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Joseph Brooks, Stephen Palmer; Attention and figure-ground status produce separate steady-state VEP effects in human cortex. Journal of Vision 2007;7(9):311. doi: https://doi.org/10.1167/7.9.311.
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Figure-ground organization determines perceived shape and relative depth along edges in visual scenes. We report a continuous neural measure that reflects the figure-ground status of image regions from image onset through behavioral response. The contrast of the texture in figure and ground regions, as determined by the presence of the extremal edges figure-ground cue (Palmer & Ghose, VSS 2006), was reversed at 6.25 Hz and 10 Hz, respectively, to cause steady-state visual evoked potential (SSVEP) responses at corresponding harmonic frequencies in the scalp electroencephalogram. Because figural regions are known to attract attention (Nelson & Palmer, in press) and attention affects the SSVEP response (Muller, et al., 1998, Nature Neuroscience), the location of spatial attention was also manipulated to dissociate these factors. While observers fixated a central point, they continuously directed their attention to either the left or right side, as instructed, and reported whether they perceived that region as figure or ground. Both attention and figural status increased SSVEP amplitude, but their effects differed in scalp distribution: Whereas the attention effect was strongest over the hemisphere contralateral to the attended location, the figural assignment effect on the SSVEP was equally strong over both hemispheres. Importantly, the time course of the figural SSVEP effect was strongly correlated with the response time of behavioral figure-ground judgments, suggesting that the neural activity causing this effect is linked to the conscious representation of figural status.
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