mental rotation as generalization-Roger Shepard is a psychologist who has studied what he calls "mental rotation." In a typical experiment (Cooper & Shepard, 1973), people were shown letters that had been rotated varying degrees from their normal, upright position and were asked whether the letters were backward (that is, mirror images of the original) or not. The result was that the greater the rotation, the longer it took people to answer. Shepard concludes from such data that people mentally rotate an "internal representation" or image of the letter until it is in its normal, upright position and then decide whether it is backward.Although Shepard refers to the mental rotation of images, his data consist of the time it takes to react to rotated figures. It is interesting that when these data are plotted graphically, the resulting curve looks remarkably like a generalization gradient (Figure 11-7). Participants respond most quickly to the "training stimulus" (the letter they were trained in school to recognize); the less the stimulus resembles the training stimulus, the slower is the response.-In one experiment, Donna Reit and Brady Phelps (1996) used a computer program to train college students to discriminate between geometric shapes that did and did not match a sample. The items were rotated from the sample position by 0, 60, 120, 180, 240, or 300 degrees. The students received feed- back after each trial. When the researchers plotted the data for reaction times, the results formed a fairly typical generalization gradient (-In a second experiment, Phelps and Reit (1997) got nearly identical results, except that with continued training the generalization gradients flattened. This is probably because students continued to receive feedback during testing and therefore improved their reaction times to rotated items. (They could not improve their performance on unrotated items much because they were already reacting to those items quite quickly.) In any case, these data clearly suggest that "mental rotation" data are generalization data.-Phelps and Reit note that most of their students, like Shepard's, reported that they solved the problems by "mentally rotating" the test stimuli. As Phelps and Reit point out, however, the subjective experience of mental rotation does not explain the differences in reaction times. A scientific explanation must point to physical features of the situation and to the learning history of the participant. The expression "mental rotation" at best identifies the covert behavior involved; it does not explain the participant's performance.