cortical involvement in the semantic processing

8/6/2019 Cortical Involvement in the Semantic Processing

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BRAIN AND LANGUAGE 7, 86- 100 (1979)

Cortical Involvement in the Semantic Processing ofCoarticulated Speech Cues

DENNIS L. MOLFESESouthern Illinois University

Auditory evoked responses (AER) were recorded from scalp locations over theleft and right temporal regions in response to CVC words and nonsense syllables.Various components of the AER were found to vary systematically with changesin stimulus meaning. One such component reflected subcortical involvement insemantic processing. Other components reflected changes in voicing and place ofarticulation as well as hemisphere differences.

Averaged evoked response (AER) techniques, which measure the elec-trical activity of the brain elicited by a specific stimulu event, have beenused increasingly in the last decade to study cognitive functions (Regan,1972; Callaway, 1975; Thatcher & John, 1977). Although certain compo-nents of the AER such as P3,,,, Callaway, 1975) have previously beenknown to reflect cognitive activities, only recently have researchers suc-cessfully identified specific components of the AER which are sensitive tolinguistic factors such as voice onset time (Molfese, 1978; Molfese &Hess, 1978), formant structure and consonant transitions (Molfese,Nunez, Seibert, & Ramanaiah, 1976), inflection and phoneme categories{Wood, Goff, & Day, 1971), and semantic properties (Molfese,Papanicoaou, Hess, & Molfese, 1979; Matsumiya, Tagiliasco, Lambroso,& Goodglass, 1972; Begleiter & Platz, 1969; Brown, Marsh, & Smith,1973; 1976; Chapman, McCrary, Bragdon, & Chapman, 1977).However, several questions concerning language perception have yetto be addressed: (1) how language listeners are able to process 40 to 50speech sounds per set while they are only able to detect 18 to 20 differentnonlanguage sounds in a comparable period of time (Foulke & Sticht,1969) and (2) at what point in the perception of a word does the individualrecognize it as meaningful and begin to process it semantically. AlthoughThe Author acknowledges the assistance of A. Papanicolaou and T. Hess during the

recording phase of the present study. Requests for reprints should be addressed to Dennis L.Molfese, Department of Psychology, Southern Illinois University, Carbondale, IL 62901.86

0093-934X/79/010086-15$02.00/0Copyright @ 1979 by Academic Press. Inc.All rights of reproduction in any form reserved.


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AERS TO COARTICULATED SPEECH CUES 87no studies employing AER procedures have yet addressed these issues,an extensive body of behavioral research suggests one possible solutionthat involves a process called coarticulation.Coarticulation refers to the notion that the shape and adjustments of thevocal tract during the production of earlier speech sounds in a word willbe altered somewhat by the place and manner of articulation for latersounds. MacNeilage and DeClerk (1969) made cineflurograms and elec-tromyograms of an individual producing a series of 36 CVC syllables.They found that the articulation of initial consonant sounds changed as afunction of the identity of successive sounds. Daniloff and Mall (1968)using similar procedures found that the lip rounding movements whichcharacterized a final vowel sound such as /u/ at the end of the utterancebegan during the closure of the initial consonant. Ali, Gallagher, Gold-stein and Daniloff (1971) have noted what may be a perceptual counter-part of this phenomenon. These investigators constructed a series of CVCand CVVC syllables in which the final sound was either a nasal (/m/, In/)or nonnasal consonant. After the final vowel-consonant and consonanttransitions were removed the resulting CV and CVV syllables were thenpresented to a group of individuals who were to indicate whether themissing final consonant was nasal or nonnasal. All subjects were able todiscriminate between the nasal and nonnasal stimuli well above chancelevels. Ali et al. (1971) argued that such findings corresponded to theproductive coarticulation effects noted above. If there is a perceptualcounterpart to productive coarticulation as Ali et al. have argued, percep-tual cues as to the identity of final consonant sounds may even be con-tained in the initial consonant of a CVC syllable. Furthermore, if suchcues are present they may enable the language listener to perceive andprocess the entire utterance before it has been fully articulated therebygreatly accelerating the speech perception process.

The present research represents an initial attempt to (1) isolate AERcomponents which might reflect the presence of such coarticulated cuesand (2) to identify the point in the perceptual process at which thelanguage listener can use coarticulated cues to differentiate meaningfulfrom nonmeaningful speech stimuli. The stimuli were divided into twogroups, words and nonwords, such that only the final consonant soundwould determine if an item was a word or not. If an AER componentsensitive to this distinction would occur prior to the production of the finalconsonant, this would be evidence for the presence of some coarticulatedperceptual cues. Furthermore, the presence of such a component wouldalso indicate at what point in the signal the listener semantically processedthe stimulus. Since the LH is generally believed to be more involved inlanguage processing than the RH (Shankweiler & Studdert-Kennedy,1967), it was anticipated that any cortical AERs which might reflect someprocessing of coarticulated information would occur primarily in the LH.


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88 DENNIS L. MOLFESETABLE IINITIAL AND FINALARTICLJLATORYFEATURESFORTHEFOLJRWORDSTIMULI (MEANING+) AND THE FOUR NONSENSESYLLABLES(MEANING -)l-st CONSONANTMEANING POSITION VOICE

KEOS +PmK + + -GaP + +BOOK + + +KwK -PaS - + -GmK - +SaeP - + +

2nd CONSONANTPOSITION

+VOICE

+-

+

-Consonants produced in the front o f the vocal tract are identified as + position whilethose produced in the back are - position. Voiced consonants are characterized as +voice while voice less consonants are - voice. Stimuli are listed in IPA notation.

To test this assumption, the AERs were recorded from scalp regionswhich corresponded approximately to the superior temporal areas of bothhemispheres. Diffential AER activity across the two hemispheres wouldreflect the involvement of these two regions in processing the semanticinformation.METHOD

SubjectsTen adults, four males and six females, enrolled in the Introductory Psychology course at

Southern Illinois University at Carbondale volunteered to participate in this experiment.The mean age of these individuals was 21.4 years (range : 18.8 years to 27.5 years). TheEdinburgh Inventory for Handedness (Oldfield, 1971)was administered to all participants inorder to determine hand preferences. The group average Laterality Quotient (LQ) was 80.57(range : 50 to lOO.00).All participants had normal hearing with no personal or family historyof hearing difficulties.Stimuli

The stimuli consisted of four consonant-vowel-consonant (CVC) words and four CVCnonsense syllables. The meaningful stimuli (M) consisted of the words /keb, pack, gasp, baWwhile /kak, preb, grek, beep! comprised the nonsense syllables (NM).The articulatory features that characterize these two groups of stimuli are presented inTable 1. The two groups are matched in terms of voicing and place of articulation for boththe initial and final consonants. The middle vowel /ml was the same for all stimuli. The eightstimuli were produced by a male adult speaker who has a general American dialect. Allstimuli were computer edited so that they were matched in peak intensity and in duration(720 msec). The sonographs for one word, /breW, and a nonsense CVC, /bzp/ are presented


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AERS TO COARTICULATED SPEECH CUES 89

ioons&FIG. 1. Speech spectrograms for a word stimulus, /b&l, and a nonsense syllable, lbaepl.Interval marker is 200 msec. Vertical markers are at 500 hertz intervals.in Fig. I. Sixteen unique random orderings of the eight stimuli were recorded on one channelof a Sony Stereo Tape Recorder (Model TC-560). A 80 Hz square wave pulse was recordedon the second channel of the tape recorder 40 msec prior to the onset of each stimulus. Thispulse identified the beginning of each stimulus presentation and the auditory evoked poten-tial for later analyses. The interstimulus interval varied randomly from 8 to 16 sec.

ProcedureAll participants were tested individually in a sound dampened, electrically shielded room.Silver electrodes were placed on the scalp over the superior temporal regions of the left andright hemispheres as T3 and T4 of the IO-20 electrode system of the International Federation(Jasper, 1958)and referred to linked ear leads (A,, A2). Electrode impedances for each sideof the head were recorded before and after each testing session. Impedances for the twotemporal sites were maintained within 1 KOhm of each other throughout the session. Themean group electrode impedances were 2.31 KOhms (range : 1.2-4.2 KOhms) immediatelybefore testing and 2.60 KOhms (range : 1.3-5.0 KOhms) at the end of the 20-min testingsession. The recording electrodes were connected to two Analogue Devices Isolation Am-plifiers (Model 2735) powered by an Analogue Devices Power Supply (Model 904). Theoutput of each amplifier was connected to two modified Tektronix AM 502 differentialamplifiers with the gain settings at 20K and the bandpass flat between 0 .1 Hz and 30 Hz. Theamplified AERs and the trigger pulses were recorded on a cassette FM tape recorder (VetterModel C-4) for later off line analyses.Two telegraph keys positioned 5 in apart were placed directly in front of the subject whowas instructed to rest one hand next to each of the keys. Subjects were then told that theywould hear a series of CVC syllables. If they heard a word, they were told to press one keyand to press the other key when they heard a syllable that was not a word. All subjects wereinstructed to wait until after the stimulus was completed before pressing a key. The positionof the keys pressed in response to the meaningful and nonsense stimuli was counterbalancedacross subjects. These responses were recorded by an observer positioned behind thesubject during the testing session. The stimuli were presented through a speaker suspendedapproximately 85cm above the subjects head. The stimulus intensity at the individuals earwas 80 db SPL.

RESULTSAll subjects responded to all stimuli correctly. The AERs recordedfrom each subject were digitized beginning with stimulus onset at 5 msec


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90 DENNIS L.MOLFESEintervals over a 550 msec period (for 110 points) and then averaged bycondition for each subject on a PDP-12 computer using a modified versionof Averager (Decus No. 12-84). Averages were obtained from the 16repetitions of each stimulus for each of the words (4) and nonsensesyllables (4) for each hemisphere (2) of each subject (IO) for a total of 160averaged evoked potentials. Following the procedures outlined by Mol-fese et al. (1976), Molfese (1978), and Chapman, McCrary, Bradgon, andChapman (1977), a 110 (time point variables) x 160 (averaged evokedpotential cases) input matrix was constructed. Intercorrelations amongthe 110 variables were submitted to a principal components analysis withthe BMD08M program that is part of a standard computer statisticalpackage (Dixon, 1972). This program first transformed the data into acorrelation matrix. The principal components analysis was then applied tothis 110 x 110 matrix which consisted of the product moment correlationscomputed for each pair of time points. Factors which met the eigen value= 1.0 criterion were then retained. In this way, only 10 factors, each ofwhich accounted for at least as much variance in the data as any one of theoriginal variables, were withheld for futher analyses. The 10 factorsaccounted for 93.2% of the total variance. These were then rotated usingthe normalized varimax criterion which preserved the orthogonalityamong the factors while improving their distinctiveness. Factor scores(gain factors) were then computed for each of the 160 original AERs foreach of the 10 rotated principal components.The centroid (the average AER of the entire data set) and the 10 factorsobtained by the principal components analysis are plotted in Fig. 2. The10 factors each consist of 110 factor loading which correspond to the 110time points. The factors loadings reflect the association of the factors tothe original variables (time points). Since the factors were orthogonal, thefactor scores for each factor were treated as dependent variables inindependent analyses of variance using the BMD08V computer program(Dixon, 1972). The Hemispheres (2) x Meaning (2) x Voicing of InitialConsonant (2) x Articulatory Position of Initial Consonant (2) analyses ofvariance were performed in order to determine if any relationships existedbetween the factors and specific levels of the independent variables. Allsignificant main effects and interactions beyond the .05 level are reportedbelow. All post hoc tests involved tests for simple main effects. Sig-nificant main effects for Hemispheres (F = 38.17, df = 1,9, p < .OOl) andMeaning (F = 11.67, u = 1,9, p < .Ol) as well as a Meaning x Voicinginteraction (F = 5.26, u = 1,9, p < .05) were found for Factor 1. A testfor simple main effects indicated that this interaction was due to differ-ences in responding to the meaningful stimuli depending on the voicing ofthe initial consonant (F = 6.89, df = 1,9, p < .OS). A main effect forHemispheres was found for Factor 2 (F = 18.93, df = 1,9,p < .Ol), whilemain effects for Hemispheres (F = 17.35, & = 1,9, p < .Ol) and Voicing(F = 14.59, d = 1,9, p < .Ol) were found for Factor 3. Main effects for


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AERS TO COARTICULATED SPEECH CUES

FIG. 2. The centroio, which reflects the activity common to all AERs for all conditions,and the 10 factors isolated with the principal components analysis. The factors are graphs ofthe factor loadings and reflect the association of the factors to the original time points of theAER. The marker at the beginning of the centroid indicates the point of stimulus onset. Thecalibration marker is 2 PV with positive up. The calibration marker does not apply to thefactors. The contribution of each factor to the AER in terms of its polarity and amplitude isdetermined by multiplying it by the factor score for that condition. Plots are based on pointsat 5 msec intervals.Hemispheres (F = 13.18, df= 1,9,p --c Ol), Meaning (F = 7.03, & = 1,9,p < .05), and interactions for Hemispheres x Meaning (F = 10.71, df =1,9, p < .Ol) and Meaning x Voicing (F = 10.27, & = 1,9, p < .025)characterized Factor 4. The Hemispheres x Meaning interaction was dueto differences in the responses of the two hemispheres to the meaningfulstimuli (F = 83.20, df = 1,9, p < .OOOl)and to the nonsense stimuli (F =


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DENNIS L. MOLFESE

.? FACTOR 4

/ t--LH-RHL MEANINSFUL NONSENSESTIMULI

FIG. 3. The mean factor scores for the Hemisphere x Meaning interaction for Factor 4.The factor scores plotted along the ordinate are the means for the dependent measures in theanalysis of variance. These means are the weights for the Factor that is represented as awaveform in Fig. 2. The polarity and size of this waveform for a particular condition isdetermined by multiplying each point of the waveform by the mean factor score for thatcondition.21.20, df = 1,9, p < .Ol). This interaction is presented in Fig. 3. TheMeaning x Voicing effect was due to differential responding to the mean-ingful stimuli as a function of initial consonant voicing (F = 19.51, df =1,9, p < .Ol) as was the case for Factor 1.Significant main effects were found for Hemispheres (F = 108.10, & =1,9, p < .OOOl)and Position (F = 30.35, df = 1,9, p < .OOl) for Factor 5.The Hemisphere x Position interaction (F = 12.54, df = 1,9, p < .Ol) andthe Hemisphere x Meaning x Voicing x Position (F = 6.45, & = 1,9,p

cortical involvement in the semantic processing

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