Masked targets trigger event-related potentials indexing shifts of attention but not error detection.

Our dynamic environment requires that we adjust how our brain processes information while we are performing tasks vital to our survival, such as hunting, avoiding predators, or operating vehicles. In the laboratory, the error-related negativity (ERN) of the event-related potential (ERP) waveform is used to measure when the brain detects errors or evaluates feedback (Gehring, Gross, Coles, Meyer, & Donchin, 1993; Gehring & Willoughby, 2002; Holroyd & Coles, 2002). The theories explaining the processes underlying the ERN have proposed that it measures error detection (Gehring et al., 1993), response conflict (Botvinick, Braver, Barch, Carter, & Cohen, 2001), and learning (Brown & Braver, 2005; Holroyd & Coles, 2002). Empirically, several studies suggest that the ERN can be found even when subjects are unaware that they have made an error. Evidence for this hypothesis has been provided using paradigms in which observers make a behavioral response at the end of the trial in a saccadic response paradigm (Endrass, Franke, & Kathmann, 2005; Endrass, Reuter, & Kathmann, 2007; Nieuwenhuis, Ridderinkhof, Blom, Band, & Kok, 2001) or on the next trial of a go/no-go paradigm (O'Connell et al., 2007) to indicate that they made an error. Using these paradigms, researchers have found similar ERNs whether or not subjects explicitly reported that they made an error. Thus, it has been proposed that the mechanism indexed by the ERN may operate beyond awareness. The question addressed here is whether the error-detection mechanism indexed by the ERN can operate on the preliminary products of visual scene processing which are sufficient for visual attention mechanisms to shift attention to target objects. If this is the case and the visual system responds as if a target is present although the subject is not aware that it was present, then a target-absent response by the subject should elicit an ERN. To test this hypothesis the study utilizes a specific type of visual masking. Object-substitution masking occurs when a visual search target is rendered unreportable by the continued presence of four-dot stimuli surrounding the target location (Di Lollo, Enns, & Rensink, 2000). Target discrimination or detection is accurate when the four-dots surrounding the possible targets offset simultaneously with the objects in the search array (see Figure 1A). However, when the four-dot masks remain visible after the search array offsets, observers approach chance at discriminating or detecting search targets in cluttered visual scenes. Previously, Woodman & Luck (2003a) showed that despite being unreportable, substitution-masked targets elicit shifts of attention as indexed by the N2pc component of the visual ERP waveform. The N2pc is a negative going potential typically elicited 200 ms following the onset of a search array, contralateral to the target or attended item (N2-posterior-contralateral Luck & Hillyard, 1994a, 1994b; Woodman & Luck, 1999, 2003b). The N2pc appears to index a mechanism of spatially focused visual attention in human and nonhuman primates (Luck, Girelli, McDermott, & Ford, 1997; Woodman, Kang, Rossi, & Schall, 2007). The previous findings indicate that during a substitution-masking paradigm the target representation is sufficient to trigger a normal shift of perceptual attention as indexed by the N2pc component (Woodman & Luck, 2003a). The present study uses the same substitution-masking paradigm to test the hypothesis that the error-detection mechanism indexed by the ERN operates prior to awareness and is as sensitive to the target information in a masked array as the N2pc component. If the mechanism indexed by the ERN operates beyond perceptual awareness, then it should be observed even when subjects fail to detect substitution-masked targets. In contrast, if the mechanism indexed by the ERN requires representations to be encoded into working memory resulting in awareness of the task-relevant information, then masking the targets by object substitution should eliminate the ERN. Figure 1 Examples of the stimuli, the behavioral performance and the stimulus-locked waveforms. A. Example stimuli for both trial types. B. The behavioral results with 95% confidence intervals shown. C. The grand average ERPs time-locked to the onset of the array ...