Testing visual short-term memory of pigeons (Columba livia) and a rhesus monkey (Macaca mulatta) with a location change detection task

Change detection is commonly used to assess capacity (number of objects) of human visual short-term memory (VSTM). Comparisons with the performance of non-human animals completing similar tasks have shown similarities and differences in object-based VSTM, which is only one aspect (“what”) of memory. Another important aspect of memory, which has received less attention, is spatial short-term memory for “where” an object is in space. In this article, we show for the first time that a monkey and pigeons can be accurately trained to identify location changes, much as humans do, in change detection tasks similar to those used to test object capacity of VSTM. The subject’s task was to identify (touch/peck) an item that changed location across a brief delay. Both the monkey and pigeons showed transfer to delays longer than the training delay, to greater and smaller distance changes than in training, and to novel colors. These results are the first to demonstrate location-change detection in any non-human species and encourage comparative investigations into the nature of spatial and visual short-term memory.     

False memory in a short-term memory task

The Deese/Roediger-McDermott (DRM; Roediger & McDermott, 1995) paradigm reliably elicits false memories for critical nonpresented words in recognition tasks. The present studies used a Sternberg (1966) task with DRM lists to determine whether false memories occur in short-term memory tasks and to assess the contribution of latency data in the measurement of false memories. Subjects studied three, five, or seven items from DRM lists and responded to a single probe (studied or nonstudied). In both experiments, critical lures were falsely recognized more often than nonpresented weak associates. Latency data indicated that correct rejections of critical lures were slower than correct rejections of weakly related items at all set sizes. False alarms to critical lures were slower than hits to list items. Latency data can distinguish veridical and false memories in a short-term memory task. Results are discussed in terms of activation-monitoring models of false memory.     

Implicit learning for probable changes in a visual change detection task

Previous research demonstrates that implicitly learned probability information can guide visual attention. We examined whether the probability of an object changing can be implicitly learned and then used to improve change detection performance. In a series of six experiments, participants completed 120–130 training change detection trials. In four of the experiments the object that changed color was the same shape (trained shape) on every trial. Participants were not explicitly aware of this change probability manipulation and change detection performance was not improved for the trained shape versus untrained shapes. In two of the experiments, the object that changed color was always in the same general location (trained location). Although participants were not explicitly aware of the change probability, implicit knowledge of it did improve change detection performance in the trained location. These results indicate that improved change detection performance through implicitly learned change probability occurs for location but not shape.     

The influence of memory for prior instances on performance in a conflict detection task

In 3 experiments, the authors examined the role of memory for prior instances for making relative judgments in conflict detection. Participants saw pairs of aircraft either repeatedly conflict with each other or pass safely before being tested on new aircraft pairs, which varied in similarity to the training pairs. Performance was influenced by the similarity between aircraft pairs. Detection time was faster when a conflict pair resembled a pair that had repeatedly conflicted. Detection time was slower, and participants missed conflicts, when a conflict pair resembled a pair that had repeatedly passed safely. The findings identify aircraft features that are used as inputs into the memory decision process and provide an indication of the processes involved in the use of memory for prior instances to make relative judgments.     

Video reminders in a representational change task: memory for cues but not beliefs or statements

Two experiments investigated the effect of video reminders on 3-year-olds' performance in a representational change task. In Experiment 1, children in a video support condition viewed videotapes of their initial incorrect statements about a misleading container prior to being asked to report their initial belief. Children in a control condition viewed an irrelevant videotape. Despite reporting what they had said on the videotape, children in the video support condition typically failed the representational change task. Experiment 2 replicated the main findings from Experiment 1 and also revealed that a video reminder failed to increase the likelihood that children would correctly report what they had said about the object. Results are discussed in terms of the processes whereby mnemonic cues might affect performance on tasks assessing theory of mind.     

Accessing long-term memory representations during visual change detection

In visual change detection tasks, providing a cue to the change location concurrent with the test image (post-cue) can improve performance, suggesting that, without a cue, not all encoded representations are automatically accessed. Our studies examined the possibility that post-cues can encourage the retrieval of representations stored in long-term memory (LTM). Participants detected changes in images composed of familiar objects. Performance was better when the cue directed attention to the post-change object. Supporting the role of LTM in the cue effect, the effect was similar regardless of whether the cue was presented during the inter-stimulus interval, concurrent with the onset of the test image, or after the onset of the test image. Furthermore, the post-cue effect and LTM performance were similarly influenced by encoding time. These findings demonstrate that monitoring the visual world for changes does not automatically engage LTM retrieval.     

Delayed signal detection, differential reinforcement, and short-term memory in the pigeon.

In two discrete-trial delayed-detection experiments, six pigeons were trained on dependent concurrent variable-interval schedules. Pecking a red side key was reinforced when the brighter of two white lights (S1) had been presented on the center key, and pecking a green side key was reinforced when the duller of two white lights (S2) had been presented on the center key. Incorrect responses were red side-key pecks following S2 presentations and green side-key pecks following S1 presentations; these resulted in three-second blackouts. In Experiment 1, the time between presentation of S1 or S2 on the center key and the onset of the red and green side keys was varied nonsystematically from 0.06 seconds to 19.69 seconds across experimental conditions. Stimulus discriminability decreased as the stimulus-choice delay increased. A rectangular-hyperbolic function better described this decrease in discriminability over time than did a negative-exponential function. In Experiment 2, at each of three stimulus-choice delays (0.06, 3.85, and 10.36 seconds), relative reinforcer frequency for correct responses to the red and green side keys was varied by changing the values of the dependent concurrent variable-interval schedules. The sensitivity of choice to relative reinforcer frequency was independent of the decrease in stimulus discriminability with increasing stimulus-choice delay.     

Monkeys exhibit prospective memory in a computerized task

Prospective memory (PM) involves forming intentions, retaining those intentions, and later executing those intended responses at the appropriate time. Few studies have investigated this capacity in animals. Monkeys performed a computerized task that assessed their ability to remember to make a particular response if they observed a PM cue embedded within an ongoing learning-set (LS) task. At a break in the LS task, monkeys selected one of two icons indicating that they had or had not encoded the occurrence of the PM cue (the latter icon resumed the LS task). Critically, during this response period, the PM response icon appeared after a delay during which monkeys could self-initiate the PM response prior to receiving any external prompt. Monkeys selected the PM and LS icons when each was the optimal response, illustrating that they could encode, store, and respond appropriately to a stimulus event in the future. Critically, some monkeys self-initiated the PM response prior to that icon's appearance, indicating that they could retrieve the PM and act on their intention to make that response without the aid of a prompt. These monkeys appeared capable of using PM in this task. Thus, this capacity appears not to be limited to humans.     

Attenuating memory distrust in a repeated checking task

Repeated checking has been demonstrated to lead to reductions in memory confidence in several previous studies using student and clinical samples. This process of reduced confidence in memory and detail for memory, are thought to arise from the inhibition of perceptual processing that develops during repeated checking. Our research investigated whether reduced memory confidence from repeated checking could be attenuated through the use of novel stimuli during the repeated checking task. Three groups were generated through random assignment of 65 undergraduate students. As seen in previous research, individuals who repeatedly checked a stimulus (a virtual stovetop) showed reduced memory confidence, vividness, and detail, when compared with individuals who repeatedly checked a different stimulus. A third group in which the colour of the repeatedly-checked stovetop changed every five trials showed no significant decline in memory confidence between the pre-test and post-test. Results suggest that increased memory distrust can be ameliorated through the use of stimuli with characteristics that are novel and distinctive. Findings are discussed in the context of the existing model of repeated checking and memory confidence, and implications for treatment methods are presented.     

Differences in target monitoring in a prospective memory task

The goal of the present study was to examine individual differences in the degree to which controlled attention is allocated towards a prospective memory (PM) task. Using a PM task that should require high levels of controlled attention in a sample of 138 young, middle-aged, and older adults, two subgroups of participants could be identified, i.e., participants who clearly demonstrated evidence for monitoring and those for whom no clear evidence for monitoring was revealed. A control group (n=95) was tested to control for practice effects in the ongoing task. Differences between subgroups were examined in terms of age, PM accuracy, baseline ongoing task performance, and general negative mood. Nonmonitorers and monitorers differed in age (more older adults being nonmonitorers), ongoing task accuracy (a nonsignificant trend was observed here), PM task accuracy (both young and middle-aged/older monitorers were more accurate than nonmonitorers), and the number of reported depressive symptoms (nonmonitorers > monitorers). Moreover, results showed that even in nonmonitorers PM accuracy was above floor level, indicating that noticing and reacting to some of PM cues is possible without strongly investing in resource demanding monitoring processes.     

Sustained change blindness to incremental scene rotation: a dissociation between explicit change detection and visual memory

In a change detection paradigm, the global orientation of a natural scene was incrementally changed in 1 degree intervals. In Experiments 1 and 2, participants demonstrated sustained change blindness to incremental rotation, often coming to consider a significantly different scene viewpoint as an unchanged continuation of the original view. Experiment 3 showed that participants who failed to detect the incremental rotation nevertheless reliably detected a single-step rotation back to the initial view. Together, these results demonstrate an important dissociation between explicit change detection and visual memory. Following a change, visual memory is updated to reflect the changed state of the environment, even if the change was not detected.     

Ear asymmetry in a dichotic detection task

Pairs of consonant-vowel (CV) syllables were presented dichotically to Ss who were instructed to monitor for the presence of a target CV which could occur in either ear. Ss responded by depressing a response button ; reaction time (RT) was also recorded. Right ear targets were detected 6.2% more frequently, on the average, than left ear targets and had an average RT 50 msec quicker than their left ear counterparts. These results demonstrate the existence of a right ear superiority in dichotic listening when a nonverbal motor response measure is used, supporting the contention that the ear asymmetry phenomenon is truly perceptual in nature and not merely due to the lateralization of verbal output. Two alternative explanations of the RT difference between left and right ear targets are offered. One attributes this difference to the time necessary for intercortical transfer of right hemisphere information, while the second holds that it is due to the longer times needed by the right hemisphere to process information projected to it.     

How to measure working memory capacity in the change detection paradigm

Although the measurement of working memory capacity is crucial to understanding working memory and its interaction with other cognitive faculties, there are inconsistencies in the literature on how to measure capacity. We address the measurement in the change detection paradigm, popularized by Luck and Vogel (Nature, 390, 279-281, 1997). Two measures for this task-from Pashler (Perception & Psychophysics, 44, 369-378, 1988) and Cowan (The Behavioral and Brain Sciences, 24, 87-114, 2001), respectively-have been used interchangeably, even though they may yield qualitatively different conclusions. We show that the choice between these two measures is not arbitrary. Although they are motivated by the same underlying discrete-slots working memory model, each is applicable only to a specific task; the two are never interchangeable. In the course of deriving these measures, we discuss subtle but consequential flaws in the underlying discrete-slots model. These flaws motivate revision in the modal model and capacity measures.     

Apparent movement detection in the pigeon

Pigeons were trained to discriminate between the presence and absence of apparent movement in visual displays. Generalization gradients obtained on the dimension of speed indicated a broad range of movement detection. Extremely accurate discrimination performance on displays varying in direction of movement suggested formation of a "movement concept".     

Involuntary aware memory enhances priming on a conceptual implicit memory task

This study investigated the role of involuntary aware memory (phenomenological awareness of the study episode) on a conceptual implicit memory task. Subjects studied words under levels of processing conditions (LOP: nonsemantic vs. semantic) and then received a category exemplar generation priming task. Subjects were either aware or unaware of the study test relationship. Subjects who were study test aware showed more priming for semantic study than subjects who were study test unaware, thus producing an LOP dissociation between them. The findings suggest that involuntary aware memory can enhance performance on conceptual implicit memory tasks, thus having theoretical implications for implicit and explicit memory.     

Environmental context-dependent recognition memory using a short-term memory task for input

Although many studies have demonstrated that recall is better when tested in the learning environmental context (EC) than in a new EC, almost all of the studies have failed to find EC-dependent memory when recognition was used to measure memory. Evidence of EC-dependent recognition, however, was found in the three experiments of the present study, in which an incidental short-term memory task was used for input of learned material. The results suggest that type of processing during input is a predictor of context-dependent recognition; material studied for a long-term memory test is not susceptible to background context effects, whereas material merely maintained for a short-term memory test is more apt to lead to EC-dependent recognition.