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a, Achromatic gratings were presented in a 1 s on, 1 s off design for 16 s followed by 10 s with only a fixation point visible. The red fixation point cued the subject to attend to orientation changes (indicated by “O”) and ignore contrast changes (“C”). b, Isoluminant chromatic gratings were used. The blue fixation point cued subjects to attend to contrast decrements. Orientation and contrast changes are exaggerated for illustrative purposes.
