Eliot Hearst Indiana University
When Kurt Koffka ( 1935) asked his readers to consider why we normally "see things and not the holes between them," individual reaction differed as to whether the question was profound, trivial, misleading, or meaningless. A similar profile of opinions would probably greet a contemporary student of learning and cognition who wondered aloud why we seem to more easily associate things with each other than with the temporal or spatial holes that surround or separate them. If he or she were a persistent person, this psychologist might argue that the selfsame kind of associative bias affects the thinking of researchers, so that certain possibilities for learning have been neglected in the laboratory. Until fairly recently, for example, the study of Pavlovian conditioning in animals has concentrated on phenomena produced by closely contiguous occurrences of conditioned stimuli (CSs) and unconditioned stimuli (USs), and not on phenomena engendered by delivering USs only in the absence of some definite stimulus or by presenting discrete CSs that are never followed shortly by another event.
On the surface, cases in which the absence of something serves as a signal, or is itself signaled by the presentation of a particular stimulus, may appear to have weak ecological validity. The international spy does not indicate the need for a secret meeting by removing a red flower from the pot on his window sill, and the baseball coach does not use a failure to touch his cap as a sign for players to swing the bat or steal a base. Billboards do not announce times when a special bargain is unavailable, and beacons do not start flashing when a danger is over. Like the figure- ground principle that Koffka was exemplifying in his question, nonoccurrences and omissions of some event or behavior seem generally less salient, memorable, or informative than occurrences or additions. Without the establishment of special conditions to overcome or replace attention to and control by positive happenings, efficient learning and performance based on absence seem comparatively hard to achieve. Often theorists find it useful to treat absence in terms of prevailing states or circumstances -- situational cues, contexts, background stimuli -- which, to continue the Gestalt metaphor, represents a way of altering or reversing figure-ground relations.