Good vibrations: Oscillatory phase shapes perception T. Neuling a , S. Rach a , S. Wagner b , C.H. Wolters b , C.S. Herrmann a, c, ⁎ a Experimental Psychology Lab, University of Oldenburg, Ammerländer Heerstr. 114-118, 26111, Oldenburg, Germany b Institute for Biomagnetism and Biosignalanalysis, University of Münster, Malmedyweg 15, 48149, Münster, Germany c Research Center Neurosensory Science, University of Oldenburg, Ammerländer Heerstr. 114-118, 26111, Oldenburg, Germany abstract article info Article history: Accepted 13 July 2012 Available online 23 July 2012 Keywords: tDCS tACS EEG Alpha Phase Perception In the current study, we provide compelling evidence to answer the long-standing question whether percep- tion is continuous or periodic. Spontaneous brain oscillations are assumed to be the underlying mechanism of periodic perception. Depending on the phase angle of the oscillations, an identical stimulus results in differ- ent perceptual outcomes. Past results, however, can only account for a correlation of perception with the phase of the ongoing brain oscillations. Therefore, it is desirable to demonstrate a causal relation between phase and perception. One way to address this question is to entrain spontaneous brain oscillations by apply- ing an external oscillation and then demonstrate behavioral consequences of this oscillation. We conducted an auditory detection experiment with humans, recorded the electroencephalogram (EEG) concurrently and simultaneously applied oscillating transcranial direct current stimulation at 10 Hz (α-tDCS). Our approach revealed that detection thresholds were dependent on the phase of the oscillation that was entrained by α-tDCS. This behavioral effect was accompanied by an electrophysiological effect: α-power was enhanced after α-tDCS as compared to a pre-stimulation period. By showing a causal relation between phase and perception, our results extend findings of previous studies that were only able to demonstrate a correlation. We found that manipulation of the phase resulted in differ- ent detection thresholds, which supports the notion that perception can be periodically modulated by oscil- latory processes. This demonstrates that tDCS can serve as a tool in neuroscience to extend the knowledge of the functional significance of brain oscillations. © 2012 Elsevier Inc. All rights reserved. Introduction It is still a matter of debate whether brain oscillations indeed have a causal role in perception, cognition, and action, or are a mere epi- phenomenon. Brain oscillations are believed to serve as an essential factor to code the temporal dynamics of our environment (Cohen, 2011). The phase of brain oscillations, in particular, carries temporal information in the millisecond range which is transferred and distrib- uted within (Lakatos et al., 2008) and across (Varela et al., 2001) brain regions. The oscillatory phase represents brain states fluctuat- ing between high and low cortical excitability (Engel et al., 2001; Lakatos et al., 2007), a concept proposed by Bishop (1932) over 75 years ago. Depending on that internal state, identical external events can result in different perceptual and behavioral outcomes. In particular, the phase of the α-rhythm (8–12 Hz) has been pro- posed to account for differences in reaction times (Callaway and Yeager, 1960), the perceptual fate of auditory (Rice and Hagstrom, 1989) and visual stimuli (Busch et al., 2009; Mathewson et al., 2009), as well as visual attention (Busch and VanRullen, 2010). The latter results especially support the notion that perception is periodic, represented as a sequence of snapshots (Busch and VanRullen, 2010; Busch et al., 2009; Mathewson et al., 2009; Varela et al., 1981) rather than as a continuous stream, supporting a hypoth- esis proposed by VanRullen and Koch (2003). These studies, however, reflect only a correlation between perception and the phase of the ongoing EEG oscillations but not for a causal relation. The question whether perception is periodic and if the periodicity is caused by the oscillatory phase can be addressed by entraining spontaneous brain oscillations through applying an external oscilla- tion and subsequently demonstrating behavioral consequences of this oscillation (Sejnowski and Paulsen, 2006). Recently, techniques to stimulate the human cortex non-invasively have been developed that are capable of modulating the ongoing rhythmic brain activity (transcranial alternating current stimulation, tACS; e.g., Kanai et al., 2008; Zaehle et al., 2010, oscillating transcranial direct current stimu- lation, otDCS; e.g., Antal et al., 2008; Marshall et al., 2006 and rhyth- mic transcranial magnetic stimulation, rTMS; e.g. Klimesch et al., 2003; Romei et al., 2010, 2012; Thut et al., 2011). Most of these stud- ies, however, are limited by only demonstrating either the effects of stimulation on behavior or changes in brain activity caused by the stimulation, but not both simultaneously. Only Marshall et al. (2006) demonstrated physiological and behavioral effects after NeuroImage 63 (2012) 771–778 ⁎ Corresponding author at: Experimental Psychology Lab, University of Oldenburg, Ammerländer Heerstr. 114-118, 26111, Oldenburg, Germany. Fax: +49 441 798 3865. E-mail address: christoph.herrmann@uni-oldenburg.de (C.S. Herrmann). 1053-8119/$ – see front matter © 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.neuroimage.2012.07.024 Contents lists available at SciVerse ScienceDirect NeuroImage journal homepage: www.elsevier.com/locate/ynimg