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Joshua Foster, Emma Bsales, Edward Awh; The N2pc does not reflect a shift of covert spatial attention. Journal of Vision 2018;18(10):1220. doi: https://doi.org/10.1167/18.10.1220.
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The N2pc is a transient negative deflection in EEG activity that is observed contralateral to visually selected stimuli. While the N2pc provides a sensitive index of visual selection, it remains unclear whether it reflects a shift of attention towards the selected stimulus, or cognitive operations that occur after spatial attention has been deployed. Kiss and colleagues (2008) demonstrated that the N2pc component was unaffected when subjects were cued to the relevant hemifield in advance, suggesting that it may not reflect a shift of attention. However, because observers can orient attention precisely within a hemifield, the N2pc may still have been driven by reorienting of attention within the relevant hemifield. To resolve this issue, we manipulated whether or not covert attention was deployed to the target location prior to the onset of a search array, and we used ongoing alpha oscillations to verify the specific locus of covert selection prior to target onset. In line with past work, alpha activity precisely tracked the selected location following informative cues, but showed no evidence of spatial orienting following neutral cues. Nevertheless, both conditions exhibited a clear target-locked N2pc with equal amplitude, suggesting that the N2pc does not reflect a shift of attention to the target. An attractive alternative explanation is that the N2pc reflects target individuation, the segregation of selected items from the background and other items in the display (Ester et al., 2012; Mazza & Caramazza, 2015). Interestingly, while N2pc amplitude was equivalent following informative and neutral cues, the onset of both the N2pc and the lateralized readiness potential (LRP) – an electrophysiological signature of response preparation – were earlier following informative cues (16 and 50 ms, respectively). Thus, our findings also show that spatial precues speed the accumulation of response-related information, rather than just facilitating the spatial selection of targets.
Meeting abstract presented at VSS 2018
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