Jan W. de Fockert

Load Theory of Selective Attention and Cognitive Control

A load theory of attention in which distractor rejection depends on the level and type of load involved in current processing was tested. A series of experiments demonstrates that whereas high perceptual load reduces distractor interference, working memory load or dual-task coordination load increases distractor interference. These findings suggest 2 selective attention mechanisms: a perceptual selection mechanism serving to reduce distractor perception in situations of high perceptual load that exhaust perceptual capacity in processing relevant stimuli and a cognitive control mechanism that reduces interference from perceived distractors as long as cognitive control functions are available to maintain current priorities (low cognitive load). This theory resolves the long-standing early versus late selection debate and clarifies the role of cognitive control in selective attention.

The Role of Working Memory in Attentional Capture

Much previous research has demonstrated that visual search is typically disrupted by the presence of a unique “singleton” distractor in the search display. Here we show that attentional capture by an irrelevant color singleton during shape search critically depends on availability of working memory to the search task: When working memory is loaded in a concurrent yet unrelated verbal short-term memory task, capture increases. These findings converge with previous demonstrations that increasing working memory load results in greater distractor interference in Stroop-like tasks (de Fockert, Rees, Frith, & Lavie, 2001; Lavie, Hirst, de Fockert, & Viding, 2004), which support the hypothesis that working memory provides goal-directed control of visual selective attention allowing to minimize interference by goal-irrelevant distractors.

Neural Correlates of Attentional Capture in Visual Search

Much behavioral research has shown that the presence of a unique singleton distractor during a task of visual search will typically capture attention and thus disrupt target search. Here we examined the neural correlates of such attentional capture using functional magnetic resonance imaging in human subjects during performance of a visual search task. The presence (vs. absence) of a salient yet irrelevant color singleton distractor was associated with activity in the superior parietal cortex and frontal cortex. These findings imply that the singleton distractor induced spatial shifts of attention despite its irrelevance, as predicted from an AC account. Moreover, behavioral interference by singleton distractors was strongly and negatively correlated with frontal activity. These findings provide direct evidence that the frontal cortex is involved in control of interference from irrelevant but attention-capturing distractors.

Contrasting effects of sensory limits and capacity limits in visual selective attention

The effects of perceptual load and those of target-stimulus degradation on distractor processing were contrasted. Targets either had to be found among several nontargets (high perceptual load) or were presented alone and were intact (low perceptual load), had reduced size and contrast (Experiment 1), had reduced duration and were followed by a mask (Experiment 2), or had reduced visual acuity owing to position eccentricity (Experiment 3) in thedegraded low-load condition. The results showed that both high perceptual load and target degradation increased general task difficulty, as is reflected by overall reaction times and accuracy. However, whereas high perceptual load reduced response-competition effects of irrelevant distractors, target degradation increased distractor effects. These results support the hypothesis that distractor processing depends on the extent to which high perceptual load exhausts attention in relevant processing, and provide a dissociation between perceptual load and general task difficulty and processing speed.

Rapid extraction of mean identity from sets of faces

Recent evidence suggests that the notion that the visual system can rapidly extract summary statistics from complex scenes extends to representing sets of faces in terms of mean emotion or gender. Here we show that observers can also extract a mean identity from a set of faces with different identities. Observers first saw a set of four faces with different identities and were subsequently asked whether or not a single test face had been present in the preceding set. They were significantly more likely to respond that the test face had been present in the set if it was the morphed mean of all four set faces than when it was an actual set member. This finding suggests that representations based on summary statistics are available for face identity.

ERP evidence for successful voluntary avoidance of conscious recollection

We investigated neurocognitive processes of voluntarily avoiding conscious recollection by asking participants to either attempt to recollect (the Think condition) or to avoid recollecting (the No-Think condition) a previously exposed paired associate. Event-related potentials (ERPs) during Think and No-Think trials were separated on the basis of previous learning success versus failure. This separation yielded temporal and topographic dissociations between early ERP effects of a Think versus No-Think strategy, which were maximal between 200 and 300 ms after stimulus presentation and independent of learning status, and a later learning-specific ERP effect maximal between 500 and 800 ms after stimulus presentation. In this later time-window, Learned Think items elicited a larger late left parietal positivity than did Not Learned Think, Learned No-Think, and Not Learned No-Think items; moreover, Learned No-Think and Not Learned Think items did not differ in late left parietal positivity. Because the late left parietal positivity indexes conscious recollection, the results provide firm evidence that conscious recollection of recollectable information can be voluntarily avoided on an item-specific basis and help to clarify previous neural evidence from the Think/No-Think procedure, which could not separate item-specific from strategic processes.

ERP and behavioural evidence for direct suppression of unwanted memories

There are some past experiences that we would prefer not to remember. Previous research has shown that repeatedly stopping retrieval of an unwanted memory increases the probability of later forgetting of that memory, and engages prefrontal control mechanisms to attenuate activity in the hippocampus. However, the mechanisms of preventing memory retrieval, and how these relate to the later forgetting, are yet to be fully understood. Here we present neural and behavioural evidence that two distinct strategies for retrieval stopping - direct memory suppression and self-distracting thought substitution - contribute to forgetting of unwanted memories in qualitatively different ways. Only direct memory suppression reduced centro-parietal positivity in the event-related potentials (ERP) between 300 and 600 ms post-stimulus, consistent with a reduction in the ERP correlate of recollection. Furthermore, only direct memory suppression produced later inhibitory forgetting that was predicted by an earlier negative ERP effect that may be associated with motor inhibition. In contrast, thought substitution produced later non-inhibitory forgetting and had no effect on the ERP correlate of recollection. Our findings demonstrate the first ERP and behavioural dissociation between inhibitory and non-inhibitory forgetting, and suggest that unwanted memories may be directly suppressed without selective retrieval of alternative memories.

Frontal control of attentional capture in visual search

Lavie and colleagues recently suggested that cognitive control functions that are mediated by frontal cortex provide goal-directed control of selective attention, serving to minimize interference by goal-irrelevant distractors. Here we provide new evidence for this claim from an attentional capture paradigm. An event-related fMRI experiment shows that the presence (vs. absence) of an irrelevant colour singleton distractor in a visual search task was not only associated with activity in superior parietal cortex, in line with a psychological attentional capture account, but was also associated with frontal cortex activity. Moreover, behavioural interference by the singleton was negatively correlated with frontal activity, suggesting that frontal cortex is involved in control of singleton interference. Behavioural tests confirmed that singleton interference depends on availability of cognitive control to the search task: Singleton interference was significantly increased by high working memory load. These results demonstrate the important role of frontal cognitive control of attention by working memory in minimizing distraction.

More Accurate Size Contrast Judgments in the Ebbinghaus Illusion by a Remote Culture

The Ebbinghaus (Titchener) illusion was examined in a remote culture (Himba) with no words for geometric shapes. The illusion was experienced less strongly by Himba compared with English participants, leading to more accurate size contrast judgments in the Himba. The study included two conditions of inducing stimuli. The illusion was weaker when the inducing stimuli were dissimilar (diamonds) to the target (circle) compared with when they were similar (circles). However, the illusion was weakened to the same extent in both cultures. It is argued that the more accurate size judgments of the Himba derive from their tendency to prioritize the analysis of local details in visual processing of multiple objects, and not from their impoverished naming.

Behavioral and ERP evidence of greater distractor processing in old age

The ability to minimize processing for irrelevant information is a central component of goal-directed behavior, which has been suggested to be compromised in old age. In this study, we investigate age differences in distractor rejection by presenting target names alongside to-be-ignored distractor faces. Older adults (mean age 70) showed greater behavioral slowing than young adults (mean age 24) when the distractor face was incompatible with the target name. That this increased interference in the older adults was indeed associated with more distractor processing, was shown by the face-related N170 component of the EEG, which had greater amplitude in older adults when faces were unattended, but not when they were attended. These findings suggest a reduced ability to prevent distractor processing in old age.