Event-related potentials were recorded in a flanker task using arrowheads pointing to the left or to the right as targets and as congruent or incongruent flanker stimuli using squares as neutral flanker stimuli. The onset of the flanker stimuli preceded that of the target stimuli by 100 ms. Lateralized readiness potentials showed response activation below execution threshold in correspondence to the information conveyed by the flanker stimuli. Exclusively, the incongruent flanker condition provoked a N2c, which evolved closely synchronized to the erroneous response. Graded response analyses separating incongruent trials with weak, medium, and strong incorrect response activation revealed that the N2c amplitude covaried with the magnitude of the erroneous response. The N2c in the incongruent compatibility condition of the flanker task thus corresponds to the avoidance of inappropriate responses, possibly reflecting the inhibition of automatically but erroneously primed responses. The results are compatible with studies of error correction, suggesting that efference monitoring is a constituent of executive control.

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A metacontrast procedure was combined with the recording of event-related potentials (ERPs) to examine the mechanisms underlying the priming effect exerted by masked visual stimuli (primes) on target processing. Participants performed spatially arranged choice responses to stimulus locations. The relationship between prime and target locations (congruity) and the mapping between target and response locations (compatibility) were factorially manipulated. Although participants were unaware of prime locations, choice responses were faster for congruent than incongruent conditions irrespective of the mapping. Visual ERP components and the onset of the lateralized readiness potential (LRP), an index of specific motor activation, revealed that neither perceptual nor preselection processes contributed to the congruity effect. However, the LRP waveform indicated that primes activated responses that fit the stimulus-response mapping. These results support the view that sensorimotor processing of masked stimuli is functionally distinct from their conscious perception.Masked stimuli that humans remain unaware of may nevertheless influence their behavior (cf. Kihlstrom, 1987). For example, when a lighted disk is presented by itself and then, after a short time interval, surrounded by a lighted ring, the ring obscures the visibility of the disk, as indicated by subjective judgments of stimulus brightness. This phenomenon is called metacontrast (Stigler, 1910; cf. Breitmeyer, 1984). Further studies have revealed that masked stimuli nonetheless trigger manual responses (e.g., Fehrer & Raab, 1962;Taylor & McCloskey, 1990). This dissociation between subjective reports and motor performance leads to the question of which mechanisms underlie the processing of masked stimuli.Neumann and Klotz (1994) modified the metacontrast paradigm in several ways to address this question. Participants performed manual choice responses to target stimuli that appeared shortly after a prime. The time interval between the onset of the prime and the onset of the target-stimulus onset asynchrony (SOA)-was 42 ms. Examples of the stimuli are shown in Figure 1. The prime display was composed of horizontally arranged rectangles displayed at fixation and to the left and right of fixation. Either the left or the right rectangle was flanked by horizontal bars defining the prime location (PL). The target display consisted of rectangles arranged so that the outer contours of rectangles in the prime display fit inside the outer contours of the rectangles in the target display, resulting in metacontrast masking of the prime. Target location (TL) was defined by bars flanking one of the rectangles in the target display. Discriminative responses about the presence and absence of the prime revealed null sensitivity ( d ' = .024) of observers, indicating an unawareness of the prime. Nonetheless, the relationship between the prime and target locations (i.e., prime-target congruity) influenced the processing of target location in two-choice reaction time (RT) tasks. F...