Electrophysiological evidence of visual encoding deficits in a cross-modal attentional blink paradigm

Two experiments are reported in which two target stimuli, T1 and T2, were presented at variable stimulus onset asynchronies (SOAs). In Experiment 1, T1 and T2 were visual stimuli embedded in a rapid serial visual presentation (RSVP) stream of distractors. Participants were asked to report T1 and T2 at the end of the stream. In Experiment 2, T1 was an auditory stimulus, and T2 a visual stimulus embedded in an RSVP stream. Participants made a speeded discriminative response to T1, and reported T2 at the end of the stream. An attentional blink (AB) effect was observed in both experiments: T2 report suffered at short SOA compared to long SOA. During the AB, the amplitude of the P300 component of the event-related potential (ERP) locked to T2 onset was sensibly reduced in both experiments. Behavioral and ERP results were very similar across the two experiments. Implications for models of the AB effect are discussed.     

Task switching mediates direct interference of intertarget distractors in the attentional blink: An event‐related potential study

The attentional blink (AB) refers to the difficulty in reporting a second target (T2) presented shortly after a first target (T1) in a stream of distractors. The goal of the present study was to investigate distractor‐based interference in the AB by recording the P3 component of the event‐related potential to both targets. An intertarget distractor was presented at lag 1 (T1+1), at lag 2 (T1+2), or at neither of these two lags (no distractor). T2 was always presented at lag 3, as the last item in the stream. In two experiments, the P3 from T1 was attenuated in the T1+1 condition compared to the two other distractor conditions. In absence of a task switch (Experiment 1), the P3 from T2 was delayed in both the T1+1 and T1+2 conditions. In the presence of a task switch (Experiment 2), the P3 from T2 was delayed only in the T1+1 condition. Results demonstrate that intertarget distractors can modulate the AB directly but only in absence of a task switch.     

Masking of a first target in the attentional blink attenuates the P3 to the first target and delays the P3 to the second target

The attentional blink (AB) refers to the decline in report accuracy of a second target (T2) when presented shortly after a first target (T1) in a rapid serial visual presentation (RSVP) of distractors. It is known that masking T1 increases the magnitude of the AB, and masking a single target (equivalent to T1) in a RSVP stream attenuates the P3 to the target in correct trials. The major purpose of the present study was to clarify how these two effects may be integrated. An intervening distractor was presented at lag 1 (T1+1), at lag 2 (T1+2), or at neither of these two lags (no distractor). T2 was always presented at lag 3, as the last item in the stream. The P3 to T1 was attenuated and the P3 to T2 delayed in the T1+1 condition compared to the two other distractor conditions. These results clearly show that masking T1 attenuates the P3 to T1 and delays the P3 to T2 in the AB. Implications for extant theories of the AB are discussed.     

The attentional blink within and across the hemispheres: Evidence from a patient with a complete section of the corpus callosum

The attentional blink (AB) refers to an impairment in the report of a second target (T2) if it closely follows the presentation of a first target (T1) in a rapid serial visual presentation (RSVP), when both targets must be reported. In the present study, a modified AB paradigm was used in which targets could appear in any of four simultaneous RSVP streams, one in each quadrant of the visual field. In half of the trials, T1 and T2 were displayed in the same visual hemifield (either left or right) and, in the other half, T1 and T2 were displayed in different visual hemifields. Using this paradigm with both neurologically intact individuals and a split-brain patient, we sought to investigate (1) possible hemispheric asymmetries in attentional processes, and (2) whether the AB would be reduced when targets are displayed in different visual hemifields. A comparable AB was found for both neurologically intact individuals and the split-brain patient, with no significant variations due to whether targets were displayed in the same or in different hemifields. A left hemisphere advantage in the processing of same and different hemifield targets was observed only in the split-brain patient.     

Semantic and repetition priming within the attentional blink: an event-related brain potential (ERP) investigation study

An attentional blink (AB) paradigm was used to directly compare semantic and repetition priming for reported words versus missed words. Three target words (T1, T2, T3) were embedded in a rapidly presented stream of non-word distractors for report at the end of each trial. Whereas T1 was not related to either T2 or T3, T2 and T3 could be unrelated words, semantically related words, or identical. Semantic and repetition priming effects were evident in both behavioral and electrophysiological measures on T3, whether T2 was accurately reported or 'blinked'. These results suggest that semantic and repetition priming effects, under rapid serial visual presentation conditions, are modulated by at least partially overlapping neural mechanisms.     

Intramodal and crossmodal processing delays in the attentional blink paradigm revealed by event-related potentials

In the attentional blink (AB), processing of a second target (T2) is impaired if it is presented shortly after the onset of a first target (T1), leading to a decrease in accurate report of T2 if T2 is masked. Some prominent theories of the AB suggest that an amodal bottleneck in working memory consolidation underlies the AB. We investigated this by factorially manipulating T1 and T2 modalities (visual or auditory) using equivalent stimuli and tasks in both modalities to minimize task switching. T2 was not masked. In all modality combinations, the electrophysiological P3 component to T2, obtained by subtracting T1 only trials from T1+T2 trials, was delayed and reduced in amplitude when T2 was presented soon after T1 relative to when T1 and T2 were presented farther apart. Results provide support for a common amodal bottleneck that underlies the AB effects observed in all visual/auditory modality combinations.     

Visual search is postponed during the period of the AB: An event‐related potential study

In the phenomenon known as the attentional blink (AB), perception of the second of two rapidly sequential targets (T2) is impaired when presented shortly after the first (T1). Studies in which T2 consisted of a pop‐out search array provided evidence suggesting that visual search is postponed during the AB. In the present work, we used behavioral and electrophysiological measures to test this postponement hypothesis. The behavioral measure was reaction time (RT) to T2; the electrophysiological measure was the onset latency of an ERP index of attentional selection, known as the N2pc. Consistent with the postponement hypothesis, both measures were delayed during the AB. The delay in N2pc was substantially shorter than that in RT, pointing to multiple sources of delay in the chain of processing events, as distinct from the single source postulated in current theories of the AB. Finally, the finding that the N2pc was delayed during the AB strongly suggests that attention is involved in the processing of pop‐out search arrays.     

Biasing the brain’s attentional set: II. Effects of selective intersensory attentional deployments on subsequent sensory processing

This study used high-density mapping of human event-related potentials to examine the brain activity associated with selective information processing when subjects were cued on a trial-by-trial basis to perform a discrimination in either the visual or auditory modality. On each trial, word-cues (S1) instructed subjects to attend to features within one sensory-modality of an impending compound auditory-visual stimulus (S2) that arrived approximately 1-second following the cue. Subjects made a discrimination within the cued modality of the S2 stimulus. The spatio-temporal patterns of activity in response to the compound S2 stimulus were examined as a function of the sensory modality being attended. The earliest effects of intersensory attention on visual processing were seen subsequent to the initial activation of visual cortex, beginning at 80 ms and continuing into the P1 and N1 components of the visual ERP. The scalp-topography of this earliest modulation was consistent with modulation of activity in ventral visual stream areas. Thus, the locus of effects on visual S2 processing differed from the anticipatory parieto-occipital biasing activity that preceded S2 presentation. This pattern of effects strongly suggests that the anticipatory activity (following the cue) associated with sustaining the focus of attention during intersensory attention, at least in the context of this paradigm, does not operate as a simple gain mechanism in early visual sensory areas. Rather, attentional biasing can operate through a higher-order process whereby parieto-occipital cortices influence the subsequent flow of visual processing in the ventral stream.     

Linguistic predictability influences auditory stimulus classification within two concurrent speech streams

Acoustic predictability has been shown to affect auditory stream segregation, while linguistic predictability is known to be an important factor in speech comprehension. We tested the effects of linguistic predictability on auditory stream segregation and target detection by assessing the event-related potentials elicited by targets and distractors in participants presented with two concurrent speech streams. The linguistic cues of predictability varied over four levels. In the three real speech conditions, natural speech was presented with intact phonotactics and sentence prosody: normal speech, word-salad (randomized word order within each sentence), and pseudo-words (randomized syllable order within each sentence). The fourth (control) condition delivered a spectrally rotated version of the normal speech condition. Participants were instructed to attend one stream and respond to the natural cough sounds embedded in it. Coughs were present in both streams, serving as targets in the attended and as distractors in the unattended stream. We expected improved target detection with increasing linguistic predictability. The target-related N2b component's amplitude monotonically increased from the pseudo-word to the word-salad and normal speech condition, while no predictability effects were observed for the P3b amplitude or for behavioral responses. The dissociation between the N2b and P3b/behavioral effect suggests that while linguistic predictability enhanced the process of classifying stimuli as potential targets, this did not affect their detection. Furthermore, the observed nonmonotonic distractor N2 (probably MMN) amplitude increase with increasing linguistic predictability is compatible with the notion that linguistic predictability can modulate auditory stream segregation and/or stream selection.     

Event‐related potential N270 delayed and enhanced by the conjunction of relevant and irrelevant perceptual mismatch

Event‐related potential studies using delayed match‐to‐sample tasks have demonstrated the presence of two components, N270 and N400, possibly reflecting the sequential processing of multiple sources of endogenous mismatch. To date, studies have only investigated mismatch between a single cue and target. In this study, we used distractor stimuli to investigate the effect of a secondary source of mismatch distinct from the task‐relevant stimulus. Subjects performed two paradigms in which the cue and target could match or mismatch. In one paradigm, task‐irrelevant distractors were added—producing a source of task‐irrelevant perceptual mismatch. A mismatch‐triggered negativity was elicited in both paradigms, but was delayed and enhanced in magnitude in the distractors present paradigm. It is suggested that the distractors may differentially affect mismatch responses through the generation of a task‐irrelevant mismatch response.     

Temporal order judgments for audiovisual targets embedded in unimodal and bimodal distractor streams

We investigated whether the presence of unimodal or bimodal (synchronous) distractors would affect temporal order judgments (TOJs) for pairs of asynchronous audiovisual target stimuli. Participants made unspeeded TOJs regarding which of a pair of auditory and visual stimuli, presented at different stimulus onset asynchronies using the method of constant stimuli, occurred first. These asynchronous target stimuli were presented in a fixed position amongst a stream of three (auditory, visual, or audiovisual) distractors in each block of trials. The largest just noticeable differences (JNDs) were reported when the target stimuli were presented in the middle (position 3) of the distractor stream. Importantly, audiovisual distractors were shown to interfere with TOJ performance far more than unimodal (auditory or visual) distractors. The point of subjective simultaneity (PSS) was also influenced by the modality of the distractors, and by the position of the target within the distractor stream. These results confirm the existence of a specifically bimodal crowding effect, with audiovisual TOJs being impaired far more by the presence of audiovisual distractors that by unimodal auditory or visual distractors.     

Neuroelectric correlates of auditory attentional blink

Attentional blink (AB) refers to the situation where the correct identification of a first target causes a processing deficit of a second target. The present study aims to clarify the stage at which the auditory AB occurs by means of scalp-recorded event-related potentials. On each trial, participants indicated whether predefined target sounds were presented in a rapid series of distractor sounds. The results showed a large AB when the presentation rate was fast. This auditory AB was paralleled by a suppression of the P3b wave to the second target. During the AB, the second target generated N1 and P2 waves, suggesting that some aspects of perceptual processing were preserved. The magnitude of the AB decreased with decreasing presentation rate. The improvement in detecting the second target was accompanied by an increased and delayed P3b. These results suggest that the auditory AB reflects a limitation of short-term consolidation and provides evidence for a common underlying processing limitation during the AB in both visual and auditory modalities.     

Toward the influence of temporal attention on the selection of targets in a visual search task: An ERP study

We used ERPs to investigate whether temporal attention interacts with spatial attention and feature‐based attention to enhance visual processing. We presented a visual search display containing one singleton stimulus among a set of homogenous distractors. Participants were asked to respond only to target singletons of a particular color and shape that were presented in an attended spatial position. We manipulated temporal attention by presenting a warning signal before each search display and varying the foreperiod (FP) between the warning signal and the search display in a blocked manner. We observed distinctive ERP effects of both spatial and temporal attention. The amplitudes for the N2pc, SPCN, and P3 were enhanced by spatial attention indicating a processing benefit of relevant stimulus features at the attended side. Temporal attention accelerated stimulus processing; this was indexed by an earlier onset of the N2pc component and a reduction in reaction times to targets. Most importantly, temporal attention did not interact with spatial attention or stimulus features to influence visual processing. Taken together, the results suggest that temporal attention fosters visual perceptual processing in a visual search task independently from spatial attention and feature‐based attention; this provides support for the nonspecific enhancement hypothesis of temporal 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.     

Failure of temporal selectivity: Electrophysiological evidence for (mis)selection of distractors during the attentional blink

The attentional blink (AB) refers to the impairment in accurate report of a second target (T2) when presented shortly after a first target (T1) in a rapid serial visual presentation of distractors. The goal of the present study was to determine whether the AB is caused by a failure in early selection processes, which leads to the selection and consolidation of the wrong item in working memory, by measuring the frequency‐related P3 to T2 and to T2 + 1. During the AB, an attenuation of the P3 to T2 was observed, as well as an increase in the amplitude of the P3 to T2 + 1. Whereas the P3 to T2 was observed only when T2 was correctly reported, the P3 to T2 + 1 was observed only in trials where T2 was incorrectly reported, and its amplitude was correlated to individual differences in misselection rate. These results support the claim that failure of temporal selection underlies the AB.     

Cross-Modal Selective Attention Effects on Retinal, Myogenic, Brainstem, and Cerebral Evoked Potentials

Short latency evoked potentials were recorded during a cross-modal selective attention task to evaluate recent proposals that sensory transmission in the peripheral auditory and visual pathways can be modified selectively by centrifugal mechanisms in humans. Twenty young adult subjects attended in turn to either left-ear tones or right-field flashes presented in a randomized sequence, in order to detect infrequent, lower-intensity targets. Attention-related enhancement of longer-latency components, including the visual P105 and the auditory N1/Nd waves and T-complex, showed that subjects were able to adopt a selective sensory set toward either modality. Neither the auditory evoked brainstem potentials nor the early visual components (electroretinogram, occipito-temporal N40, P50, N70 waves) were significantly affected by attention. Measures of retinal B-waves were significantly reduced in amplitude when attention was directed to the flashes, but concurrent recordings of eyelid electromyographic activity and the electro-oculogram indicated that this effect may have resulted from contamination of the retinal recordings by blink microreflex activity. A trend toward greater positivity in the 15-50 ms latency range for auditory evoked potentials to attended tones was observed. These results provide further evidence that the earliest levels of sensory transmission are unaffected by cross-modal selective attention, but that longer latency exogenous and endogenous potentials are enhanced to stimuli in the attended modality.     

Editorial: Neural Systems and Psychiatric Disorders

Introduction: Patients with schizophrenia demonstrate a wide range of information processing deficits. Most recent studies argue in favour of high level deficits, including attention and context processing, whereas fewer studies have demonstrated deficits at earlier stages of processing, such as perceptual discrimination and organisation. This is the first study to investigate both high and low level processing, within a single paradigm, in the case of auditory temporal processing in schizophrenia. Methods: Patients with schizophrenia were compared to controls on a series of tasks involving three auditory temporal processes varying from low to higher level: (1) segregation of a complex sequence into component auditory streams; (2) detection of local temporal irregularities within a stream; (3) attentional focusing on one stream by the use of a cue preceding the complex sequence. Results: The lowest level of processing examined here--stream segregation--appeared to function equally well in patients as in controls. However, the higher level processes--irregularity detection and attentional focus--functioned in both groups, but less efficiently in patients with schizophrenia. Conclusions: Results demonstrate abnormal auditory temporal processing in schizophrenia. Abnormal performances only in Processes 2 and 3 support and hypothesis of higher level rather than lower level processing deficits in schizophrenia.     

Effects of perceptual context on event-related brain potentials during auditory spatial attention

The effects of auditory spatial attention on event-related brain potentials (ERPs) were examined in situations that promoted stream segregation. Short and long noise bursts were presented at three azimuth locations and listeners were asked to respond to the longer sounds occurring at either the right- or left-most location. In the baseline condition, the three sound sources were evenly spaced apart. In the distractor clustering conditions, middle and far sounds were clustered. In the attended clustering conditions, middle and attended sounds were clustered. ERP indices of attention, isolated as negative difference (Nd) waves, were greater over the hemisphere contralateral to the attended location. Nd waves were also larger when the middle sounds were moved toward the far distractors, consistent with an object-based gradient of auditory attention in which higher order information provided by the perceptual context influences selective processing.     

Preferential neural processing of attended stimuli in attention-deficit hyperactivity disorder and normal boys

Event-related auditory and visual potentials were recorded from 36 attention-deficit hyperactivity disorder (ADHD) and 35 normal 6-year-old subjects engaged in a two-choice discrimination task. When normal subjects attended to stimuli in a given modality, enhanced negative (N2) and positive (P3b) responses (as compared with responses to nonattended stimuli) were found for auditory and visual target stimuli. In contrast, when ADHD subjects attended, little or no enhanced negative responses were found in either modality, and enhanced positive P3b responses were found only in response to visual target stimuli. Auditory N1, N2, and P3b and visual N2 amplitudes to attended target stimuli were significantly reduced in ADHD subjects as compared with normal subjects. No between-group differences were found for responses to nonattended stimuli. Both amplitude and latency abnormalities indicate that ADHD boys suffer from deficient preferential processing of attended stimuli. P3b and N2 abnormalities found here suggest deficiencies in two independent cognitive processes thought to be crucial to what we perceive, learn, and remember.     

Cross-modal links in endogenous spatial attention are mediated by common external locations: evidence from event-related brain potentials

Recent behavioural and event-related potential (ERP) studies reported cross-modal links in spatial attention between vision, audition and touch. Such links could reflect differences in hemispheric-activation levels associated with spatial attention to one side, or more abstract spatial reference-frames mediating selectivity across modalities. To distinguish these hypotheses, ERPs were recorded to lateral tactile stimuli, plus visual (experiment 1) or auditory stimuli (experiment 2), while participants attended to the left or right hand to detect infrequent tactile targets, and ignored other modalities. In separate blocks, hands were either in a crossed or uncrossed posture. With uncrossed hands, visual stimuli on the tactually attended side elicited enhanced N1 and P2 components at occipital sites, and an enhanced negativity at midline electrodes, reflecting cross-modal links in spatial attention from touch to vision. Auditory stimuli at tactually attended locations elicited an enhanced negativity overlapping with the N1 component, reflecting cross-modal links from touch to audition. An analogous pattern of results arose for crossed hands, with tactile attention enhancing auditory or visual responses on the side where the attended hand now lay (i.e. in the opposite visual or auditory hemifield to that enhanced by attending the same hand when uncrossed). This suggests that cross-modal attentional links are not determined by hemispheric projections, but by common external locations. Unexpectedly, somatosensory ERPs were strongly affected by hand posture in both experiments, with attentional effects delayed and smaller for crossed hands. This may reflect the combined influence of anatomical and external spatial codes within the tactile modality, while cross-modal links depend only on the latter codes. Electronic Publication