Participants performed a Go/NoGo version of the Continuous Performance Test (CPT) described in previous studies [34], [35]. This task consisted of a series of letters presented sequentially, one at a time, for 0.2 s with interstimulus interval of 2 s. The subject was instructed to respond as quickly as possible to letter X preceded by letter O by pressing a button on a response pad using the right index finger and to withhold the response in the case of any O-not-X combination. Response speed and accuracy were equally emphasized. A total of 400 letters were presented, including 40 O-X (Go) and 40 O-not-X (NoGo) combinations occurring in a pseudo-random order. The response prepotency and, hence, the degree or processing conflict was increased in this task by the relative rareness of the Go and NoGo stimuli and by the fact that the letter O served as a warning cue informing the subject that the next letter is likely to be a Go signal requiring a speeded response. All Xs were preceded by O, and the O-X contingency was explicitly emphasized in
