Task-irrelevant filler items alter the dynamics of electrical brain activity during visual search.

In electroencephalography (EEG) studies of visual search, task-irrelevant fillers are included in displays to balance bottom-up stimulation across the visual field and generally considered as inconsequential for performance or EEG results. We examined the impact of fillers on target and distractor processing using lateralised event-related potentials (ERPs). Two task-relevant items (TRIs) were presented, with or without fillers. One TRI (target or target-colour distractor) was on the vertical midline and the other in a lateral position (left or right visual field) on an imaginary circle around fixation. An N2pc was elicited by lateral targets and task-relevant distractors, suggesting that attention was allocated to the lateral TRI because it possessed a target defining feature (colour). A Ptc was only elicited by lateral task-relevant distractors, in line with previous research suggesting that this component is associated with distractor processing. When fillers were also in the circular arrangement, alterations in performance and neural activity occurred. Fillers enhanced and delayed attentional deployment (N2pc) and delayed distractor processing (Ptc). Critically, we observed no difference in Ptc amplitude according to filler presence. Thus, if the Ptc reflects active suppression (or attentional disengagement), it was not required for fillers. ERPs were also modulated by the distance between TRIs (which could be separated by one or four filler positions), but differently according to the colour scheme (blue TRIs with grey fillers or vice versa). Our results suggest that fillers affect lateralised electrophysiological activity at multiple loci during visual search and should not be considered inconsequential.

Dual task interference on early perceptual processing.

When two tasks, Task 1 and Task 2, are conducted in close temporal proximity and a separate speeded response is required for each target (T1 and T2), T2 report performance decreases as a function of its temporal proximity to T1. This so-called psychological refractory period (PRP) effect on T2 processing is largely assumed to reflect interference from T1 response selection on T2 response selection. However, interference on early perceptual processing of T2 has been observed in a modified paradigm, which required changes in visual-spatial attention, sensory modality, task modality, and response modality across targets. The goal of the present study was to investigate the possibility of early perceptual interference by systematically and iteratively removing each of these possible non perceptual confounds, in a series of four experiments. To assess T2 visual memory consolidation success, T2 was presented for a varying duration and immediately masked. T2 report accuracy, which was taken as a measure of perceptual-encoding or consolidation-success, decreased across all experimental control conditions as T1-T2 onset proximity increased. We argue that our results, in light of previous studies, show that central processing of a first target, responsible for the classical PRP effect, also interferes with early perceptual processing of a second target. We end with a discussion of broader implications for psychological refractory period and attentional blink effects.

Effects of task-irrelevant or filler items on brain mechanisms of visual spatial attention.

Many visual search paradigms use color to distinguish task-relevant items from those considered fillers (e.g., blue task-relevant items and grey fillers). Hilimire and colleagues suggested that the N2pc, a lateralized electrophysiological component typically observed in visual attention, is a neural correlate for localized attentional interference, which postulates that target selection is degraded by nearby competing stimuli. In their study, N2pc amplitude decreased with decreasing distance between task-relevant items presented among fillers. With an increase in distance, however, there was also an increase in the number of fillers between task-relevant items. We tested whether this distance effect could be explained by the presence of fillers near task-relevant items rather than their proximity per se. We manipulated the distance between task-relevant items (adjacent, separated by two, or by four positions) and the presence/absence of fillers orthogonally. We used two color schemes: blue task-relevant items and grey fillers or grey task-relevant items and blue fillers (manipulated between-subjects) to control for color interactions. N2pc amplitude increased with increasing distance, but only when fillers were present, suggesting that the results of Hilimire et al. may be due to increasing fillers interference. Exploratory analyses also suggested that the colors selected to be task-relevant and task-irrelevant could play a role in our ability to filter task-irrelevant information. Our results suggest that fillers are not as inconsequential as sometimes assumed and generally support the Ambiguity Resolution Theory, where nearby items increase N2pc amplitude because of a greater need for focused attention.

Electrophysiological evidence of low salience distractor interference during visual search.

Visual search displays often include distractors of lesser salience in addition to a target and one or more salient distractors. We investigated low salience distractor effects on the N2pc, an ERP component indexing the deployment of attention, and the Ptc, a component purported to reflect attentional disengagement. We hypothesized that salient distractors pull the attentional focus away from the target, which could lead to increased attentional processing of low salience distractors close to the target and salient distractor. Participants looked for a colored inverted T during a visual search task while ignoring an L of the same color at a fixed distance on an imaginary circle around fixation. There were four conditions: no additional gray (low salience) distractors, two additional gray distractors between color items, two additional gray distractors just outside the area delimited by the colored items, and four additional gray distractors inside and outside the attended region. The gray distractors impacted N2pc and Ptc amplitude and latency, indicating an effect of gray distractors on attentional processing. Also, additional gray distractors led to increasingly more deviation of the N2pc and Ptc waveforms from the baseline offered by the condition with no additional gray distractors. When we increased the difficulty to individuate the target, we observed more displacement of lateralized activity from the N2pc to the Ptc time window. We argue that distractor-related modulations likely result from increased variance in the latency of attentional engagement activity to the target instead of distractor inhibition or attentional disengagement.

Colour-specific differences in attentional deployment for equiluminant pop-out colours: evidence from lateralised potentials.

We investigated how target colour affected behavioural and electrophysiological results in a visual search task. Perceptual and attentional mechanisms were tracked using the N2pc component of the event-related potential and other lateralised components. Four colours (red, green, blue, or yellow) were calibrated for each participant for luminance through heterochromatic flicker photometry and equated to the luminance of grey distracters. Each visual display contained 10 circles, 1 colored and 9 grey, each of which contained an oriented line segment. The task required deploying attention to the colored circle, which was either in the left or right visual hemifield. Three lateralised ERP components relative to the side of the lateral coloured circle were examined: a posterior contralateral positivity (Ppc) prior to N2pc, the N2pc, reflecting the deployment of visual spatial attention, and a temporal and contralateral positivity (Ptc) following N2pc. Red or blue stimuli, as compared to green or yellow, had an earlier N2pc. Both the Ppc and Ptc had higher amplitudes to red stimuli, suggesting particular selectivity for red. The results suggest that attention may be deployed to red and blue more quickly than to other colours and suggests special caution when designing ERP experiments involving stimuli in different colours, even when all colours are equiluminant.

The attentional blink freezes spatial attention allocation to targets, not distractors: evidence from human electrophysiology.

Previous work found a significant reduction of the amplitude of the N2pc ERP component during the attentional blink in response to lateral visual targets, suggesting that the allocation of attention to visual targets is impaired during the attentional blink. Recent theorizing on the processes reflected by the N2pc suggests the possibility of distinct sets of neural mechanisms underlying its generation, one responsible for target activation, and one for distractor inhibition. To disentangle whether either or both of these mechanisms are impaired during the attentional blink, an RSVP sequence of circles, equidistant from fixation was used. The first target frame (T1) contained the same repeated target colour circle and target whereas the second target frame (T2) contained a distractor colour singleton as well as a target colour singleton. Only the target or only the distractor was presented at a lateral position; the other singleton was presented on the vertical midline so as not to elicit any event-related lateralization. Impaired T2 report accuracy at a short stimulus-onset asynchrony (SOA) was accompanied by a significant delay of the N2pc to lateral T2 targets when compared to a long SOA condition. No such delay was found when the lateralized stimulus was a distractor, suggesting that the attentional blink impacts attention allocation to targets, not distractors. We also observed a lateralized component earlier than the N2pc, a posterior contralateral positivity (Ppc) that did not depend on T1-T2 SOA and that was elicited by both lateral targets and distractors. We conclude that, contrary to N2pc, the Ppc likely reflects activity of bottom-up mechanisms responding unselectively to asymmetrical visual displays.

The "red-alert" effect in visual search: evidence from human electrophysiology.

Participants had to determine the orientation of a segment inside a target color circle among other gray distractor circles. The target circle was either red or green and was accompanied in the display by a distractor in the other color. To dissociate event-related potentials of target and distractor processing, one of them was on the vertical meridian and the other in a lateral position. In Experiment 1, the target color was indicated on a per-trial basis and, in Experiment 2, on a per-block basis. The results revealed the N2pc elicited by red targets had an earlier latency relative to the N2pc elicited by green targets. Contralateral responses of positive polarity linked to distractor inhibition were found only with red lateral distractors. The results suggest that the choice of colors to distinguish targets from distractors may play a role in visual search performance and in the functional characterization of event-related lateralizations.

N1pc reversal following repeated eccentric visual stimulation.

Early event-related potential (ERP) hemispheric asymmetries recorded at occipitoparietal sites are usually observed following the sudden onset of a lateral peripheral stimulus. This is usually reflected in an onset-locked larger N1 over the posterior contralateral hemisphere relative to the ipsilateral hemisphere, an early ERP asymmetry labeled N1pc. When the peripheral sudden onset is followed by a central stimulus, or by a bilaterally balanced visual array of stimuli, these events evoke a reversed N1pc, that is, a larger N1 over the hemisphere ipsilateral to the peripheral sudden onset. This N1pc reversal has been taken as evidence for a remapping of the visual space from an absolute, retinally based frame of reference to a relative, attentionally based frame of reference that codes the spatial positions of objects relative to the peripheral sudden onset, rather than relative to the fovea. Here, we pit the reference frame-remapping account against an alternative account based on reduced neural reactivity following the peripheral sudden onset. In three experiments, we varied the spatial location of an object relative to a preceding sudden onset, and tested the opposite predictions generated by the frame-remapping and the reduced neural reactivity accounts. Taken together, the results from the present experiments were consistent with the reduced neural reactivity account and inconsistent with the frame-remapping account.

Contralateral cortical organisation of information in visual short-term memory: evidence from lateralized brain activity during retrieval.

We studied brain activity during retention and retrieval phases of two visual short-term memory (VSTM) experiments. Experiment 1 used a balanced memory array, with one color stimulus in each hemifield, followed by a retention interval and a central probe, at the fixation point that designated the target stimulus in memory about which to make a determination of orientation. Retrieval of information from VSTM was associated with an event-related lateralization (ERL) with a contralateral negativity relative to the visual field from which the probed stimulus was originally encoded, suggesting a lateralized organization of VSTM. The scalp distribution of the retrieval ERL was more anterior than what is usually associated with simple maintenance activity, which is consistent with the involvement of different brain structures for these distinct visual memory mechanisms. Experiment 2 was like Experiment 1, but used an unbalanced memory array consisting of one lateral color stimulus in a hemifield and one color stimulus on the vertical mid-line. This design enabled us to separate lateralized activity related to target retrieval from distractor processing. Target retrieval was found to generate a negative-going ERL at electrode sites found in Experiment 1, and suggested representations were retrieved from anterior cortical structures. Distractor processing elicited a positive-going ERL at posterior electrodes sites, which could be indicative of a return to baseline of retention activity for the discarded memory of the now-irrelevant stimulus, or an active inhibition mechanism mediating distractor suppression.

Electrophysiological evidence of multitasking impairment of attentional deployment reflects target-specific processing, not distractor inhibition.

We studied the interaction between the control mechanisms subserving spatial attention and central attention using the psychological refractory period (PRP) paradigm. Two stimuli, a pure tone (T(1)) and a circular visual array (T(2)), including a salient target and a salient distractor, were presented at varying stimulus onset asynchronies, each requiring a speeded response. Target-specific and distractor-specific lateralized event-related potentials were isolated by placing one of them at a lateral position and the other on the vertical midline. As SOA was decreased, a progressive reduction and postponement of a T(2)-locked N2pc component was observed with a lateral target and a central distractor. No lateralized potentials were associated with a lateral distractor and a central target. The sustained posterior contralateral negativity (SPCN) was observed independently of SOA modulation, only with a lateral target. We also observed an earlier positive deflection, the Ppc (positivity posterior contralateral), that was contralateral to both lateral targets and distractors, whose amplitude and latency were not affected by SOA variations. We conclude that central processing interferes specifically with target processing reflected by the N2pc and SPCN. We propose that the Ppc reflects an initial, bottom-up response to the presence of a salient stimulus, whereas the N2pc and SPCN reflect the controlled deployment of spatial attention to targets and maintenance of target information in visual short-term memory, respectively.