A signal detection analysis of memory for nonoccurrence in pigeons.

Abstract

In a relatively unexplored memory procedure, pigeons indicated whether or not an event recently occurred. On positive trials, a sample stimulus was presented, followed by a delay, followed by a choice between the yes and no alternatives (with yes being the correct choice). On negative trials, the yes and no choice alternatives were presented without a preceding sample, and on these trials no was the correct choice. Recent research on this topic seems to suggest that performance on negative trials is governed by a memory-free default response strategy, whereas performance on positive trials is governed by memory for the sample stimulus. However, a signal detection analysis leads to a different conclusion. In both cases, according to this account, performance was determined by the strength of a memory trace in relation to a decision criterion. For various reasons, investigations of pigeon short-term memory have often involved sample stimuli consisting of presentations of food versus no food. Typically, the presentation of one of these samples is followed, after some delay, by a choice between two comparison stimuli (e.g., red and green). A response to one comparison is reinforced following samples of food, and a response to the other comparison is reinforced following samples of no food. The precise manner in which these sample stimuli are presented varies from study to study. In some cases, trials are initiated by a warning stimulus (e.g., a white light) followed either by the presentation of food or by nothing at all (i.e., no food). In other cases, no warning stimulus is used, and the samples consist of the brief presentation of food or a brief blackout (i.e., no food). In still other cases, no exteroceptive stimulus change is correlated with no-food samples. In spite of these procedural differences, the consistent finding is that performance following samples of food declines as the retention interval increases, whereas performance following samples of no food does not (Colwill, 1984; Colwill & Dickinson, 1980; Grant, 1991; Wilson & Boakes, 1985). The obvious question raised by these results is why performance is unaffected by the size of the retention interval on no-food trials. Although food versus no-food samples clearly differ in motivational value, recent research suggests that a more important difference is that one involves the presence of an event (i.e., the occurrence of food), whereas the other involves the absence of an event (i.e., the nonoccurrence of food). Indeed, the same asymmetrical decay functions were observed by Grant (1991) when samples consisted of the presence versus absence of a variety of stimuli (including colors, shapes, and food). In each case, performance following the presence of an event declined as the retention interval increased, but performance following the absence of an event did not.