Extracting a stimulus-unlocked component from EEG during NoGo trials of a Go/NoGo task Yusuke Takeda, Kentaro Yamanaka, Daichi Nozaki, and Yoshiharu Yamamoto ⁎ Educational Physiology Laboratory, Graduate School of Education, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Received 22 October 2007; revised 5 March 2008; accepted 10 March 2008 Available online 20 March 2008 Like electroencephalographic (EEG) activity during reaction time tasks, EEG activity during tasks without overt responses may also consist of two components: stimulus-locked and -unlocked compo- nents. The extraction of such stimulus-unlocked components has been difficult owing to the unknown delays. Here, we propose a novel method to extract both of the two components from single-channel EEG epochs. In this method, we initially set random values for the delays and extract uncontaminated stimulus-locked and -unlocked components using the preset delays and a discrete Fourier transform. Then, we reconstruct the EEG by overlapping the extracted com- ponents with the preset delays, and calculate the residual errors between the reconstructed and original EEG. This procedure is repeated by updating the delays until the residual errors become adequately small. After verifying the performance of this method by two kinds of simulations with artificial and EEG data, we apply the method to EEG during NoGo trials of a Go/NoGo task, and obtain the stimulus-unlocked components, the magnitudes of which are compar- able with those of the stimulus-locked components. By applying this method, it is possible to study internal and subjective brain activity, which occurs with variable delays. © 2008 Elsevier Inc. All rights reserved. Keywords: Stimulus-locked component; Stimulus-unlocked component; NoGo trial; Go/NoGo task; Electroencephalography (EEG) Introduction When a subject responds overtly to a stimulus, brain activity consists of two components: a component time-locked to the stimulus, i.e., a stimulus-locked component, and a component not time-locked to the stimulus but to the response, i.e., a response- locked component (Braun et al., 2002; Endo et al., 1999; Goodin et al., 1986; Jung et al., 2001; Lamarre et al., 1983; Nelson, 1987; Nelson et al., 1991; Perfiliev, 1998; Tanji and Kurata, 1982). In the human scalp electroencephalography (EEG) studies, these compo- nents are conventionally extracted by averaging EEG epochs with respect to either stimulus or response onset to increase the signal to noise ratio (SNR). However, when the two components are tem- porally overlapping, the two components are mutually contami- nated by the averaging procedure (Kok, 1988; Verleger, 1988; Verleger et al., 2006). To solve this problem, we recently proposed a method to extract the uncontaminated components from each channel of EEG epochs and reaction times (RTs) (described in the Methods section), and applied the method to the EEG during an auditory simple reaction time task (Takeda et al., 2008). However, even when a subject does not respond overtly to a stimulus, EEG activity may exhibit two components: a stimulus- locked component, and a component not time-locked to the stimulus but to a certain event in the brain (David et al., 2006; Haenschel et al., 2000; Schürmann and Başar, 2001; Tallon-Baudry et al., 1996; Tallon-Baudry and Bertrand, 1999); here, the latter is called the stimulus-unlocked component, defined as the EEG component which has a fixed waveform and of which the delay fluctuates from trial to trial. However, such a stimulus-unlocked component cannot be extracted by the averaging procedure, or even by our previous method (Takeda et al., 2008), because its delays in individual trials are of unknown length. Although, using time-frequency analyses, some studies indicate that EEG activity during various cognitive tasks involves some stimulus-unlocked oscillatory components together with stimulus-locked components (David et al., 2006; Haenschel et al., 2000; Jung-Beeman et al., 2004; Kirmizi-Alsan et al., 2006; Tallon-Baudry et al., 1996; Tallon-Baudry and Bertrand, 1999), suggestive of the presence of the stimulus-unlocked components, their waveforms and the delays of individual trials have not been identified. Because the brain activity related to a complex function, e.g., attention, memory and problem solving, occurs in a stimulus-unlocked way, it is important to develop a method for extracting the waveform and delays of the stimulus-unlocked brain activity. In this paper, we thus propose a novel method to estimate the delays of the stimulus-unlocked component, and to extract the www.elsevier.com/locate/ynimg NeuroImage 41 (2008) 777 – 788 ⁎ Corresponding author. Fax: +81 3 5689 8069. E-mail address: yamamoto@p.u-tokyo.ac.jp (Y. Yamamoto). Available online on ScienceDirect (www.sciencedirect.com). 1053-8119/$ - see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.neuroimage.2008.03.012