Posterior N1 Research Articles

ERP evidence for distinct mechanisms of fast and slow visual perceptual learning

Perceptual learning (PL) occurs not only within the first training session but also between sessions. Once acquired, the learning effects can last for a long time. By examining the time course of learning-associated ERP changes, this study explores whether fast and slow visual PL contribute to long-term preservation. Subjects first participated in a visual task for three training sessions, and were then given one test session six months later. ERP results showed that fast learning effects, as reflected by the decrement of posterior N1 and increment of posterior P2 within session 1, were preserved in session 3 but not in the test session. However, slow learning effects, as reflected by the increment of posterior N1 and decrement of frontal P170 between sessions 1 and 3, were retained completely in the test session. This study indicates that PL induces different changes in the human adult brain during and after active training, and only the delayed changes of brain activity can be preserved for a long period of six months.

Perceiving the writing sequence of Chinese characters: An ERP investigation

The neural dynamics in perceiving well-learned sequences and its modulation by task demand were investigated in this study in which participants were asked to observe stroke-by-stroke display of Chinese characters composed of two radicals while their brain activity was monitored with the event-related potential (ERP) technique. Experiment 1 used an accuracy judgment task that would draw participants' attention to the violation of the writing sequence whereas Experiment 2 required participants to judge the completion of the display and thus the more automatic aspects of sequence processing could be revealed. In Experiment 1, the within-radical boundary reversal produced bilateral posterior N2 enhancement and the cross-boundary reversal elicited a left N2 effect and right posterior N2 reduction on the critical stroke. Both types of reversal elicited P3 effects on the critical stroke and sustained negativity effects on the following stroke, with the size being larger for the cross-boundary reversal. In Experiment 2, in addition to the P3 effects, the within-boundary reversal elicited a left posterior N2 effect and the cross-boundary reversal elicited right posterior N2 reduction on the critical stroke. Moreover, on the following stroke, the cross-boundary reversal elicited a right N2 effect and both types of reversal elicited sustained positivity effects. These findings demonstrate that native Chinese readers use their sequential knowledge to predict upcoming strokes in perceiving the writing of characters and to construct appropriate representations for the action sequence regardless of whether such predictions and constructions are required by the task.

Task difficulty modulates electrophysiological correlates of perceptual learning

To investigate the influence of task difficulty on the neural mechanisms of visual perceptual learning, we recorded event-related potentials (ERPs) from human adults while they were trained with either an easy or a difficult orientation discrimination task. The discrimination thresholds of participants trained with the difficult task decreased after training, but thresholds of participants with the easy task did not. More importantly, although posterior N1 reduction and anterior P2 decrement were obtained in both conditions, only the difficult task training showed an enhancement in posterior P1, an increase in N2 and P3, and a broader scalp distribution of posterior N1 attenuation. These differences indicated that the difficult task training affected the early visual processing stage, the later ERP components, and the broader visual cortical regions, respectively. The results in the present study provide direct electrophysiological evidence for task difficulty modulation of perceptual learning-related neuroplasticity.

N1 WAVE IN THE P300 BCI IS NOT SENSITIVE TO THE PHYSICAL CHARACTERISTICS OF STIMULI

One of the widely used paradigms for the brain-computer interface (BCI), the P300 BCI, was proposed by Farwell and Donchin as a variation of the classical visual oddball paradigm, known to elicit the P300 component of the brain event-related potentials (ERP). We show that this paradigm, unlike the standard oddball paradigm, elicit not only the P300 wave but also a strong posterior N1 wave. Moreover, we present evidence that the sensitivity of this ERP component to targets cannot be explained by the variations of the perceived stimuli energy. This evidence is based on comparing the ERP obtained for usual P300 BCI stimuli and for the "inverted" stimulation scheme with low stimulus related variations of light energy (gray letters on the light gray background, "highlighted" by very light darkening). Despite the dramatic difference between the stimuli in the standard and "inverted" schemes, no difference between N1 amplitudes were found, supporting the view that this component's sensitivity to targets cannot be based simply on "foveating" the target, but may be related to spatial attention mechanisms, which involvement is natural for the P300 BCI. Efforts to optimize the P300 BCI should address better use of both P300 and N1 waves.

Event-related potentials findings differ between children and adults during arithmetic-fact retrieval

Some cognitive abilities of arithmetical calculation depend on retrieval of arithmetic facts from long-term memory. Arithmetic-fact retrieval has been studied in adults through Event-Related Potentials (ERP) experiments. Such information in children, however, has been scarce. It has been reported that from the age of 9 years, children employ a memory retrieval strategy for solving simple multiplication problems. The present study compared arithmetical-fact retrieval in children and adults while they were being subjected to ERP recording. The subjects were asked to make judgments about solutions to simple multiplication problems. Both groups of participants displayed the so-called arithmetic N400 effect for incorrect solutions relative to correct solutions. Adults showed a posterior N400 effect, while children showed a widely distributed N400 effect. Children displayed a larger amplitude and longer latency arithmetic N400 component than adults; this observation could be due to children exerting greater effort involving more widespread cortical activation than adults to solve the experimental problems. The Late Positive Component (LPC), which follows the arithmetic N400 and has been described previously in adult subjects, was observed in the present adult subjects, but was present in children only for correct solutions. These results may indicate that, relative to adults, children showed slower memory retrieval and a different pattern of a verification mechanism for correct and incorrect solutions.

Neurophysiological correlates of comprehending emotional meaning in context.

Although the neurocognitive mechanisms of nonaffective language comprehension have been studied extensively, relatively less is known about how the emotional meaning of language is processed. In this study, electrophysiological responses to affectively positive, negative, and neutral words, presented within nonconstraining, neutral contexts, were evaluated under conditions of explicit evaluation of emotional content (Experiment 1) and passive reading (Experiment 2). In both experiments, a widely distributed Late Positivity was found to be larger to negative than to positive words (a "negativity bias"). In addition, in Experiment 2, a small, posterior N400 effect to negative and positive (relative to neutral) words was detected, with no differences found between N400 magnitudes to negative and positive words. Taken together, these results suggest that comprehending the emotional meaning of words following a neutral context requires an initial semantic analysis that is relatively more engaged for emotional than for nonemotional words, whereas a later, more extended, attention-modulated process distinguishes the specific emotional valence (positive vs. negative) of words. Thus, emotional processing networks within the brain appear to exert a continuous influence, evident at several stages, on the construction of the emotional meaning of language.

Electrophysiological indices of spatial attention during global/local processing in good and poor phonological decoders

Previous research suggests a relationship between spatial attention and phonological decoding in developmental dyslexia. The aim of this study was to examine differences between good and poor phonological decoders in the allocation of spatial attention to global and local levels of hierarchical stimuli. A further aim was to investigate the relationship between global/local processing and electrophysiological indices (N1, N2) of spatial attention in these groups. Good ( n = 18) and poor ( n = 16) phonological decoders were selected on the basis of non-word reading ability. Participants responded to either the global or local level of hierarchical stimuli presented in the left or right visual field in a sustained attention task. Poor phonological decoders showed slower RT relative to good phonological decoders regardless of whether attention was directed to either global or local processing levels. This was accompanied by a lack of task-related modulation of the posterior N1 and N2 Event-Related Potential (ERP) components, suggesting differences in the early allocation of spatial attention and later perceptual processing respectively. Poor decoders also showed greater N2 amplitude overall, suggestive of compensatory processing at later perceptual stages. There was preliminary evidence for sex differences in hemispheric lateralisation, with a reversal of hemispheric lateralisation observed among male and female poor phonological decoders. These findings have important implications for the understanding of the relationship between spatial attention and phonological decoding in developmental dyslexia.

Selecting and ignoring salient objects within and across dimensions in visual search

There are two strategies for selecting relevant information from a visual scene: according to either its salience or its relevance for behavioral goals. Although there is broad evidence for the existence of both mechanisms, there has been a debate concerning the impact of top-down control on salience-based selection. We investigated whether salient but irrelevant information is filtered by the visual system and what role the organization of the visual system plays in selection. Using event-related brain potentials (ERPs), we compared processing of the same salient objects when relevant versus irrelevant to the task at hand. Observers had to detect a target singleton defined in a specific dimension (e.g., orientation), while ignoring a singleton non-target defined in either another dimension (e.g., color; Experiment 1, requiring dimension-based search) or the same dimension (Experiment 2, requiring feature-based search). For dimension-based search, the results revealed an enhanced posterior N2, indicative of singleton selection, and an enhanced P3 for target singletons, but no difference between non-target singletons defined in another dimension and blank trials. Also, N2pc results indicated that the allocation of attention was modulated by the task. In contrast, for feature-based search, some ERP enhancement was also observed for non-target singletons (defined in the same dimension as the target) in the N2, N2pc, and P3 latency ranges. This pattern argues in favor of a strong top-down influence on singleton selection that operates on dimensions rather than features.

Inhibition of Return in the Covert Deployment of Attention: Evidence from Human Electrophysiology

People are slow to react to objects that appear at recently attended locations. This delay-known as inhibition of return (IOR)-is believed to aid search of the visual environment by discouraging inspection of recently inspected objects. However, after two decades of research, there is no evidence that IOR reflects an inhibition in the covert deployment of attention. Here, observers participated in a modified visual-search task that enabled us to measure IOR and an ERP component called the posterior contralateral N2 (N2pc) that reflects the covert deployment of attention. The N2pc was smaller when a target appeared at a recently attended location than when it appeared at a recently unattended location. This reduction was due to modulation of neural processing in the visual cortex and the right parietal lobe. Importantly, there was no evidence for a delay in the N2pc. We conclude that in our task, the inhibitory processes underlying IOR reduce the probability of shifting attention to recently attended locations but do not delay the covert deployment of attention itself.

Salience detection and attentional capture

There is an ongoing debate to what extent irrelevant salient information attracts an observer's attention and is processed without the observer intending to do so. The present experiment investigated attentional capture of salient but irrelevant objects and compared target processing in target-and-distractor to target-only trials. Both form and color singletons were used and their target-distractor assignment was interchanged. Thus the general impact of the presence of a salient distractor on target processing could be separated from the impact of the specific target-distractor salience relation. Response latencies and event-related brain potentials (ERPs) were registered. Results showed a strong influence of the mere presence of an irrelevant distractor on target processing: both the visual N1 and the posterior N2 showed better attention focusing in target-only trials compared to target-and-distractor trials. Response times and N2pc results, on the other hand, showed evidence in favor of salience-specific attention allocation. N2pc results indicated that the distractor affected the allocation of attention in trials with form targets and color distractors but not in the opposite condition. Taken together, results showed a general impact of irrelevant salient singletons on search behavior when they were presented simultaneously with relevant singletons. The allocation of focal attention (as mirrored by the N2pc), however, was also influenced by the specific target-distractor salience relation.

Differentiating spatial and object-based effects on attention: An event-related brain potential study with peripheral cueing

Do spatial attention and object attention modulate visual processing in similar ways? Previously we have found a dissociation between these two forms of attention on ERP measures of sensory processing under conditions of peripheral cueing, with spatial attention effects associated with changes over anterior scalp regions and object attention effects associated with changes over posterior regions (He, X., Fan, S., Zhou, K., Chen, L., 2004. Cue validity and object-based attention. J. Cogn. Neurosci. 16, 1085–1097). However, under conditions of central cueing recent data suggest that spatial and object attention have similar effects over posterior cortical areas (e.g., Martínez, A., Teder-Sälejärvi, W., Hillyard, S.A., 2007. Spatial attention facilitates selection of illusory objects: evidence from event-related brain potentials. Brain Res. 1139, 143–152). In the present study we present further evidence for dissociation between spatial and object-based attention under conditions in which spatial attention effects were enhanced by increasing the cue validity and the task load. The data replicated our previous results, with the effects of spatial attention found in an enhanced anterior N1, while the effects of object-based attention emerged in an enhanced posterior N1. Analyses of attention effect maps and current source density maps confirmed the distinct scalp distributions. These results support the proposal that, under peripheral cueing, spatial attention and object attention are associated with activity respectively in anterior and posterior brain structures, and further suggest a distinction between how attention modulates processing under conditions of central cueing and peripheral cueing.

Evidence for automatic sentence priming in the fusiform semantic area: Convergent ERP and fMRI findings

Event-related potential (ERP) studies of semantic processing have generally focused on the N400, a component that peaks at about 400 ms in response to words and which is larger when words are incongruent with the preceding sentence context. An earlier left-lateralized posterior N2 p3 has also been found to be correlated with an “unexpectedness” rating for incongruent sentence endings [Dien, Frishkoff, Cerbone, and Tucker, 2003, Parametric analysis of event-related potentials in semantic comprehension: evidence for parallel brain mechanisms, Cognitive Brain Research, 15: 137–153]. Because the incongruent endings were too odd to be explicitly predicted, we here hypothesize that this rating, and hence the N2 p3, reflects the degree of automatic spreading activation (ASA) in the visual lexical network rather than semantic expectancy, an interpretation also consistent with the early latency of this ERP (208 ms). In order to identify the brain systems involved in these linguistic processes, functional magnetic resonance imaging (fMRI) was utilized in a replication of the ERP study [Dien, Frishkoff, Cerbone, and Tucker, 2003, Parametric analysis of event-related potentials in semantic comprehension: evidence for parallel brain mechanisms, Cognitive Brain Research, 15: 137–153]. We found that activation in the fusiform semantic area (FSA), an area that converges with the source solution for the N2 p3, responded to the “unexpectedness” parameter in the same manner as the N2 p3 component. These findings suggest that the FSA helps mediate ASA processes and that the N2 p3 can serve as an index of ASA. Furthermore, cloze effects were found in the superior frontal gyrus and the inferior frontal gyrus that could reflect subvocalization and semantic selection processes respectively.

ERP evidence that surface-based attention biases interocular competition during rivalry

J. F. Mitchell, G. R. Stoner, and J. H. Reynolds (2004) observed that exogenously cuing one of two superimposed transparent surfaces resulted in an enhanced perceptual bias for the cued surface during binocular rivalry. We investigated the neural bases of this effect by recording event-related potentials (ERPs). Subjects viewed two superimposed rotating transparent surfaces and compared the directions of two successive translations, either both of the same surface or one of each surface. Following the first translation, which cued attention to the translating surface, two surface images were removed-one from each eye (dichoptic viewing) or both from one eye (monocular viewing). Subjects were impaired at comparing the first and second translations when they occurred on different surfaces, and the impairment was greater during dichoptic viewing (rivalry). The P1 component (110-160 ms) of the ERP elicited by the second translation of the same surface was larger than for the different surface during dichoptic but not monocular viewing. Larger cueing effects were also observed for the subsequent posterior N1 (160-220 ms) and P2 (250-300 ms) components during rivalry than during monocular viewing. These results are in line with a hybrid model of rivalry whereby cuing one surface initiates an earlier interocular selection process when the competing surfaces are presented to separate eyes.

Involvement of memory‐comparison‐based change detection in visual distraction

The involvement of memory-comparison-based change detection in visual distraction was elucidated. Not only luminance increments that engaged memory-comparison-based change detection and refractoriness-based rareness detection but also luminance decrements that engaged only memory-comparison-based change detection caused behavioral distraction, which was mirrored by a posterior negativity (240-260 ms, posterior N2) and a broad positivity (420-460 ms, P3a) that reflected attentional capture. Preceding these effects, luminance increments elicited a posterior positivity (100-120 ms, change-related positivity) and a posterior negativity (120-140 ms, change-related negativity), whereas luminance decrements elicited only a posterior positivity (160-180 ms, change-related positivity). These results suggest that memory-comparison-based change detection indexed by change-related positivity is involved in visual distraction as a result of attentional capture.

Attention switching function of memory-comparison-based change detection system in the visual modality

To elucidate the attention switching function of a memory-comparison-based change detection system in the visual modality, the effects of task-irrelevant infrequent stimulus-size decrements that engaged memory-comparison-based change detection as well as stimulus-size increments that engaged memory-comparison-based change detection and refractoriness-based rareness detection on behavioral and event-related brain potential (ERP) measures were assessed using the distraction paradigm. Both size increments and size decrements caused distraction in forced-choice task performance, which was mirrored by a posterior negativity (peaking at around 240–260 ms, posterior N2) and a broad positivity (420–460 ms, P3a) that reflected attentional capture. Preceding these effects, size increments elicited a posterior negativity (120–140 ms, change-related negativity), while size decrements elicited a posterior positivity (140–160 ms, change-related positivity) and an anterior positivity (160–180 ms, frontal positivity). Taken together, these results indicate an attention switching function of a memory-comparison-based change detection system in the visual modality, which is most probably indexed by change-related positivity.

When and where perceptual load interacts with voluntary visuospatial attention: An event-related potential and dipole modeling study

Perceptual load is recognized to affect visual selective attention, but at an unknown spatiotemporal locus in the brain. To examine this issue, event-related potentials (ERPs) were recorded while participants performed an orientation discrimination task, under conditions of low or high perceptual load. Participants were required to respond to targets (10% of trials) presented in the attended visual field while ignoring all stimuli in the unattended visual field. The interaction between voluntary attention and perceptual load was significant for the posterior N1 component (190 ms) but not for the earlier C1 (84 ms) or P1 (100 ms) components. This load by attention interaction for N1 was localized to the temporoparietal–occipital (TPO) gyrus by dipole modeling analysis. Dipole modeling also showed that a reversed attentional effect in the C1 time range was due to ERP overlap from the subsequent attention-sensitive P1 component. Results suggest that perceptual load affects voluntary visuospatial attention at an early (but not the earliest) processing stage and that the TPO gyrus mediates target selection at the discrimination stage.

The emergence of semantic categorization in early visual processing: ERP indices of animal vs. artifact recognition.

BackgroundNeuroimaging and neuropsychological literature show functional dissociations in brain activity during processing of stimuli belonging to different semantic categories (e.g., animals, tools, faces, places), but little information is available about the time course of object perceptual categorization. The aim of the study was to provide information about the timing of processing stimuli from different semantic domains, without using verbal or naming paradigms, in order to observe the emergence of non-linguistic conceptual knowledge in the ventral stream visual pathway. Event related potentials (ERPs) were recorded in 18 healthy right-handed individuals as they performed a perceptual categorization task on 672 pairs of images of animals and man-made objects (i.e., artifacts).ResultsBehavioral responses to animal stimuli were ~50 ms faster and more accurate than those to artifacts. At early processing stages (120–180 ms) the right occipital-temporal cortex was more activated in response to animals than to artifacts as indexed by posterior N1 response, while frontal/central N1 (130–160) showed the opposite pattern. In the next processing stage (200–260) the response was stronger to artifacts and usable items at anterior temporal sites. The P300 component was smaller, and the central/parietal N400 component was larger to artifacts than to animals.ConclusionThe effect of animal and artifact categorization emerged at ~150 ms over the right occipital-temporal area as a stronger response of the ventral stream to animate, homomorphic, entities with faces and legs. The larger frontal/central N1 and the subsequent temporal activation for inanimate objects might reflect the prevalence of a functional rather than perceptual representation of manipulable tools compared to animals. Late ERP effects might reflect semantic integration and cognitive updating processes. Overall, the data are compatible with a modality-specific semantic memory account, in which sensory and action-related semantic features are represented in modality-specific brain areas.

The semantic processing of syntactic structure in sentence comprehension: An ERP study

Previous studies have demonstrated that verbs violating selectional constraints of their arguments elicit N400 effects in the event-related potentials (ERPs) in sentence comprehension. The present study examined brain responses to verbs violating semantic constraints specified by syntactic structures (i.e., phrasal constructions), contrasting them with those elicited by lexical–semantic violations between verbs and their arguments. The construction-based semantic violations gave rise to a posterior N400, while the lexical-based semantic violations produced a much stronger N400 with a broader scalp distribution. These findings suggested that the integration of verb meaning with prior sentence context is influenced not only by semantic features of preceding content words with which the verb co-occurs, but also by semantic properties of the syntactic structure in which the verb appears. This study provides online evidence supporting the constructionist approaches to language, which claim that syntactic structures may have their own (abstract) meanings, independent of lexical meanings of their constituent content words.

Spatial attention facilitates selection of illusory objects: Evidence from event-related brain potentials

The relationship between spatial attention and object-based attention has long been debated. On the basis of behavioral evidence it has been hypothesized that these two forms of attention share a common mechanism, such that directing spatial attention to one part of an object facilitates the selection of the entire object. In a previous study (Martinez, A., Teder-Salejarvi, W., Vazquez, M., Molholm, S., Foxe, J.J., Javitt, D.C., Di Russo, F., Worden, M.S., Hillyard, S.A., 2006. “Objects are highlighted by spatial attention.” J. Cogn. Neurosci. 18(2): 298–310) we used recordings of event-related potentials (ERPs) during a paradigm modeled after that of Egly et al. (Egly, R., Driver, J., Rafal, D.R., 1994. Shifting visual attention between objects and locations: evidence from normal and parietal lesion subjects. J. Exp. Psychol. Gen. 123(2) 161–77) to investigate this relationship. As reported in numerous studies of spatial attention, we found the typical pattern of enhanced neural activity in visual cortex elicited by attended stimuli. Unattended stimuli belonging to the same object as the attended stimuli elicited a very similar spatiotemporal pattern of enhanced neural activity that was localized to lateral occipital cortex (LOC). This similarity was taken as evidence that spatial- and object-selective attention share, at least in part, a common neural mechanism. In the present study we further investigate this relationship by examining whether this spread of spatial attention within attended objects can be guided by objects defined by illusory contours. Subjects viewed a display consisting of two illusory rectangular objects and directed attention to continuous sequences of stimuli (brief onsets) at one end of one of the objects. Stimuli occurring at irrelevant locations but belonging to the same attended object elicited larger posterior N1 amplitudes than that elicited by unattended objects forming part of a different object. This object-selective N1 enhancement was localized to lateral occipital cortex. The present data support the hypothesis that the allocation of spatial attention can be guided by illusory object boundaries and that this allocation strengthens the perceptual representations of attended objects at the level of visual area LOC.

Drosophila Anterior-Posterior Polarity Requires Actin-Dependent PAR-1 Recruitment to the Oocyte Posterior

The Drosophila anterior-posterior axis is established at stage 7 of oogenesis when the posterior follicle cells signal to polarize the oocyte microtubule cytoskeleton. This requires the conserved PAR-1 kinase, which can be detected at the posterior of the oocyte in immunostainings from stage 9. However, this localization depends on Oskar localization, which requires the earlier PAR-1-dependent microtubule reorganization, indicating that Oskar-associated PAR-1 cannot establish oocyte polarity. Here we analyze the function of the different PAR-1 isoforms and find that only PAR-1 N1 isoforms can completely rescue the oocyte polarity phenotype. Furthermore, PAR-1 N1 is recruited to the posterior cortex of the oocyte at stage 7 in response to the polarizing follicle cell signal, and this requires actin, but not microtubules. This suggests that posterior PAR-1 N1 polarizes the microtubule cytoskeleton. PAR-1 N1 localization is mediated by a cortical targeting domain and a conserved anterior-lateral exclusion signal in its C-terminal linker domain. PAR-1 is also required for the polarization of the C. elegans zygote and is recruited to the posterior cortex in an actin-dependent manner. Our results therefore identify a molecular parallel between axis formation in Drosophila and C. elegans and make Drosophila PAR-1 N1 the earliest known marker for the polarization of the oocyte.