Syntactic processing with aging: an event-related potential study

Effect of Healthy Aging and Gender on Syntactic Input Processing: A P600 Event-Related Potential Study

PURPOSE

This study aimed to investigate the effect of healthy aging and gender, as well as the interaction, thereof, on syntactic input processing during sentence comprehension. This was achieved through the recording of the P600 event-related potential.

METHOD

Sixty Flemish (native speakers of Dutch) participants (30 men and 30 women), equally distributed into three age groups (young, middle-aged, and older adults), were subjected to a visually presented word order violation task under simultaneous electro-encephalography recording. The task contained 60 sentences, of which half were grammatical and half contained a word order violation. P600 responses were analyzed for amplitude, latency, topographical distribution, and source localization.

RESULTS

Regarding the effect of healthy aging, no age-related differences were found for the amplitude, onset latency, and topographical distribution of the P600 effect (difference wave). Although aging effects on the P600 effect amplitude were absent, a reduced P600 amplitude in response to both the grammatical and ungrammatical sentences was found, next to a reduced overall degree of source activation in linguistic regions of interest. Also, a reduced behavioral accuracy in response to the word order violation was observed in the older adults group. Regarding the effect of gender, females exhibited a larger P600 effect amplitude and a reduced behavioral accuracy compared to males. No gender-related differences were found for P600 effect onset latency, topographical distribution, and source activation.

CONCLUSIONS

While this study demonstrates no effect of aging on the P600 effect, the lower behavioral response and absence of any activation shift argues against functional compensation. Moreover, although increased neural activation in women combined with their reduced behavioral accuracy may indicate the use of different cognitive strategies in men and women, source localization analysis could not objectify this hypothesis.

Musical and linguistic syntactic processing in agrammatic aphasia: An ERP study

Language and music rely on complex sequences organized according to syntactic principles that are implicitly understood by enculturated listeners. Across both domains, syntactic processing involves predicting and integrating incoming elements into higher-order structures. According to the Shared Syntactic Integration Resource Hypothesis (SSIRH; Patel, 2003), musical and linguistic syntactic processing rely on shared resources for integrating incoming elements (e.g., chords, words) into unfolding sequences. One prediction of the SSIRH is that people with agrammatic aphasia (whose deficits are due to syntactic integration problems) should present with deficits in processing musical syntax. We report the first neural study to test this prediction: event-related potentials (ERPs) were measured in response to musical and linguistic syntactic violations in a group of people with agrammatic aphasia (n=7) compared to a group of healthy controls (n=14) using an acceptability judgement task. The groups were matched with respect to age, education, and extent of musical training. Violations were based on morpho-syntactic relations in sentences and harmonic relations in chord sequences. Both groups presented with a significant P600 response to syntactic violations across both domains. The aphasic participants presented with a reduced-amplitude posterior P600 compared to the healthy adults in response to linguistic, but not musical, violations. Participants with aphasia did however present with larger frontal positivities in response to violations in both domains. Intriguingly, extent of musical training was associated with larger posterior P600 responses to syntactic violations of language and music in both groups. Overall, these findings are not consistent with the predictions of the SSIRH, and instead suggest that linguistic, but not musical, syntactic processing may be selectively impaired in stroke-induced agrammatic aphasia. However, the findings also suggest a relationship between musical training and linguistic syntactic processing, which may have clinical implications for people with aphasia, and motivates more research on the relationship between these two domains.

Altered event processing in persons with Parkinson's disease

Persons with Parkinson's disease (PD) often show particular problems in seemingly simple routines despite relatively preserved cognitive function. We therefore investigated the processing of everyday events on behavioral and neurophysiological levels in a PD and control group. The participants had to indicate via button press whether three sequentially presented sub-events described a previously defined event (e.g., going grocery shopping). Sub-event sequences were either correct or included an event that did not belong to the event (content violation), or events were chronologically wrong (temporal violation). During task execution event-related potentials (ERPs) were recorded. Generally, the PD group showed less accurate performance independently from task conditions, and reaction times to temporal violations were particularly slow compared to the control group. Regarding ERP results, the control group showed a right lateralized N400 effect in response to content violations, which was absent in the PD group indicating altered content event processing. Concerning the reanalysis of content event violations, the expression of late positive components (LPCs) was similar between both groups. Upon temporal violations, both groups also showed a LPC with a tendentially earlier onset in the PD group, resembling positive components indicative of novelty processing. Together, these findings suggest poor event prediction in PD, which may originate from weak event representation or retrieval and possibly relate to prevalent behavioral dysfunctions in everyday life in PD.

ERP evidence of age-related differences in emotional processing

The present study used event-related potentials (ERPs) to examine differences in the temporal dynamics of emotion processing in young and older adults, with a specific focus on the positivity effect, that is, the preferential processing of positive over negative information. To this aim, we used a language paradigm that allowed us to investigate early ERP components as well as later components, namely the N400 and the late positive complex (LPC). Young and older adults were presented with neutral sentence stems with positive, negative or neutral/semantically-incongruent critical word endings while their electrical brain activity was recorded. There were no effects of emotional valence on early ERP components. Instead, a positivity effect was evident in young adults indexed by reduced N400s for positive sentence endings. Perhaps due to reduced semantic processing abilities, older adults did not show any N400 effect. ERP effects in this group were evident at a later processing stage and took the form of larger LPCs for neutral/incongruent information. Overall, there was no effect of emotional valence on either the N400 or the LPC in older adults. Our data suggest that with age, more effortful semantic processing may deplete resources for emotional processing.

Online sentence processing impairments in agrammatic and logopenic primary progressive aphasia: Evidence from ERP

Evidence from psycholinguistic research indicates that sentence processing is impaired in Primary Progressive Aphasia (PPA), and more so in individuals with agrammatic (PPA-G) than logopenic (PPA-L) subtypes. Studies have mostly focused on offline sentence production ability, reporting impaired production of verb morphology (e.g., tense, agreement) and verb-argument structure (VAS) in PPA-G, and mixed findings in PPA-L. However, little is known about real-time sentence comprehension in PPA. The present study is the first to compare real-time semantic, morphosyntactic and VAS processing in individuals with PPA (10 with PPA-G and 9 with PPA-L), and in two groups of healthy (22 young and 19 older) individuals, using event-related potentials (ERP). Participants were instructed to listen to sentences that were either well-formed (n = 150) or contained a violation of semantics (e.g., *Owen was mentoring pumpkins at the party, n = 50), morphosyntax (e.g., *The actors was singing in the theatre, n = 50) or VAS (*Ryan was devouring on the couch, n = 50), and were required to perform a sentence acceptability judgment task while EEG was recorded. Results indicated that in the semantic task both healthy and PPA groups showed an N400 response to semantic violations, which was delayed in PPA and older (vs. younger) groups. Morphosyntactic violations elicited a P600 in both groups of healthy individuals and in PPA-L, but not in PPA-G. A similar P600 response was also found only in healthy individuals for VAS violations; whereas, abnormal ERP responses were observed in both PPA groups, with PPA-G showing no evidence of VAS violation detection and PPA-L showing a delayed and abnormally-distributed positive component that was negatively associated with offline sentence comprehension scores. These findings support characterizations of sentence processing impairments in PPA-G, by providing online evidence that VAS and morphosyntactic processing are impaired, in the face of substantially preserved semantic processing. In addition, the results indicate that on-line processing of VAS information may also be impaired in PPA-L, despite their near-normal accuracy on standardized language tests of argument structure production.

Electrophysiological differences in older and younger adults' anaphoric but not cataphoric pronoun processing in the absence of age-related behavioural slowdown

This study reports on an event-related potentials experiment to uncover whether per-millisecond electrophysiological brain activity and analogous behavioural responses are age-sensitive when comprehending anaphoric (referent-first) and cataphoric (pronoun-first) pronouns. Two groups of French speakers were recruited (young n = 18; aged 19-35 and older adults n = 15; aged 57-88) to read sentences where the anaphoric/cataphoric pronouns and their potential referents either matched or mismatched in gender. Our findings indicate that (1) the older adults were not less accurate or slower in their behavioural responses to the mismatches than the younger adults, (2) both anaphoric and cataphoric conditions evoked a central/parietally distributed P600 component with similar timing and amplitude in both the groups. Importantly, mean amplitudes of the P600 effect were modulated by verbal short-term memory span in the older adults but not in the younger adults, (3) nevertheless, the older but not the younger adults displayed an additional anterior negativity emerging on the frontal regions in response to the anaphoric mismatches. These results suggest that pronoun processing is resilient in healthy ageing individuals, but that functional recruitment of additional brain regions, evidenced with the anterior negativity, compensates for increased processing demands in the older adults' anaphora processing.

Effects of incidental physical activity on morphosyntactic processing in aging

Older adults have difficulties in sentence comprehension when working memory (WM) load increases (e.g., multiple embedded clauses). Structured physical activity has been related to improvements in cognition; however, incidental physical activity (PA, i.e., unstructured daily physical activities), particularly incidental vigorous activity has been poorly studied in relation to its effects on behavior. Furthermore, no positive effect on language has been reported in either form of physical activity. The aim of this study was to evaluate how two levels of PA (high or low) affect WM processing and how this, in turn, may affect morphosyntactic processing in older adults. Individuals with high PA (n = 18) had a higher WM load effect than those with low PA (n = 18), both behaviorally (greater differences between high and low WM loads in correct responses) and in terms of event-related potentials (only subjects with high PA showed LAN and P600b amplitude differences between high and low WM loads). These findings suggest that PA promotes cognitive strategies to face WM loads and morphosyntactic processing.

The oscillatory mechanisms associated with syntactic binding in healthy ageing

Older adults frequently display differential patterns of brain activity compared to young adults in the same task, alongside widespread neuroanatomical changes. Differing functional activity patterns in older adults are commonly interpreted as being compensatory (e.g., Cabeza et al., 2002). We examined the oscillatory activity in the EEG during syntactic binding in young and older adults, as well as the relationship between oscillatory activity and behavioural performance on a syntactic judgement task within the older adults. 19 young and 41 older adults listened to two-word sentences that differentially load onto morpho-syntactic binding: correct syntactic binding (morpho-syntactically correct, e.g., "I dotch"); incorrect syntactic binding (morpho-syntactic agreement violation, e.g., "they dotches") and no syntactic binding (minimizing morpho-syntactic binding, e.g., "dotches spuff"). Behavioural performance, assessed in a syntactic judgement task, was characterized by inter-individual variability especially in older adults, with accuracy ranging from 76 to 100% in young adults and 58-100% in older adults. Compared to young adults, older adults were slower, but not less accurate. Functional neural signatures for syntactic binding were assessed as the difference in oscillatory power between the correct and no syntactic binding condition. In older adults, syntactic binding was associated with a smaller increase in theta (4-7 Hz), alpha (8-12 Hz) and beta (15-20 Hz) power in a time window surrounding the second word. There was a significant difference between the older and young adults: in the alpha range, the condition difference seemed to be in the opposite direction for older versus young adults. Our findings thus suggest that the neural signature associated with syntactic binding in older adults is different from young adults. However, we found no evidence of a significant association between behavioural performance and the neural signatures of syntactic binding for older adults, which does not readily support the predictions of compensatory models of language and ageing.

Poor working memory performance in healthy elderly adults with electroencephalographic risk of cognitive decline affects syntactic processing

OBJECTIVE

To examine the effects of working memory (WM) load and gender agreement on sentence processing as a function of the electroencephalographic risk (i.e., abnormally high values of theta absolute power) of cognitive decline in older adults.

METHODS

Event-related potentials (ERPs) were collected from Spanish speakers (22 older adults belonged to the Risk group, mean age = 67.7 years; 22 older adults belonged to the Control group, mean age = 65.2 years) while reading sentences to detect grammatical errors. Sentences varied with regard to (1) the gender agreement of the noun and adjective, where gender of the adjective either agreed or disagreed with the noun, and (2) WM load (i.e., the number of words between the noun and adjective in the sentence).

RESULTS

The Risk group showed a lower percentage of correct answers and longer reaction times than the Control group. The Risk group also showed a different pattern of ERP components, which was characterized by smaller amplitude and longer latency of the P600a component under high WM load conditions.

CONCLUSION

The findings suggest that the Risk group shows difficulties integrating information associated with the previous sentence context.

SIGNIFICANCE

The electroencephalographic risk factor of cognitive decline might be not only a predictor of but also an indicator of current decline.

The P3b and P600(s): Positive contributions to language comprehension

Since its discovery in the 1960s, the P300 has been contributing both directly and indirectly to language research. Perhaps most notably, it has been suggested that the P600, an ERP component that was first characterized in the context of syntactic processing, could be a variant of the P3b subcomponent of the P300. Here, we review studies on both sides of the debate. We also review the "semantic P600," a positivity with a similar time course and distribution to the P600 seen for syntactic manipulations but that is obtained in response to some types of semantic anomalies. Because most current theories of the P600 try to account for both the syntactic and the semantic variant, linking the syntactic P600 to the P3b might also imply a similar link for the semantic P600. However, we describe emerging research in our lab that casts doubt on the idea that the syntactic P600 and the semantic P600 are the same effect. We argue that grouping ERP responses primarily by domain (language vs. nonlanguage) is likely to be misleading and suggest alternative ways of determining whether ERP effects reflect similar or different processing mechanisms.

Electrophysiological evidence for preserved primacy of lexical prediction in aging

Young adults show consistent neural benefits of predictable contexts when processing upcoming words, but these benefits are less clear-cut in older adults. Here we disentangle the neural correlates of prediction accuracy and contextual support during word processing, in order to test current theories that suggest that neural mechanisms underlying predictive processing are specifically impaired in older adults. During a sentence comprehension task, older and younger readers were asked to predict passage-final words and report the accuracy of these predictions. Age-related reductions were observed for N250 and N400 effects of prediction accuracy, as well as for N400 effects of contextual support independent of prediction accuracy. Furthermore, temporal primacy of predictive processing (i.e., earlier facilitation for successful predictions) was preserved across the lifespan, suggesting that predictive mechanisms are unlikely to be uniquely impaired in older adults. In addition, older adults showed prediction effects on frontal post-N400 positivities (PNPs) that were similar in amplitude to PNPs in young adults. Previous research has shown correlations between verbal fluency and lexical prediction in older adult readers, suggesting that the production system may be linked to capacity for lexical prediction, especially in aging. The current study suggests that verbal fluency modulates PNP effects of contextual support, but not prediction accuracy. Taken together, our findings suggest that aging does not result in specific declines in lexical prediction.

Effects of Age and Working Memory Load on Syntactic Processing: An Event-Related Potential Study

Cognitive changes in aging include working memory (WM) decline, which may hamper language comprehension. An increase in WM demands in older adults would probably provoke a poorer sentence processing performance in this age group. A way to increase the WM load is to separate two lexical units in an agreement relation (i.e., adjective and noun), in a given sentence. To test this hypothesis, event-related potentials (ERPs) were collected from Spanish speakers (30 older adults, mean age = 66.06 years old; and 30 young adults, mean age = 25.7 years old) who read sentences to detect grammatical errors. The sentences varied with regard to (1) the gender agreement of the noun and adjective, where the gender of the adjective either agreed or disagreed with the noun, and (2) the WM load (i.e., the number of words between the noun and adjective in the sentence). No significant behavioral differences between groups were observed in the accuracy of the response, but older adults showed longer reaction times regardless of WM load condition. Compared with young participants, older adults showed a different pattern of ERP components characterized by smaller amplitudes of LAN, P600a, and P600b effects when the WM load was increased. A smaller LAN effect probably reflects greater difficulties in processing the morpho-syntactic features of the sentence, while smaller P600a and P600b effects could be related to difficulties in recovering and mapping all sentence constituents. We concluded that the ERP pattern in older adults showed subtle problems in syntactic processing when the WM load was increased, which was not sufficient to affect response accuracy but was only observed to result in a longer reaction time.

Reduced Syntactic Processing Efficiency in Older Adults During Sentence Comprehension

Researchers have frequently reported an age-related decline in semantic processing during sentence comprehension. However, it remains unclear whether syntactic processing also declines or whether it remains constant as people age. In the present study, 26 younger adults and 20 older adults were recruited and matched in terms of working memory, general intelligence, verbal intelligence and fluency. They were then asked to make semantic acceptability judgments while completing a Chinese sentence reading task. The behavioral results revealed that the older adults had significantly lower accuracy on measures of semantic and syntactic processing compared to younger adults. Event-related potential (ERP) results showed that during semantic processing, older adults had a significantly reduced amplitude and delayed peak latency of the N400 compared to the younger adults. During syntactic processing, older adults also showed delayed peak latency of the P600 relative to younger adults. Moreover, while P600 amplitude was comparable between the two age groups, larger P600 amplitude was associated with worse performance only in the older adults. Together, the behavioral and ERP data suggest that there is an age-related decline in both semantic and syntactic processing, with a trend toward lower efficiency in syntactic ability.

Neurophysiological and Behavioral Differences between Older and Younger Adults When Processing Violations of Tonal Structure in Music

Aging is associated with decline in both cognitive and auditory abilities. However, evidence suggests that music perception is relatively spared, despite relying on auditory and cognitive abilities that tend to decline with age. It is therefore likely that older adults engage compensatory mechanisms which should be evident in the underlying functional neurophysiology related to processing music. In other words, the perception of musical structure would be similar or enhanced in older compared to younger adults, while the underlying functional neurophysiology would be different. The present study aimed to compare the electrophysiological brain responses of younger and older adults to melodic incongruities during a passive and active listening task. Older and younger adults had a similar ability to detect an out-of-tune incongruity (i.e., non-chromatic), while the amplitudes of the ERAN and P600 were reduced in older adults compared to younger adults. On the other hand, out-of-key incongruities (i.e., non-diatonic), were better detected by older adults compared to younger adults, while the ERAN and P600 were comparable between the two age groups. This pattern of results indicates that perception of tonal structure is preserved in older adults, despite age-related neurophysiological changes in how melodic violations are processed.

Age-related shifts in hemispheric dominance for syntactic processing

Recent ERP data from young adults have revealed that simple syntactic anomalies elicit different patterns of lateralization in right-handed participants depending upon their familial sinistrality profile (whether or not they have left-handed biological relatives). Right-handed participants who do not have left-handed relatives showed a strongly lateralized response pattern, with P600 responses following left-hemisphere-biased presentations and N400 responses following right-hemisphere-biased presentations. Given that the literature on aging has documented a tendency to change across adulthood from asymmetry of function to a more bilateral pattern, we tested the stability of this asymmetric response to syntactic violations by recording ERPs as 24 older adults (age 60+) with no history of familial sinistrality made grammaticality judgments on simple two-word phrases. Results showed that the asymmetric pattern observed in right-handed adults without familial sinistrality indeed changes with age, such that P600 responses come to be elicited not only with left-hemisphere-biased but also with right-hemisphere-biased presentations in older adults. These findings suggest that, as with many other cognitive functions, syntactic processing becomes more bilateral with age.

Age-related changes in feature-based object memory retrieval as measured by event-related potentials

To investigate neural mechanisms that support semantic functions in aging, we recorded scalp EEG during an object retrieval task in 22 younger and 22 older adults. The task required determining if a particular object could be retrieved when two visual words representing object features were presented. Both age groups had comparable accuracy although response times were longer in older adults. In both groups a left fronto-temporal negative potential occurred at around 750ms during object retrieval, consistent with previous findings (Brier, Maguire, Tillman, Hart, & Kraut, 2008). In only older adults, a later positive frontal potential was found peaking between 800 and 1000ms during no retrieval. These findings suggest younger and older adults employ comparable neural mechanisms when features clearly facilitate retrieval of an object memory, but when features yield no retrieval, older adults use additional neural resources to engage in a more effortful and exhaustive search prior to making a decision.

Grammatical number agreement processing using the visual half-field paradigm: an event-related brain potential study

Despite indications in the split-brain and lesion literatures that the right hemisphere is capable of some syntactic analysis, few studies have investigated right hemisphere contributions to syntactic processing in people with intact brains. Here we used the visual half-field paradigm in healthy adults to examine each hemisphere's processing of correct and incorrect grammatical number agreement marked either lexically, e.g., antecedent/reflexive pronoun ("The grateful niece asked herself/*themselves…") or morphologically, e.g., subject/verb ("Industrial scientists develop/*develops…"). For reflexives, response times and accuracy of grammaticality decisions suggested similar processing regardless of visual field of presentation. In the subject/verb condition, we observed similar response times and accuracies for central and right visual field (RVF) presentations. For left visual field (LVF) presentation, response times were longer and accuracy rates were reduced relative to RVF presentation. An event-related brain potential (ERP) study using the same materials revealed similar ERP responses to the reflexive pronouns in the two visual fields, but very different ERP effects to the subject/verb violations. For lexically marked violations on reflexives, P600 was elicited by stimuli in both the LVF and RVF; for morphologically marked violations on verbs, P600 was elicited only by RVF stimuli. These data suggest that both hemispheres can process lexically marked pronoun agreement violations, and do so in a similar fashion. Morphologically marked subject/verb agreement errors, however, showed a distinct LH advantage.

Reading aloud: a psychophysiological investigation in children

This study investigated the electrophysiological responses to single-letter reading in children (reading-related potentials) and explored the morphological differences between covert and overt reading conditions. Sixty-five healthy children (6-13 years) participated in this study. Reading-related potentials were recorded during visual stimulation with single Italian alphabetic letters. Stimuli were displayed for 5 ms either automatically at a randomly jittered time lag or upon voluntary self-paced button press by children. In the covert conditions, children had to passively look at single letters, while in the overt conditions children were required to read aloud the letters. Electromyographic activity of the forearm and lips was additionally recorded during all tasks. Superimposition of reading-related potentials with the electromyographic activity of forearm and lips during self-paced reading aloud allowed to segregate the reading-related components into four periods: preparatory, pre-lexical, lexical and post-lexical. Reading-related potentials of the preparatory period can be related to preparation/intention to read, those of the pre-lexical period to visual-perceptual processes, those of the lexical period to the external/internal reafferent activity and those of the post-lexical period to the feedback processes following task completion. Analysis of variance showed a significant interaction of reading-related components with electrode locations and task conditions in all periods. The systematic characterization of the neurophysiological correlates of the elementary association between letters and sounds is helpful to highlight the neurobiological and functional basis of reading in healthy as well as impaired readers, for possibly developing neurophysiologically grounded rehabilitation therapies and further improving the explanatory models of dyslexia.

Ambiguity's aftermath: how age differences in resolving lexical ambiguity affect subsequent comprehension

When ambiguity resolution is difficult, younger adults recruit selection-related neural resources that older adults do not. To elucidate the nature of those resources and the consequences of their recruitment for subsequent comprehension, we embedded noun/verb homographs and matched unambiguous words in syntactically well-specified but semantically neutral sentences. Target words were followed by a prepositional phrase whose head noun was plausible for only one meaning of the homograph. Replicating past findings, younger but not older adults elicited sustained frontal negativity to homographs compared to unambiguous words. On the subsequent head nouns, younger adults showed plausibility effects in all conditions, attesting to successful meaning selection through suppression. In contrast, older adults showed smaller plausibility effects following ambiguous words and failed to show plausibility effects when the context picked out the homograph's non-dominant meaning (i.e., they did not suppress the contextually-irrelevant dominant meaning). Meaning suppression processes, reflected in the frontal negativity, thus become less available with age, with consequences for subsequent comprehension.

Language of the aging brain: Event-related potential studies of comprehension in older adults

Normal aging brings increased richness in knowledge and experience as well as declines in cognitive abilities. Event-related brain potential (ERP) studies of language comprehension corroborate findings showing that the structure and organization of semantic knowledge remains relatively stable with age. Highlighting the advantages of the temporal and functional specificity of ERPs, this survey focuses on age-related changes in higher-level processes required for the successful comprehension of meaning representations built from multiple words. Older adults rely on different neural pathways and cognitive processes during normal, everyday comprehension, including a shift away from the predictive use of sentential context, differential recruitment of neural resources, and reduced engagement of controlled processing. Within age groups, however, there are important individual differences that, for example, differentiate a subset of older adults whose processing patterns more closely resemble that of young adults, providing a window into cognitive skills and abilities that may mediate or moderate age-related declines.

Some effects of aging on event-related potentials during a linguistic monitoring task

Two groups of adults, one young and one elderly, were compared on a dichotic task in which continuous speech was monitored for grammatically and semantically anomalous words. Event-related potentials (ERPs) elicited by these anomalies were analyzed in terms of peak amplitude and peak latency of the evoked late-positive component (LPC). Results showed that while LPC peak amplitudes were overall reduced for the group of seniors, LPC peak latencies were similar between both groups of listeners. These findings are consistent with the hypothesis that elderly persons can offset declines in memory, perceptual processes, and speed of mental processing by taking advantage of contextual and prosodic cues in running speech. Additionally, an apparently asymmetric processing negativity was observed in the waveforms of young adults but not seniors. Possible mechanisms include the N400 response to semantic incongruity and a processing negativity component associated with attentional bias toward right hemispace.

Identifying reliable independent components via split-half comparisons

Independent component analysis (ICA) is a family of unsupervised learning algorithms that have proven useful for the analysis of the electroencephalogram (EEG) and magnetoencephalogram (MEG). ICA decomposes an EEG/MEG data set into a basis of maximally temporally independent components (ICs) that are learned from the data. As with any statistic, a concern with using ICA is the degree to which the estimated ICs are reliable. An IC may not be reliable if ICA was trained on insufficient data, if ICA training was stopped prematurely or at a local minimum (for some algorithms), or if multiple global minima were present. Consequently, evidence of ICA reliability is critical for the credibility of ICA results. In this paper, we present a new algorithm for assessing the reliability of ICs based on applying ICA separately to split-halves of a data set. This algorithm improves upon existing methods in that it considers both IC scalp topographies and activations, uses a probabilistically interpretable threshold for accepting ICs as reliable, and requires applying ICA only three times per data set. As evidence of the method's validity, we show that the method can perform comparably to more time intensive bootstrap resampling and depends in a reasonable manner on the amount of training data. Finally, using the method we illustrate the importance of checking the reliability of ICs by demonstrating that IC reliability is dramatically increased by removing the mean EEG at each channel for each epoch of data rather than the mean EEG in a prestimulus baseline.

Effects of advancing age on the processing of semantic anomalies in adults: evidence from event-related brain potentials

Age-related changes in the processing of sentence-embedded semantic anomalies were examined using auditory event-related potentials (ERPs). Semantically incongruous words elicited an N400 effect in middle-aged (50s: 55.6 years) and elderly (60s: 64.1 years) subjects, whereas in older elderly adults (70s: 74.9 years) this effect approached significance. N400 peak latencies were not delayed with advancing age; however, there was a reliable linear decrease in mean and peak amplitude, with slopes being similar to those previously reported for the visual N400 effect. A P600 effect was obtained in response to semantic anomalies and it was neither delayed in latency nor reduced in amplitude with advancing age. However, it was found to be larger over anterior sites in elderly and older elderly subjects.

Stem allomorphy in the Spanish mental lexicon: evidence from behavioral and ERP experiments

This study presents results from a nonce-word elicited production task and a reading experiment using event-related brain potentials (ERPs) investigating finite forms of Spanish verbs which consist of marked stems and regular person and number agreement suffixes. The first experiment showed that unmarked stems are productively extended to nonce words, whereas marked stems generalize more restrictively to nonce words, based on lexical similarity to existing stem forms. The second experiment yielded a lexical ERP signature for stem violations and an ERP pattern signaling morpho-syntactic (rule-based) processing for suffix violations. We argue that stem allomorphy is lexically represented in the Spanish mental lexicon, with marked stems forming subnodes of structured lexical entries.

Effects of self-evaluation on the P300 event-related potential

The effects of self evaluation on the P300 event-related potential (ERP) were explored with 56 participants (16 men, 40 women; M age = 23.4 yr., SD = 1.2) across three conditions. The conditions included (1) a standard ERP auditory oddball discrimination between a random target (15% occurrence) and standard stimuli (85% occurrence), (2) the oddball task followed by the additional cognitive task of maintaining a mental count of the target tones, and (3) the oddball task followed by the additional cognitive task of self-evaluating whether they felt surprised by the current occurrence of the target tone. The added cognitive requirements for Conditions 2 and 3 required the subjects to maintain a cognitive readiness for the secondary stimulus-related task during their sensory discrimination response for the standard oddball task. During the self-evaluation condition, the P300 amplitude was significantly larger across all recording locations than the regular oddball condition and the cognitive count condition.

Aging in context: Age-related changes in context use during language comprehension

Effects of normal aging on the use of sentence context information during language comprehension were examined by measuring younger and older adults' event-related potential (ERP) responses to congruent sentence-final words as a function of contextual constraint. Half of the sentence contexts were strongly constraining, rendering the target word very predictable, whereas the other half were weakly constraining. Both age groups elicited smaller N400 responses to target words in strongly than in weakly constrained contexts, but this effect was significantly smaller and later for older adults. Older adults with lower reading spans showed greater delays. Age-related changes were driven primarily by decreases in older adults' ability to make use of the richer information available from strongly constraining contexts to guide semantic processing, as word processing (N400s) in weak contexts was qualitatively and quantitatively similar in the two age groups.