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Some preliminary electrophysiological studies on chronic neuronally isolated cerebral cortex
SOME PRELIMINARY ELECTROPHYSIOLOGICAL STUDIES ON N E U R O N A L L Y I S O L A T E D CEREBRAL C O R T E X
CHRONIC
B. GRAFSTEIN, Ph.D. and P. BRAHMAYYA SASTRY,1 Ph.D.
Department o[ Physiology, McGill University, Montreal, Canada (Received for publication: June 8, 1957) It is possible to sever completely the neuronal connections from a given region of cerebral cortex to the adjacent grey matter and underlying white matter, without destroying its pial blood supply. In such an isolated are't of cortex shortly after isolation, spontaneous electrical activity may sometimes be observed (Kristiansen and Courtois 1949), and activity may be elicited by direct electrical stimulation (Burns 1950, 1:)51). Even several months after isolation, the isolated tissue may still be viable and electrical activity may be recorded from it (Echlin, Arnett and Zoll ]952). The preseut paper is a report of a brief surv~,y which has been made to compare the electrical :wtivit5 of acute and chronic isolated cortex. As the basis for this comparison, four types of ~.ortical responses to direct electrical stimulation have bee~ examined. These responses, which may be elicited from the unanaesthetized cortex, have been defined . s follows: (1) A local surface-negative response, lasting about 20 msec., is produced when u single we,k stimulus is applied to the surface of the cortex (Burns 1950). I f the s t i m u h s strength is increased this response is followed by (2) a burst of activity which is synaptically transmitted throughout the isolated region (Burns 1951). This burst response lasts 0.5-2 see., and during it the surface of the active tissue becomes positive with respect to an indifferent cortical point. (3) With repetitive stimulation at a rate of 5-10 per sec. for 5-10 sec. an afterdischarge may be produced, lasting for some seconds after stimulation has ceased. This activity characteristically consists of a train of brief~ high voltage discharges, which spread out from the stimulated point and may involve large areas of cortex. Activity of this kind in the motor cortex has been seen to be accompanied by epileptiform clonic limb movements (Adrian 1936), and this response is therefore referred to as an epileptiform afterdiseharge. (4) Repetitive stimulation stronger than that required to produce the ~pilepliform activity may give rise to spreading cortical depression (Lego 1944). This response is readily detectable even in those preparations of isolated cortex which are not spontaneously active, since it is accompanied by a slow negative shift of the potential of the cortical surface (Grafstein 1956). 1 Research :Fellow from India under Colombo Plan. Present address : Andhra Medical College, Yisakhapatnam. India.
These 4 types of activity were examined in cortex which hail been isolated for 2 to 8 weeks, and in acutely isolated cortex. METHODS Ten cats were used for the chronic preparations. Under Nembutal anaesthesia and with aseptic precautions, an area of cortex about 0.5 cm. X 2 cm. was isolated in the suprasylvian gyrus, in the manner described by Burns (1951). A f t e r 2 to 8 weeks the cat was decerebratcd under ether anaesthesia, then taken off anaesthetic. The chronic isolated cortical slab was exposed, and a corresponding acute slab was usually prepared in the suprasylvian gyrus on the other side. When the electrical activity was to be examined the cortex was immersed in a pool of paraffin oil. 5Ionopolar recording was usually used, with the reference electrode on an area of dead cortex at one end of the isolated region. Recording and stimulating arrangements have been described elsewhere (Burns and Grafstein 1952; Burns 1954). RESULTS A table summarizing the results obtained is given below. Very little difference was observed in the responses obtained from cortex which had been isolated for 2 weeks, 4 weeks, or 8 weeks, so the results from all the chronic preparations have been included together.
(a) Spontaneous activity. I n 9 out of 10 cats the chronically isolated cortex exhibited spontaneous activity, which was in some cases continuous, but in most cases consisted of intermittent bursts s i m i h r in character to the burst responses produced by direct stimulation. I n the acute preparations spontaneous activity was not usually observed, but Burns (1954) has shown that even silent isolated cortex may become spontaneously active a f t e r only a few stimuli. For this reason it is very difficult to decide whether this difference between the acute and chronic preparations represents a significant alteration in the properties of the cortex. (b) Surface-negative response. Although the threshold stimulus for elieiting this response was considerably higher in the chronic preparations, the distance over which the response could be recorded
[ 723 ]
724
B. G R A F S T E I N
a n d P. B R A I t M A Y Y A
SASTRY
very m u c h higher. I n 5 o u t of 8 chronic p r e p a r a t i o n s t e s t e d depression w a s n o t p r o d u c e d by repetitive slim u l a t i o n in t h e r a n g e of s t r e n g t h s tested (see footnote to table I ) , a n d s u r f a c e - n e g a t i v e p o l a r i z a t i o n with c u r r e n t s of u p to 2000 gA. w a s also i n e f f e c t i v e . E v e n in t h e s e cases, however, as in the acute p r e p a r a t i o n s , depression could be i n i t i a t e d b y m e c h a n i c a l s t i m u l a t i o n or by t h e topical a p p l i c a t i o n of 1 p e r cent KCI.
was n o t s i g n i f i c a n t l y d i f f e r e n t in tile two k i n d s of preparations. (c) Burst response. T h e t h r e s h o l d f o r t h i s response did n o t c h a n g e as a r e s u l t of t h e chronic isolation, b u t the a m p l i t u d e of i t s s u r f a c e - p o s i t i v e " D . C . " c o m p o n e n t w a s a p p a r e n t l y reduced to some extent, t h e d i f f e r e n c e b e t w e e n the responses in t h e a c u t e a n d chronic p r e p a r a t i o n s b e i n g s t a t i s t i c a l l y s i g n i f i c a n t a t a 5 p e r cent level of confidence. I n two chronic p r e p a r a t i o n s it w a s observed t h a t in each b u r s t a n initial b r i e f s u r f a c e - n e g a t i v e p h a s e preceded the onset of the surface-positive potential.
COMMENT A f t e r 2 to 8 weeks the n e u r o n a l l y isolated cerebral cortex showed relatively little alter:ltion in its electro-
TABLE
I
Acute
Chronic
P
T h r e s h o l d for s u r f a c e - n e g a t i v e response with 1.2 msee. s t i m u l u s (V.).
1.5 ___ 0.4 (5)
2.9 _+ o.l (7)
D i s t a n c e over which s u r f a c e - n e g a t i v e response m a y be recorded (ram.).
9.8 -4- 0.4 (2)
7.8 ~
0.6 (5)
~.:~
T h r e s h o l d for b u r s t response with 1.2 msee. s t i n m l u s (V.).
2.7 _
3.0 ~
0.5 (7)
t~..~
A m p l i t u d e of s u r f a c e - p o s i t i v e c o m p o n e n t of b u r s t ( g V . ) .
193
Tllreshold for e p i l e p t i f o r m a f t e r d i s c h a r g e (40 stimuli, 8/see. 1.2 reset.) (V.). l ) u r a t i o n of e p i ] e p t i f o r m a f t e r d i s e h a r g e at t h r e s h o l d s t i m u l u s (see.). T h r e s h o l d for s p r e a d i n g depression (stimuli 1.2 msee. at 10/see. for 10 see.) (V.).
0.6 (7)
___ 80
(4"1
11.0 -4- 4.9 (5)
7.4 ~
6.6 ( 7 )
11.3 -4- 3.8 (6)
95
___ 29
6.l ~
(;4
< (~.(,(*1
(5)
q).o5
l.tl (7/
(*.o5
-4- 3 5
> 17.(!
(I;~
,~.2
(,.02
(~.olll
'~ (8)
A m p l i t u d e of slow n e g a t i v e p o t e n t i a l d u r i n g depression (inV.).
8.0 ___ 3.2 (5/
2.5 -4- 1.2 (7)
o.(,2
I~.Ol
R a t e of s p r e a d of depression ( m l n . / m i n . ) .
4.5 ~
3.6 -4- 1.2 (8~
~4
0.3
~ I n 5 o u t of 8 e a t s d e p r e s s i o n
15 and 25 V.
was ilot produced
11.5 (3)
with
tile s t r o n g e s t
stimuli
tested,
which
/'allged be|weetl
C o m p a r i s o n of solne p r o p e r t i e s of acute a n d chronic neuronally isolated rerebrai rortex. In each case the m e a n a n d its s t a n d a r d deviation are given, the nunll)er (If ],rcl)',r:~ti
(d) Epileptiform aft~,rdiseharge. A lowered t h r e s h o l d ~md considerably p r o l o n g e d d u r a t i o n were ch'm~eteristic of this response in the chronic prepar.~tions. (c) Spreadi~g depression. W h e n depression was i n i t i a t e d in the chronic isolated cortex, its rate of s p r e a d was c o m p a r a b l e to t h a t in t h e acute p r e p a r a tions. However, the a m p l i t u d e of the a c c o m p a n y i n g slgw p o t e n t i a l c h a n g e was smaller, a n d the t h r e s h o l d for depression, tested with repetitive s t i m u l a t i o n , was
physiological properties. All of the responses which were p r o d u c e d by electrical s t i n m l a t i e n ill the acute p r e p a r a t i o n could also be elieitt.d in the chronic one, ~flthough t h e r e were some ehantzcs in the t h r e s h o l d s a n d a m p l i t u d e s of the v a r i o u s resplmses. P o s s i b l y the m o s t i n t e r e s t i n g f e a t u r e of the eleetrieal behaviour of the chronic isolqted cortex was the h m g d u r a t i o n of the e p i l e p t i f o r m a f t e r d i s e l m r g e which eimhl be produced in it. I n cortex which h a s been isolated for longer periods t h a n those in this study, this after~
CHRONIC ISOLATED d i s c h a r g e h a s been f o u n d to be even more prolonged. :For example, in cortex isolated for 6 m o n t h s , a n a f t e r d i s c h a r g e could 1)e i n i t i a t e d which lasted f o r several hours (Barns and Sastry, unpublished results). I t is n o t possible, on the b a s i s of this b r i e f survey, 1o m a k e a n y s p e c u l a t i o n s on the n a t u r e of the s t r u c t u r a l c h a n g e s u n d e r l y i n g the observed c h a n g e s in the electrophysiological p r o p e r t i e s of the isolated cortex. We hope, however, t h a t these p r e l i m i n a r y ol)servations will be of use ill the f u r t h e r exldoration of this problem. Our t h a n k s are due to P r o f e s s o r B. D. B u r n s for his valuable advice, to P r o f e s s o r F. C. 5 I a c I n t o s h f o r his i n t e r e s t in this s t u d y a n d to Mr. Carl Holizek for his a s s i s t a n c e ill the aseptic operations. One of us ( S a s t r y ) wishes to e x p r e s s his t h a n k s to t h e Technical Co-operation Service of tile C a n a d i a n Governm e n t for t h e a w a r d of the fellowship u n d e r tile Colombo P l a n . T h i s research was s u p p o r t e d 1)y a g r a n t to P r o f e s s o r B u r n s f r o m the N a t i o n a l R e s e a r c h Council ,if ('an'td'L
725
CORTEX REFERENCES
ADRIAN, E. D. The s p r e a d of a c t i v i t y in t h e cerebral cortex. J. Physiol., 1936, 88: 127-161. BURNS, B. D. Some p r o p e r t i e s of the c a t ' s isolated cerebral cortex. J. Physiol., 1950, 111: 50-68. BURNS, B. D. Some properties of isolated cerebral cortex in the u n a n a e s t h e t i z c d cat. J. Physiol.. 1951, i12: 156-175. B v R x s , B. D. The p r o d u c t i o n of a f t e r - b u r s t s in isolated u n a n a e s t h e t i z e d cerebral cortex. J. Physiol., 1954, 125: 427-446. BURNS, B. D. a n d GRAFSTEIN, B. Tile f u n c t i o n a n d s t r u c t u r e of solne neurolles ill the c a t ' s cerebl*al cortex. J. Physiol., 1952, 113:412 433. KRISTIANSEN, S . a n d COURTOIS~ G. R h y t h m i c electrical activity f r o m isolated cerebral cortex. EEG Clin. Yeurophysiol., 1949, 1: 265-272. ECHLIN, F. A., ARNETT, ~r. a n d ZOLL, J. P a r o x y s m a l high voltage d i s c h a r g e s f r o m isolated a n d partially isolated h u n m n a n d a n i m a l cerebral cortex. EEG Clin. Ncurophysiol., 1952, ~l: 147-164. GRAFSTEIN, B. M e c h a n i s m of s p r e a d i n g cortical depression. J. Neurophysiol., 1956, 19: 154-171. LEX0, A. A. P. S p r e a d i n g depression of a c t i v i t y in the cerebral cortex. J. Ncurophysiol., 1944, 7: 359-390.
Refere~we: GRAFSTEIN, B. and SASTRY, P. B. Some preliminary eleetrophysiological studies on chronic neuronalty isolated cerebral cortex. EEG Clin. Neurophysiol., 1957, 9: 723-725.
3
CHRONIC
B. GRAFSTEIN, Ph.D. and P. BRAHMAYYA SASTRY,1 Ph.D.
Department o[ Physiology, McGill University, Montreal, Canada (Received for publication: June 8, 1957) It is possible to sever completely the neuronal connections from a given region of cerebral cortex to the adjacent grey matter and underlying white matter, without destroying its pial blood supply. In such an isolated are't of cortex shortly after isolation, spontaneous electrical activity may sometimes be observed (Kristiansen and Courtois 1949), and activity may be elicited by direct electrical stimulation (Burns 1950, 1:)51). Even several months after isolation, the isolated tissue may still be viable and electrical activity may be recorded from it (Echlin, Arnett and Zoll ]952). The preseut paper is a report of a brief surv~,y which has been made to compare the electrical :wtivit5 of acute and chronic isolated cortex. As the basis for this comparison, four types of ~.ortical responses to direct electrical stimulation have bee~ examined. These responses, which may be elicited from the unanaesthetized cortex, have been defined . s follows: (1) A local surface-negative response, lasting about 20 msec., is produced when u single we,k stimulus is applied to the surface of the cortex (Burns 1950). I f the s t i m u h s strength is increased this response is followed by (2) a burst of activity which is synaptically transmitted throughout the isolated region (Burns 1951). This burst response lasts 0.5-2 see., and during it the surface of the active tissue becomes positive with respect to an indifferent cortical point. (3) With repetitive stimulation at a rate of 5-10 per sec. for 5-10 sec. an afterdischarge may be produced, lasting for some seconds after stimulation has ceased. This activity characteristically consists of a train of brief~ high voltage discharges, which spread out from the stimulated point and may involve large areas of cortex. Activity of this kind in the motor cortex has been seen to be accompanied by epileptiform clonic limb movements (Adrian 1936), and this response is therefore referred to as an epileptiform afterdiseharge. (4) Repetitive stimulation stronger than that required to produce the ~pilepliform activity may give rise to spreading cortical depression (Lego 1944). This response is readily detectable even in those preparations of isolated cortex which are not spontaneously active, since it is accompanied by a slow negative shift of the potential of the cortical surface (Grafstein 1956). 1 Research :Fellow from India under Colombo Plan. Present address : Andhra Medical College, Yisakhapatnam. India.
These 4 types of activity were examined in cortex which hail been isolated for 2 to 8 weeks, and in acutely isolated cortex. METHODS Ten cats were used for the chronic preparations. Under Nembutal anaesthesia and with aseptic precautions, an area of cortex about 0.5 cm. X 2 cm. was isolated in the suprasylvian gyrus, in the manner described by Burns (1951). A f t e r 2 to 8 weeks the cat was decerebratcd under ether anaesthesia, then taken off anaesthetic. The chronic isolated cortical slab was exposed, and a corresponding acute slab was usually prepared in the suprasylvian gyrus on the other side. When the electrical activity was to be examined the cortex was immersed in a pool of paraffin oil. 5Ionopolar recording was usually used, with the reference electrode on an area of dead cortex at one end of the isolated region. Recording and stimulating arrangements have been described elsewhere (Burns and Grafstein 1952; Burns 1954). RESULTS A table summarizing the results obtained is given below. Very little difference was observed in the responses obtained from cortex which had been isolated for 2 weeks, 4 weeks, or 8 weeks, so the results from all the chronic preparations have been included together.
(a) Spontaneous activity. I n 9 out of 10 cats the chronically isolated cortex exhibited spontaneous activity, which was in some cases continuous, but in most cases consisted of intermittent bursts s i m i h r in character to the burst responses produced by direct stimulation. I n the acute preparations spontaneous activity was not usually observed, but Burns (1954) has shown that even silent isolated cortex may become spontaneously active a f t e r only a few stimuli. For this reason it is very difficult to decide whether this difference between the acute and chronic preparations represents a significant alteration in the properties of the cortex. (b) Surface-negative response. Although the threshold stimulus for elieiting this response was considerably higher in the chronic preparations, the distance over which the response could be recorded
[ 723 ]
724
B. G R A F S T E I N
a n d P. B R A I t M A Y Y A
SASTRY
very m u c h higher. I n 5 o u t of 8 chronic p r e p a r a t i o n s t e s t e d depression w a s n o t p r o d u c e d by repetitive slim u l a t i o n in t h e r a n g e of s t r e n g t h s tested (see footnote to table I ) , a n d s u r f a c e - n e g a t i v e p o l a r i z a t i o n with c u r r e n t s of u p to 2000 gA. w a s also i n e f f e c t i v e . E v e n in t h e s e cases, however, as in the acute p r e p a r a t i o n s , depression could be i n i t i a t e d b y m e c h a n i c a l s t i m u l a t i o n or by t h e topical a p p l i c a t i o n of 1 p e r cent KCI.
was n o t s i g n i f i c a n t l y d i f f e r e n t in tile two k i n d s of preparations. (c) Burst response. T h e t h r e s h o l d f o r t h i s response did n o t c h a n g e as a r e s u l t of t h e chronic isolation, b u t the a m p l i t u d e of i t s s u r f a c e - p o s i t i v e " D . C . " c o m p o n e n t w a s a p p a r e n t l y reduced to some extent, t h e d i f f e r e n c e b e t w e e n the responses in t h e a c u t e a n d chronic p r e p a r a t i o n s b e i n g s t a t i s t i c a l l y s i g n i f i c a n t a t a 5 p e r cent level of confidence. I n two chronic p r e p a r a t i o n s it w a s observed t h a t in each b u r s t a n initial b r i e f s u r f a c e - n e g a t i v e p h a s e preceded the onset of the surface-positive potential.
COMMENT A f t e r 2 to 8 weeks the n e u r o n a l l y isolated cerebral cortex showed relatively little alter:ltion in its electro-
TABLE
I
Acute
Chronic
P
T h r e s h o l d for s u r f a c e - n e g a t i v e response with 1.2 msee. s t i m u l u s (V.).
1.5 ___ 0.4 (5)
2.9 _+ o.l (7)
D i s t a n c e over which s u r f a c e - n e g a t i v e response m a y be recorded (ram.).
9.8 -4- 0.4 (2)
7.8 ~
0.6 (5)
~.:~
T h r e s h o l d for b u r s t response with 1.2 msee. s t i n m l u s (V.).
2.7 _
3.0 ~
0.5 (7)
t~..~
A m p l i t u d e of s u r f a c e - p o s i t i v e c o m p o n e n t of b u r s t ( g V . ) .
193
Tllreshold for e p i l e p t i f o r m a f t e r d i s c h a r g e (40 stimuli, 8/see. 1.2 reset.) (V.). l ) u r a t i o n of e p i ] e p t i f o r m a f t e r d i s e h a r g e at t h r e s h o l d s t i m u l u s (see.). T h r e s h o l d for s p r e a d i n g depression (stimuli 1.2 msee. at 10/see. for 10 see.) (V.).
0.6 (7)
___ 80
(4"1
11.0 -4- 4.9 (5)
7.4 ~
6.6 ( 7 )
11.3 -4- 3.8 (6)
95
___ 29
6.l ~
(;4
< (~.(,(*1
(5)
q).o5
l.tl (7/
(*.o5
-4- 3 5
> 17.(!
(I;~
,~.2
(,.02
(~.olll
'~ (8)
A m p l i t u d e of slow n e g a t i v e p o t e n t i a l d u r i n g depression (inV.).
8.0 ___ 3.2 (5/
2.5 -4- 1.2 (7)
o.(,2
I~.Ol
R a t e of s p r e a d of depression ( m l n . / m i n . ) .
4.5 ~
3.6 -4- 1.2 (8~
~4
0.3
~ I n 5 o u t of 8 e a t s d e p r e s s i o n
15 and 25 V.
was ilot produced
11.5 (3)
with
tile s t r o n g e s t
stimuli
tested,
which
/'allged be|weetl
C o m p a r i s o n of solne p r o p e r t i e s of acute a n d chronic neuronally isolated rerebrai rortex. In each case the m e a n a n d its s t a n d a r d deviation are given, the nunll)er (If ],rcl)',r:~ti
(d) Epileptiform aft~,rdiseharge. A lowered t h r e s h o l d ~md considerably p r o l o n g e d d u r a t i o n were ch'm~eteristic of this response in the chronic prepar.~tions. (c) Spreadi~g depression. W h e n depression was i n i t i a t e d in the chronic isolated cortex, its rate of s p r e a d was c o m p a r a b l e to t h a t in t h e acute p r e p a r a tions. However, the a m p l i t u d e of the a c c o m p a n y i n g slgw p o t e n t i a l c h a n g e was smaller, a n d the t h r e s h o l d for depression, tested with repetitive s t i m u l a t i o n , was
physiological properties. All of the responses which were p r o d u c e d by electrical s t i n m l a t i e n ill the acute p r e p a r a t i o n could also be elieitt.d in the chronic one, ~flthough t h e r e were some ehantzcs in the t h r e s h o l d s a n d a m p l i t u d e s of the v a r i o u s resplmses. P o s s i b l y the m o s t i n t e r e s t i n g f e a t u r e of the eleetrieal behaviour of the chronic isolqted cortex was the h m g d u r a t i o n of the e p i l e p t i f o r m a f t e r d i s e l m r g e which eimhl be produced in it. I n cortex which h a s been isolated for longer periods t h a n those in this study, this after~
CHRONIC ISOLATED d i s c h a r g e h a s been f o u n d to be even more prolonged. :For example, in cortex isolated for 6 m o n t h s , a n a f t e r d i s c h a r g e could 1)e i n i t i a t e d which lasted f o r several hours (Barns and Sastry, unpublished results). I t is n o t possible, on the b a s i s of this b r i e f survey, 1o m a k e a n y s p e c u l a t i o n s on the n a t u r e of the s t r u c t u r a l c h a n g e s u n d e r l y i n g the observed c h a n g e s in the electrophysiological p r o p e r t i e s of the isolated cortex. We hope, however, t h a t these p r e l i m i n a r y ol)servations will be of use ill the f u r t h e r exldoration of this problem. Our t h a n k s are due to P r o f e s s o r B. D. B u r n s for his valuable advice, to P r o f e s s o r F. C. 5 I a c I n t o s h f o r his i n t e r e s t in this s t u d y a n d to Mr. Carl Holizek for his a s s i s t a n c e ill the aseptic operations. One of us ( S a s t r y ) wishes to e x p r e s s his t h a n k s to t h e Technical Co-operation Service of tile C a n a d i a n Governm e n t for t h e a w a r d of the fellowship u n d e r tile Colombo P l a n . T h i s research was s u p p o r t e d 1)y a g r a n t to P r o f e s s o r B u r n s f r o m the N a t i o n a l R e s e a r c h Council ,if ('an'td'L
725
CORTEX REFERENCES
ADRIAN, E. D. The s p r e a d of a c t i v i t y in t h e cerebral cortex. J. Physiol., 1936, 88: 127-161. BURNS, B. D. Some p r o p e r t i e s of the c a t ' s isolated cerebral cortex. J. Physiol., 1950, 111: 50-68. BURNS, B. D. Some properties of isolated cerebral cortex in the u n a n a e s t h e t i z c d cat. J. Physiol.. 1951, i12: 156-175. B v R x s , B. D. The p r o d u c t i o n of a f t e r - b u r s t s in isolated u n a n a e s t h e t i z e d cerebral cortex. J. Physiol., 1954, 125: 427-446. BURNS, B. D. a n d GRAFSTEIN, B. Tile f u n c t i o n a n d s t r u c t u r e of solne neurolles ill the c a t ' s cerebl*al cortex. J. Physiol., 1952, 113:412 433. KRISTIANSEN, S . a n d COURTOIS~ G. R h y t h m i c electrical activity f r o m isolated cerebral cortex. EEG Clin. Yeurophysiol., 1949, 1: 265-272. ECHLIN, F. A., ARNETT, ~r. a n d ZOLL, J. P a r o x y s m a l high voltage d i s c h a r g e s f r o m isolated a n d partially isolated h u n m n a n d a n i m a l cerebral cortex. EEG Clin. Ncurophysiol., 1952, ~l: 147-164. GRAFSTEIN, B. M e c h a n i s m of s p r e a d i n g cortical depression. J. Neurophysiol., 1956, 19: 154-171. LEX0, A. A. P. S p r e a d i n g depression of a c t i v i t y in the cerebral cortex. J. Ncurophysiol., 1944, 7: 359-390.
Refere~we: GRAFSTEIN, B. and SASTRY, P. B. Some preliminary eleetrophysiological studies on chronic neuronalty isolated cerebral cortex. EEG Clin. Neurophysiol., 1957, 9: 723-725.
3