You are in: Home » Research at the Unit » EEG/MEG Research on Language
Neurophysiological activity reflecting word-category-specific processes
Neurophysiological activity reflecting word-category-specific processes
A prediction of a neurobiological model of word processing in the brain is that words referring to actions are cortically realized by distributed neuronal ensembles that include neurons related to the execution of the relevant motor programs (PulvermŸller, 2001). A word such as "(to) kick" should be realized not only by neurons housed in the perisylvian language areas related to the articulation and acoustic perception of the word, but its neuronal representation should incorporate additional neurons involved in the coordination of the movements involved in actual kicking. This is a necessary postulate for any neurobiological language model that takes seriously the principle of association learning. If neurons that frequently fire together also wire together by strengthening their synaptic connections, the disjoint neuronal populations related to the word form and the referent actions should link into a strongly connected neuronal group when word and action frequently occur at the same time or in close temporal succession. The argument made for the leg-related word "(to) kick" can be extended to other action words, for examples hand/arm-related words, such as "(to) pick", and face-related words, such as "(to) lick". These considerations suggest that action words of different types are cortically realized as cell assemblies with different cortical distributions. It is well-known that the motor and premotor cortices in the frontal lobe are organized topographically with the leg representation located dorsal to the arm representation, which, in turn, is dorsal to the representation of the face (He, Dum, & Strick, 1993; Penfield & Rasmussen, 1950). Therefore, the word-related neuron ensembles should be differentially distributed over fronto-central areas (PulvermŸller, 2001). Allthough all word networks would include neurons in the perisylvian language areas, the leg-related words would accordingly be realized as networks that include additional neurons in dorso-medial primary motor and pre-motor areas, hand/arm-word representations would include more inferior action-related neurons, and face word representations neurons in inferior fronto-central areas. The referent actions of action words would be woven into the word-related neuronal ensemble (PulvermŸller, 2001).
According to classical neurological models of language, words are processed in two restricted language centers in the dominant hemisphere, usually the left in right-handers. Modern brain imaging studies, however, showed that various other areas can also become active during word processing. Retrieving word meaning, for example, certainly involves additional areas, although there is, at present, much discussion about their exact definition (for an overview, see PulvermŸller, 1999). Category-specific patterns of brain activity have been found in numerous neurophysiological and metabolic imaging studies (for an overview, see PulvermŸller, 2001), although not all studies have observed category differences over varying tasks and stimulus sets (Devlin et al., 2002). In our recent neurophysiological studies, we found consistently that well-matched words from different categories (function vs. content words, nouns vs. verbs, visually-associated vs. action associated nouns) led to distinct patterns of cortical activation, and that the neurophysiological differences are best explained by semantic differences between word types (PulvermŸller, Assadollahi, & Elbert, 2001; PulvermŸller, HŠrle, & Hummel, 2000; PulvermŸller, Lutzenberger, & Preissl, 1999; PulvermŸller, Mohr, & Schleichert, 1999).
Behavioral and EEG studies of action semantics
In behavioral and neurophysiological experiments, we tested the predictions, outlined above, arising from a Hebbian model of word processing. Because the predictions relate associations at the cognitive level (for example, whether a word refers to, and reminds one of, actions performed with the leg) to neurophysiological events in the brain, it was necessary to establish the cognitive properties of the stimulus words used in the neuroimaging studies. This is only possible in behavioural experiments. We asked subjects to judge words in English and in German (in separate studies) with regard to their action associations. The ratings allowed us to pick groups of words primarily associated with one of the three body parts under examination, face, arm/hand, and leg/foot. The words were matched for physical and psycholinguistic variables, such as length, familiarity, word frequency, and imageability. This led to well-matched stimulus groups, which significantly differed in their action associations (face, arm, and leg words). These were used in subsequent imaging studies.
In neurophysiological studies, the EEG was recorded while subjects made lexical decision responses to words shown briefly at fixation. Response times were slightly slower for leg words as compared with the other two word categories. Importantly, event-related brain potentials reliably differed between subcategories of action words. As Current Source Density Analyses (CSDA) indicated, leg words elicited stronger activity (as indicated by stronger outward flow of electrons) at the top of the head, near the cortical representation of the leg, as compared with face-related words. Face words, in contrast, produced stronger activity signs at left-anterior sites, close to the cortical representation of the face. Although these data do not allow for an exact localization of the cortical sources of the word-category-related differences in brain activation, the results argue that subcategories of action verbs related to actions performed with different body parts are neurophysiologically distinct (PulvermŸller, Hummel, & HŠrle, 2001).
In the earlier experiment, subjects had to respond overtly to the stimulus words. Since differences in response times were found between action word subcategories, differential motor preparation is a possible confound. Therefore, we are investigating putative neurophysiological differences between leg- and face-related words with a different set of stimuli and in a silent reading task in which no motor response is required. To obtain a better estimate of the cortical locus of the word category differences in brain activation, Olaf Hauk calculated Minimum Norm Estimates (MNE). The MNE is a method for solving the so-called Inverse Problem (von Helmholtz, 1853) which selects the unique solution that explains the scalp topography by the least amount of overall current (HŠmŠlŠinen, Hari, Ilmoniemi, Knuutila, & Lounasmaa, 1993). Our preliminary MNE of the cortical generators indicate that a central dorsal area including the primary motor and premotor areas representing the leg were more strongly activated for leg words as compared with other word groups, and that an inferior frontal area was most active for face words (Hauk & PulvermŸller, 2002). We take these results as evidence that areas in the frontal lobes are differentially activated by words referring to actions performed with different body parts. The plan is to continue this line of research in the future proposal, project SL4.1.
When is word meaning reflected in the brain response?
The great strength of neurophysiological data is the exact localization of the effects in time. Time-wise, our results were surprising, because several experiments unequivocally demonstrate that word category differences were present early in the brain response. The earliest meaning-related differences (to frequently repeated word stimuli presented in a memory task) were already observed 100 ms after the onset of visual word stimuli (PulvermŸller, Assadollahi et al., 2001). In this MEG study carried out with Ramin Assadollahi and Thomas Elbert at the University of Konstanz, we found a high correlation between the magnitude of an early magnetic brain wave (latency ~100 ms) elicited by matched written words and our subjects' ratings of the strength with which individual words reminded them of referent perceptions and actions. This suggests that meaning-related aspects of a word are reflected in the brain response already 100 ms after the information in the input allows for word identification.
While some studies suggested a very early onset of the neurophysiological differences between word categories, the latest word category differences we saw in our series of experiments were present at ~200 ms after visual word onset (PulvermŸller, Hummel et al., 2001). Clearly, future research is required to exactly define the point in time where specific brain responses possibly related to the meaning of a word arise. Some of the variance may be accounted for by physical word properties, such as the length of written words and their sound structure and recognition point (Marslen-Wilson & Tyler, 1980) of spoken words (see also L2.3 and L4.3).
The fact that the brain distinguishes early between word categories, at ~100-200 ms, after delivery of the information necessary for word identification, may be important for theories of language in the brain. It is consistent with proposals based on psycholinguistic reaction time experiments according to which access to the lexical and semantic information occur early in word processing (Marslen-Wilson & Tyler, 1980). Our findings speak against the view that the meaning of words is accessed only at around 400 ms after the critical information is presented (e.g, Friederici, 2002). Instead, they suggest that meaning access is an early neurophysiological process occurring near-simultaneously with the access to lexical and phonological information. To further clarify these issues, we plan to look more closely at the specific brain activity patterns elicited by different kinds of action words. With the use of advanced neurophysiological source localization techniques (see MR12, MR15), it should become possible to separate the perisylvian phonologically- and lexically-related activity patterns from those in dorsal motor and premotor areas so that conclusions about the spatio-temporal pattern of activation of phonological and semantic activity will become possible.
Methodological issues in the investigation of word-evoked brain activity
To draw conclusions about the neurophysiological reflections of word semantics (see L4.1 and SL4.1), it is important to study other properties of words that become manifest in the brain response and which therefore can confound the results of EEG/MEG activity related to word processing. Further, as noted, there is the open question of why some of the word category differences surfaced quite early, whereas in other research, they appeared with longer delays (for an overview, see Assadollahi & PulvermŸller, 2001). In a series of studies, we investigated the influence of the factors of word length and word frequency on the EEG and MEG response. In contrast to earlier studies, we kept the variance in length and frequency of our stimuli minimal. This is important methodologically, because variability in these stimulus properties adds noise to the early ERP responses (P70, N100), which are known to be short-lasting and focal, so that potential early physiological category differences may be masked (for discussion, see PulvermŸller, 1999).
A main finding of our studies (Assadollahi & PulvermŸller, 2001; Hauk & PulvermŸller, submitted) is that word length effects became manifest already around 100 ms after word presentation onset, whereas word frequency modulates the neurophysiological brain response slightly later, at ~150 ms. This makes it clear that early neurophysiological differences between words can be related to word length or frequency. However, the differences between long and short and between common and unusual words were always widespread, making it unlikely that topographically specific differences between subcategories of action words (L4.1.1) are a by-product of differences in length or frequency. Nevertheless, due to the modulation of neurophysiological activity related to word length and frequency, it appears advisable in neurophysiological studies of word-category processes to exactly match the word material for these variables. In contrast to earlier studies (King & Kutas, 1998), we could not replicate a correlation of word frequency with the latency of components of the word-evoked neurophysiological response.
In the context of the debate about latencies of word category effects, we note that long words usually elicited relatively large early neurophysiological responses, whereas short words elicited relatively large responses at a later point in time. This indicates that the mixing of long and short words can account for why some investigations found early word-category differences (100-200 ms; Sereno, Rayner & Posner, 1997) and others only found them later (~400 ms; Polich & Donchin, 1987). A similar confound may occur for the variable word frequency. Some of the latency variance found in earlier studies on category-specific word-evoked activity may be accounted for in terms of physical and psycholingusitic stimulus properties.