Studying semantics in the brain: the rapid stream stimulation paradigm

Lexical Processing as Revealed by Lateralized Event-Related Brain Potentials

Abstract. Neurocognitive models of written-word processing from low-level perceptual up to semantic analysis include the notion of a strongly left-lateralized posterior-to-anterior stream of activation. Two left-lateralized components in the event-related brain potential (ERP), N170 and temporo-parietal PSA (posterior semantic asymmetry; peak at 300 ms), have been suggested to reflect sublexical analysis and semantic processing, respectively. However, for intermediate processing steps, such as lexical access, no posterior left-lateralized ERP signature has yet been observed under single-word reading conditions. In combination with a recognition task, lexicality and depth of processing were varied. Left-minus-right difference ERPs optimally suited to accentuate left-lateralized language processes revealed an occipito-temporal processing negativity (210–270 ms) for all stimuli except alphanumerical strings. This asymmetry showed greater sensitivity to the combined effects of attention and lexicality than other ERPs in this time range (i.e., N170, P1, and P2). It is therefore introduced as “lexical asymmetry.”

Left cortical specialization for visual letter strings predicts rudimentary knowledge of letter-sound association in preschoolers

Reading, one of the most important cultural inventions of human society, critically depends on posterior brain areas of the left hemisphere in proficient adult readers. In children, this left hemispheric cortical specialization for letter strings is typically detected only after approximately 1 y of formal schooling and reading acquisition. Here, we recorded scalp electrophysiological (EEG) brain responses in 5-y-old (n = 40) prereaders presented with letter strings appearing every five items in rapid streams of pseudofonts (6 items per second). Within 2 min of recording only, letter strings evoked a robust specific response over the left occipito-temporal cortex at the predefined frequency of 1.2 Hz (i.e., 6 Hz/5). Interindividual differences in the amplitude of this electrophysiological response are significantly related to letter knowledge, a preschool predictor of later reading ability. These results point to the high potential of this rapidly collected behavior-free measure to assess reading ability in developmental populations. These findings were replicated in a second experiment (n = 26 preschool children), where familiar symbols and line drawings of objects evoked right-lateralized and bilaterally specific responses, respectively, showing the specificity of the early left hemispheric dominance for letter strings. Collectively, these findings indicate that limited knowledge of print in young children, before formal education, is sufficient to develop specialized left lateralized neuronal circuits, thereby pointing to an early onset and rapid impact of left hemispheric reentrant sound mapping on posterior cortical development.

Detecting self-produced speech errors before and after articulation: an ERP investigation

It has been argued that speech production errors are monitored by the same neural system involved in monitoring other types of action errors. Behavioral evidence has shown that speech errors can be detected and corrected prior to articulation, yet the neural basis for such pre-articulatory speech error monitoring is poorly understood. The current study investigated speech error monitoring using a phoneme-substitution task known to elicit speech errors. Stimulus-locked event-related potential (ERP) analyses comparing correct and incorrect utterances were used to assess pre-articulatory error monitoring and response-locked ERP analyses were used to assess post-articulatory monitoring. Our novel finding in the stimulus-locked analysis revealed that words that ultimately led to a speech error were associated with a larger P2 component at midline sites (FCz, Cz, and CPz). This early positivity may reflect the detection of an error in speech formulation, or a predictive mechanism to signal the potential for an upcoming speech error. The data also revealed that general conflict monitoring mechanisms are involved during this task as both correct and incorrect responses elicited an anterior N2 component typically associated with conflict monitoring. The response-locked analyses corroborated previous observations that self-produced speech errors led to a fronto-central error-related negativity (ERN). These results demonstrate that speech errors can be detected prior to articulation, and that speech error monitoring relies on a central error monitoring mechanism.

Effects of long-time reading experience on reaction time and the recognition potential

The proposition that long-time experience in reading a language gradually builds up rapidly acting neural processes that facilitate the processing of words in that language and speed them into conscious awareness was examined. Behavioral reaction time (RT) and electrophysiological responsiveness to visually displayed words and non-language images were measured in persons who differed in how much experience they had in reading English. The electrophysiological response was the recognition potential (RP). Behavioral RT and the latency of the RP to English words were both expected to depend upon how much English reading experience a person had. The short latency of the RP was expected to free it from the influence of non-perceptual factors that affect RT, such as speed/accuracy tradeoff. This expectation yielded the prediction that the behavioral and electrophysiological results would differ in a specific way. Long-time readers of English were expected to show shorter RP latency to English words than less experienced (China-educated) readers of English but no RP latency difference for non-language images, with which neither group had greater experience. In contrast, due to speed accuracy tradeoff, the China-educated subjects were expected to show longer RT for both the words and the non-language images. The prediction was confirmed. The amount of language experience that a person had showed a stronger relationship to RP latency than it did to RT. This helped to validate the use of the RP as a tool for investigating perception and demonstrated definite advantages that it has for studying acquired perceptual processes in humans.

Looking at emotional words is not the same as reading emotional words: Behavioral and neural correlates

Recent research suggests that the allocation of attentional resources to emotional content during word processing might be sensitive to task requirements. This question was investigated in two tasks with similar instructions. The stimuli were positive, negative, and neutral nouns. Participants had to identify meaningful words embedded in a stream of non-recognizable stimuli (task 1) or pseudowords (task 2). Task 1 could be successfully performed on the basis of the perceptual features whereas a lexico-semantic analysis was required in task 2. Effects were found only in task 2. Positive nouns were identified faster, with fewer errors and elicited larger amplitude in an early negativity. Also, the amplitude of a late positivity was larger for both positive and negative nouns than for neutral nouns. It is concluded that some degree of linguistic processing is needed to direct attention to the affective content during word processing.

Neural activations correlated with reading speed during reading novels

Functional magnetic resonance imaging was used to measure neural activations in subjects instructed to silently read novels at ordinary and rapid speeds. Among the 19 subjects, 8 were experts in a rapid reading technique. Subjects pressed a button to turn pages during reading, and the interval between turning pages was recorded to evaluate the reading speed. For each subject, we evaluated activations in 14 areas and at 2 instructed reading speeds. Neural activations decreased with increasing reading speed in the left middle and posterior superior temporal area, left inferior frontal area, left precentral area, and the anterior temporal areas of both hemispheres, which have been reported to be active for linguistic processes, while neural activation increased with increasing reading speed in the right intraparietal sulcus, which is considered to reflect visuo-spatial processes. Despite the considerable reading speed differences, correlation analysis showed no significant difference in activation dependence on reading speed with respect to the subject groups and instructed reading speeds. The activation reduction with speed increase in language-related areas was opposite to the previous reports for low reading speeds. The present results suggest that subjects reduced linguistic processes with reading speed increase from ordinary to rapid speed.

The recognition potential reflects an intermediate level of visual representation

A well-known event-related potential (ERP) component-the recognition potential (RP) is a response to recognizable stimuli, peaking at around 200-250 ms. The aim of the present study is to put to the test the contrasting perceptual and semantic accounts of the RP, with manipulation of word meaning and presentation format of stimuli. The hypothesis was that RP latencies would be observed to increase with the difficulty of identification in presentation formats in accord with both accounts and the present study would replicate the semantic effect independently of the perceptual manipulations. The stimuli were Chinese characters and presented to native speakers. The data yielded a negative component (RP) at around 250 ms for all formats of real characters and pseudocharacters at parietooccipital electrodes. The RP for real characters was larger in normal formats than in mirror formats and the RP for pseudocharacters was greater in normal-upright format than other formats. However, a significant RP latency difference was observed between real characters and pseudocharacters. It seems that our data could not be taken as evidence for semantic processing account. We suggest that the RP is a reflection of intermediate visual representation which integrates information from various pathways and then achieve word form analysis. When word form analysis becomes difficult, the categorical feature of word form contributes to word recognition.

Wiener filter deconvolution of overlapping evoked potentials

Evoked potentials (EPs) typically contain components that last up to several seconds. In order to save time and study adaptation effects, stimuli are often presented at high stimulation rates. Unfortunately, such protocols often suffer from a response overlap problem. Techniques based on Continuous Loop Averaging Deconvolution (CLAD) have been developed to formulate circular convolution to restore the source EP. These techniques, however, are sensitive to the additive noise and do not perform well when stimulus sequences with poor noise attenuation properties are chosen. This study offers a solution to this problem by applying Wiener theory to obtain an optimal filter that adapts the ratio of noise-to-signal by estimating the power spectra of both noise and signal using non-parametric or parametric methods. Experiments are conducted on simulation data and Auditory Evoked potentials (AEPs) acquired from human subjects to demonstrate the validity of the proposed theory. The results show that the Wiener deconvolution method improves the quality of the responses acquired with sequences with poor noise attenuation in the presence of high noise levels compared to the straight CLAD method. The results are comparable to responses obtained by conventional and Maximum Length Sequence (MLS) methods.

Language outside the focus of attention: The mismatch negativity as a tool for studying higher cognitive processes

Which aspects of language and cognitive processing take place irrespective of whether subjects focus their attention on incoming stimuli and are, in this sense, automatic? The Mismatch Negativity (MMN), a neurophysiological brain response recorded in the EEG and MEG, is elicited by attended and unattended stimuli alike. Recent studies investigating the cognitive processes underlying spoken language processing found that even under attentional withdrawal, MMN size and topography reflect the activation of memory traces for language elements in the human brain. Familiar sounds of one's native language elicit a larger MMN than unfamiliar sounds, and at the level of meaningful language units, words elicit a larger MMN than meaningless pseudowords. This suggests that the MMN reflects the activation of memory networks for language sounds and spoken words. Unattended word stimuli elicit an activation sequence starting in superior-temporal cortex and rapidly progressing to left-inferior-frontal lobe. The spatio-temporal patterns of cortical activation depend on lexical and semantic properties of word stems and affixes, thus indicating that the MMN can give clues about lexico-semantic information processing stored in long term memory. At the syntactic level, MMN size was found to reflect whether a word string conforms to abstract grammatical rules. This growing body of results suggests that lexical, semantic and syntactic information can be processed by the central nervous system outside the focus of attention in a largely automatic manner. Analysis of spatio-temporal patterns of generator activations underlying the MMN to speech may be an important tool for investigating the brain dynamics of spoken language processing and the activated distributed cortical circuits acting at long-term memory traces.

Syntactic expectancy: an event-related potentials study

Although extensive work has been conducted in order to study expectancies about semantic information, little effort has been dedicated to the study of the influence of expectancies in the processing of forthcoming syntactic information. The present study tries to examine the issue by presenting participants with grammatically correct sentences of two types. In the first type the critical word of the sentence belonged to the most expected word category type on the basis of the previous context (an article following a verb). In the second sentence type, the critical word was an unexpected but correct word category (an article following an adjective) when a verb is highly expected. Event-related potentials (ERPs) were measured to critical words in both sentence types. Brain waves evoked by the correct but syntactically unexpected word revealed the presence of a negativity with a central distribution around 300-500 ms after stimuli onset, an N400, that was absent in the case of syntactically expected words. No differences were present in previous time windows. These results support models that differentiate between the processing of expected and unexpected syntactic structures.

ERP components reflecting stimulus identification: contrasting the recognition potential and the early repetition effect (N250r)

The recognition potential component (RP) in the event-related brain potential (ERP) appears during rapid stream stimulation and has been related to the activation of word form or word meaning. The early repetition effect (ERE/N250r) is observed in repetition priming designs and has been linked to the access to stored representations of the structure of familiar faces and names. Because of the apparent similarities in latency, topography and theoretical interpretation we compared the RP and ERE/N250r within the same rapid stream stimulation design and for the same type of stimulus material: names and faces of famous persons and names and pictures of common objects. Contrasting with RP, the ERE/N250r occurred later and differed in both scalp topography and amplitude patterns across stimulus conditions. Therefore, the ERE/N250r seems to reflect a separate and content-specific stage of information processing, following the RP, which appears to reflect domain-general processes of structural analyses.

The recognition potential during sentence presentation: stimulus probability, background stimuli, and SOA

Recognition potential (RP) is an electrical brain response that has proved its usefulness for studying semantic processing of isolated words, and appears when subjects view meaningful stimuli embedded in a stream of background images at a high rate of presentation: the rapid stream stimulation paradigm (RSS). The present technical study is aimed at testing the validity of this procedure in the study of words within sentences. For this purpose, we varied word and background probability of appearance, the number of background stimuli preceding each word, and stimulus onset asynchrony. Probability did not have significant effects on RP, but it was found that a minimum number of two background stimuli preceding each word and a high rate (250 ms) of presentation are preferable for enhancing RP amplitude. The RSS paradigm would therefore improve the visibility-and, hence, refine the analysis-of a component that can nevertheless be obtained with more standard paradigms, such as rapid serial visual presentation, devoid of interspersed background stimuli.