Fundamental properties of the N2pc as an index of spatial attention: effects of masking.

Masking is an important tool in many paradigms used to study the cognitive architecture. The N2pc is an electrophysiological event-related potential (ERP) that has been used as a tool to study the deployment of visual spatial attention. The aim of this paper was to study the effects of masking on the N2pc. Two stimuli were presented on the screen, one to left and one to right of fixation, and subjects reported the identity of one of them. The targets could be discriminated both by their category (letters vs. digit) and by their colour (pink vs. green). Backward masking was produced by presenting a second pair of bilateral stimuli after the offset of the first pair. The second pair of stimuli consisted of characters of the same colour and category as in the first pair. Forward masking was produced by using the very same stimuli as in the backward masking condition, but by instructing subjects to report the second stimulus. The forward mask trials had longer response times compared to no-mask trials, and backward mask trials had even longer response times, and also a higher error rate. Although the different masking procedures lead to clear behavioural effects, the N2pc was not affected, suggesting that the deployment of visual spatial attention, per se, was not affected by pattern masking. A sustained posterior contralateral negativity (SPCN) following the N2pc was also found (300 ms post-target, and beyond), and the amplitude of the SPCN was strongly modulated by the number of presented stimuli and the duration of the SPCN was positively correlated with RT in the behavioural task. We hypothesize that the SPCN reflects neural activity associated with the passage of information through visual short-term memory.     

Time-course of hemispheric preference for processing contralateral relevant shapes: P1pc, N1pc, N2pc, N3pc

A most sensitive and specific electrophysiological indicator of selective processing of visual stimuli is the N2pc component. N2pc is a negative EEG potential peaking 250 ms after stimulus onset, recorded from posterior sites contralateral to relevant stimuli. Additional deflections preceding or following N2pc have been obtained in previous studies, possibly produced by specific stimulus features or specific prime-target sequences. To clarify the entire time-course of the contralateral- ipsilateral (C-I) difference recorded from the scalp above visual cortex in response to left-right pairs of targets and distracters, C-I differences were here compared between two types of stimuli and between stimuli that were or were not preceded by masked neutral primes. The C-I difference waveform consisted of several peaks, termed here P1pc (60-100 ms after target onset), N1pc (120-160 ms), N2pc (220-280 ms), and N3pc (360-400 ms). Being markedly enhanced when stimuli were preceded by the neutral primes, P1pc may indicate a response to stimulus change. Also, when stimuli were primed, N2pc reached its peak earlier, thereby tending to merge with N1pc. N3pc seemed to increase when target discrimination was difficult. N1pc, N2pc, and N3pc appear as three periods of one process. N3pc probably corresponds to L400 or SPCN as described in other studies. These observations suggest that the neurophysiological basis of stimulus-driven focusing of attention on target stimuli is a process that lasts for hundreds of milliseconds, with the relevant hemisphere being activated in an oscillating manner as long as required by the task.     

Electrophysiological evidence for cognitive control during conflict processing in visual spatial attention

Event-related potentials were measured to investigate the role of visual spatial attention mechanisms in conflict processing. We suggested that a more difficult target selection leads to stronger attentional top-down control, thereby reducing the effects of arising conflicts. This hypothesis was tested by varying the selection difficulty in a location negative priming (NP) paradigm. The difficult task resulted in prolonged responses as compared to the easy task. A behavioral NP effect was only evident in the easy task. Psychophysiologically the easy task was associated with reduced parietal N1, enhanced frontocentral N2 and N2pc components and a prolonged P3 latency for the conflict as compared to the control condition. The N2pc effect was also obvious in the difficult task. Additionally frontocentral N2 amplitudes increased and latencies of N2pc and P3 were delayed compared to the easy task. The differences at frontocentral and parietal electrodes are consistent with previous studies ascribing activity in the prefrontal and parietal cortex as the source of top-down attentional control. Thus, we propose that stronger cognitive control is involved in the difficult task, resulting in a reduced behavioral NP conflict.     

The role of spatial switching in the attentional blink

The attentional blink (AB) is a well-established paradigm in which identification of a target T2 is reduced shortly after presentation of an earlier target T1. An important question concerns the importance of backward masking during the AB. While task switching has been found to be a strong modulator mediating the AB without any masking of T2, the present study investigated whether spatial switching could similarly produce an AB without masking. Using a spatial AB paradigm in which items appeared at different locations; we found (a) a significant AB without backward masking of T2 but no AB when no distractors followed T2, (b) no evidence for Lag 1 sparing. These findings show that when there is a spatial switch between the targets, presenting the distractor following T2 at the same location than T2 (backward masking) is not a necessary condition for the AB to occur, but T2 has to be followed by surrounding distractors (appearing at different locations than T2). This pattern of data confirms that spatial switching is a robust modulator of the AB, but to a less extent than task switching.     

Masked-target recovery requires focused attention on the target object

Flashing a homogeneous light mask after the presentation of a masked target reduces the deleterious effects of the mask, a phenomenon often called target recovery. Target recovery has been studied using masking paradigms in which a target object is presented in isolation prior to the presentation of a mask, thus capturing attention. In the present study, we examined whether target recovery is possible when a target does not benefit from attentional capture. We hypothesized that target recovery would be eliminated when a target must compete with distractors for perceptual attention. Replicating classic studies, we observed target recovery when pattern and light masks followed an isolated target. However, target recovery was not observed when a light mask followed a masked visual search target. Furthermore, using an attentional-capture paradigm we found that sudden onset search targets were recoverable whereas nononset targets were not. The present findings indicate that attentional capture by a target prior to masking plays a critical role in the subsequent recovery of the target.     

Representational masking and the attentional blink

After successful detection of a target item within a stream of rapidly displayed visual stimuli, subsequent detection of an additional target is impaired for roughly 500 ms. This impairment is known as the “attentional blink” (AB). Previous studies have found that if either the first (Tl; Raymond, Shapiro, & Arnell, 1992) or second target (T2; Giesbrecht & Di Lollo, 1998) is not followed by a mask, the AB impairment is significantly reduced. Whereas low-level perceptual factors have been found to influence the efficacy of masking in the AB (e.g., Seiffert & Di Lollo, 1997), the current experiment used a higher level (representational) manipulation, i.e., repetition blindness to reduce the efficacy of target masking in the AB. These findings contribute to the growing literature suggesting that masking plays more than a perceptual role in the AB phenomenon.     

On the labile memory buffer in the attentional blink: masking the T2 representation by onset transients mediates the AB

Report of a second target (T2) is impaired when presented within 500 ms of the first (T1). This attentional blink (AB) is known to cause a delay in T2 processing during which T2 must be stored in a labile memory buffer. We explored the buffer's characteristics using different types of masks after T2. These characteristics were inferred by determining what attributes of T2 are hindered by a given form of masking. In Experiments 1-3, trailing metacontrast and four-dot masks produced ABs of equal magnitudes, implicating the onset-transient triggered by the mask as the mechanism underlying the AB and strongly suggesting a locus in early vision. In Experiment 4, metacontrast and four-dot masks were presented in a common-onset masking (COM) paradigm in which a brief, combined display of T2 and the mask was followed by a longer display of the mask alone. COM is thought to occur late in the sequence of processing events. No AB occurred with COM, confirming the critical role of the mask's onset transients and ruling out a high-level locus for the labile memory buffer.     

Masking T1 difficulty: processing time and the attenional blink

When observers are presented with 2 targets in rapid succession, identification of the 1st is highly accurate, whereas identification of the 2nd is impaired at brief intertarget intervals (i.e., 200-500 ms). This 2nd-target deficit is known as the attentional blink (AB). According to bottleneck models, the AB arises because attending to the 1st target delays allocation of attention to the 2nd target. Thus, these models predict that increasing 1st-target processing time will increase the magnitude of the AB. Previous tests of this prediction have yielded mixed results. The present work suggests that one factor contributing to this uncertainty is masking of the 1st target: When this mask is omitted, processing time and AB magnitude are reliably related. These findings clarify the role of 1st-target masking in the AB and support the validity of the bottleneck account.     

A recognition masking study of consonant processing

The processing of consonants was investigated in a series of experiments using a recognition masking paradigm. Experiment I investigated the effects of target duration, interstimulus interval, forward vs. backward masking, and the phonetic feature composition of the target and mask on accuracy of target identification. Experiment II assessed consonant processing when the target and mask were presented dichotically in order to separate central and peripheral components of consonant masking. Experiment III investigated the effects of mask duration on consonant processing. Substantial masking was found in backward and forward diotic and dichotic conditions. Evidence for target-mask interaction at the level of phonetic features was also found.     

基于N2pc成分的视觉空间注意功能老化机制

视觉空间注意是人的一种基本认知功能,对老年人的生活有重要影响。N2pc作为一种与注意选择相关的ERP成分,可以反映注意资源在空间上的分配,是研究视觉空间注意的一个良好指标。本文对此前基于N2pc成分的注意功能老化研究进行了综述。前人研究主要采用视觉搜索范式与空间线索范式。采用视觉搜索范式的研究发现,当目标周围有干扰子时,目标诱发的N2pc在老年人中振幅减小,潜伏期延迟,同时在行为上老年人与年轻人相比,正确率更低,反应时更长; 当目标周围无干扰子时,目标诱发的N2pc振幅老年人与年轻人相当或比年轻人更大,老年人的正确率与年轻人无显著差异。采用空间线索范式的研究发现,与目标无相同特征的线索奇异子(singleton cue)不会诱发N2pc,与目标有相同特征的线索奇异子诱发的N2pc未表现出年龄差异,差异体现在一种反映抑制的ERP成分—Pd上(年轻人有Pd而老年人没有),此时老年人在行为上表现出比年轻人更强的空间线索效应。这些研究共同说明,视觉空间注意功能的老化是一个复杂的过程,涉及到搜索目标时注意分配速度的减慢,对干扰子抑制能力的下降以及基于特征的注意捕获能力的良好保持。     

The chronometry of mental ability: An event-related potential analysis of an auditory oddball discrimination task

The relation between intelligence and speed of auditory discrimination was investigated during an auditory oddball task with backward masking. In target discrimination conditions that varied in the interval between the target and the masking stimuli and in the tonal frequency of the target and masking stimuli, higher ability participants (HA) displayed more accurate discriminations, faster response time, larger P300 amplitude, and shorter P300 and mismatch negativity (MMN) latency than lower ability participants (LA). Task difficulty effects demonstrated with variation in mask type indicate that the mask does not interfere with the detection of the deviant target stimulus, but rather that the target and mask are integrated as a single compound stimulus. The temporal effects suggest that the speed of accessing short-term memory is faster for HA than LA and, on the basis of the MMN latency, the effect is accomplished automatically, without focused attention. Moreover, the pattern of results obtained with these data support the view that the accuracy effects are determined by processing speed rather than discrimination ability. Comparator models that accommodate these effects are discussed.     

Recently attended masks are less effective

Much work has been done to investigate participants' ability to detect repeated targets, but the presentstudy examined the influence of recently attended stimuli on target masking. Participants performed a target identification task in which the item that masked the target was either a recently attended item or a novel item. When it was identical to a previously attended stimulus, the mask was rendered considerably less effective. We have termed this effect a repeated mask reduction (RMR). This simple manipulation resulted in a large, reliable effect on the efficacy of visual masking in a single-target identification paradigm. In Experiments 1, 2, and 3, we demonstrated the basic effect and noted that the RMR increased as task difficulty increased. Experiments 4 and 5 suggested that attention to the first instance of the mask was crucial to this effect by showing that the magnitude of the RMR was unaffected by repetition of nonmask distractors and that the magnitude of the effect was reduced when less attention was paid to the first instance of the mask.     

Object substitution masking and the object updating hypothesis

The object updating hypothesis of object substitution masking proposes that the phenomenon arises when the visual system fails to individuate target and mask at the level of object token representations. This hypothesis is tested in two experiments using modifications of the dot mask paradigm developed by Lleras and Moore (2003). Target—mask individuation is manipulated by the presentation of additional display items that influence the linking apparent motion seen between a target and a spatially separated mask (Experiment 1), and by the use of placeholders that maintain the target object’s presence during mask presentation (Experiment 2). Results in both cases are consistent with the updating hypothesis in showing significantly reduced masking when the conditions promoted target—mask individuation. However, in both experiments, some masking was still present under conditions of individuation, an effect we attribute to attentional capture by the mask.     

注意瞬脱的神经机制及其理论解释

对一个目标的准确报告使得很难报告之后500ms内呈现的另一个目标,这种加工削弱被定义为注意瞬脱。注意瞬脱范式可以用来研究注意的时间进程。对于注意瞬脱现象已经有了多种理论解释,许多研究者利用功能核磁共振和事件相关电位等神经成像技术考察了注意瞬脱的神经机制。相关研究结果支持自上而下的控制过程与自下而上过程的交互作用导致了瞬脱现象的产生。最后对这些研究存在的问题和尚未解决的问题做了简要的说明     

T1 difficulty and the attentional blink: expectancy versus backward masking

According to bottleneck models of the attentional blink (AB), first-target (T1) processing difficulty should be related to AB magnitude. Tests of this prediction that have varied T1 difficulty in the context of a standard AB paradigm, however, have yielded mixed results. The present work examines two factors that may mediate the relationship between T1 difficulty and the AB: observer expectancy and backward masking of T1. In two experiments, omission of the backward mask consistently yielded the predicted relationship between T1 difficulty and the AB. In contrast, observer expectancy influenced target identification accuracy but did not mediate the relationship between T1 difficulty and the AB.     

T1 difficulty and the attentional blink: Expectancy versus backward masking

According to bottleneck models of the attentional blink (AB), first-target (T1) processing difficulty should be related to AB magnitude. Tests of this prediction that have varied T1 difficulty in the context of a standard AB paradigm, however, have yielded mixed results. The present work examines two factors that may mediate the relationship between T1 difficulty and the AB: observer expectancy and backward masking of T1. In two experiments, omission of the backward mask consistently yielded the predicted relationship between T1 difficulty and the AB. In contrast, observer expectancy influenced target identification accuracy but did not mediate the relationship between T1 difficulty and the AB.     

Active suppression after involuntary capture of attention

After attention has been involuntarily captured by a distractor, how is it reoriented toward a target? One possibility is that attention to the distractor passively fades over time, allowing the target to become attended. Another possibility is that the captured location is actively suppressed so that attention can be directed toward the target location. The present study investigated this issue with event-related potentials (ERPs), focusing on the N2pc component (a neural measure of attentional deployment) and the Pd component (a neural measure of attentional suppression). Observers identified a color-defined target in a search array, which was preceded by a task-irrelevant cue array. When the cue array contained an item that matched the target color, this item captured attention (as measured both behaviorally and with the N2pc component). This capture of attention was followed by active suppression (indexed by the Pd component), and this was then followed by a reorienting of attention toward the target in the search array (indexed by the N2pc component). These findings indicate that the involuntary capture of attention by a distractor is followed by an active suppression process that presumably facilitates the subsequent voluntary orienting of attention to the target.     

Object substitution masking interferes with semantic processing: evidence from event-related potentials

Object substitution masking (OSM) refers to impaired target identification caused by common onset, but delayed offset, of a surrounding dot mask. This effect has been hypothesized to reflect reentrant processes that result in the mask replacing the target representation. However, little is known about the depth of processing associated with masked targets in this paradigm. We investigated this issue by examining the effect of OSM on the N400 component of the event-related potential, which reflects the degree of semantic mismatch between a target and its context. Participants read a context word followed by a semantically related or unrelated target word surrounded by dots. As expected, delayed dot offset significantly reduced accuracy in identifying the target. The N400 amplitude was also diminished by OSM. These findings offer the first evidence that substitution interferes with target processing prior to semantic analysis, demonstrating an important difference between OSM and other visual phenomena, such as the attentional blink, in which semantic processing is independent of awareness.     

Spatial frequency masking in positive- and negative-symptom schizophrenia

The role of transient and sustained channels in masking was investigated in groups with positive and negative symptoms in schizophrenia and in a control group. The target stimulus was a 3.0 c/deg sinusoidal grating, which was masked at 11 stimulus-onset asynchronies between -40 to 360 ms by a 1.0 c/deg mask or an 11.0 c/deg mask. The results showed that there was no difference between the control and positive-symptom groups in the perception of the 3 c/deg target stimulus, nor was there a difference when the target was masked by 1 or 11 c/deg masking stimuli. In comparison with the control and positive-symptom groups, the negative-symptom group showed a significantly higher threshold for the perception of the 3 c/deg target stimulus and more masking with a 1 c/deg mask, but not with an 11 c/deg mask. The results provide evidence for distinguishable differences in visual masking between groups with positive and negative symptoms in schizophrenia.     

Does failure to mask T1 cause lag-1 sparing in the attentional blink?

The attentional blink (AB) effect demonstrates that when participants are instructed to report two targets presented in a rapid visual stimuli stream, the second target (T2) is often unable to be reported correctly if presented 200-500 msec after the onset of the first target (T1). However, if T2 is presented immediately after T1, in the conventional lag-1 position (100-msec stimulus onset asynchrony; SOA), little or no performance deficit occurs. The present experiments add to the growing literature relating the "lag-1 sparing" effect to T1 masking. Using a canonical AB paradigm, our results demonstrate that T2 performance at lag 1 is significantly reduced in the presence of T1 masking. The implications of this outcome are discussed in relation to theories of the AB.