Locational representation in imagery: a moving spot task.

Subjects with eyes closed imagined a spot moving from cell to cell in a matrix in response to spoken instructions: Up, Down, Left, and Right. After 12 such moves from a given starting point, the subject indicated the final position of the spot. Accuracy was studied as a function of matrix order, which varied from 3 X 3 to 8 X 8. (Visual angles of the displayed matrices were also varied, from 4.6 degrees/cell in one experiment to 1 degree/cell in another, but with little or no effect on results.) The greatest decrement in accuracy occurred between 3 X 3 and 4 X 4 cells. Moreover, showing the subjects a systematic way of structuring matrices (i.e., of dividing them into parts that could be visualized more or less individually) improved performance markedly on all but the 3 X 3 matrix. These results, which agree fairly well with the verbal reports of the subjects, indicate that some capacity limitation of the image system is exceeded in going from a 3 X 3 to a 4 X 4 grid. Implications concerning the nature of imagery are discussed.

Accuracy of extrapolating a pointer into perceived and imagined space.

The authors investigated the ability of people to extrapolate lines in tridimensional space: an experimenter pointed at variously located targets, and the subjects tried to identify them. In an experimental condition, the subjects sat with their backs turned to the target objects, which they had previously inspected during a brief learning period; in two control conditions the targets were in front of the subjects and clearly visible. Overall error (difference in degrees of pointer angle between correct and judged targets) was no worse for objects imagined behind the head than for visible targets, about 2.7 degrees (root-mean-square) in both cases. Variable error was 29% greater for the imagined objects, but constant error was greater on the matched control targets. Some qualitative differences, in constant error patterns, were also found. With due allowance for these differences, the results strongly indicate that imagined space is functionally continuous with perceived space in the representational system.

The visual world behind the head.

After a 5-minute inspection of 7 objects laid out on a shelf, subjects were seated with the objects behind them and answered questions about the locations and orientations of objects by throwing a switch left or right. The "visual image" subjects were told to imagine that the objects were still in front of them and to respond accordingly. The "real space" (RS) subjects were told to respond in terms of the positions of the objects in real space behind them. Thus correct responses (left vs. right) were completely opposite for the 2 groups. A control group responded while facing a curtain concealing the objects. The task was harder, by time and error criteria, for group RS than for the other 2 groups, but not dramatically so. All RS subjects denied using a response-reversal strategy. Some reported translating the objects from back to front and thus responding as to a mirror-image of the array. When this evasion was discouraged, RS subjects typically reported responding in terms of visual images located behind them and viewed as if by "eyes in the back of the head." The paradox of a visual image that corresponds to no possible visual input is discussed.