Acquisition of shape information in working memory, as a function of viewing time and number of consecutive images: evidence for a succession of discrete storage classes

Abstract

The capacity of visual working memory was investigated using abstract images that were slightly distorted N× N (with generally N=8) square lattices of black or white randomly selected elements. After viewing an image, or a sequence of images, the subjects viewed couples of images containing the test image and a distractor image derived from the first one by changing the black or white value of q randomly selected elements. The number q was adjusted in each experiment to the difficulty of the task and the abilities of the subject. The fraction of recognition errors, given q and N was used to evaluate the number M of bits memorized by the subject. For untrained subjects, this number M varied in a biphasic manner as a function of the time t of presentation of the test image: it was on average 13 bits for 1 s, 16 bits for 2 to 5 s, and 20 bits for 8 s. The slow pace of acquisition, from 1 to 8 s, seems due to encoding difficulties, and not to channel capacity limitations. Beyond 8 s, M( t), accurately determined for one subject, followed a square root law, in agreement with 19th century observations on the memorization of lists of digits. When two consecutive 8×8 images were viewed and tested in the same order, the number of memorized bits was downshifted by a nearly constant amount, independent of t, and equal on average to 6–7 bits. Across the subjects, the shift was independent of M. When two consecutive test images were related, the recognition errors decreased for both images, whether the testing was performed in the presentation or the reverse order. Studies involving three subjects, indicate that, when viewing m consecutive images, the average amount of information captured per image varies with m in a stepwise fashion. The first two step boundaries were around m=3 and m=9–12. The data are compatible with a model of organization of working memory in several successive layers containing increasing numbers of units, the more remote a unit, the lower the rate at which it may acquire encoded information.