Distinct patterns of neural activity during memory formation of nonwords versus words

Research into the neural underpinnings of memory formation has focused on the encoding of familiar verbal information. Here, we address how the brain supports the encoding of novel information that does not have meaning. Electrical brain activity was recorded from the scalps of healthy young adults while they performed an incidental encoding task (syllable judgments) on separate series of words and "nonwords" (nonsense letter strings that are orthographically legal and pronounceable). Memory for the items was then probed with a recognition memory test. For words as well as nonwords, event-related potentials differed depending on whether an item would subsequently be remembered or forgotten. However, the polarity and timing of the effect varied across item type. For words, subsequently remembered items showed the Usually observed positive-going, frontally distributed modulation from around 600 msec after word onset. For nonwords, by contrast, a negative-going, spatially widespread modulation predicted encoding success from 1000 rnsec onward. Nonwords also showed a modulation shortly after item onset. These findings imply that the brain supports the encoding of familiar and unfamiliar letter strings in qualitatively different ways, including the engagement of distinct neural activity at different points in time. The processing of semantic attributes plays an important role in the encoding of words and the associated positive frontal modulation

A visual M170 effect of morphological complexity

Recent masked priming studies on visual word recognition have suggested that morphological decomposition is performed prelexically, purely on the basis of the orthographic properties of the word form. Given this, one might expect morphological complexity to modulate early visual evoked activity in electromagnetic measures. We investigated the neural bases of morphological decomposition with magnetoencephalography (MEG). In two experiments, we manipulated morphological complexity in single word lexical decision without priming, once using suffixed words and once using prefixed words. We found that morphologically complex forms display larger amplitudes in the M170, the same component that has been implicated for letterstring and face effects in previous MEG studies. Although letterstring effects have been reported to be left-lateral, we found a right-lateral effect of morphological complexity, suggesting that both hemispheres may be involved in early analysis of word forms

ERPs and their brain sources in perceptual and conceptual prospective memory tasks: commonalities and differences between the two tasks

The present study examined whether Event-Related Potential (ERP) components and their neural generators are common to perceptual and conceptual prospective memory (PM) tasks or specific to the form of PM cue involved. We used Independent Component Analysis (ICA) to study the contributions of brain source activities to scalp ERPs across the different phases of two event-based PM-tasks: (1) holding intentions during a delay (monitoring) (2) detecting the correct context to perform the delayed intention (cue detection) and (3) carrying out the action (realisation of delayed intentions). Results showed that monitoring for both perceptual and conceptual PM-tasks was characterised by an enhanced early occipital negativity (N200). In addition the conceptual PM-task showed a long-lasting effect of monitoring significant around 700 ms. Perceptual PM-task cues elicited an N300 enhancement associated with cue detection, whereas a midline N400-like response was evoked by conceptual PM-task cues. The Prospective Positivity associated with realisation of delayed intentions was observed in both conceptual and perceptual tasks. A common frontal-midline brain source contributed to the Prospective Positivity in both tasks and a strong contribution from parieto-frontal brain sources was observed only for the perceptually cued PM-task. These findings support the idea that: (1) The enhanced N200 can be understood as a neural correlate of a ‘retrieval mode’ for perceptual and conceptual PM-tasks, and additional strategic monitoring is implemented according the nature of the PM task; (2) ERPs associated with cue detection are specific to the nature of the PM cues; (3) Prospective Positivity reflects a general PM process, but the specific brain sources contributing to it depend upon the nature of the PM task

Event-related brain potentials in the study of inhibition: cognitive control, source localization and age-related modulations

In the previous 15 years, a variety of experimental paradigms and methods have been employed to study inhibition. In the current review, we analyze studies that have used the high temporal resolution of the event-related potential (ERP) technique to identify the temporal course of inhibition to understand the various processes that contribute to inhibition. ERP studies with a focus on normal aging are specifically analyzed because they contribute to a deeper understanding of inhibition. Three time windows are proposed to organize the ERP data collected using inhibition paradigms: the 200 ms period following stimulus onset; the period between 200 and 400 ms after stimulus onset; and the period between 400 and 800 ms after stimulus onset. In the first 200 ms, ERP inhibition research has primarily focused on N1 and P1 as the ERP components associated with inhibition. The inhibitory processing in the second time window has been associated with the N2 and P3 ERP components. Finally, in the third time window, inhibition has primarily been associated with the N400 and N450 ERP components. Source localization studies are analyzed to examine the association between the inhibition processes that are indexed by the ERP components and their functional brain areas. Inhibition can be organized in a complex functional structure that is not constrained to a specific time point but, rather, extends its activity through different time windows. This review characterizes inhibition as a set of processes rather than a unitary process

Brain asymmetry and visual word recognition: do we have a split fovea?

In this chapter we discuss how the anatomical divide between the left and the right brain half has implications for visual word recognition. In particular, it introduces the need for massive interhemispheric communication. Unlike what was believed in the traditional view, it looks increasingly likely that interhemispheric integration is already needed from the very first stages of word processing, when the letter information is combined to activate stored word representations. Taking into account these insights not only improves our understanding of the neurophysiological and cognitive mechanisms of reading, it also gives us new ideas to look at individual differences in reading

Brain plasticity in aphasic patients: Intra- and inter-hemispheric reorganisation of the whole linguistic network probed by N150 and N350 components

The present study examined linguistic plastic reorganization of language through Evoked Potentials in a group of 17 non-fluent aphasic patients who had suffered left perisylvian focal lesions, and showed a good linguistic recovery. Language reorganisation was probed with three linguistic tasks (Phonological, Semantic, Orthographic), the early word recognition potential (N150) and the later phonological-related component (N350). Results showed the typical left-lateralised posterior N150 in healthy controls (source: left Fusiform Gyrus), that was bilateral (Semantic) or right sided (Phonological task) in patients (sources: right Inferior/Middle Temporal and Fusiform Gyri). As regards N350, controls revealed different intra- and inter-hemispheric linguistic activation across linguistic tasks, whereas patients exhibited greater activity in left intact sites, anterior and posterior to the damaged area, in all tasks (sources: Superior Frontal Gyri). A comprehensive neurofunctional model is presented, describing how complete intra- and inter-hemispheric reorganisation of the linguistic networks occurs after aphasic damage in the strategically dominant left perisylvian linguistic centres

Neural Correlates of Letter Reversal in Children and Adults

Children often make letter reversal errors when first learning to read and write, even for letters whose reversed forms do not appear in normal print. However, the brain basis of such letter reversal in children learning to read is unknown. The present study compared the neuroanatomical correlates (via functional magnetic resonance imaging) and the electrophysiological correlates (via event-related potentials or ERPs) of this phenomenon in children, ages 5–12, relative to young adults. When viewing reversed letters relative to typically oriented letters, adults exhibited widespread occipital, parietal, and temporal lobe activations, including activation in the functionally localized visual word form area (VWFA) in left occipito-temporal cortex. Adults exhibited significantly greater activation than children in all of these regions; children only exhibited such activation in a limited frontal region. Similarly, on the P1 and N170 ERP components, adults exhibited significantly greater differences between typical and reversed letters than children, who failed to exhibit significant differences between typical and reversed letters. These findings indicate that adults distinguish typical and reversed letters in the early stages of specialized brain processing of print, but that children do not recognize this distinction during the early stages of processing. Specialized brain processes responsible for early stages of letter perception that distinguish between typical and reversed letters may develop slowly and remain immature even in older children who no longer produce letter reversals in their writing

During visual word recognition, phonology is accessed within 100 ms and may be mediated by a speech production code: evidence from magnetoencephalography

Debate surrounds the precise cortical location and timing of access to phonological information during visual word recognition. Therefore, using whole-head magnetoencephalography (MEG), we investigated the spatiotemporal pattern of brain responses induced by a masked pseudohomophone priming task. Twenty healthy adults read target words that were preceded by one of three kinds of nonword prime: pseudohomophones (e.g., brein–BRAIN), where four of five letters are shared between prime and target, and the pronunciation is the same; matched orthographic controls (e.g., broin–BRAIN), where the same four of five letters are shared between prime and target but pronunciation differs; and unrelated controls (e.g., lopus–BRAIN), where neither letters nor pronunciation are shared between prime and target. All three priming conditions induced activation in the pars opercularis of the left inferior frontal gyrus (IFGpo) and the left precentral gyrus (PCG) within 100 ms of target word onset. However, for the critical comparison that reveals a processing difference specific to phonology, we found that the induced pseudohomophone priming response was significantly stronger than the orthographic priming response in left IFG/PCG at ∼100 ms. This spatiotemporal concurrence demonstrates early phonological influences during visual word recognition and is consistent with phonological access being mediated by a speech production code

Testing the domain-general nature of monitoring in the spatial and verbal cognitive domains

While it is well-established that monitoring the environment for the occurrence of relevant events represents a key executive function, it is still unclear whether such a function is mediated by domain-general or domain-specific mechanisms. We investigated this issue by combining event-related potentials (ERPs) with a behavioral paradigm in which monitoring processes (non-monitoring vs. monitoring) and cognitive domains (spatial vs. verbal) were orthogonally manipulated in the same group of participants. They had to categorize 3-dimensional visually presented words on the basis of either spatial or verbal rules. In monitoring blocks, they additionally had to check whether the word displayed a specific spatial configuration or whether it contained a certain consonant. The behavioral results showed slower responses for both spatial and verbal monitoring trials compared to non-monitoring trials. The ERP results revealed that monitoring did not interact with domain, thus suggesting the involvement of common underlying mechanisms. Specifically, monitoring acted on lower-level perceptual processes (as expressed by an enhanced visual N1 wave and a sustained posterior negativity for monitoring trials) and on higher-level cognitive processes (involving larger positive modulations by monitoring trials over frontal and parietal scalp regions). The source reconstruction analysis of the ERP data confirmed that monitoring was associated with increased activity in visual areas and in right prefrontal and parietal regions (i.e., superior and inferior frontal gyri and posterior parietal cortex), which previous studies have linked to spatial and temporal monitoring. Our findings extend this research by supporting the domain-general nature of monitoring in the spatial and verbal domains