Brain potentials reflect meaning in language

T I N S - March 1985

91

,Research News Brain potentials reflect meaning in language Over the past twenty years much has been learned about the acuvities of the human brain that are trine-locked to specific cognitive processes. These studies have employed computerized averaging techniques to extract the phasic voltage fluctuattons in the EEG (termed event-related brain potennals or ERPs) that accompany focused attentton, aspects of perception, recogn,tlon and dectstor making 1-~ The focus of thts article is a recent series of studtes that suggest the semantic context m which a word ts presented influences the brain's response to the word. These results provide clues about the neural mechamsms that influence the rate and accuracy wtth whzch words are comprehended 4. Several E R P components have been shown to be mvariant across different tasks that b n n g into play sirmlar facets of information processing. Whale such observations strengthen the interpretation of the functional s~gnificance of particular E R P components, there are reasons to predict that the processing

of different types of stlmuh might be mediated by non-identical neural systems. For example, considerable controversy exists as to w h e t h e r linguistic reformation engages neural systems that are distinct from those associated with other cognitwe functions Reports that bear on this issue have come

from Kutas and Hdlyard 5-s who recorded ERPs from subjects as they read sentences that were presented one word at a time E R P s to the final words of sentences that were meaningful (e.g 'he sr~read the warm bread with butter)' were characterized by a positive shift_ By contrast E R P s to final words that were semantically anomalous (e.g. 'he spread the warm bread with socks') were dominated by a large negative potential that peaked at 400 ms (N400). O t h e r incongruities m the sentences did not elicit the N400. Thus neither unpredictable changes in the type or size of p n n t (e g. 'he spread the warm bread with B UTTER'), nor anomalies of grammar (e.g 'he spread the warm bread with buttered'), elicited the N400 (see Fig l a ) Apparently only semantic mcon-

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92 grmtles, 1 e those related to the meaning of the sentence, were sufficient to elicit the N400 These results have been replicated several times m the visual modallty, and recently have b e e n reported for the auditory presentation of sentences as well 9 The identification of the N400 response sheds hght on the role of semantic expectancies in the processlng of language. Virtually all previous investigations of ERPs in relation to expectancy had focused on the P300 component, whose amplitude had been reported to be inversely proportional to the predictablhty of a given stimulus 2. E v e n E R P s from language paxadlgms which required lexlcal or semantic decisions about unpredictable words were dominated by the apparently ubiquitous P300 component. However, a systematic and controlled investigation of the brain events associated with semantic expectancies had not been performed until Kutas and Hdlyard's study Since their initial reports, a series of studies have further clarified the functional relevance of the N400 wave A provocative study by FischleL Bloom, Childers, Roucos and Perry l° reported that both false affirmative sentences (e.g. 'a canary is a fish') and true negative statements (e.g 'a canary is not a fish') elicited an N400-1ike wave These results suggested that, while sufficient, the occurrence of semantic a n o m a h e s were not necessary for the elicltation of the N400 Instead it appeared that the degree of semantic relatedness between two words may be indexed by N400 Confirmation of this view was reported by Kutas and Hlllyard 11 who observed a strong negative correlation between the semantic predictablhty of words that completed meaningful sentences and the N400 assocmted with them (Fig l b ) These results and others since then 12,13 suggest that the N400 may be an online physiological index of the degree to which the representation of a word has been activated or primed by the preceding semantic context_ Further research is underway to relate the N400 to other negative E R P components occurnng in the same time window, and to explore the possibility that one or more of these may reflect aspects of an expected words phonology or orthography TM. Perhaps more difficult to address is the extent to which the N400 indexes all types of semantic activation or w h e t h e r it is uniquely tied to the

processing of m e a m n g in language For example, will the presentation of films portraying sequences of events violating known physical laws eliot N400 components ~ Will the perception of notes or passages of music that are unexpected e h o t N400~ A tantahzlng presentation at the last Neurosciences Meeting reported that incongruous notes in a famihar melody do not ehclt the N400 (Ref 15)_ Further basic research along these hnes is currently underway A t the same time investigators have begun to use the N400 to chart the ontogeny of semantic processes both m normal chddren and in those displaying specific language disorders Additionally, ongoing studies of patients with circumscribed brain lesions wdl contribute important information bearing on the nature of the processes underlying the N400 wave and the m a n n e r in which their disruption can affect language skills Valuable to each of these enterprises will be knowledge about the neural structures that generate the N400. mformatlon that is now accumulating from lntracramal E R P studies of humans ~6 Future research along each of these lines will help to clarify the nature of the neural activity underlying meaning in language

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Selected references 1 Donchm, E , Ratter, W and MeCallum, W C (1978) ]n Event-Related Bram Potennals m Man (Callaway, E , Tueting, P and

Koslow, S , eds), pp 349-q41~ AcademlL Press, New York Hlllyard, S A and Kutas, M 11983) Anm, Rev Psychol 34, 334il Kutas, M and Hlllyard, b A (1984) m Cognmve Neurosclence (Gazzamga, M ed ), pp 387-409, Plenum Press, New York Schvaneveldt, R W and Meyer, D E (1973) m 4ttentlon and Performance IV (Kornblum S , ed ), Academl~ Press_ Nev, York Kutas, M and Hdlyard, ~, A (1980a) Science 207 203-205 Kutas, M and HIIlyard, S A II980b) Brain Lang 1l, 354-373 Kutas, M and Hdlyard, S A (1980c) B~ol Psychol 11 99-116 Kutas, M and Hdlyard, S A (1983) Memory and Cognmon 11,539-550 McCanum, W C , Farmer, S F and Pocock, P V (1984)Electroencephatogr Clm Neurophvstol 59(6), 477--489 Flschler, I , Bloom, P A , Chllders, D A Roucos, S E and Perry, Jr , N W (1983) Psycbophystology 20, 400-4(0 Kutas, M and Hlllyard, S A (1984) Nature (London) 307, 161-163 Bentm, S McCarthy, G and Wood, C C Electroencephalogr Chn Neurophys]ol (m press) Holcomb, P Electroencephalogr Chn Neurophys]ol (Proceedings of Seventh International Congress on F',oked Potentmls) (]n press) Rugg, M D (1984) Neuropsychologla 22, 435-JA4 Besson, M , Macar, F and Pynte, J (1984) Soc Neurosct Abstr 847 McCarthy, G and Wood, C C (1984) Soc.

Neuroscl Abstr 847 HELEN J NEVILLE

Neuropsychology Laboratory, The Salk Insatute, PO Box 85800, San D:ego, CA 92138-9216,

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Co-localtz of 6,# A and neurons In the ceret. c rtex Stewart Hendry and his colleagues have discovered that all or almost all of certain neuropepttde-contaming neurons within the cerebral cortex also contain a synthetw enzyme for y-ammobutyrw actd ( G A B A ) . Accordingly, these neurons are hkely to be GABAerglc in addiuon to being pepttderglc Further, some neuropepttdes are localized m the same termmal-hke puncta as the G A B A synthettc enzyme, suggesting that neuropepttdes may be released at some of the same sites as G A B A Although the funcnon of neuropepndes In lntracorncal information processing :s not known, their linkage with presumed inhibitory mterneurons may provide important clues to the mechamsms of reformation processing and storage m the cerebral cortex Stewart H e n d r y and his collaborators, E. G Jones, J DeFihpe, D Schmechel, C. B r a n d o n , and P. C Emson, have provided dramatic lmmunocytochenucal evidence that certain neuropeptide-containing neurons w~thln the cerebral cortex also produce the classical neurotransmttter, ¥-amlnobutync acid ( G A B A ) Their findings were published in the

(~ 1985 ElsevierS,cwncePubhshers~ V Amsterdam 0378- Sg12/85/S02I1~)

October 1984 Issue of the Procee&ngs o f the National Academy o[ Sciences, USA l, and they mark a sudden advance m understanding the chemical anatomy of the cerebral cortex It was possible, for example, for a leadmg expert to conclude qmte recently 2 that 'there are no indications yet of the presence of G A B A and a pepUde in the same cortical neuron' Now such