Confirmation is provided for the belief that evoked potentials may reflect differences in hemispheric functioning that are marginal at best. Subjects were right-handed and audiologically normal men and women, and responses were recorded using standard EEG techniques. Subjects were instructed to listen for the targets while laying in a darkened sound booth. Different stimuli, speech and tone signals, were used. Speech sounds were shown to evoke a response pattern that resembles that to tone or clicks. Analysis of variances on peak amplitude and latency measures showed no significant differences between hemispheres, however, a Wilcoxon test showed significant differences in hemispheres for certain target tasks

Benson, P.

Galambos, R.

Osier, H.

Schulman-Galambos, C.

Smith, T. S.

English

NASA Technical Reports Server

General Disclaimer 
One or more of the Following Statements may affect this Document 
 
 This document has been reproduced from the best copy furnished by the 
organizational source. It is being released in the interest of making available as 
much information as possible. 
 
 This document may contain data, which exceeds the sheet parameters. It was 
furnished in this condition by the organizational source and is the best copy 
available. 
 
 This document may contain tone-on-tone or color graphs, charts and/or pictures, 
which have been reproduced in black and white. 
 
 This document is paginated as submitted by the original source. 
 
 Portions of this document are not fully legible due to the historical nature of some 
of the material. However, it is the best reproduction available from the original 
submission. 
 
 
 
 
 
 
 
Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19750019672 2020-03-22T20:10:15+00:00Z
Vaasa-Ca • lal^f^l c^^ sa + ts ►DLfiC Di ► frJis^ [ s	 r^S-2T7z^
s• !^oat0 wTfKYats TO S ► taCd STInULI
(Colltotate 0619.)
	
11 P NC $1.4%	 CSCL OAP
nncleq
G3/S 2 ? y [ 12
On Hemispheric Differences in Evoked
Potentials to Speech Stimulil
1
Robert Galambos, Peter Benson, 2
 'Timothy S. Smith,
Carol Schulman-Gal-limbos and Helene Osier
Departments of Neurosciences and Litiguistico
University of California at San Diego, La Jolla, California 92037
(
i
This research was j
 ointl supported b
	 (J
NIH 11DO869 h (CS-G)and :1S-1173` (TSS)y grants
 NGR v 059-1y8 (HO,
?NINDS Postdoctoral Fellow.
a
9
W
i;
a	 F
r
li
A recent publi..tion in this Journal (Friedman et al. 1975) reviews the
history of the search for evoked potential correlates of hemispheric asymmetries
for speech signals, presents new data and concludes that "while evoked potentials
way som.times reflect differences in hemispheric ,unctioning, this effect is
marginal at best" (p. 18). The purpose of this note is `o provide confirmation
of that conclusion from a study of our own. Our report can be brief because we
have covered mush the same ground as Friedman et al; a full manuscript that
includes all details omitted here (on methods used, data obtained, and statis-
tical proced •res) is available on reque^t.
Methods
Sub ectn were young men ( 14) and women (4), who volunteered and were paid
for their services. All were right-handed and audiologically normal.
Standard EEG recording techniques were used, with gold disc electrodes
located at C  and over the presumed left and right parietal association areas
(mid-way between T 3 and P5 , and Ti; and T6, respectively). Reference leads
were on the mf,stoids, linked. AF;-AgC1 electrodes at the corner aid beneath one
eye monitored eye movements. Each of the three EEG signals was amplified by a
Grass P-15 (gain: 100X) in series with a TektroniX r74 122 (100X) and ba ;id pass
filtered between 0.2 and 1.00 Rz. Evoked responses were monitored on-line with
a Mnemctron CAT jOA computer and also recorded on an Ampex SP 300 instrumenta-
tion rc!order for off-line averaging by a PDP-12 computer.
Stimuli consisted of binaural, 250 msec, 65 dB SPI, natural speech syllables
(pa, ba) or pure tones with 5 msec rise-fall times (250 Iiz, 600 }iz). Each
stimulus was accompanied by a computer-generated synchronous trigger pulse.
Randomized lists of either speech or tone signals were constructed so than, one
stimulus, the frequent one, occurred 100 times while the other, the infrequent
i
,or target stimulus, occurred at random ir ►tcrvnis 22 to 35 times in the list.
The interstimulus interval wits 1. 9 seconds.
Procedure
The subject wearing earphones (`i'D11-39) lay on a cot in a darkened IAC
sound booth, with eyes fixed on a point on the ceiling. Subjects were instructed
to listen for the '^.argets, count them (without overt motor gestures), and report
the count at the er.d of the run. They first listened to four lists--two speech
and two t.ne; each stimulus (e.g. pa or 250 Hr) appeared in one of the lists
as the target and in the other as the frequent. The four lists were then repeated,
in counterbalanced order, to Frovidc a replication.
Response averaging and analysis
The tape-recorded EEG signals were sampled by the PDP-12 at 1 kliz for 512
MCC. A true average was computed for each channel. The averaging program
also compared the amplitude of calibration pulses in each channel and corrected
these averages so that they were stored at identical gain. Evoked respone.es at
each electrode site for each subject in each condition were plotted on an
analogue X-Y plotter (BBN 715) and measured by hand. Composite curves over all
8 subjects for cacti electrode site in each condition were also computed and
plotted.
RESUL'T'S
Fig. 1 shows the composite of the averages of all eight subjects for each
of the four conditions of the study. Both tone and syllable stimuli elicit the
two deflections widely reported to result from clicks (see, e.g. Pict.on et al.
197 10: N1
 (latency about 100 msec) and P 2
 (190). 1'3 (abo ►tt 350 msec) is small
for the frequent syllables but large (as expected, e.g. from Picton arid Hillyard
I"
319"(4) for the targets. The vertex response closely resembles that seen at the
hcmiapheres, but it is larger in amplitude (as expected f'rom mapping studies,
e.g. Picton ct al. 1974). The vertex P 3
 shows notable differences in the four
conditions which in part ar -shared at the hemispheric sites; for instance:
1 ►
 there is a prominent nc- ive peak (at about 250 mbec) in the target speech
response, and 2) target tones evoked a P 3
 that is larger, "sharper" and shorts:
in latency (by some 13 msee) than that evoked by target speech.
D6 a analysis
The vexing question of how to estimate the significance of the rather small
differences rioted in Fig. 1 has been thoughtfully addressed by Friedman et al.
(1975). We have employed three methods, as follows:
Method 1. Meaningful differences in response waveshapes, latencies, etc.
are likely to be evident to the eye. We therefore asked a jury of 10 persons
skilled in recordinC and measuring human evoked responses to match the averages
obtained from each individual subject with the composites shown in Fig. 1.
Since there were 8 subjects and 1+ conditions, each judge performed a total of
32 such matches. Together they correctly matched 182 records (57%) with the
composite to which it belonged. Their discrimination of frequent from target
responses was even better (9?,% correct). Their ability to distinguish a syllable
from a tone response, however, was poor (for the frequents, 84 correct vs. 64
incorrect identifications; for targets, 9), vs. 52). Thus '.:he naked eye readily
sees differences between the upper and lower rows of Fig. 1 but has difficulty
detecting differences in the columns.
Methud 2. Every response (8 subjects, 4 conditions, 3 electrode sites)
latency and. amplitude (for N i , P22 and when present, P3 ) was measured arid their
mean and standard deviation calculated. A series of repeated-measures ANOVA
tests showed m,j difference a,t p < . 05 among conditions or subjects in the peak
r
4c►mplitude measures. In particular there wits no significant difference in peak
amplitude between the right. and left hemisphere responses within (or between)
any of the conditions. In the latency measures two interesting results emerged.
1) The latency difference in the N I peak between the y speech (frequent or target)
and the tone (frequent or target) conditions is significant (F = 12.39 for
1,7 degrees of freedom); and 2) the P 3 peak latency at the two hemisphere sites
differed between the speech and tone conditions (F = 7.96 for 1,'( degrees of
freedom) .
As Friedman et al. (1975) have argued, sore statistical. procedures are
less conservative than others for evaluating the significance of differences
between evoked potentials. Repeated tests of significance increase the proba-
bility of obtaining "significant" differences by chance. Our findings with
respect to hem:-ipheric differences are computed for an uncorrected alpha of
.05. Were we to correct this criterion as Friedman et al. suggest, it would
only underscore the lack of significant differences.
Method 3. Following Wood et al. (1971; see also Wood 19'(5), we comPax ed
,-oss subjects and conditions using the Wilcoxon match-pair signed ranks test
at each digitized point in the respon3es. significant points of difference
(D < .02) in tone vs. speech responses at left and right hemisphere were rare
for frequent tones, but reasonably common for targets, principally in the P3
region, with somewhat larger areas of difference on the left. These signifi-
cant differences mainly reflect an approximately 30 msec P 3 latency difference
measurable in the responses to speech vs. tone targets. However, on the
average, these P3 latency differences are greater on the right, and so the
larger number of points on the left cannot be wholly explained by such latency
differences.
5Since 512 tests were perfe.zned with each comparison, some small number,
an average of approximately 10, might be expected to differ by chance. TO
test this the Wilcoxon test was separately applied to the first and second
halves (replications) of the data collected from each subject. The comparisons
were made over left and right electrode sites, speech and tone stimulus condi-
tions, and frequent and target stimuli. The results show no more points of
significance than might be expected !)y chance, and thus lead to two inter-
locking conclusions: 1) this particular test does not produce spuriously
significant points more often than would be expected by chance; and 2) the
first acid second halves of our experiment (purpooel) designed to be identical)
are ind— 1 good replications.
When each of the replications was separately analyzed as described above
for the combined data, they agreed with each other and with the conclusions
giv n above for the combined data. A smaller number of significant differences
was noted in the replications, a fact which :effects the greater variability
in them, since combining them increases the SIN ratio by 2 (Vaughan 19740.
DISCUSSION
The main purpose of this experiment was to crt 	 she optimal conditions
for demonstrating an electrophysiological difference due to cerebral dominance,
if any exists. To do this, we 1) chose the cortical evoked response and re-
corded this over :scalp areas which are implicated in the processing of auditory
signals and which, on the left side, are known to be involved iii the perception
of language. 2) We presented two different stimulus sets: n. speech set and
a tone set, and tested the hypothesis that, of the two, the speech set would
evoke reliably larger potentials on the left. 3) To minimize the effects of
the unavoidable physical differences in speech and non-speech signal:.;, we
Grequired the subjectu to perfc.< •m a vigilanot- tack it ► which they counted in-
1're(l ►jent. (1.arget.) sti.mull, a task which produce • u u P 3 wave irrespective of
modeLlity of preuentation or signal charnete • istics. 10 The mea-"Ures of
Interest were the response wave shape to the different stimuli and the peak
amplitude and latency of its pj . We conclude:
1) Waveshape. Speech sounds evoked a response pattern that remarkably
resembles that to tones or clicks. Similar 1) 3 waves were obtained to both
tone and speech targets, b e lt the latency of P3 to speech is the longer of the
two. This latency s' ►ift may mean that different processes occur in the identi-
fication of the npeec,h and non-speech stimuli. Additionally, a small P 3 wave
occurred in four out of eight subjects for the unattended, frequent speech
stimuli, a result that supports Friedman et al. ;1975) who reported similar
findings and speculated that all speech stimuli may invoke the 1' 3 producing
mechanism.
2) Hemispheric differences. Analyc;is of variances on peak amplitude and
latency measures showed no significant differences between hemispheres. 'rho
Wilcoxon test showed significant differences between the hemispheres for the
target t&sks in the P3 region of the response; some but not all of these
differences can be explained by P 3 latency differences. Hence it is possible
that P3 to speech is larger on the left than P3 to tones.
If one credits, as we do, the overwhelming clinical and behavioral evi-
dence for hemispheric asymmetry during processing of speech signals, it is
not unreasonable to expect a corresponding lack of symmetry in electrophysio-
l.ogical measures of hemispheric responsivity. Available recording and
analyzing techniques would appear to be precise enough to detect even very
small evoked-response differences, as our results and those of other groups
(summarized by Friedman et al. 1975) can be said to demonstrate. The
r
•	 r
interhem1upheric electrophys^ -.logical differences reported to date, however,
ure so tiny u: ► to be barely believable. Hence, either the evoked response
method is virtually blind to the cruclul events we believe muBt be there, or
the hemispheric differences tire barely pre:ient in the conditions under which
the measurements are currently being made.
I
._.^
	
_ _—	
_._.._. 	
.^..^.._	
-	
^ - "°•7w n rim
8	
I
SUMMARY
Fright subjects listened to lists of speech sounds (pct or ba) or F.•re
tones (250 or 600 Hz). WittO n each list one of the sounds (the "frequent")
occurred more often than the otter (the "target") in it ratio of approximately
h:l. Subjects were required to count the targets in each list; concurrently,
evoked responses to both target: and frequents were being separately averaged
from. electrodes at vertex and at : ,-yzJmetrical left and right parietal locations.
The evoked re:sporses show the expected sequence of deflections at all three
electrode sites, including large P 3 waves (about 350 nssec latency) to the
target stimuli. However, the left and right hemispheric responses to speech
or tones, either frequent or target, were strikingly similar, both to the eye
and by statistical tests intended to reveal differences between them.
JI4
l.
tHEFDUICES
Friedman, U., Simson, R., Hitter, W., and Rapin, I. Cortical evoked potentials
I
elicited by real speech words and human sounds. Fleet roenceph. clip.
Neuroj) viol. 1975, 3r 13-19.
• Picton, T. W., Hillyard, S. A., Krausz, 11. I., and Galambos, H. human auditory
evoked potentials I: Evaluation of components. F.lectroenceph. clin.
Neurophysiol. 197 1+, 36:179-190.
Picton, T. W. and hillyard, S. A. Human auditory evoked potentials II: Effects
1
of attention. Electroenceph. clip. Neurophysiol. 1974, 3:191-199•
Vaughan, H. G., Jr. "The analysis of scalp-recorded brain potenti+Lls," in
H. F. Thompson and M. M. Patterson (eds.) Bioelectrie Hecording Techniques
(B) 197 11, 158-209. New York: Academic Press.
Wood, C. C., Goff, W. R. and bay, H. S. Auditory evoked potentials during
speech perception. Science. 1971, 133:12);8-1.251.
Wood, C. C. Auditory and phonetic levels of processing in speech perception:
Neurophysiological and informr+tion-processing analyses. J. k^Kp. Psychol:
Human Perception and Performance. 1975, 1011:3-20.
FIGURE LEGENDS
Figure 1. Composite of the average evoked potentials for eight
subjects. N w 704 for the target ntlmuli, N a 3040 for the frequent.
SPEECH
N,
2AXI N
Vertex
P2
P3
^^	 Right '-'
left ^^•	 ^'
5µv
100 msec +
^'' -^'	 ---- Right
— Left


On hemispheric differences in evoked potentials to speech stimuli

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server