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

The Mind's Eye 283once a second. The perception of flipping was so obvious that we didn'tbother to recruit volunteers to confirm it. When the 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 the axisevery time. The subjects in our experiment were smarter than we were.The clincher came from Tarr's thesis. He had replicated our experimentsusing three-dimensional shapes and their mirror images, rotatedin the picture plane (shown below) and in depth:0° 45° 90° 135° 180°Everything came out the same as for the 2-D shapes, except whatpeople did with the mirror images. Just as a misoriented 2-D shape canbe matched to the standard orientation by a rotation in the 2-D pictureplane, and its mirror image can be rotated to the standard orientation bya 180-degree flip in the third dimension, a misoriented 3-D shape (toprow) can be rotated to the standard orientation in 3-D space, and its mirrorimage (bottom row) can be rotated to the standard by a 180-degreeflip in the fourth dimension. (In H. G. Wells' "The Plattner Story," anexplosion blows the hero into four-dimensional space. When he returns,his heart is on the 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 the fourth dimension, our mental space being strictly3-D. All the versions should show an effect of tilt, unlike what wehad found for 2-D shapes, where the mirror images did not. That's whathappened. The subtle difference between two- and three-dimensional