Bettina Mohr

Lexical decision after left, right and bilateral presentation of function words, content words and non-words: evidence for interhemispheric interaction.

Function words, content words and pronounceable non-words (pseudowords) were presented tachistoscopically either in the left or the right visual field or with identical copies flashed simultaneously to both visual half-fields. Consistent with earlier studies [10], function words were found to show a right visual field advantage, whereas for content words the right visual field advantage was absent. Compared to either of the unilateral modes of presentation, bilateral presentation of identical word stimuli improved accuracy and latency significantly. The bilateral (Bi) advantage was largest for content words, and was also highly significant for function words in both latency and accuracy. The Bi gain was absent for non-words (significant interaction of Wordness x Visual Field). These results indicate that the lexicons of the left and right hemisphere can collaborate rather than inhibit each other or act independently when processing the same linguistic stimuli. Our findings are consistent with the view that the neuronal counterparts of words are Hebbian cell assemblies consisting of strongly connected excitatory neurons of both hemispheres. Since function words show a right visual field advantage in addition to their Bi gain, their assemblies are likely to have most of their neurons located in the left hemisphere. Neuronal assemblies corresponding to content words may be less strongly lateralized.

High-frequency brain activity: Its possible role in attention, perception and language processing

Coherent high-frequency neuronal activity has been proposed as a physiological indicator of perceptual and higher cognitive processes. Some of these processes can only be investigated in humans and the use of non-invasive recording techniques appears to be a prerequisite for investigating their physiological substrate in the healthy human brain. After addressing methodological issues in the non-invasive recording of high-frequency responses, we summarize studies indicating co-occurrence of neuronal synchrony of single cells exhibiting rhythmic activity at high frequencies, oscillations in the local field potential and dynamics in high frequencies recorded using high-resolution electroencephalography (EEG) and magnetoencephalography (MEG). We then review EEG and MEG studies of attention, perception, and language processing in humans indicating that dynamics in the high-frequency range > 20 Hz reflect specific cognitive processes. Types of high-frequency (HF) activity can be distinguished according to their latency after stimulus onset, stimulus-locking, cortical topography and frequency. There appears to be a systematic relationship between specific cognitive processes and types of HF activity. The findings are related to recent theories about the generation of HF activity and their possible role in binding of stimulus features. Dynamics of HF cortical activity reflecting higher cognitive processes can be accounted for based on the assumption that the elements of cognitive processing, e.g. visual objects and words, are organized in the brain as distributed neuronal assemblies with defined cortical topographies generating well-timed spatio-temporal activity patterns.

High-frequency cortical responses reflect lexical processing: an MEG study

Meaningful words and matched pseudowords, such as moon vs. noom, are of equal perceptual complexity, but invoke different cognitive processes. To investigate high-frequency cortical responses to these stimuli, biomagnetic signals were recorded simultaneously over both hemispheres of right-handed individuals listening to words and pseudowords. Consistent with earlier EEG studies, evoked spectral responses recorded from the left hemisphere revealed depression of spectral power in the low gamma band (around 30 Hz) after pseudowords but not after words. Similar differences between stimulus categories were present in the beta range. These results indicate that distinct patterns of high-frequency cortical responses correspond to the different cognitive processes invoked by words and pseudowords. It is hypothesized that differential high-frequency cortical responses signal the activation or activation failure of distributed Hebbian cell assemblies representing words and other elements of cognitive processing.

Interhemispheric cooperation for face recognition but not for affective facial expressions.

Interhemispheric cooperation can be indicated by enhanced performance when stimuli are presented to both visual fields relative to one visual field alone. This bilateral gain is seen for words but not pseudowords in lexical decision tasks, and has been attributed to the operation of interhemispheric cell assemblies that exist only for meaningful words with acquired cortical representations. Recently, a bilateral gain has been reported for famous but not unfamiliar faces in a face recognition task [Neuropsychologia 40 (2002) 1841]. In Experiment 1 of the present paper, participants performed familiarity decisions for faces that were presented to the left (LVF), the right (RVF), or to both visual fields (BVF). An advantage for BVF relative to both LVF and RVF stimuli was seen in reaction times (RTs) to famous faces, but this bilateral advantage was absent for unfamiliar faces. In Experiment 2, participants classified the expression (happy or neutral) of unfamiliar faces. No bilateral advantage was seen for expressions, although a right hemisphere superiority was seen in terms of higher accuracy for LVF and BVF trials relative to the RVF. Recognition of famous faces (but not of facial expressions) require access to acquired memory representations that may be instantiated via cortical cell assemblies, and it is suggested that interhemispheric cooperation depends on these acquired cortical representations.

The concept of transcortical cell assemblies : a key to the understanding of cortical lateralization and interhemispheric interaction

According to Hebb, elements of higher cognitive processes, such as concepts, words and mental images, are realized in the brain as cortical cell assemblies, i.e. large and strongly connected neuron populations that form functional units. Neurons belonging to such assemblies may be scattered over wide cortical areas, and some cell assemblies may even comprise neurons of both hemispheres (transcortical assemblies). If full activation (ignition) of an assembly leads to fast circulation of neuronal activity in the assembly, this process should be visible in high-frequency cortical responses. Some evidence will be reviewed that cell assembly ignition indeed leads to changes in high-frequency cortical responses which can be recorded in the EEG and MEG. Within the cell assembly-framework, the question of cortical laterality translates into the question of how neurons of transcortical assemblies are balanced between the hemispheres. This approach allows for different degrees of laterality. Recent evidence is summarized that the degree of laterality indeed differs between language units. For example, the cortical representation of certain words appears to be strongly lateralized to the left hemisphere while those of others are less lateralized. If neurons of both hemispheres are part of one assembly bihemispheric processing should lead to a processing advantage compared to processing in the dominant hemisphere alone. The latter appears to be the case for lexical processing, as revealed by recent behavioral studies. In conclusion, the cell assembly-framework suggests a more fine-grained description of the issue of cortical laterality; it is not appropriate to ask whether modules supporting higher cortical functions are located either in the left or right hemisphere. Rather, it appears fruitful to ask how the neurons of transcortical cell assemblies are balanced between the hemispheres.

Semantic or lexico-syntactic factors: what determines word-class specific activity in the human brain?

Words from different classes have been found to activate different brain areas. However, it is unclear whether grammatical word properties, for example their being part of different lexical categories (e.g. nouns vs. verbs) or semantic features of the words (e.g. that they refer to visually perceived entities or to actions) are relevant for eliciting differential brain responses. We tested this by comparing brain potentials elicited by nouns and verbs that varied in their action and visual associations. Naturally spoken word stimuli were from three categories: (1) nouns with strong visual associations; (2) action verbs with strong associations of actions, and (3) nouns with strong action associations. Word-category specific differences became apparent around 500 ms after stimulus onset, approximately 150-200 ms after the average recognition point of the words involved. Brain responses to visual nouns and action verbs differed at central and occipital recording sites. A very similar topographical difference emerged from the comparison of visual vs. action nouns, whereas no significant difference was found between action-related nouns and verbs. These results indicate that grammatical differences alone, e.g. between two lexical classes such as action verbs and action-related nouns, are not sufficient for eliciting differential brain responses. In contrast, semantic differences between items from the same lexical category can be sufficient for changing the topography of cortical processes induced by word stimuli. This is support for associative theories of word processing.

Interhemispheric cooperation during lexical processing is mediated by the corpus callosum: Evidence from the split-brain

If two copies of a meaningful word are tachistoscopically presented simultaneously in both visual half-fields of normal subjects the word will be processed more rapidly and more accurately compared to unilateral presentation (bilateral gain). The word-specific bilateral gain may be due to excitatory transcallosal connections within interhemispheric cell assemblies corresponding to words. In this case, the bilateral gain should be absent in split-brain patients. L.B., a split-brain patient, performed a lexical decision task with words and non-words presented in the left visual field, the right visual field, or in both visual fields simultaneously. As predicted, bilateral presentations did not improve performance compared to unilateral presentation in the right visual field. This result suggests that transcallosal connections play a significant role in lexical processing.

Interhemispheric cooperation for familiar but not unfamiliar face processing

Evidence for interhemispheric cooperation during language processing has been demonstrated for words, but not for meaningless pseudowords. Specifically, responses were found to be faster and more accurate when identical copies of a word were presented bilaterally to both hemispheres, relative to unilateral single presentations. This bilateral advantage for words seems to be a robust effect in normals. The present study addressed the question of whether the bilateral advantage is restricted to lexical material or whether it is a more global phenomenon occurring for meaningful material in general. Thirty healthy participants performed a familiarity decision in which one copy of familiar and unfamiliar faces was presented tachistoscopically to the right visual hemifield (RVF), the left visual hemifield (LVF) or simultaneously to both visual hemifields (bilateral condition, BVF). We obtained a highly significant familiarity by visual field interaction(P < 0.006) showing that only for familiar faces, a bilateral advantage was obtained. Unfamiliar face processing did not yield a bilateral advantage. We conclude that interhemispheric cooperation only occurs for meaningful material for which cortical representations can be assumed.

Interhemispheric cooperation during word processing: evidence for callosal transfer dysfunction in schizophrenic patients

Functional lateralization and interhemispheric interaction during word processing were investigated in schizophrenic patients (n=12) and matched healthy controls (n=18). Words and phonologically regular pseudowords were presented tachistoscopically either in the left or right visual field (unilateral conditions), or simultaneously in both visual hemifields (bilateral condition). Consistent with earlier findings, healthy controls showed a right visual field advantage (RVFA), indicating left-hemispheric dominance for language. The patients showed a RVFA similar to that of controls, consistent with normal left-hemispheric language dominance. Importantly, controls performed much better on words presented in the bilateral condition, when two copies of the same word appeared twice, compared to stimulation in only one of the visual hemifields. This bilateral advantage, which has been interpreted as evidence for cooperation between the hemispheres, was absent in schizophrenics. These data show that schizophrenic patients can exhibit similar lateralization patterns as healthy controls. Their specific functional deficit may be a lack of cooperation between the hemispheres.

Distributed cell assemblies for general lexical and category-specific semantic processing as revealed by fMRI cluster analysis.

Here, we ask whether frontotemporal cortex is functionally dissociated into distributed lexical and category‐specific semantic networks. To this end, fMRI activation patterns elicited during the processing of words from different semantic categories were categorized using k‐means cluster algorithms. Results showed a distributed pattern of inferiorfrontal, superiortemporal, and fusiform activation shared by different word categories. This shared activation contrasted with patterns of category‐specific semantic activation in widely distributed neural systems. Clustering revealed congruent functional specificity of focal area activations in frontal and temporal cortex; thus suggesting a correspondence between functional partitionings of frontocentral mirror neuron systems and those of inferiortemporal lexical and semantic circuits. Action words related to the face, arms, and legs specifically activated the motor system in a somatotopic manner, whereas form‐related words activated prefrontal areas. Similar functional specificity was evident in temporal cortex, where a different semantic topography emerged for form‐ and action‐related words. Results were replicated in a separate data set, therefore recommending fMRI cluster analysis as a reliable method for scrutinizing the brain basis of lexical, semantic, and conceptual systems in humans. As focal modules do not explain the distributed character of functionally specific clusters and their distinct topographies are at variance with general distributed processing accounts, the functionally‐homogenous distributed clusters specific to semantic types are best explained by specifically‐distributed cortical circuits which, similar to Hebbian cell assemblies, represent functional units with specific roles in cognitive processing, especially in lexical and semantic access and memory. Hum Brain Mapp, 2009.