E F F E C T S O F S T I M U L A T I O N I N T E N S I T Y , G E N D E R A N D
H A N D E D N E S S U P O N A U D I T O R Y E V O K E D P O T E N T I A L S
SUSANA CAMPOSANO * — FERNANDO LOLAS *
SUMMARY — Left handers and women show less anatomical brain asymmetry, larger corpus
callosum and more bilateral representation of specific functions. Sensory and cognitive
components of cortical auditory evoked potentials (AEF) have been shown to be asymmetric
in right handed males and to be influenced by stimulus intensity. In this study the
influence of sex, handedness and stimulus intensity upon AEP components is investigated
under basal conditions of passive attention. 14 right handed mtales, 14 right handed females,
14 left handed males, and 14 left handed females were studied while lying awake and
paying passive attention to auditory stimulation (series of 100 binaural clicks, duration
1 msec, rate 1/sec, at four intensities). Cz, C3 and C4 referenced to linked mastoids and
right EOG were recorded. Analysis time was 400 msec, average evoked potentials were
based on 100 clicks. Stimulus intensity and gender affect early sensory components (P1N1
and N1P2) at central leads, asymmetry is influenced only by handedness, right handers
showing larger P1N1 amplitudes over the right hemisphere.
KEY WORDS: auditory evoked potential, sex difference, handedness difference.
Efectos de la intensidad de la estimulación, sexo y manualidade sobre el potencial evocado
auditivo.
RESUMEN — Los sujetos zurdos y de sexo femenino tienden a exhibir menores grados de
asimetría anatómica, mayores diámetros del cuerpo calloso y mayor representación bilateral
de funciones específicas. Los componentes sensoriales y cognitivos del potencial evocado
auditivo (PEA) cortical han demostrado sensibilidad a la intensidad de estimulación y
asimetría en sujetos masculinos. En este estudio se investiga la influencia de sexo,
ma-nualidad e intensidad de la estimulación sobre los componentes corticales del PEA. 56
suje-tos voluntarios fueron sometidos a estimulación auditiva (series de 100 clicks binaurales,
duración de 1 mseg, frecuencia de 1/seg, a cuatro diferentes intensidades) instruyéndoseles
a prestar atención pasiva. Se registró derivaciones Cz, C3 y C4 referidas a mastoides
bila-teral y electrooculograma derecho, el tiempo de análisis fue de 400 mseg, los promedios
fueron obtenidos sobre 100 clicks. La intensidad de la estimulación y el sexo afectan los
componentes sesoriales corticales P1N1 y N1P2 en la derivación central. La asimetría sólo
es influenciada por la manualidad; los Ss diestros exhiben mayores amplitudes P1N1 en
el hemisferio derecho.
PALAVRAS LLAVE: potencial evocado auditivo, diferencia sexual, diferencia de
ma-nualidad.
PALAVRAS CHAVE: potencial evocado auditivo, diferença sexual, diferença
manuali-dade.
Behavioral and physiological hemispheric asymmetry of function has been related
to organism variables such as sex and handedness. Left-handers and women tend
to show less anatomical asymmetry
5,8,13.
The corpus callosum is larger in both
groups, in men at the splenium and in left handers at the central region
740,16.
Early
* Psychophysiology Unit, Department of Physiology and Biophysics, Faculty of
Medi-cine, University of Chile. This research is supported by FONDECYT Project 1126/90, and
the University of Chile (D.T.I.).
sensory components df cortical auditory potential ( A E P ) have been shown to be
asymmetric in right handed males and to be influenced by stimulus intensity 4 , n .
It is investigated in this study the influence of sex, handedness and stimulus
intensity upon A E P components and their hemispheric asymmetry in basal conditions.
Symmetrical (binaural) stimulation with low informational content (click) and passive
attention conditions were used in order to avoid selecting a lateralized processing
mode.
M E T H O D S
52 u n p a i d v o l u n t e e r s w i t h o u t n e u r o l o g i c a l o r h e a r i n g i m p a i r m e n t w e r e r e c r u i t e d f r o m
t h e u n i v e r s i t y c o m m u n i t y ( 1 4 r i g h t h a n d e d m e n , 14 l e f t h a n d e d m e n , 14 r i g h t h a n d e d
w o m e n a n d 14 l e f t h a n d e d w o m e n ) . H a n d e d n e s s w a s a s s e s s e d u s i n g s o m e i t e m s of t h e
E d i n b u r g h I n v e n t o r y 12. C o n c o r d a n t m a n u a l p r e f e r e n c e f o r w r i t i n g , e a t i n g w i t h a s p o o n ,
h o l d i n g a h a m m e r to h i t a n a i l , a n d t h r o w i n g a b a l l w a s r e q u e s t e d ( S s . w h o s h o w e d m i x e d
p r e f e r e n c e w e r e n o t i n c l u d e d ) . F o r r i g h t h a n d e r s i t w a s a l s o r e q u e s t e d n o t t o h a v e a
h i s t o r y o f d i s l e x i a o r s t u t t e r i n g a n d n o l e f t h a n d e d r e l a t i v e s .
S u b j e c t s l y i n g i n a n a c o u s t i c a l l y a n d e l e c t r i c a l l y i s o l a t e d r o o m w e r e i n s t r u c t e d t o
s t a y a w a k e a n d l i s t e n t o a u d i t o r y s t i m u l a t i o n . S t i m u l a t i o n c o n s i s t e d of f o u r s e r i e s of 100
b i n a u r a l c l i c k s , 1 m s e c i n d u r a t i o n , r a t e 1 / sec, a t f o u r i n t e n s i t i e s ( 6 3 , 74, 87 a n d 97 d B
S P L ) . I n t e n s i t y s e q u e n c e w a s r a n d o m .
R e c o r d i n g s w e r e m a d e f r o m C z , C 3 a n d C 4 r e f e r e n c e d t o l i n k e d m a s t o i d s w i t h s u r f a c e
s i l v e r - s i l v e r c h l o r i d e e l e c t r o d e s . R i g h t E O G w a s s i m u l t a n e o u s l y r e c o r d e d f r o m i n f r a a n d
s u p r a o r b i t a l e l e c t r o d e s . S i g n a l s w e r e a m p l i f i e d 2 0 , 0 0 0 t i m e s a n d b a n d p a s s f i l t e r e d b e t w e e n
1 a n d 100 H z u s i n g B i o d a t a P A 4 0 0 a m p l i f i e r s . A v e r a g e s w e r e b a s e d o n 1 0 0 c l i c k s a t e a c h
i n t e n s i t y , w i t h o u t a r t i f a c t r e j e c t i o n ; a n a l y s i s t i m e w a s 4 0 0 m s e c s p a n n e d b y 6 4 0 d a t a p o i n t s .
L a t e n c y of P I , N I , P 2 a n d N 2 c o m p o n e n t s a n d p e a k t o p e a k a m p l i u t d e s of P 1 N 1 ,
N 1 P 2 a n d P 2 N 2 c o m p l e x e s w e r e m e a s u r e d u s i n g a c u r s o r p r o g r a m . I f t h e m a x i m a l e x c u r s i o n
a t t h e E O G e x c e e d e d m a x i m a l p e a k t o p e a k a m p l i t u d e a t C z , a v e r a g e s w e r e r e j e c t e d . C o m
-p o n e n t s w e r e i d e n t i f i e d a c c o r d i n g t o m a x i m a l d e f l e c t i o n in t h e f o l l o w i n g t i m e w i n d o w s :
P I , 4 0 - 8 0 m s ; N l , 6 0 - 1 2 0 m s ; P 2 , 1 0 0 - 1 9 0 m s ; N 2 , 1 5 0 - 3 0 0 m s .
A n a s y m m e t r y c o e f f i c i e n t f o r t h e p r o p o r t i o n a l a c t i v a t i o n of t h e l e f t h e m i s p h e r e ( L e f t
P r o p o r t i o n a l A m p l i t u d e C o e f f i c i e n t , L P A C ) w a s o b t a i n e d a s f o l l o w s : L P A O = A m p l i t u d e at
C 3 / A m p l . a t C 3 + A m p l . a t C 4 .
A m p l i t u d e a n d l a t e n c y a v e r a g e s a r e s h o w n i n T a b l e s 1 a n d 2 . A n a l y s i s of v a r i a n c e
( A N O V A , B a l a n c e d ) w a s p e r f o r m e d f o r C z l a t e n c i e s a n d a m p l i t u d e s a n d f o r L P A C , f a c t o r s
b e i n g s e x , h a n d e d n e s s a n d i n t e n s i t y . C 3 a n d C 4 a m p l i t u d e a n d l a t e n c y d i f f e r e n c e s w e r e
a n a l y z e d u s i n g n o n p a r a m e t r i c p r o c e d u r e s ( W i l c o x o n S i g n R a n k T e s t ) .
R E S U L T S
A t e n d e n c y t o l a r g e r a m p l i t u d e s in w o m e n a n d r i g h t h a n d e r s c a n b e a p p r e c i a t e d in
F i g u r e 1. A m p l i t u d e a n d l a t e n c y a v e r a g e s a n d s t a n d a r d d e v i a t i o n s a r e s h o w n i n T a b l e s
1 a n d 2.
A n a l y s i s o f v a r i a n c e f o r c e n t r a l l e a d P 1 N 1 , N 1 P 2 a n d P 2 N 2 p e a k - t o - p e a k a m p l i t u d e s
s h o w s d i f f e r e n t i n f l u e n c e of t h e v a r i a b l e s s t u d i e d o n t h e c o m p o n e n t s ( T a b l e 3 ) . L a t e n c y
w a s l e s s i n f l u e n c e d b y t h e s e f a c t o r s ( T a b l e 4 ) .
R i g h t - l e f t d i f f e r e n c e s i n a m p l i t u d e w e r e f o u n d f o r PINT, i n r i g h t h a n d e r s , W i l c o x o n :
C 4 > C 3 in r i g h t h a n d e d m a l e s a t 7 4 a n d 8 7 d B ( p < 0 . 0 5 ) ; C 4 > C 3 i n r i g h t h a n d e d f e m a l e s
a t 6 3 d B ( p < 0 . 0 2 ) . P 1 N 1 a m p l i t u d e a t C 3 a n d C 4 i s s h o w n i n T a b l e 5 .
A m p l i t u d e a s y m m e t r y a s m e a s u r e d b y L P A C i s o n l y i n f l u e n c e d b y h a n d e d n e s s ( T a b l e
6 ) . V a l u e s f o r L P A C of r i g h t a n d l e f t h a n d e r s a r e s h o w n i n T a b l e 4 . D i f f e r e n c e s c a n b e
C O M M E N T S
Stimulus intensity and gender influence early sensory components of A E P at
central leads but exert no influence on hemispheric asymmetry of electrocortical
signals. Asymmetry is influenced only by handedness, with right handers showing
larger amplitudes over the right hemisphere.
Amplitude at central lead is influenced by the three factors studied: P1N1 by
intensity; N 1 P 2 by intensity, gender and handedness; and P 2 N 2 only by handedness.
The influence of intensity on early cortical components is well known, therefore
they are called «stimulus d e p e n d e n t s
1 4. Sexual differences are also known, women
tend to have larger amplitudes 3 .
T h e influence of handedness on central lead amplitude is a new finding and
could be explained if left handers had more bilateral activation of supratemporal
regions. Only right handed subjects show greater C 4 amplitudes, in left handers no
significant asymmetry could be demonstrated. Amplitude depends on previous
acti-vation of generator sites, the greater the previous actiacti-vation, the lower is the evoked
amplitude ! 5 . This view would be concordant with greater left side activation in
right handers (thus showing lower amplitudes at C 3 ) and with bilateral activation
in left handers (thus showing less asymmetry and lower amplitudes at C z ) . T h i s
bilateral activation can be related to the thicker central part of corpus callosum,
which includes auditory cortex callosal fibers 1. Anova shows asymmetry as measured
by L P A C of P 1 N 1 and P 2 N 2 to be influenced only by handedness.
R E F E R E N C E S
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