Thorpe et al., 1996; VanRullen and Thorpe, 2001b). This first rapid categoriza- tion appears to be similar for diverse categories such as means of transportation or living objects (Thorpe and Fabre-Thorpe, 2001; VanRullen and Thorpe, 2001a). An open issue regards the capacity of invisible stimuli to influ- ence visual categorization and to activate different areas of the visual cortex. Experiments employing change blindness and inat- tentional blindness have clearly documented that important visual events that impinge on our retina can go widely unseen when atten- tion is diverted from them (Mack and Rock, 1998; Simons and Chabris, 1999). On the other side, it has been also demonstrated that visual category detection can be rapidly achieved even in the near absence of visual attention (Fei-Fei et al., 2002). Psychophysical studies using interocular suppression as well as neuroimaging stud- ies have given conflicting reports on the degree to which suppressed information activates areas of the brain (Blake and Logothetis, 2002; Alais and Melcher, 2007). Visual information arriving to the ventral Intercepting the first pass: rapid categorization is suppressed for unseen stimuli Lisandro Nicolas Kaunitz 1 *, Juan Esteban Kamienkowski 2 , Emanuele Olivetti 1,3 , Brian Murphy 1 , Paolo Avesani 1,3 and David Paul Melcher 1,4 1 Center for Mind/Brain Sciences, University of Trento, Trento, Italy 2 Laboratory of Integrative Neuroscience, School of Exact and Natural Sciences, University of Buenos Aires, Buenos Aires, Argentina 3 Neuroinformatics Laboratory, Bruno Kessler Foundation, Trento, Italy 4 Department of Cognitive Sciences, University of Trento, Rovereto, Italy The operations and processes that the human brain employs to achieve fast visual categorization remain a matter of debate. A first issue concerns the timing and place of rapid visual categorization and to what extent it can be performed with an early feed-forward pass of information through the visual system. A second issue involves the categorization of stimuli that do not reach visual awareness. There is disagreement over the degree to which these stimuli activate the same early mechanisms as stimuli that are consciously perceived. We employed continuous flash suppression (CFS), EEG recordings, and machine learning techniques to study visual categorization of seen and unseen stimuli. Our classifiers were able to predict from the EEG recordings the category of stimuli on seen trials but not on unseen trials. Rapid categorization of conscious images could be detected around 100 ms on the occipital electrodes, consistent with a fast, feed-forward mechanism of target detection. For the invisible stimuli, however, CFS eliminated all traces of early processing. Our results support the idea of a fast mechanism of categorization and suggest that this early categorization process plays an important role in later, more subtle categorizations, and perceptual processes. Keywords: visual awareness, rapid categorization, continuous flash suppression, EEG Edited by: Rufin VanRullen, Centre de Recherche Cerveau et Cognition, France Reviewed by: Thomas Carlson, University of Maryland, USA Thomas Serre, Brown University, USA Naotsugu Tsuchiya, RIKEN, Japan *Correspondence: Lisandro Nicolas Kaunitz, Center for Mind/Brain Sciences (CIMeC), University of Trento, Via delle Regole 101, 38060 Mattarello (TN), Italy. e-mail: lkaunitz@gmail.com www.frontiersin.org August 2011 | Volume 2 | Article 198 | 1 ORIGINAL RESEARCH ARTICLE published: 23 August 2011 doi: 10.3389/fpsyg.2011.00198