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Michelle R. Greene, Michael C. Mangini, Irving Biederman; Trying your best to ignore a face does little to diminish the N170. Journal of Vision 2003;3(9):828. doi: 10.1167/3.9.828.
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Faces produce a significantly greater N170 response in the EEG waveform than objects. However, faces are interesting and invite attention. To what extent is the N170 response modulated by attention? In separate blocks of trials, subjects viewed images of faces alone, butterflies alone, and composites of faces and butterflies in which an image of a face and an image of a butterfly were superimposed. Subjects performed a 1-back task in which they had to judge whether the face or butterfly on the previous trial was identical to the face or butterfly that they were currently viewing. For the composite stimuli, this meant that subjects were sometimes attending to the faces on some blocks and to the butterflies on other blocks for the identical images. EEG was continuously recorded from two sites, T5 and T6. An N170 response was observed for the faces alone but not for the butterflies alone, consistent with previous accounts. Surprisingly, the largest N170 was found for face-attended composite images. This increase over baseline can be analyzed into two components, one associated with the mere presence of the face in the stimulus (whether by itself or in a composite with a butterfly, as these produced highly similar responses) and the other with whether the task required or deflected attention to the face. The largest component, 82.7% of the total, could be attributed to adding a face to the stimulus. Only 17.3% of the difference in the increase in the magnitude of the N170 when attending to a face in a composite over the butterfly alone condition was due to attention to the face. Thus, the N170 appears to be reflecting a face processing mechanism that is engaged automatically when one looks at a face, with only a modest increase in magnitude when that face is attended.
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