Slow potentials of the brain related to speech processing in normal speakers and stutterers

Elec~roencephalography and Clinical Neurophysiology, 1974, 37:599 607 (~) Elsevier Scientific Publishing Company. Amsterdam-Printed in The Netherlands

SLOW POTENTIALS PROCESSING

OF THE BRAIN RELATED

IN NORMAL

SPEAKERS

599

TO SPEECH

AND STUTTERERS

GERALD N. ZIMMERMANN1 AND J. R. KNOTT Division of Electroencephaloyraphy and Neurophysiology, Department of Psychiatry, University (~f Iowa, Iowa City. la. 52242 (U.S.A.)

(Accepted for publication: June 18, 1974)

In the past, major theories of stuttering have implicated : (1) the role of hemispheric dominance; and/or (2) the role of "anxiety". Both of these constructs may now be investigated by slow potential EEG techniques, utilizing the contingent negative variation (CNV). Theories positing "anxiety" as a causal factor in stuttering, inilially introduced as a replacement of the dominance theory by Johnson and Knott (1936) have, in their more extensively developed forms, regarded anxiety-eliciting stimuli in speaking situations as leading to either a breakdown in fluent speech (Wischner 1950, 1952; Brutten and Shoemaker 1967), or as evoking avoidant behaviors which interfere with fluent speech (Johnson 1955; Bloodstein 1969). Cerebral dominance theories, originally put forth by Stier (1911), Sachs (1924) and fully developed by Orton and Travis (Orton 1927; Travis 1931) posited a lack of lateral cerebral dominance which created a mistiming of neuromotor impulses to the bilateral speech musculature. This theory found support in early studies of peripheral sidedness" and of handedness. More recent studies using dichotic listening tasks (Curry and Gregory 1969) and articulatory tracking (Sussman and MacNeilage 1974) have lent further support to this position. Slow-potential recording techniques have been used to study both "anxiety" and lateral cerebral dominance. Knott and Irwin (1967,

1968, 1973) showed that anxiety-prone normal speakers, when under stress, had smaller CNVs than nonanxiety-prone subjects. This has been extensively supported by Knott et al (1973). Low and Swift (1971) showed that CNV amplitude was inversely related to anxiety by progressively incrementing levels of stress. However, Knott and Peters (1974) suggested that this is sexrelated, appearing to develop clearly in females and in another form in males. McAdam and Whitaker (1971), Morrell and Huntington (1971) and Low et al. (1974) have shown that anticipatory slow potential shifts preceding speech are larger over the left hemisphere than over the right hemisphere in righthanded subjects. Low et al. went on to find a significant correlation between hemispheric dominance determined by the Wada sodium amytal test and dominance derived from CNV amplitudes in the left and right motor speech areas. Thus, recording of slow brain potentials appears useful to test some of the hypotheses raised by the foregoing theories of stuttering, especially the "dominance" theory, since the direct use of the Wada test of dominance has some degree of morbidity, while slow potential recording is benign. The purpose of the present study was to investigate the relationships between slow potential shifts measured at the vertex and over inferior frontal positions, which presumably involve primary speech motor areas, in relation to speech onset.

1Present address: Research Fellow, Division ot Electroencephalography and Neurophysiology, Department of Psychiatry, University of Iowa, Iowa City, Ia. 52242, U.S.A.

METHODS

Subjects were 9 male stutterers who were

600 enrolled in the University of Iowa Speech and Hearing Clinic and 5 nonstuttering students enrolled at the University 2. All subjects wrote with their right hands. They ranged in age from 20 to 29. All subjects were free from any known neurological defect and none had any prior contact with CNV recording procedures. Slow potential changes in scalp EEG were recorded using Beckman "Biopotential" electrodes attached with collodion-impregnated gauze patches at Cz, iF;, and iF~. The latter were located 11 cm laterally from Cz and from that point 4 cm anteriorly. According to McAdam and Whitaker (1971), and subsequently verified in our own laboratory by neuroanatomical survey in a cadaver, these placements lie over the presumed Broca's area (iF'3) and its contralateral homologue (iF4). Linked electrodes on the left and right mastoid processes served as a "reference". Eye movements were recorded using one lead above and one below the right eye, referred to an electrode on the right ear lobe (Peters et al. 1970). The AC impedance (measured with a Grass EZM) of each electrode was determined to be less than 5000 fL A Grass Model 7 polygraph with 7P-1 chopper-stabilized preamplifiers operating in the 1 Mf~ DC mode, with companion Grass 7DA driver amplifiers, was used to record the slow potentials and eye movements. The upper limit of the system, which is determined by the chopper frequency of the preamplifiers, was less than 60 c/sec. Data were stored on a Precision Instrument Model 207 F M tape recorder. Those trials which the on-line paper record showed to be free of eye movement or other contaminating artifacts were subsequently fed into a PDP-12 computer and averaged over a 4 sec epoch (BNI Series DECUS 12-1 )3. No fewer than 6, nor more than 12 trials, were averaged per subject in each condition. The loss of trials (40 were given in each condition) was due primarily to eye movement artifacts. In order to allow amplitude measurements of the average waveforms, calibration pulses of 25/~V were placed on the F M tape 2Eleven stutterers and seven normal speakers were run in the experiment. Two from each group were eliminated due to excessive artifacts in their records. 3Our thanks to Maurice Mendel, Department of Otolaryngology for the use of the PDP-12 computer.

G. ZIMMERMANN AND J. R. KNOTT

on line before each trial (Emde 1964) and were thus averaged together with the CNVs. Subjects were seated comfortably in a soundtreated room, 6 ft. in front of a television monitor above which were a loud speaker and a Grass PS-2 photostimulator lamp. A microphone was placed in front of the subject to monitor his utterances. Condition 1, a nonverbal control condition, involved a.differential reaction-time paradigm in which S~ was either a high (800 c/sec) or low (300 c/sec) tone, 250 msec in duration and with an intensity of 75 dB SPL (measured at the subject's ear), and $2 was a flash of light (PS-2) 1500 msec after the onset of S t. The subject was instructed to press a telegraph key as soon as possible after S 2 following the high frequency tone but not to press when $2 followed the low tone. The response and nonresponse trials were randomly intermixed and delivered in two segments of twenty blocks each. The intertrial interval was from 15 to 30 sec. Condition 2 was a verbal, nonspeech, "expectancy" condition. S~ was one word, from a list of 40, flashed silently on the TV monitor via a closed circuit TV-slide system. The words were familiar mono- and polysyllabic words with no control for initial sounds. Each word appeared on the screen for 250 msec. Subjects were instructed to press one of two telegraph keys after a brief light flash appeared above the TV monitor 1500 msec after the onset of the word. One key was designated "Yes", and the other " N o " . The subject was instructed to press the "Yes" key after the light flash if he thought he would have "difficulty" (i.e., would "stutter") if required to speak the word out loud, and the " N o " key if he thought he would have no trouble. The subjects were instructed not to speak the word and to breathe through the nose during the $l $2 interval. Condition 3, the experimental verbal speech condition, was similar to condition 2 except that, instead of pressing a key after S 2, the subjects were instructed to speak the word after (not before) the light flash. Subjects were instructed to breathe through the nose in the S 1-$2 interval in order to achieve a constant rest position prior to S z. Trials in which subjects spoke or manifested observable movement in the $l-$2 interval

SLOW POTENTIALSAND SPEECHPROCESSING

601 tion preceding or during the production of the utterance, as judged by the experimenter (G.Z.) on the basis of visual and auditory observation at the time of recording. RESULTS

r" ~' "S;yV150V~'omS TS ~ I

I

4000 mS Fig. 1. Tinting and averaging for one subject. Amplitude measured at "A". S1 and S2 represent onset of stimuli. Calibrator pulse= 25 I~V.Negative up in all figures.

Because of the simplicity of the task, very little dysfluent speech behavior was present: thus, results for only nonstuttered speech and " N o " expectancy responses will be considered for the verbal conditions. Descriptive and statistical analyses were performed. Vertex data

were eliminated from the off-line data analysis. C o n d i t i o n 4, a second control condition, was identical to condition 1. Trials were in all conditions initiated by the experimenter. Instructions were given before each condition. Five rain rest periods were allowed between conditions, as well as in the middle of each condition. During all conditions subjects were asked to fixate their gaze on a red dot in the middle of the TV monitor. The experimental chamber was comfortably illuminated. Average waveforms were plotted on a Houston Instrument incremental plotter. Fig. 1 presents an X Y plot of an average from a sample condition for one subject and illustrates the scoring method used. Baseline for the vertex CNVs was determined from the 800 msec period prior to and following the calibration pulse. Tl~e measure of CNV was the voltage difference between baseline and the peak negativity (smoothed by visual evaluation) occurring at any point in an interval beginning 400 msec after St and ending just prior to $2. Each measurement was made by E without knowledge of the condition or subject represented by the average. • Polarity of shifts at the lateral derivations iF3 and iF,;, were judged by two independent judges. It was required that both judges must agree on existence and direction of shift in order for it to be evaluated as occurring. Subjects were monitored auditorily, and also visually through a window separating the experimental chamber and the control room. For the purpose of this study "stuttering" was defined as any dysfluency, hesitation, or repeti-

The Mann-Whitney U-test was employed to assess differences in v e r t e x amplitudes for the two groups and four conditions, and the values are shown in Tables I-IV. Table I sl~ows that the v e r t e x CNV amplitude was significantly different for both samples on the response and nonresponse trials for control TABLE I Mann-Whitney U values for comparisons within control conditions 1 and 4. Response vs. Nonresponse trials. R = response, NR = nonresponse. Stutterers

Nonstutterers

U= 5*

U=0*

U=21

U =6

Condition 1 Rvs. N R

Condition 4 R vs. NR

* Significant at 0.05 level for two-tailed test.

TABLE II Mann-Whitney U values for comparisons between control conditions within each group. R=response, NR=nonresponse. Stutterers

Nonstutterers

U=21

U=I0

U=I0

U= 6

Condition 1 R VS,

Condition 4 R Condition 1 NR Condition 4 NR * Significant at 0.05 level for two-tailed test.

602

G.

ZIMMERMANN AND J. R . KNOTT

CONDITION 2

T A B L E III

(EXPECTANCY)

NORMAL SPEAKERS M a n n Whitney U values for within groups comparisons between conditions.

CZ

J?

Conditions

V~ v V"

M lvs. 4

lvs. 2

t vs. 3

2vs. 3

U=21 U=10

U=9* U=0*

U=II* U = 0*

U=34 U=9

•"v

Stutterers Normals

~---~j

v

• W

"3~ r" II",J

* Significant at 0.05 level for two-tailed test.

~ . ~ ^ ^ o,.

'VV

d

T A B L E IV Mann-Whitney U values for between groups comparisons within conditions.

-.,,..-~v $1

t

S2

$1

S2

$1

Conditions

Stutterers Normals

vs.

S2

CAL.= 25JJV

1

2

3

4

U = 17

U = 13

U = 15

U = 17

condition 1. The response trials showed greater amplitude CNV than the nonresponse trials. Table II shows that there were no significant differences between control condition 1 and control condition 4. Thus, the subjects were capable of generating the classical CNV, and no habituation occurred over time. Table III shows that the U values reached significance for both groups when comparing control condition 1 with each of the verbal conditions, 2 and 3. The vertex C N V amplitudes were consistently lower in the verbal conditions than in the nonverbal control condition for both groups. Table IV shows that there were no significant differences in vertex CNV between groups in any of the 4 conditions.

Fig. 2. Averages at each electrode derivation for normal speakers in condition 2 (expectancy).

CONDITION 2 (EXPECTANCY) STUTTERERS

'? v~

~A

.JL.

r. "V V

.....

N

v.~/~f~

^; Wv A.~J

a

;ivy2 W[.~- " 7

Laterafity data For laterality effects in the experimental conditions, an individual descriptive analysis of waveforms recorded from iF~ and iF~, was preferred over a statistical analysis, for the following reasons : 1. A statistical analysis based on a peak amplitude or area measure would be misleading

i: vL

, .a42L, ~ N

f'~' " f ' $1

t $2

~''T

t SI

$2

~-v.

-- I' . . . . .

$I

t•

$2

CAL.=25JJV Fig. 3. Averages at each electrode derivation for stutterers in condition 2 (expectancy).

603

S L O W P O T E N T I A L S A N D SPEECH P R O C E S S I N G

and camouflage the variability in waveforms and polarity at the lateral electrode derivations. 2. A descriptive individual analysis lends itself better to future studies on individual assessment (e.9., of diagnosis and therapy) than do statistical analyses, 3. One need not make certain assumptions that are implicit in chosing one measure to quantify complex waveforms (Peters et al. 1974). Figs. 2-5 show the averaged waveforms (for each subject in each group) recorded at each derivation, iF; iF 4 and Cz, for the verbal conditions 2 and 3. Table V shows the polarity of these shifts as evaluated by two independent judges. (The 3 samples on which the judges did not agree, were scored as "zero" shifts.) Fig. 2

CONDITION NORMAL

"~

S1

3

(SPEECH)

SPEAKERS

"'a2

"~

$2

"

V

S1 CAL.:

st2 2 5)dv'

Fig. 4. A v e r a g e s o f each electrode derivation for normal speakers in condition 3 (speech).

TABLE V

Polarity of shifts at each electrode or derivation for each subject in conditions 2 and 3. - = negative shift, + = positive shift, 0 = no discernible shift. SuNect

Stutterers 1

2 3 4

5 6 7 8 9

Normal speakers

3

CONDITION

3

(SPEECH)

STUTTERERS

Condition Electrode derivation iF~

Cz

iF~

2

0

0

-

3

0

-

-

2

0

-

-

3

-

-

-

2

0

--

-

3

+

-

-

2

0

0

0

3

0

-

0

2

-

-

-

3

+

0

0

2 3 2 3

+ + + 0

0 + 0

+ + 0 0 +

3 2 3

0 -

+

+

-

0 0

2

-

-

-

2

-

-

0

2 3 2

-

-

-

-

0

-

0 0 0

,)

_

_

_

3

-

-

0

if*

V~'L

u-~

~,, L a l L /k V " v

$I

S2

$I

S2

~

't

t

SI

S2

CAL.= 251JV

5

Fig. 5. Averages at each electrode derivation for stutterers in

condition 3 (speech).

604

G. ZIMMERMANN AND J. R. KNOTT

TABLE VI Bilateral polarity relationships for subjects in both groups.

Electrode derivation

Condition

"~ Subjects Normal

iF;

Stutterers

iF~

-

-

2

40

11.1

-

0

3

60

11.1

2 3 2 3 2 3 2 3

40 40 0 0 0 0 0 0 0 0 0 0 0 0 20 0

11 .l 11.1 0 11.1 ll.l 11.1 11.1 11.1 33.3 11.1 0 11.1 11.l 11.1 11.1 11.1

+

+

0

+

+

0 0

+

-

+

0

0

3 2 3 2 3 2 3

shows that four of the five normal "speakers showed a larger negative slow potential shift in the left hemisphere than in the right preceding speech. Table VI presents the bilateral polarity relationships for the subjects in the two groups in the verbal conditions. It is evident in Fig. 2 and Table VI that the inter- and intrahemispheric variability, with respect to polarity and presence or absence of a slow potential shift, is greater in both verbal conditions in stutterers than in normal speakers. Table VI shows the percentage of subjects in each group showing particular interhemispheric relationships. If a post-hoc categorization of "normal" is established, consisting of a negative shift in the left hemisphere and a lesser negative or no shift in the right hemisphere, then only 2 ~~.~/o ~o/ of the stutterers fall into this category in the speech conditions and 22.2°/0 in the expectancy condition, as opposed to 80°o in the speech condition and 60%1~in the nonspeech condition in the normal group. DISCUSSION

The foregoing suggests that stutterers and

normal speakers are not different in their vertex CNV response amplitudes in the nonverbal control conditions. Furthermore, there are no differences in vertex CNV amplitudes between the normal speakers and stutterers in the verbal conditions, although both groups show significantly smaller vertex CNV amplitudes in the verbal conditions than in control condition 1.~. Neither are there any differences in vertex data between the groups in the verbal-expectancy and verbal-speech conditions. However, when CNVs are recorded at iF'3 and iF~ and interhemisperic comparisons are made, the results support the conclusions of McAdam and Whitaker (1971) and Low et al. (1974), that time-locked EEG recording preceding speech can be a useful tool in measuring lateral asymmetries of the brain. As expected from previous results, four of the five normal right-handed speakers showed larger DC shifts in the left hemisphere than in the right preceding speech. The present results suggest that the motor aspect of speech is not a necessary condition for this effect, since in the "'expectancy" condition "verbal processing" without motor speech was sufficient to show this asymmetry. Furthermore, the results show that most of the stutterers (82'! ;) in our sample showed variable relationships in interhemispheric activity in the verbal conditions. In short, these data suggest that the left and right inferior frontal areas of nonstutterers and stutterers perform differently not only preceding fluent speech but also when they are making decisions about the expectancy to have difficulty speaking a word, even though no overt motor speech act occurs. When processing verbal stimuli stutterers appear to show more variable interhemispheric relationships and do not show a shift that is consistently larger in the left hemisphere than in the right. In a previous report Zimmermann and Knott (l"974) alluded to an imbalance of cortical areas in stutterers and found support for Travis' hypothesis, which posits an interaction between stress, as a nociceptive stimulus, and the loss of a critical balance in the hemispheric relationship in stutterers, leading to disintegration of the speech process and "stuttering". This inter4All subjects showed negative shifts over right and left hemispheres in control conditions 1 and 4.

605

SLOW POTENTIALS AND SPEECH PROCESSING

pretation led to further hypotheses concerning presumably critical cortical areas for speech. If the interaction between "arousal", as measured one accepts Benton's (1970) definition of cerebral by the CNV, and lateral dominance as measured dominance as: " . . . a state of affairs in which one by the DC shift over Broca's area and its contra- hemisphere possesses functional properties or subserves behavioral functions that are not lateral homologue. The effect of arousal on the bilateral shared by the other hemisphere either at all or to relationship was suggested by data which showed the same degree", and if one assumes that the that preceding s t u t t e r e d s p e e c h no CNV appeared larger slow negative shifts over the left hemiat the vertex in the $1 -S: interval, and that there sphere in normal right-handed speakers (also was, on the average, a negative shift in the left shown by McAdam and Whitaker (1971), hemisphere and a positive shift in the right and Low et al. (1974)) represent specialized hemisphere. Fig. 6 shows the relationship for activity of the left hemisphere for verbal prostuttered and nonstuttered speech reported by cessing, it follows that stutterers and normal Zimmermann and Knott (1973, 1974). In the speakers differ in their cerebral dominance. It is not, however, enough to describe the small sample of subjects who displayed stuttered speech there appeared to be an interaction differences found in terms of cerebral dominance, between the CNV amplitude at the vertex and speech set (Marsh et al. 1974), or speech mode presence or absence and polarity of shifts over (Liberman et al. 1967). The complex perceptualmotor integration necessary for adequate speech the left and right hemispheres. The present data, which show no difference in production necessitates further investigation of vertex CNV in the verbal conditions for the two underlying processes which contribute to the groups of speakers, suggest that differences in differences in lateralized cerebral activity. It is interhemispheric relationships may not be a necessary to define and describe the perceptual function of an interaction between the "arousal and expressive parameters of speech and language system", as seen by CNV at vertex, and the motor that underlie these constructs in order to give output system. These data suggest that when functional significance to them. Thus, the present results suggest that the stutterers are speaking "fluently", or not motor output systems are behaving differently speaking at all, there is no measurable difference during speech processing preceding fluent speech in their level of arousal compared to normals, but that there is in stutterers an imbalance in the

IF"4

CZ

11:'3

V

t

t

"

"

T

T

T

'

_,

E%J

'--F t

T

S1

$2

"'"-v'wV T S1

T S2

v~,

n

.,J[

T

.

SI

/~

S2

Fig. 6. Averages of averaged waveforms preceding stuttered (ST) and nonstuttered (NS) speech of stutterers. From Zimmermann and Knott (1974).

606

G. ZIMMERMANN AND J. R. KNOTT

in stutterers and normal speakers. This, coupled with the finding that preceding nonstuttered speech there are no differences in CNV amplitude at the vertex which presumably should be affected by stress and anxiety-proneness, further suggests that hypotheses implicating stress and anxiety as primary causal factors in stuttering should be re-evaluated and the perceptual and neuromotor systems involved in speech activity be further investigated. SUMMARY

A CNV paradigm was employed to study slow potential shifts of the brain in stutterers and normal speakers during speech and nonspeech verbal tasks and in a nonverbal manual task. Slow potential shifts were recorded from Cz, iF~ and iF~. It was found that vertex CNVs were similar in normal speakers and stutterers in the verbal and nonverbal tasks. Differences between groups were found at the lateral electrode derivations in both verbal tasks. Preceding speech, four of five of the normal speakers showed a larger shift in the left hemisphere than in the right, While only 22% of the stutterers showed a left greater than right asymmetry. In the verbal condition in which the subjects did not speak (the "expectancy" condition) similar relationships were found. It is concluded that the left and right inferior frontal areas of nonstutterers and stutterers performed differently even when the stutterers werc not approaching a moment of stuttering, i.e,, when both groups appeared to have equal speech performance, and also when they were making identical decisions about the "expectancy" of having difficulty speaking a word, although no overt motor speech was required. The interhemispheric differences in the two groups are not attributed to increased "anxiety" since the vertex CNV was unaffected. RESUME POTENTIELS

LENTS

CEREBRAUX

L'ACQUISITION DU LANGAGE CHEZ AVEC LANGAGE NORMAL ET CHEZ

LIES

A

DES SUJETS DES SUJETS

BEGUES

Un paradygme de VCN a 6t6 utilis6 pour

6tudier les d6flections lentes de potentiel du cerveau chez des b6gues et des sujets fi langage normal au cours des tfiches verbales parl6es et non-parl6es et dans une tSche manuelle non verbale. Les variations lentes de potentiels sont enregistr6es ~tpartir des 61ectrodes Cz, iF~ et iFS,. Les auteurs ont trouv6 que la VCN recueillie au vertex 6tait similaire chez les sujets "fi langage normal et chez les b6gues aussi bien dans les t'~ches verbales que non verbales. Des diff6rences entre ces groupes ont 6t6 observ6es au niveau des d6rivations lat6rales dans les deux tfiches verbales. Pr6c6dant la parole, 4 des 5 sujets ~t langage normal ont montr6 une d6flection plus large dans l'h6misph6re gauche que dans l'h6misph6re droit alors que 22 o~, seulement des b6gues ont montr6 une asym6trie de m6me nature. Dans la condition verbale dans laquelle les sujets ne parlent pas (condition d'expectative) des relations similaires ont 6t6 observ6es. Les auteurs concluent que les aires frontales inf6rieures gauches et droites des b6gues et des non-b6gues fonctionnent de faqon diff6rente m6me lorsque les b6gues ne sont pas dans l'imminence d'un moment de b6gaiement, c'est~t-dire quand les 2 groupes paraissent avoir une performance de parole 6gale, et aussi lorsqu'ils prennent des d6cisions identiques au sujet de l'expectative. REFERENCES BENTON, A. L. Hemispheric cerebral dominance. Israel J. med. Sei., 1970, 6: 294-303. BLOODSTEIN, 0. A handbook on stuttering. National Easter Seal Society for Crippled Children and Adults, Chicago, Ill., 1969, 300 p. BRUTTEN, E. J. and SHOEMAKER, D. J. The modification o f stuttering. Prentice-Hall, Englewood Cliffs, N.J., 1967, 148 p. CURRY, F. K. W. and GREGORY, H. The performance of stutterers on dichotic listening tasks thought to reflect cerebral dominance. J. Speech Res., 1969,12 : 73 82. EMDE, J. W. A time-locked low level calibrator. Electroenceph, clin. Neurophysiol., 1964, /6: 61(~618. JOHNSON, W. A study of the onset and development of stuttering. In W. JOHNSON and R. LENTENEGGER (Eds.), Stuttering in children and adults. Univ. of Minnesota P~'ess, Minneapolis, 1955, 562 p. JOHNSON, W. and KNOTT, J. R. The m o m e n t of stuttering. J. 9enet. Psyehol., 1936, 48: 475~,79. KNOTT, J. R. and IRWl~. D. A. Anxiety, stress and the contingent negative variation. Electroenceph. clin. Neurophysiol., 1967, 22: 188.

SLOW POTENTIALS AND SPEECH PROCESSING KNOTT, J. R. and IRWIN, D. A. Anxiety, stress and the contingent negative variation (CNV). Electroenceph. clin. Neurophysiol., 1968, 24: 286287. KNOTT, J. R. and IRWIN, D. A. Anxiety, stress and the contingent negative variation. Arch. gen. Psyehiat, 1973, 29 : 538-541. KNOTT, J. R. and PETERS, J. F. Changes in CNV amplitude with progressive induction of stress as a function of sex. Eh, etroenceph, clin. Neurophysiol., 1974, 36: 47-51. KNOTT, J. R., VAN VEEN, W., MILLER, L. H., PETERS, J. F. and COHEN, S. I. Perceptual mode, anxiety, sex and the contingent negative variation. Biol. Psychiat., 1973, 7: 43 52. LIBERMAN, A. M., COOPER, F. S., SHANKWEILER, D, and STUDDERT-KENNEDY, M. Perception of the speech code. Psyehol. Rev., 1967, 74: 431~61. Low, M. D. and SWIFT, S. J. The contingent negative variation and the "resting" D.C. potential of the human brain : effects of situational anxiety. Neuropsychologia, 1971, 9: 203 208. Low, M. D., WADA, J. A. and Fox, M. Electroencephalographic localization of conative aspects of language production in the human brain. In W. C. MCCALLUMand J. R. KNOTT (Eds.), Proc. 3rd Int. Congr. on Event Related Slow Potentials t~f the Brain, 1974, in press. MARSH, G. R., POON, L. W. and THOMPSON, L. W. Some relationships between CNV, P300 and task demands. In W. C. MCCALLUM and J. R. KNOTT (Eds.), Proc. 3rd Int. Congr. on Event Related Slow Potentials of the Brain, 1974, in press. McADAM, D. W. and WHITAKER, H. A. Language production: electroencephalographic localization in the normal brain. Science, 1971, 172: 499-502. MORELL. U K. and HUNTINGTON, D. A. Electrocortical

607 localization of language production. Science, 1971, 174: 1359-1360. ORTON, S. T. Studies in stuttering. Arch. Neurol. Psychiat. (Chic.) 1927, 18: 671-672. PETERS, J. F., KNOTT, J. R. and HAMILTON, C. E. Further thoughts on measurement of "The" CNV. In W. C. MCCALLUM and J. R. KNOTT (Eds.), Proc. 3rd Int. Congr. on Event Related Slow Potential,; of the Brain, 1974, in press. PETERS, J. F., KNOTT, J. R., MILLER, L. H., VAN VEEN, W. J. and COHEN, S. 1. Response variables and magnitude of the contingent negative variation, Electroenceph. olin. Neurophysiol., 1970, 29: 608-611. SACHS, M. W. Zur Aetiologie des Stottern. Klin. Wschr., 1924,37:113 115. ST1ER, E. Untersuchen uber Linkshandigkeit und die funktionellen D(fferenzen der HirnhM/ten. Fischer, Jena, 1911. SUSSMAN, H. and MACNEILAGE, P. Studies of hemispheric specialization for speech production. Langua~3e and the brain, 1974, in press. TRAVlS, L. E. Speech pathology. Appleton-Century, New York, 1931, 331 p. WISCHNER, G. J. Stuttering behavior and learning. J. Speech Dis., 1950, 15: 324-335. WISCHNER, G. J. Experimental approach to expectancy and anxiety in stuttering behavior, J. Speech Dis., 1952, 17: 134-154. ZIMMERMANN, G. N. and KNOTT, J. R. Slow potentials preceding speech in stutterers and normal speakers. Electroeneeph. olin. Neurophysiol., 1973, 36: 216. Z1MMERMANN,G. N. and KNOTT, J. R. CNV and stuttering. In W. C. MCCALLUMand J. R. KNOTT (Eds.), Proc. 3rd Int. Congr. on Event Related Slow Potentials' o['the Brain, 1974, in press.