Árni Kristjánsson

Where perception meets memory: A review of repetition priming in visual search tasks

What we have recently seen and attended to strongly influences how we subsequently allocate visual attention. A clear example is how repeated presentation of an object’s features or location in visual search tasks facilitates subsequent detection or identification of that item, a phenomenon known as priming. Here, we review a large body of results from priming studies that suggest that a short-term implicit memory system guides our attention to recently viewed items. The nature of this memory system and the processing level at which visual priming occurs are still debated. Priming might be due to activity modulations of low-level areas coding simple stimulus characteristics or to higher level episodic memory representations of whole objects or visual scenes. Indeed, recent evidence indicates that only minor changes to the stimuli used in priming studies may alter the processing level at which priming occurs. We also review recent behavioral, neuropsychological, and neurophysiological evidence that indicates that the priming patterns are reflected in activity modulations at multiple sites along the visual pathways. We furthermore suggest that studies of priming in visual search may potentially shed important light on the nature of cortical visual representations. Our conclusion is that priming occurs at many different levels of the perceptual hierarchy, reflecting activity modulations ranging from lower to higher levels, depending on the stimulus, task, and context—in fact, the neural loci that are involved in the analysis of the stimuli for which priming effects are seen.

In Search of Remembrance: Evidence for Memory in Visual Search

Observers searched for a target among distractors while the display items traded places every 110 ms. Search was slower when the target was always relocated to a position previously occupied by a distractor than when the items remained in place, showing the importance of memory for locations in a visual search task. Experiment 2 repeated a previous study in which items could move to any location within the display, but used a larger range of set sizes than tested in the earlier study. A cost in search times to relocating items was found at the larger set sizes, most likely reflecting that the probability that the target would replace a distractor increased with the set size. The findings provide strong evidence for the role of memory for locations within trials in a visual search task.

Fortune and reversals of fortune in visual search: Reward contingencies for pop-out targets affect search efficiency and target repetition effects

Rewards have long been known to modulate overt behavior. But their possible impact on attentional and perceptual processes is less well documented. Here, we study whether the (changeable) reward level associated with two different pop-out targets might affect visual search and trial-to-trial target repetition effects (see Maljkovic & Nakayama, 1994). Observers searched for a target diamond shape with a singleton color among distractor diamond shapes of another color (e.g., green among red or vice versa) and then judged whether the target had a notch at its top or bottom. Correct judgments led to reward, with symbolic feedback indicating this immediately; actual rewards accumulated for receipt at study end. One particular target color led to a higher (10:1) reward for 75% of its correct judgments, whereas the other singleton target color (counterbalanced over participants) yielded the higher reward on only 25% of the trials. We measured search performance in terms of inverse efficiency (response time/proportion correct). The reward schedules not only led to better performance overall for the more rewarding target color, but also increased trial-to-trial priming for successively repeated targets in that color. The actual level of reward received on the preceding trial affected this, as did (orthogonally) the likely level. When reward schedules were reversed within blocks, without explicit instruction, corresponding reversal of the impact on search performance emerged within around 6 trials, asymptoting at around 15 trials, apparently without the observers’ explicit knowledge of the contingency. These results establish that pop-out search and target repetition effects can be influenced by target reward levels, with search performance and repetition effects dynamically tracking changes in reward contingency.

Priming in visual search: Separating the effects of target repetition, distractor repetition and role-reversal

Recent studies have identified between-trial priming effects in visual search tasks, but often with constraints on the possible similarities or changes across successive trials, and usually with the main emphasis on effects of target repetition. Here we sought to obtain a more thorough characterization of between-trial priming effects in speeded visual search, where observers determined target presence or absence among a set of distractors. The results show that various separable priming effects have a major influence on visual search performance. Facilitation was evident when a target was repeated between-trials, but there was also strong priming due to repetition of distractor types, even between successive trials for which no target was presented on either trial. Search also proceeded faster if the same distractor types were repeated, even when the current target was different from the preceding target. We also investigated the possible impact of role-reversals for particular display items, from being a target on one trial to becoming a distractor on the next, and vice-versa. We find that such role-reversals substantially affect search performance, over and above the effects of repetition per se when those were held constant.

Rapid learning in attention shifts: A review

Many lines of evidence show that the human visual system does not simply passively register whatever appears in the visual field. The visual system seems to preferentially “choose” stimuli according to what is most relevant for the task at hand, a process called attentional selection. Given the large amount of information in any given visual scene, and well-documented capacity limitations for the representation of visual stimuli, such a strategy seems only reasonable. Consistent with this, human observers are surprisingly insensitive to large changes in their visual environment when they attend to something else in the visual scene. Here I argue that attentional selection of pertinent information is heavily influenced by the stimuli most recently viewed that were important for behaviour. I will describe recent evidence for the existence of a powerful memory system, not under any form of voluntary control, which aids observers in orienting quickly and effectively to behaviourally relevant stimuli in the visual environment, in particular the stimuli that have been important in the immediate past. I will also discuss research into the potential neural mechanisms involved in these learning effects. Finally, I will discuss how these putative memory mechanisms may help in maintaining the apparent stability and continuity of the ever-changing visual environment, which is such a crucial component of our everyday visual experience.

Efficient visual search without top-down or bottom-up guidance

Two types of mechanisms have dominated theoretical accounts of efficient visual search. The first are bottom-up processes related to the characteristics of retinotopic feature maps. The second are topdown mechanisms related to feature selection. To expose the potential involvement of other mechanisms, we introduce a new search paradigm whereby a target is defined only in a context-dependent manner by multiple conjunctions of feature dimensions. Because targets in a multiconjunction task cannot be distinguished from distractors either by bottom-up guidance or top-down guidance, current theories of visual search predict inefficient search. While inefficient search does occur for the multiple conjunctions of orientation with color or luminance, we find efficient search for multiple conjunctions of luminance/size, luminance/shape, and luminance/topology. We also show that repeated presentations of either targets or a set of distractors result in much faster performance and that bottom-up feature extraction and top-down selection cannot account for efficient search on their own. In light of this, we discuss the possible role of perceptual organization in visual search. Furthermore, multiconjunction search could provide a new method for investigating perceptual grouping in visual search.

A primitive memory system for the deployment of transient attention

When transient attention is summoned by the sudden appearance of a large cue, it can be deployed to a small portion of the cue where a target appeared on previous trials (Kristjánsson, Mackeben, & Nakayama, 2001). This result runs counter to the view that transient, or exogenous, attention is summoned automatically and indiscriminately to abruptly appearing stimuli. To further characterize the short-term learning mediating this phenomenon, we report the following results. (1) When there was a consistent relationship between a small identifying portion of the cue and the target, learning occurred rapidly. Thus, transient attention can be summoned to a distinctively colored or distinctively shaped portion of the cue as a consequence of repeated pairing (Experiments 1 and 2). (2) When there was a consistent relation between a given position on the object and itsoverall color or shape, no learning occurred (Experiments 3 and 4). Thus, transient attention cannot learn a complex relation between target position and shape or color. (3)We confirmed the fast object-centered learning of position shown in Kristjánsson et al. (2001). (4) Explicit knowledge of the cue-target relationship had no effect on the performance of the task. The results provide evidence for the existence of a primitive object-centered learning mechanism beneficial for the rapid deployment of transient attention. The possible role of such a mechanism in the maintenance of representations of the visual environment is discussed.

Less attention is more in the preparation of antisaccades, but not prosaccades

To make a saccadic eye movement to a target we must first attend to it. It is therefore not surprising that diverting attention increases saccade latency, but is latency increased in all cases? We show that attending to a peripheral discrimination task has a paradoxical effect. If the stimulus to be attended appears shortly (100 to 300 ms) before an eye movement is made in a direction opposite to that of a presented stimulus (an antisaccade), its latency is reduced to well below baseline performance. In contrast, latencies for saccades toward the stimulus (prosaccades) are increased under similar conditions. This paradoxical effect may arise from competition between the processes mediating prosaccades and antisaccades. When the discrimination task is presented at the critical moment, it interferes with a reflexive prosaccade, allowing faster antisaccades. The results suggest that the suppression of sensorimotor reflexes can facilitate volitional motor acts.

Rapid, object-based learning in the deployment of transient attention

We show that transient attention summoned by an exogenous cue shows rapid learning of the relationship between the cue and a subsequent target in a discrimination task. In experiment 1, performance was unaffected when a target always appeared in the same position on a large cue, but was degraded when the target could appear anywhere within the extent of the larger cue. Experiment 2 shows that it was not the predictability of where the target appeared within the cue that aided performance, but rather a consistent location mapping of cue and target, since predictably alternating the target location relative to the cue led to worse performance than when the target was presented in the same location relative to the cue from trial to trial. Further analysis of the results of experiment 2 shows that the learning is rapid, evident after one trial, and has a cumulative influence over four consecutive trials. Possible neural correlates of this form of learning are discussed, with a focus on the supplementary eye fields in the prefrontal cortex. The reported experiments show that transient attention is not a simple reflexive mechanism but can show rapid visuospatial learning, in object-based coordinates.

Simultaneous priming along multiple feature dimensions in a visual search task

What we have recently seen generally has a large effect on how we consequently perceive our visual environment. Such priming effects play a surprisingly large role in visual search tasks, for example. It is unclear, however, whether different features of an object show independent but simultaneous priming. For example, if the color and orientation of a target item are the same as on a previous trial, is performance better than if only one of those features is repeated? In other words this paper presents an attempt at assessing the capacity of priming for different feature dimensions. Observers searched for a three featured object (a gabor patch that was either redscale or greenscale, oriented either to the left or right of vertical and of high or low spatial frequency) among distractors with different values along these feature dimensions. Which feature was the target defining feature; which was the response defining feature and which was the irrelevant feature, was varied between the different experiments. Task relevant features (target defining, or response defining) always resulted in priming effects, while when spatial frequency or orientation were task irrelevant neither resulted in priming, but color always did, even when task irrelevant. Further experiments showed that priming from spatial frequency and orientation could occur when they were task irrelevant but only when the other feature of the two was kept constant across all display items. The results show that simultaneous priming for different features can occur simultaneously, but also that task relevance has a strong modulatory effect on the priming.