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Steven Pinker -- How the Mind Works - Hampshire High Italian ...

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The <strong>Mind</strong>'s Eye 283once a second. The perception of flipping was so obvious that we didn'tbo<strong>the</strong>r to recruit volunteers to confirm it. When <strong>the</strong> shape alternatedwith its upright reflection, it seemed to pivot like a washing machine agitator.When it alternated with its upside-down reflection, it did backflips.When it alternated with its sideways reflection, it swooped backand forth around a diagonal axis, and so on. The brain finds <strong>the</strong> axisevery time. The subjects in our experiment were smarter than we were.The clincher came from Tarr's <strong>the</strong>sis. He had replicated our experimentsusing three-dimensional shapes and <strong>the</strong>ir mirror images, rotatedin <strong>the</strong> picture plane (shown below) and in depth:0° 45° 90° 135° 180°Everything came out <strong>the</strong> same as for <strong>the</strong> 2-D shapes, except whatpeople did with <strong>the</strong> mirror images. Just as a misoriented 2-D shape canbe matched to <strong>the</strong> standard orientation by a rotation in <strong>the</strong> 2-D pictureplane, and its mirror image can be rotated to <strong>the</strong> standard orientation bya 180-degree flip in <strong>the</strong> third dimension, a misoriented 3-D shape (toprow) can be rotated to <strong>the</strong> standard orientation in 3-D space, and its mirrorimage (bottom row) can be rotated to <strong>the</strong> standard by a 180-degreeflip in <strong>the</strong> fourth dimension. (In H. G. Wells' "The Plattner Story," anexplosion blows <strong>the</strong> hero into four-dimensional space. When he returns,his heart is on <strong>the</strong> right side and he writes backwards with his left hand.)The only difference is that mere mortals should not be able to mentallyrotate a shape in <strong>the</strong> fourth dimension, our mental space being strictly3-D. All <strong>the</strong> versions should show an effect of tilt, unlike what wehad found for 2-D shapes, where <strong>the</strong> mirror images did not. That's whathappened. The subtle difference between two- and three-dimensional

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