An investigation into the composite or colinear form of the Müller-Lyer illusion, with one shaft adjustable by subjects, was carried out using black-on-white and CRT displays. Three fin-angle and two fin-length conditions were examined for both configurations of this figure, i.e., arrow-headed and feather-headed standards. Adaptations of the Müller-Lyer figure were also studied. Framing effects influenced the extent of this illusion. A highly consistent illusion error for balanced-field conditions, based on the formula k L cos A can be predicted which relates closely to previous experimenters' observed illusory errors and gives a remarkably simple explanation for this illusion.

What strategies does human vision use to attend to faces and their features? How are such strategies altered by 2-D inversion or photographic negation? We report two experiments in which these questions were studied with the flicker task of the change-blindness literature. In experiment 1 we studied detection of configural changes to the eyes or mouth, and found that upright faces receive more efficient attention than inverted faces, and that faces shown with normal contrast receive more efficient attention than faces shown in photographic negative. Moreover, eyes receive greater attention than the mouth. In experiment 2 we studied detection of local changes to the eyes or mouth, and found the same results. It is well known that inversion and negation impair the perception and recognition of faces. The experiments presented here extend previous findings by showing that inversion and negation also impair attention to faces.

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Williams argues that humans have evolved special purpose adaptations for eliciting medical attention from others, such as a specific facial expression of pain. She also recognises that such adaptations would almost certainly have coevolved with adaptations for providing and responding to medical care. The placebo response may be one such adaptation, and any evolutionary account of pain must also address this important phenomenon.