Transcranial magnetic stimulation of early visual cortex interferes with subjective visual awareness and objective forced-choice performance Mika Koivisto ⇑ , Henry Railo, Niina Salminen-Vaparanta Department of Behavioural Sciences and Philosophy, Centre for Cognitive Neuroscience, University of Turku, Finland article info Article history: Received 3 May 2010 Available online 21 September 2010 Keywords: Consciousness Perception Primary visual cortex TMS abstract In order to study whether there exist a period of activity in the human early visual cortex that contributes exclusively to visual awareness, we applied transcranial magnetic stimu- lation (TMS) over the early visual cortex and measured subjective visual awareness during visual forced-choice symbol or orientation discrimination tasks. TMS produced one dip in awareness 60–120 ms after stimulus onset, while forced-choice orientation discrimination was suppressed between 60 and 90 ms and symbol discrimination between 60 and 120 ms. Thus, a time window specific to visual awareness was found only in the orientation condi- tion at 120 ms. The results imply that both conscious and unconscious perception depend on activity in early visual areas. On the basis of previous estimates of neural processing speed, we suggest that the late part of the activity period most likely involve local extras- triate–striate interactions which provide the contents for visual awareness but are not themselves sufficient for awareness to arise. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Our understanding of the organization of visual cortex and its function at neuron level has grown rapidly during recent decades, but at system level it is not clear how the populations of neurons interact and give rise to conscious percepts. One important question concerns the contribution of the primary visual cortex (V1) on visual awareness. According to views based on the classical feedforward conception, visual processing occurs hierarchically starting from V1 and proceeds in feed- forward manner to higher cortical areas where the necessary and sufficient activity producing awareness takes place (e.g., Crick & Koch, 1995; Zeki & Bartels, 1999). Alternative models assume that feedforward processing may only be sufficient for unconscious processes, but visual awareness would require also recurrent processing where the higher areas interact with the earlier ones (Bullier, 2001; Hochstein & Ahissar, 2002; Lamme, Super, & Spekreijse, 1998; Pollen, 2008). This view as- sumes that early visual areas participate in conscious perception during two periods: during an ‘‘early” feedforward phase and during a later feedbackward phase. Only the late V1 activity is assumed to be specific to visual awareness (Block, 2005; Lamme, 2006). Although there is evidence from neuroimaging studies that the activity level of early visual cortex determines the perceptual contents that will enter visual awareness (Kamitani & Tong, 2005; Ress & Heeger, 2003), it remains to be explicitly tested whether the late V1 activity is specific to visual awareness or whether it is necessary also for unconscious perception. Here we study whether there is a distinct period during which the activity of early visual cortex is specific to visual awareness by making use of transcranial magnetic stimulation (TMS). It allows one to stimulate a cortical region through the skull and to interfere with cortical processing (Walsh & Pascual-Leone, 2003). By applying single TMS pulses at varying 1053-8100/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.concog.2010.09.001 ⇑ Corresponding author. Address: Center for Cognitive Neuroscience, University of Turku, 20014 Turku, Finland. Fax: +358 2 333 5060. E-mail address: mika.koivisto@utu.fi (M. Koivisto). Consciousness and Cognition 20 (2011) 288–298 Contents lists available at ScienceDirect Consciousness and Cognition journal homepage: www.elsevier.com/locate/concog