Electroencephalography and Clinical Neurophysiology, 1 9 7 7 , 4 2 : 1 2 5 - - 1 2 7
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© E l s e v i e r / N o r t h - H o l l a n d Scientific P u b l i s h e r s L t d
Laboratory note LATENCY OF THE STEADY STATE VISUAL EVOKED POTENTIAL A. L E O N A R D D I A M O N D *
Department of Psychology, Simon Fraser University, Burnaby, B.C. V5A 1S6 (Canada) ( A c c e p t e d for p u b l i c a t i o n : April 12, 1 9 7 6 )
One m e a s u r e o f t h e h u m a n b r a i n r e s p o n s e t o s e n s o r y stimuli has b e e n called t h e s t e a d y state e v o k e d p o t e n t i a l (EP) (see R e g a n 1 9 7 2 , p. 74). This involves t h e averaging o f t h e c e r e b r a l p o t e n t i a l s t h a t result f r o m t h e successive pulses o f a s t i m u l u s rep e a t i n g at f r e q u e n c i e s usually faster t h a n o n c e p e r sec. T h e p r e s e n t e x p e r i m e n t is c o n c e r n e d w i t h this s t e a d y s t a t e m e a s u r e m e n t a n d specifically w i t h a n e w m e t h o d for d e t e r m i n i n g t h e l a t e n c y of the EP to a r e p e a t i n g visual flash. With r e s p e c t to l a t e n c y d e t e r m i n a t i o n f r o m a s t e a d y s t a t e record, R e g a n ( 1 9 7 2 , pp. 7 5 - - 7 7 ) does n o t favor such a " t i m e d o m a i n " analysis o f the response. He argues t h a t at " s u f f i c i e n t l y h i g h r e p e t i t i o n f r e q u e n c i e s , n o individual r e s p o n s e cycle can be assoc i a t e d w i t h a p a r t i c u l a r s t i m u l u s cycle. W h e n this ' s t e a d y - s t a t e ' regime is e s t a b l i s h e d it is n o longer a p p r o p r i a t e t o d e s c r i b e t h e r e s p o n s e in t e r m s o f time, t h a t is in t e r m s o f t h e l a t e n c i e s o f d i f f e r e n t c o m p o n e n t s " . A l o n g w i t h V a n der Tweel a n d V e r d u y n L u n e l ( 1 9 6 5 ) a n d Spekreijse ( 1 9 6 6 ) , h e suggests a calc u l a t i o n o f " a p p a r e n t l a t e n c y " w h i c h is e q u a l t o actual l a t e n c y w h e n t h e a s s u m p t i o n is m e t t h a t " t h e s y s t e m is a linear, m i n i m u m p h a s e - s h i f t s y s t e m plus a fixed d e l a y " . He does n o t e , h o w e v e r , t h e " m a r k e d l y n o n l i n e a r b e h a v i o u r " o f s t e a d y s t a t e EPs. O n e m i g h t conclude, t h e r e f o r e , t h a t this n o n - l i n e a r i t y is o n e o f t h e reasons h e does n o t favor a t i m e d o m a i n , or l a t e n c y , analysis o f t h e s t e a d y s t a t e EP. T h e l a t e n c y c a l c u l a t i o n suggested in t h e p r e s e n t paper, o n t h e o t h e r h a n d , does n o t require t h e ass u m p t i o n o u t l i n e d b y R e g a n and, t h e r e f o r e , c a n validly p r o v i d e s u c h a l a t e n c y analysis. This e x p e r i m e n t m e a s u r e s t h e EPs to a t r a i n o f biref, square-wave light pulses a n d f r o m t h e r e c o r d e d p o t e n t i a l s calculates t h e l a t e n c y o f t h e EP to a n y o n e o f t h e light pulses.
Method
Procedure T h e a p p a r a t u s a n d p r o c e d u r e were d e s i g n e d to m e a s u r e t h e EP to a flickering circular t e s t field, 7.5 ° in visual angle. Each flash, p r o d u c e d b y a s t r o b o s c o p e b e h i n d a diffusing screen, was a 50 psec s q u a r e wave in d u r a t i o n . T h e average c a l i b r a t e d l u m i n a n c e of t h e s e flashes, flickering at intervals of 20 msec, or 50 f/sec ( n e a r t h e fusion f r e q u e n c y for cones), was 0.60 log m i l l i l a m b e r t s . A s t i m u l a t i o n - - a d a p t a t i o n cycle o b t a i n e d t h r o u g h o u t t h e e x p e r i m e n t . This cycle i n v o l v e d 22 sec of flickering light s t i m u l a t i o n , 90 sec o f d a r k - a d a p t a t i o n to a s t e a d y , 12.1 ° circular field (---0.44 log ml in l u m i n a n c e ) , 22 sec s t i m u l a t i o n , etc. T h e 90 sec darka d a p t a t i o n t i m e f o l l o w i n g each 22 sec flash stimulat i o n was s u f f i c i e n t t o e n s u r e t h a t each m e a s u r e m e n t was n o t a f f e c t e d b y a p r e c e d i n g m e a s u r e m e n t . F o r t h e 22 sec s t i m u l a t i o n time, t h e ISI (inters t i m u l u s interval) was set at o n e o f f o u r values in e i t h e r a low range ( 3 0 - - 4 5 m s e c ) or a h i g h range ( 9 0 - - 1 0 5 msec). A t each ISI value, a t o t a l o f 128 EPs (in t w o sets of 64 each} was averaged b y a F a b r i t e k 1 0 7 0 averager over a s w e e p t i m e o f 200 msec. R e c o r d i n g s were t a k e n f r o m an active e l e c t r o d e 2 c m a b o v e t h e i n i o n along t h e m i d l i n e scalp, a reference e l e c t r o d e o n t h e right e a r l o b e a n d a g r o u n d elect r o d e o n t h e left earlobe. T h e s u b j e c t viewed t h e c e n t e r s of the test or a d a p t a t i o n fields b i n o c u l a r l y a n d n o artificial pupils were u s e d : V a n d e r Tweel a n d V e r d u y n L u n e l ( 1 9 6 5 ) f o u n d m i n i m a l EP d i f f e r e n c e b e t w e e n artificial a n d n a t u r a l pupils at m e d i u m l u m i n a n c e s for t h e s t e a d y s t a t e EP t o light flashes.
Results
* This e x p e r i m e n t was s u p p o r t e d in p a r t b y N R C Grant A9940.
T h e d a t a for o n e s u b j e c t , AD, are p l o t t e d in Fig. 1 a n d 2. T w o o t h e r s u b j e c t s s h o w similar results. As c a n be seen in Fig. 1, a a n d 2, a, for m o s t o f the ISI
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(a) MIDLINE OCCIPITAL RESPONSE
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© E l s e v i e r / N o r t h - H o l l a n d Scientific P u b l i s h e r s L t d
Laboratory note LATENCY OF THE STEADY STATE VISUAL EVOKED POTENTIAL A. L E O N A R D D I A M O N D *
Department of Psychology, Simon Fraser University, Burnaby, B.C. V5A 1S6 (Canada) ( A c c e p t e d for p u b l i c a t i o n : April 12, 1 9 7 6 )
One m e a s u r e o f t h e h u m a n b r a i n r e s p o n s e t o s e n s o r y stimuli has b e e n called t h e s t e a d y state e v o k e d p o t e n t i a l (EP) (see R e g a n 1 9 7 2 , p. 74). This involves t h e averaging o f t h e c e r e b r a l p o t e n t i a l s t h a t result f r o m t h e successive pulses o f a s t i m u l u s rep e a t i n g at f r e q u e n c i e s usually faster t h a n o n c e p e r sec. T h e p r e s e n t e x p e r i m e n t is c o n c e r n e d w i t h this s t e a d y s t a t e m e a s u r e m e n t a n d specifically w i t h a n e w m e t h o d for d e t e r m i n i n g t h e l a t e n c y of the EP to a r e p e a t i n g visual flash. With r e s p e c t to l a t e n c y d e t e r m i n a t i o n f r o m a s t e a d y s t a t e record, R e g a n ( 1 9 7 2 , pp. 7 5 - - 7 7 ) does n o t favor such a " t i m e d o m a i n " analysis o f the response. He argues t h a t at " s u f f i c i e n t l y h i g h r e p e t i t i o n f r e q u e n c i e s , n o individual r e s p o n s e cycle can be assoc i a t e d w i t h a p a r t i c u l a r s t i m u l u s cycle. W h e n this ' s t e a d y - s t a t e ' regime is e s t a b l i s h e d it is n o longer a p p r o p r i a t e t o d e s c r i b e t h e r e s p o n s e in t e r m s o f time, t h a t is in t e r m s o f t h e l a t e n c i e s o f d i f f e r e n t c o m p o n e n t s " . A l o n g w i t h V a n der Tweel a n d V e r d u y n L u n e l ( 1 9 6 5 ) a n d Spekreijse ( 1 9 6 6 ) , h e suggests a calc u l a t i o n o f " a p p a r e n t l a t e n c y " w h i c h is e q u a l t o actual l a t e n c y w h e n t h e a s s u m p t i o n is m e t t h a t " t h e s y s t e m is a linear, m i n i m u m p h a s e - s h i f t s y s t e m plus a fixed d e l a y " . He does n o t e , h o w e v e r , t h e " m a r k e d l y n o n l i n e a r b e h a v i o u r " o f s t e a d y s t a t e EPs. O n e m i g h t conclude, t h e r e f o r e , t h a t this n o n - l i n e a r i t y is o n e o f t h e reasons h e does n o t favor a t i m e d o m a i n , or l a t e n c y , analysis o f t h e s t e a d y s t a t e EP. T h e l a t e n c y c a l c u l a t i o n suggested in t h e p r e s e n t paper, o n t h e o t h e r h a n d , does n o t require t h e ass u m p t i o n o u t l i n e d b y R e g a n and, t h e r e f o r e , c a n validly p r o v i d e s u c h a l a t e n c y analysis. This e x p e r i m e n t m e a s u r e s t h e EPs to a t r a i n o f biref, square-wave light pulses a n d f r o m t h e r e c o r d e d p o t e n t i a l s calculates t h e l a t e n c y o f t h e EP to a n y o n e o f t h e light pulses.
Method
Procedure T h e a p p a r a t u s a n d p r o c e d u r e were d e s i g n e d to m e a s u r e t h e EP to a flickering circular t e s t field, 7.5 ° in visual angle. Each flash, p r o d u c e d b y a s t r o b o s c o p e b e h i n d a diffusing screen, was a 50 psec s q u a r e wave in d u r a t i o n . T h e average c a l i b r a t e d l u m i n a n c e of t h e s e flashes, flickering at intervals of 20 msec, or 50 f/sec ( n e a r t h e fusion f r e q u e n c y for cones), was 0.60 log m i l l i l a m b e r t s . A s t i m u l a t i o n - - a d a p t a t i o n cycle o b t a i n e d t h r o u g h o u t t h e e x p e r i m e n t . This cycle i n v o l v e d 22 sec of flickering light s t i m u l a t i o n , 90 sec o f d a r k - a d a p t a t i o n to a s t e a d y , 12.1 ° circular field (---0.44 log ml in l u m i n a n c e ) , 22 sec s t i m u l a t i o n , etc. T h e 90 sec darka d a p t a t i o n t i m e f o l l o w i n g each 22 sec flash stimulat i o n was s u f f i c i e n t t o e n s u r e t h a t each m e a s u r e m e n t was n o t a f f e c t e d b y a p r e c e d i n g m e a s u r e m e n t . F o r t h e 22 sec s t i m u l a t i o n time, t h e ISI (inters t i m u l u s interval) was set at o n e o f f o u r values in e i t h e r a low range ( 3 0 - - 4 5 m s e c ) or a h i g h range ( 9 0 - - 1 0 5 msec). A t each ISI value, a t o t a l o f 128 EPs (in t w o sets of 64 each} was averaged b y a F a b r i t e k 1 0 7 0 averager over a s w e e p t i m e o f 200 msec. R e c o r d i n g s were t a k e n f r o m an active e l e c t r o d e 2 c m a b o v e t h e i n i o n along t h e m i d l i n e scalp, a reference e l e c t r o d e o n t h e right e a r l o b e a n d a g r o u n d elect r o d e o n t h e left earlobe. T h e s u b j e c t viewed t h e c e n t e r s of the test or a d a p t a t i o n fields b i n o c u l a r l y a n d n o artificial pupils were u s e d : V a n d e r Tweel a n d V e r d u y n L u n e l ( 1 9 6 5 ) f o u n d m i n i m a l EP d i f f e r e n c e b e t w e e n artificial a n d n a t u r a l pupils at m e d i u m l u m i n a n c e s for t h e s t e a d y s t a t e EP t o light flashes.
Results
* This e x p e r i m e n t was s u p p o r t e d in p a r t b y N R C Grant A9940.
T h e d a t a for o n e s u b j e c t , AD, are p l o t t e d in Fig. 1 a n d 2. T w o o t h e r s u b j e c t s s h o w similar results. As c a n be seen in Fig. 1, a a n d 2, a, for m o s t o f the ISI
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