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Corentin Jacques, Bruno Rossion; Spatio-temporal dissociation between low- and high-level effects of stimulus inversion on early face-sensitive electrophysiological responses. Journal of Vision 2009;9(8):533. doi: https://doi.org/10.1167/9.8.533.
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Event-related potential studies in humans have shown that presenting a face upside-down increases the amplitude and latency of the face-sensitive N170 component recorded over occipito-temporal scalp regions. A similar effect on the preceding P1 component has also been reported. These observations have been taken by some authors as evidence that the effect of face inversion so detrimental for face perception - takes place in visual cortex as early as 100 ms following stimulus onset. However, previous studies have not investigated a possible bias in the position of fixation between upright and inverted faces that may contribute to this electrophysiological face inversion effect. Here we measured the ERP response to upright and inverted faces while instructing subjects to fixate either the upper or the lower part of the image, independently of orientation. The manipulation of orientation and fixation position affected differently the latency and amplitude of the P1 and N170. While fixation position modulated the latency effect of inversion similarly for the P1 and N170, manipulating fixation position influenced the amplitude effect of inversion differently for the P1 and N170. First, both the P1 and N170 latency delay due to face inversion was larger when subjects fixated the upper part of the stimulus. Second, while the effect of inversion on the P1 amplitude was sensitive to fixation position at all posterior electrodes, the amplitude increase of the N170 due to inversion was insensitive to fixation position at right anterior occipito-temporal electrodes. These observations suggest that while the inversion effect on the N170 amplitude observed on right anterior occipito-temporal electrodes is related to the activation of high-level face representations, the inversion effect on the P1 amplitude is mostly due to low-level differences between upright and inverted face stimulations (e.g. location of high contrast regions, position of the face in the visual field).
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