Event-related potentials, lexical decision and semantic priming

ERPs were recorded during a lexical decision task in order to investigate electrophysiological concomitants of semantic priming. The stimuli were 240 words and 240 nonwords presented one per trial at a fixed intertrial interval. Subjects were required to classify each stimulus as a word or nonword by pressing one of two response buttons. ERPs were recorded from 14 scalp locations, the right suborbital ridge, and the left earlobe, all referred to a balanced non-cephalic reference. RT and error data confirmed that semantic priming occurred under the conditions employed: primed words (those preceded by a semantically related word) were identified as words faster and more accurately than were unprimed words (those preceded by semantically unrelated words or nonwords). ERPs for all stimulus types were characterized by a large positivity peaking between 550 and 650 msec, preceded by a negative-going deflection peaking at approximately 400 msec. ERPs for primed and unprimed words were shown to differ significantly, diverging 200-250 msec following stimulus onset, reaching a maximum near the peak of the negative-going deflection at 400 msec. These differences were observed at locations over both hemispheres and were maximal in the centroparietal region. Although P300 latency differences between primed and unprimed words were also obtained, the priming effect on ERPs at shorter latencies could not be explained solely by P300 latency effects. Possible relationships between these ERP concomitants of semantic priming and P300, N200, and N400 were discussed.     

Attentional modulation of unconscious "automatic" processes: evidence from event-related potentials in a masked priming paradigm

Automatic processes are usually thought to occur independently of any cognitive resources. This traditional view has been recently challenged by showing that temporal attention to a target stimulus is a prerequisite for "automatic" response priming. The event-related potential (ERP) study reported here extends this research by pursuing a somewhat different approach. In two experiments, it was investigated whether masked semantic priming effects can be modulated by temporal attention to the prime using a cueing procedure. We hypothesized that masked priming is amplified when attention is directed to the stimulus stream in the time window of masked prime presentation, even in the absence of any prime awareness. ERPs were recorded while subjects performed a primed lexical decision task. Target words were preceded by semantically related or unrelated masked prime words, which were not consciously identified. A cue stimulus prompted subjects to direct their attention to the stimulus stream either shortly before the masked prime (short cue interval) or a long time interval before. Priming affected the amplitude of the N400 ERP component, an electrophysiological index of semantic processing. Unrelated prime-target pairs elicited a larger N400 than related pairs (N400 priming effect). Most importantly, this masked N400 priming effect was strongest when the cue interval and the stimulus onset asynchronies were short. The present results show that temporal attention to the prime is a prerequisite for obtaining masked N400 priming effects. They also demonstrate that unconscious automatic processes are susceptible to attentional modulation.     

The effects of prime visibility on ERP measures of masked priming

In two experiments, the effect of the duration (40, 80 and 120 ms) of pattern masked prime words on subsequent target word processing was measured using event-related potentials. In Experiment 1, target words were either repetitions of the prior masked prime (car-CAR) or were another unrelated word (job-CAR). In Experiment 2, primes and targets were either semantically related (cap-HAT) or were unrelated (car-HAT). Unrelated target words produced larger N400s than did repeated (Exp 1) or semantically related (Exp 2) words across the different prime durations and these N400 priming effects tended to be smaller overall for semantic than repetition priming. Moreover, there was only a modest decline in the size of N400 repetition priming at the shortest prime durations, and there was no relationship between this N400 effect and a measure of prime categorization performance. However, the size of semantic priming at the shortest durations was relatively smaller than at longer durations and was correlated with prime categorization performance. The findings are discussed in the context of the functional significance of the N400 as well as a model that argues for different mechanisms during masked repetition and semantic priming.     

Priming illusory words: an ERP approach

Repetition blindness (RB) was used to investigate whether illusory words emerge at a lexical-perceptual or a semantic-reconstructional level. Illusory words were evoked by the rapid serial visual presentation (RSVP) of two real words and a word fragment. The initial words share the same string of letters ("CREEP"-"SHEEP"), producing a free-floating word fragment ("SH"). This fragment is likely to be linked to a subsequently presented fragment ("IFT") if both combine to a meaningful word ("SHIFT"). The processing level of the illusions was probed by prime words preceding the RSVP sequence which were semantically related or unrelated to the second real word or to the illusion. Behavioural and electrophysiological correlates of the semantic priming effect were recorded in 14 subjects. Real words related to the prime were perceived more frequently, and evoked widespread N400-like effect in the event-related brain potentials (ERPs). An ERP effect of the same polarity was obtained for illusory words, however, its latency was delayed and the topographical distribution was restricted to left posterior electrode positions. These differences suggest that priming might affect real and illusory words at different levels of word processing: access to real words is facilitated at a semantic level, whereas lexical activation apparently accompanies the generation of illusory words.     

Strategy-based semantic priming in Alzheimer's dementia

The ability to engage semantic search strategies was assessed in a group of patients with dementia of the Alzheimer's type (DAT) (n = 11) and a group of age-matched control subjects (n = 13). The subjects performed a semantic priming task in which attentional priming was induced by manipulating the expected relationship between the primes and targets. Older control subjects were able to predict target words on the basis of expectancy, regardless of the semantic relationship between the prime and target. The DAT patients were also able to predict targets on the basis of expectancy, but only when the prime and target were semantically related. These results suggest that the structure of semantic memory remains intact in DAT, and that semantic memory retrieval may be facilitated by providing DAT patients with appropriate search strategies.     

Verbal Association for Simple Common Words in High-Functioning Autism

We investigated conceptual relationships in semantic memory using an indirect priming technique in high-functioning autistic adolescents and their controls who were matched for age, verbal IQ, performance IQ, and nonverbal reasoning ability. The prime was a single word and the target task was completing a word fragment that was semantically related or unrelated to the prime word. The autistic subjects and controls showed similar semantic priming effects, indicating intact conceptual relationships for simple common words in those with autism. Only in the autistic group was a significant correlation found between performance for the related items and two nonverbal cognitive measures, which suggests a possibility that semantic processing in individuals with autism might be qualitatively different from that in controls.     

Electrophysiological evidence for primary semantic memory functional organization deficits in schizophrenia

N400, an event-related brain potential (ERP) waveform elicited by meaningful stimuli, is normally reduced by stimulus repetition (N400 repetition priming), and relatedness between the eliciting stimulus and preceding ones (relatedness priming). Schizophrenia patients' N400 relatedness priming deficits suggest impairment in using meaningful prime stimuli to facilitate processing of related concepts in semantic memory. To examine whether this deficiency arises from difficulty activating the prime concept per se, as indexed by reduced N400 repetition priming; or from impaired functional connections among concepts in semantic memory, as reflected by reduced relatedness priming but normal repetition priming; we recorded ERPs from 16 schizophrenia patients and 16 controls who viewed prime words each followed at 300- or 750-ms stimulus-onset asynchrony (SOA) by an unrelated, related or repeated target word, or a nonword, in a lexical-decision task. In both groups, N400s were largest (most negative) for unrelated, intermediate for related, and smallest for repeated targets. Schizophrenia patients exhibited subnormal N400 relatedness priming at the 300-ms SOA, but normal repetition priming at both SOAs, suggesting that their impairment in using prime words to activate related concepts results from abnormal functional connections among concepts within semantic memory, rather than inability to activate the prime concept itself.     

Top-down influences on lexical selection during spoken word production: A 4T fMRI investigation of refractory effects in picture naming

Spoken word production is assumed to involve stages of processing in which activation spreads through layers of units comprising lexical-conceptual knowledge and their corresponding phonological word forms. Using high-field (4T) functional magnetic resonance imaging (fMRI), we assessed whether the relationship between these stages is strictly serial or involves cascaded-interactive processing, and whether central (decision/control) processing mechanisms are involved in lexical selection. Participants performed the competitor priming paradigm in which distractor words, named from a definition and semantically related to a subsequently presented target picture, slow picture-naming latency compared to that with unrelated words. The paradigm intersperses two trials between the definition and the picture to be named, temporally separating activation in the word perception and production networks. Priming semantic competitors of target picture names significantly increased activation in the left posterior temporal cortex, and to a lesser extent the left middle temporal cortex, consistent with the predictions of cascaded-interactive models of lexical access. In addition, extensive activation was detected in the anterior cingulate and pars orbitalis of the inferior frontal gyrus. The findings indicate that lexical selection during competitor priming is biased by top-down mechanisms to reverse associations between primed distractor words and target pictures to select words that meet the current goal of speech.     

Metaphorical and non-metaphorical links: a behavioral and ERP study in young and elderly adults.

Event-related potentials (ERPs) and reaction times were recorded in a semantic decision task with isolated word pairs. The two words in each pair had either a metaphorical semantic link or a non-metaphorical semantic link. The metaphorical semantic link implied familiar expressions. Two conditions without any semantic link were also used, where the two words had a phonological relationship or not. Normal young adults and elderly people participated in this task. Subjects had to decide whether a target word presented 700 ms after a prime word was or was not semantically linked to the prime. Among word pairs with semantic relation, some pairs presented a metaphorical link, and the others presented a non-metaphorical link. Among word pairs without semantic relation, some pairs had a phonological relationship, and the others had no specific relationship. Familiar metaphorical expressions were used. In the two groups, target words elicited larger N400 components when they were related to the prime word by a metaphorical semantic link than when the semantic link was not metaphorical. No significant differences were found between the metaphorical semantic link condition and the two conditions without semantic link. The reaction times analysis showed that young and elderly people took more time to respond to the metaphorical semantic link than to the non-metaphorical semantic link. These results suggest that understanding metaphorical semantic links requires a lexical controlled process and that metaphors have a special status in the semantic memory.     

N400 and automatic semantic processing abnormalities in patients with schizophrenia

BACKGROUND: One factor hypothesized to underlie thinking disturbance in patients with schizophrenia is abnormal or disinhibited automatic spreading activation of semantic networks, which can be assessed using the N400 event-related potential. N400 is a negative-going component elicited at about 400 milliseconds following semantic stimuli that are not primed by the preceding context. Semantic priming refers to facilitated semantic processing gained through preexposure to semantic context, which can happen automatically or strategically. Using N400, inferences can be drawn regarding the extent to which a given context primes a word. METHODS: During a picture-word matching task, N400s to primed (exact match) and unprimed (remotely related) words were recorded from 18 healthy control subjects and 18 patients with schizophrenia performing a picture-word matching task. A short interval (325 milliseconds) between picture and word onset (stimulus-onset asynchrony) was used to optimize the role of automatic spreading semantic activation and to minimize the role of attention, expectancy, preparation, and working memory. RESULTS: Despite behavioral evidence of normal semantic priming, patients generated an abnormally small N400 (ie, less negative) to unprimed words. The N400 to primed words was neither larger nor smaller in patients than in controls, suggesting normal use of context. CONCLUSIONS: A reduced N400 to unprimed words in patients with schizophrenia suggests that there was inappropriate priming of words by remotely related semantic contexts. This is consistent with an overly broad automatic spread of activation through semantic networks in patients with schizophrenia.     

Semantic and Cross-Case Identity Priming in Patients With Parkinson's Disease

Semantic and cross-case identity priming were investigated in nondemented patients with Parkinson's disease (PD) and controls using the Lexical Decision Task. Three conditions were administered that consisted of the presentation of prime and target word pairs. In the semantic priming condition the word pairs were semantically related (e.g., table-CHAIR), in the cross-case identity priming condition the word pairs consisted of the same word (e.g., noise-NOISE), and in the unrelated condition the word pairs were not related semantically (e.g., guns-DEEP). A fourth condition was also administered that consisted of the presentation of a prime word and a pronounceable nonword target (e.g., starved-FORVE). Participants were asked to indicate whether the target was a real word or a nonword. The prime and target were separated by either a short or long (250 ms or 1000 ms) stimulus onset asynchrony (SOA). Results indicated that PD patients displayed normal semantic priming (i.e., faster responding to the target in the semantic condition as compared to the unrelated condition) at both the short and long SOA. Similarly, PD patients displayed normal cross-case identity priming (i.e., faster responding to the target in the identity condition relative to the unrelated condition) at the long SOA. At the short SOA, however, PD patients displayed hyper identity priming relative to controls (134 ms vs. 50 ms). These results suggest that semantic processes are normal in nondemented PD patients but that the processes involved in accessing lexical information may be overly activated in these patients.     

Priming summation in the cerebral hemispheres: evidence from semantically convergent and semantically divergent primes

The ability to activate and to maintain a large and relatively undifferentiated semantic field has been thought to be an important component of lexical semantic processing by the right hemisphere (RH). An implication of this unique propensity of the RH was examined in the present study that included two divided visual field priming experiments with SOAs of 800 and 2500 ms. The experiments investigated the ability of the RH and the left hemisphere (LH) to summate activation from multiple primes followed by a laterally presented ambiguous target word. The priming words either converged onto the same semantic representation (i.e. all three words related to either the dominant or to the subordinate meaning of the target) or diverged onto distinct semantic representations (i.e. two words related to the dominant and one to the subordinate meaning of the target, or vice versa). Results indicated that for either an 800 or 2500 ms stimulus onset asynchrony (SOA) the LH benefited most from three semantically convergent primes that converged onto the dominant meaning of the ambiguous target word. There was no facilitation when three subordinate primes preceded the target. When the primes diverged onto different meanings, there was significant facilitation for the 800 ms SOA only. In contrast, with an 800 ms SOA, the RH benefited only from semantically divergent primes, that diverged onto alternate meanings of the ambiguous target word. With a 2500 ms SOA, the RH benefited from all combinations of primes. The discussion focuses on the implications for language processing of the differences between the two hemispheres in the scope and temporal pattern of the multiple prime effect.     

Semantic and cross-case identity priming in patients with Parkinson's disease

Semantic and cross-case identity priming were investigated in nondemented patients with Parkinson's disease (PD) and controls using the Lexical Decision Task. Three conditions were administered that consisted of the presentation of prime and target word pairs. In the semantic priming condition the word pairs were semantically related (e.g., table-CHAIR), in the cross-case identity priming condition the word pairs consisted of the same word (e.g., noise-NOISE), and in the unrelated condition the word pairs were not related semantically (e.g., guns-DEEP). A fourth condition was also administered that consisted of the presentation of a prime word and a pronounceable nonword target (e.g., starved-FORVE). Participants were asked to indicate whether the target was a real word or a nonword. The prime and target were separated by either a short or long (250 ms or 1000 ms) stimulus onset asynchrony (SOA). Results indicated that PD patients displayed normal semantic priming (i.e., faster responding to the target in the semantic condition as compared to the unrelated condition) at both the short and long SOA. Similarly, PD patients displayed normal cross-case identity priming (i.e., faster responding to the target in the identity condition relative to the unrelated condition) at the long SOA. At the short SOA, however, PD patients displayed hyper identity priming relative to controls (134 ms vs. 50 ms). These results suggest that semantic processes are normal in nondemented PD patients but that the processes involved in accessing lexical information may be overly activated in these patients.     

Cross-modal semantic priming in schizophrenia.

Work on implicit memory in normal subjects has demonstrated the influence of stimulus modality on the retrieval of semantic information. The present study examined the effects of auditory and visual semantic priming on the recognition of visual words using a lexical decision task. Performance was studied in a group of 20 patients with DSM-IV schizophrenia and 26 normal volunteers of similar age and sex. There were two versions of the task: ipsimodal, in which the word or nonword visual target followed 400 ms after the onset of a visual word prime which may or may not be semantically related to the target; and cross-modal, in which the visual target followed 400 ms after the onset of an auditory word prime. Both groups showed significant priming in both modality conditions, although the schizophrenia patients exhibited significantly greater priming in the cross-modal condition. Priming effects in the ipsimodal condition did not differ substantially between patients and controls. The priming effects in the two conditions correlated with each other in the schizophrenia patients only. The results suggest that priming may occur through amodal semantic representations. In schizophrenia, there appears to be increased cross-modal connectivity (reduced modality modularity and informational encapsulation) between lexical representations that could result in impaired language, particularly speech, processing.     

The N400 is modulated by unconsciously perceived masked words: further evidence for an automatic spreading activation account of N400 priming effects

It is a matter of debate whether the N400 component of the event-related brain potential (ERP) is sensitive to unconscious automatic priming mechanisms or to strategic mechanisms only. Recent studies demonstrated N400 modulation by masked primes at a short SOA supporting an automatic spreading activation account. However, it cannot be ruled out that strategic mechanisms based upon partial prime identification contributed to the observed priming effects. The present study was set up to substantiate masked N400 priming effects as an index of automatic spreading activation. It was assessed whether partial identification of the masked words due to backward priming could have supported strategic priming to occur. In experiment 1, ERPs were recorded while subjects performed lexical decisions on targets preceded by masked and unmasked primes at an SOA of 67 ms. Masked words, which were not consciously perceived, as well as visible words were shown to modulate the N400 to meaningfully related target words. Experiment 2 required subjects to perform decisions on visual, lexical and semantic features of masked words presented with or without semantically related context words. Subjects performed at chance in all tasks. Furthermore, the results exclude the possibility that backward priming has rendered the masked words partially visible. The present study therefore demonstrates that N400 priming effects can be reliably obtained from unconsciously perceived masked words at a very short SOA and strengthens the notion that the N400 is modulated by automatic spreading activation and not exclusively by strategic semantic processes.     

ERP analyses of task effects on semantic processing from words

Semantic (positive) priming refers to the facilitated processing of a probe word when preceded by a related prime word, and is a widely used technique for investigating semantic activation. However, the effect is interrupted or eliminated when attention is directed to low-level features of the prime word, such as its letters, a result which has been used to question the automaticity of semantic processing. We investigated this issue using both behavioural [reaction time (RT)] and electrophysiological measures [event-related potentials (ERPs)]. Subjects performed semantic categorization (living vs. nonliving) and letter search ("A" or "E") tasks on prime words followed by lexical decision on the probe. RT results showed the expected elimination of semantic priming following letter search. However, both prime tasks were affected by the semantic category of the prime, indicating that the meaning was processed. The ERP results supported this conclusion: an early component previously associated with automatic semantic processing [the Recognition Potential (RP)] was sensitive to the category of the prime word irrespective of the prime task. However, a later component (N400) was significantly affected by the task, in both the prime (categorization task) and probe words (semantic priming). The results dissociate rapid, automatic semantic processing from semantic priming. We suggest that a later inhibitory control mechanism suppresses this semantic activation when it is not relevant to the task, and that this produces the loss of semantic priming.     

Neural correlates of semantic priming for ambiguous words: an event-related fMRI study

We investigated the neural correlates of semantic priming by using event-related fMRI to record blood oxygen level dependent (BOLD) responses while participants performed speeded lexical decisions (word/nonword) on visually presented related versus unrelated prime-target pairs. A long stimulus onset asynchrony of 1000 ms was employed, which allowed for increased controlled processing and selective frequency-based ambiguity priming. Conditions included an ambiguous word prime (e.g. bank) and a target related to its dominant (e.g. money) or subordinate meaning (e.g. river). Compared to an unrelated condition, primed dominant targets were associated with increased activity in the LIFG, the right anterior cingulate and superior temporal gyrus, suggesting postlexical semantic integrative mechanisms, while increased right supramarginal activity for the unrelated condition was consistent with expectancy based priming. Subordinate targets were not primed and were associated with reduced activity primarily in occipitotemporal regions associated with word recognition, which may be consistent with frequency-based meaning suppression. These findings provide new insights into the neural substrates of semantic priming and the functional-anatomic correlates of lexical ambiguity suppression mechanisms.     

Automatic activation of the semantic network in schizophrenia: evidence from event-related brain potentials.

BACKGROUND: Language disorder associated with schizophrenia might be due to disturbances in both automatic activation and mechanisms of controlled attention. The contribution of each process to semantic memory dysfunction has not been determined for schizophrenia, and the semantic priming paradigm is well-suited for addressing this question. In the present report, event-related potentials (ERPs) elicited under conditions assumed to reveal automatic activation (short prime-target interval and low proportion of related words) are compared directly with ERPs elicited under conditions associated with controlled processing (long prime-target interval and high proportion of related words). METHODS: Visual ERPs were recorded during a lexical decision task, in which semantic relationship (associated and unassociated words), expectancy (relatedness proportions), and prime-target interval (250- and 850-msec inter-stimulus intervals [ISIs]) were varied. Diagnosis and expectancy were between-subjects factors; semantic relationship and ISI were repeated measures. The N400 priming effect (enhanced negativity to unassociated words) was compared between 34 male normal control subjects tested once and 37 male schizophrenia inpatients evaluated during their participation in a double-blind haloperidol maintenance therapy and placebo replacement protocol. RESULTS: The N400 priming effect for patients was significantly reduced during both pharmacologic phases, compared with controls. During haloperidol treatment, however, patients showed a significant N400 priming effect over the anterior scalp region and additionally under the automatic activation condition. The N400 priming effect was enhanced under the controlled processing condition for control subjects; this effect was not observed for patients. N400 amplitude elicited under the rapid presentation rate (250-msec ISI) differed between medicated patients and controls; groups did not differ for the 850-msec ISI. CONCLUSIONS: Findings suggest that automatic activation and mechanisms of controlled attention are both disrupted during semantic memory access for schizophrenia patients. Pharmacologic agents, such as haloperidol, might enhance automatic activation of the semantic network in this patient population, as indexed by the N400 component of the ERP.     

Testing a model for bilingual semantic priming with interlingual homographs: RT and N400 effects

Using a semantic priming paradigm, this study examines the effects of semantic and lexical-orthographic context on reaction times (RTs) and event-related potentials (ERPs) for interlingual homographs. Dutch-English bilinguals performed an English lexical decision task in which homographs like STEM (meaning "voice" in Dutch) were preceded by primes like ROOT or FOOL that were semantically related or unrelated to the English reading of the target word. Homographs were responded to faster following semantically related primes than following unrelated primes. The responses in both conditions were modulated by the relative frequencies of the two readings of the homographs: responses were faster when their English word frequency was high or when their Dutch word frequency was low. In the ERPs, N400 effects, taken to reflect processes of semantic integration, were found for homographs preceded by related primes. Remarkably, the amplitude of the N400 effect was also modulated by word frequency in both the first (Dutch, L1) and the second (English, L2) language. The observed relationship between lexical and semantic variables supports a model for bilingual semantic priming that extends the language nonselective BIA+ model for bilingual word recognition.     

Lack of inhibition in Parkinson's disease: evidence from a lexical decision task

Persons affected by Parkinson's disease (PD) often show an increased semantic priming effect from target words in lexical decision tasks (hyper-priming) as compared to age-matched controls. In this study, a lexical decision task was used to investigate both semantic priming (Experiment 1) and repetition priming (Experiment 2) from distractor words in PD patients and age-matched controls. With this negative priming procedure, target words in successive trials are never related, and therefore participants always have to switch between unrelated target words. Instead, it is the distractor prime word that is either related or unrelated to the subsequent target, giving the measure of priming. Results showed that PD patients demonstrated a robust effect of positive semantic priming from distractor words. Participants from the control group did not show any semantic priming effect (positive or negative) from distractors. Similarly, PD patients showed positive repetition priming from distractor words, but the control group showed significant repetition negative priming. These results support the view that the hyper-priming effect typically shown by persons with Parkinson's disease is the result of impaired inhibitory processes required to control word activation during reading.