AFFERENT LEVELS I.A.

SIGNAL OF

THE

TRANSFORMATION

VISUAL

AT SUBCORTICAL

SYSTEM

Shevelev

UDC612.84

A c o m p a r i s o n was made of the c h a r a c t e r i s t i c s of the background and light-induced impulse activity of 350 neurons of the r e t i n a and l a t e r i a l geniculate body (LGB) of an i m m o b i l i z e d cat. In the LGB, in c o n t r a s t to the ganglion c e i l s of the retina, t h e r e a r e r e l a t i v e l y m o r e neurons with high-light sensitivity, low latency of r e s p o n s e , s h o r t - i n i t i a l d i s c h a r g e , and low s u m m a t i o n t i m e , as well as r a p i d r e c o v e r y of r e a c t i v i t y . It was found that these changes a r e c h a r a c t e r i s t i c of cells connected with the p e r i p h e r a l region of the retina. In the c e n t r a l cha~mel and the s y s t e m of l o n g - l a t e n c y cells, according to a n u m b e r of p a r a m e t e r s , no such t r a n s f o r m a t i o n s a r e found, while other c h a r a c t e r i s t i c s change in the opposite direction. In the central channel at the p o s t g e n i c u l a r level, the frequency of the background and induced impulsation d e c r e a s e s , while in the p e r i p h e r a l channel it i n c r e a s e s considerably. The r e sults obtained indicate d i f f e r e n c e s in the organization of the synaptic inputs of LGB neurons of different groups. The intensification and time c o m p r e s s i o n of the s e n s o r y t r a n s m i s s i o n in the p e r i p h e r a l channel a r e explained by the r e t i c u l a r p r o p e r t i e s of the s t r u c t u r e s f o r m e d by the branching and overlapping f i b e r s coming f r o m the r e t i n a , as well as by the g r e a t effect i v e n e s s of the s u c c e s s i v e inhibition in the LGB which blocks the late group of initial d i s c h a r g e s . T h e s e t r a n s f o r m a t i o n s of the afferentation can provide reliability and rapidity of the detection of weak, slowly i n c r e a s i n g , and moving light signals. INTRODUCTION An evaluation of s p a c e - t i m e and frequency t r a n s f o r m a t i o n s of afferentat[on on its pathways f r o m the r e c e p t o r s to the cortex is n e c e s s a r y for an a n a l y s i s of the neurophysiological m e c h a n i s m s of higher s e n s o r y functions. T h e r e is information in the l i t e r a t u r e on the nature of the spatial t r a n s f o r m a t i o n s : a change in the size and shape of the r e c e p t i v e fields [3, 15], as well as a d e c r e a s e in the frequency of the background and light-induced impulsation of LGB neurons in c o m p a r i s o n w i t h r e t i n a l ganglion c e l l s [3, 12, 16, 17]. It is i m p o r t a n t to e m p h a s i z e that such c o m p a r i s o n s a r e made, as a rule, either on individual e x a m p l e s or on the b a s i s of a v e r a g e d data for groups of cells independently of their functional differences. T h e r e a r e no quantitative data on changes in sensitivity to light, duration of r e a c t i o n s , t e m p o r a l s u m m a t i o n , and r a t e s of r e c o v e r y of r e a c t i v i t y of retinal and LGB neurons. In our p r e v i o u s a r t i c l e s [5, 19, 20], data was p r e s e n t e d on a n u m b e r of the principal p r o p e r t i e s of retinal ganglion cells of LGB neurons which made it p o s s i b l e to divide them into subgroups which differ in sensitivity, rapidity of action, capacity for s u m m a t i o n , and f r e quency of d i s c h a r g e s in the light and dark. The specific t a s k of the p r e s e n t investigation was to c o m p a r e the p a r a m e t e r s of the e x t r a e e l l u l a r i m pulse activity of 150 retinal ganglion cells and 200 LGB neurons belong to r e l a t e d functional groups. METHOD Acute e x p e r i m e n t s w e r e c a r r i e d out on 40 cats i m m o b i l i z e d with d - t u b o c u r a r i n e . The impulsation of neurons of the d o r s a l nucleus of the LGB (or f i b e r s of the optic radiation), as well as f i b r e s of the optical t r ac t (OT) (axoms of retinal ganglion cells) was r e c o r d e d with the help of tungsten m i c r o e l e c t r o d e s , s t e r o t a x i c al ly embedded in the OT or LGB c o n t r a l a t e r a l l y to the site of stimulation. Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences Of the USSR, Moscow. T r a n s l a t e d f r o m Neirofiziolog[ya, Vol. 3, No. 1, pp. 13-21, J a n u a r y - F e b r u a r y , 1971. Original a r t [ cle submitted June 8, 1970. O 1971 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.

%r-~-7 2O /V

20

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c I

10 0

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-60-#0-20 0 +20

-60 "]¢0-20 0+20

it, dB

0 It, dB Fig. 2

Fig. i

Fig. 1. Differences in light sensitivity of retinal ganglion cells (1) and LGB neurons (2). The threshold intensity (in decibels of light alternation) is indicated on the a b s c i s s a ; the r e l a t i v e n u m b e r of neurons (the total number of investigated cells of a given section is take as 100%) is indicated on the ordinate. Data f r o m 137 neurons (cell groups 2 and 3 a r e excluded). Fig. 2. C o m p a r i s o n of light sensitivity of retinal ganglion cells (1) and LGB neurons (2) belonging to g r o u p s l a (A), l b (B), and 3 (C). A b s c i s s a s a m e as in Fig. 1. Number of cells (N = 72 in A, 64 in B, and 35 in C) is indicated on the ordinate. 3) F r a c t i o n of cells for which d e t e r m i n a t i o n of threshold was not significant.

An atropinized eye (eyelids extended, o n e - t h i r d of the eyelid removed) wad diffusely and totally illuminated with flashes of white light ¢Sylvania modulator lamp). The length of the light flash was v a r i e d f r o m 0.1 to 500 ms and its intensity was v a r i e d by 70 dB (100 million times) using c a l i b r a t e d neutral f i l t e r s and a cuneus. It was found that in a m a j o r i t y of the neurons examined the r e s p o n s e s to light had the s h o r t e s t latent period, highest frequency, and g r e a t e s t n u m b e r of i m p u l s e s at a flash duration of 25 m s and an intensity of about 100 Ix (in the pupil). A stimulus with these p a r a m e t e r s was c o n s i d e r e d optimal. The activity p a r a m e t e r s of the e l e m e n t s w e r e evaluated on the b a s i s of p o s t s t i m u l u s and A t - h i s t o g r a m s [9], obtained on an automatic analyzer. The m i n i m u m intensity n e c e s s a r y for generating a cellular r e s p o n s e (threshold of reaction), the m i n i m u m (at optimum stimulation) and m a x i m u m tnear threshold) latency of the d i s c h a r g e , the duration of a group of i m p u l s e s , the c r i t i c a l s u m m a t i o n t i m e , the t i m e of r e c o v e r y of r e a c t i v i t y , and the a v e r a g e frequency of the background and induced impulsation w e r e m e a s u r e d at all flash lengths. RESULTS M u l t i p a r a m e t e r c o r r e l a t i o n and r e g r e s s i o n analysis made it p o s s i b l e to divide the neurons investigated into t r a n s m i t t i n g (relay) cells (first and third groups) and inhibitary e l e m e n t s (second group) which block their d i s c h a r g e s . H e r e we shall dwell on a c o m p a r i s o n of the p r o p e r t i e s of the p r i n c i p a l r e l a y cells of the r e t i n a and LGB which are subdivided into the following groups and subgroups [19, 20]: subgroup l a , high-threshold s h o r t - l a t e n t neurons with a s h o r t (on the average) d i s c h a r g e , s h o r t s u m m a t i o n , and v i g o r ous r e c o v e r y of reactivity a f t e r a stimulus; subgroup l b , l o w - t h r e s h o l d s h o r t - l a t e n t cells with a longer d i s c h a r g e and summation, and sluggish r e c o v e r y of reactivity; group 3, l o n g - l a t e n t e l e m e n t s responding according to the type of "output" with r e a c t i o n s of the inhibiting-exiting type. L o n g e r than in neurons of the f i r s t group but the lowest frequency d i s c h a r g e s and prolonged s u m m a t i o n a r e c h a r a c t e r i s t i c of the l a t t e r . All subsequent c o m p a r i s o n s of v a r i o u s indices of neuronal activity in the r e t i n a and LGB w e r e made f r o m the mean data for these groups (Table 1). 1.

Relative

Sensitivity

to Light

The g e n e r a l nature of the distribution of LGB neurons by their r e a c t i o n t h r e s h o l d s (Fig. 1, 2) did not change in c o m p a r i s o n with the r e t i n a (Fig. 1, 1). Two c l e a r l y divided groups of cells with different s e n s i tivities a r e shown in Fig. 1. However, in the LGB t h e r e a r e r e l a t i v e l y m o r e e l e m e n t s with low r e a c t i o n thresholds (57% in c o m p a r i s o n with 42% in the retina). In the principal, f i r s t group of r e l a y s h o r t - l a t e n t n e u r o n s , the s e n s i t i v i t y i n c r e a s e d in the LGB by an a v e r a g e of 6.3 dB (p > 0.95) (Table 1). However, as seen f r o m Fig. 2, the s e n s i t i v i t y of neurons of different groups changes in different ways: in cells with high s e n s i t i v i t y (lb; Fig. 2B), the l a t t e r i n c r e a s e s even m o r e in the LGB, in neurons of the LGB with high r e a c t i o n t h r e s h o l d s (la, Fig.2A),

10

T A B L E 1. C o m p a r i s o n of P r i n c i p a l C h a r a c t e r i s t i c s N e u r o n s of the L G B * ( M e a n v a l u e s and e r r o r s ) 'l Sections of ! Cell groups and I system subgroups

Minimum

of t h e A c t i v i t y of R e t i n a l G a n g l i o n C e l l s and R e l a y

latency

Reaction threshof response, old'~, It, dB Tlmin, msec ...............

Maximum latency Time of cessation . 1[criticai IDutation or group, [ summation i of response, l of group of ira- [z~ T msec Tlmax, msec ]pulses, Tz, msec } ' [ time' Tcr' I msec$ t

Retina

LGB

la

--: 24,05 -- 2,82

lb

30,55 +---1,63

55,44 -- 3,46

60,57 ± 2,63

29,65 +--2,25

!

7,94

-- 45,2 ~ 1,7

34,19 --+,1,86

130,46 _+ 10,81

123,1 ± 6,43

91,56 _ 6,4

28,8

la-l-lb

--31,94 ± 2,14

31,89 ± 1,40

89,06 ± 7,14

84,31 +- 6,11

53,16 ± 4,71

14,1

3

--21,46 ± 3,98

99,51 +__5,39

166,33 ± 12,51

224,3 _+ 15,57

111,5 _ 12,2

12,0

la

--25,21 +-- 1,22

30,84 +_ 1,3

61,45 -- 3,89

65,1 +--3,6

33,48 ----.2,75

7,95

Ib

--49,85 ± 1,2

23,99 +_ 0,9

96,2 _ 9,26

80,97 ± 4,26

57,3 =i=6,74

I1,5

la+lb

- - 38,27 __ 2,1

27,76 ± 0,89

77,34 --- 5,21

72,57 ± 3,99

44,83 ± 3,73

10,0

-- 12,62 ± 6,12 131,36 ± 6,29 183,2 -+ 12,94 230,0 ± 84,85 104,12 ----32,6 13,8 3 * Data for inhibitory LGB neurons (group 2) are not included. "~Threshold light intensity necessary for generation of minimum reaction found from poststimulus histogram (in relative logarithmic units of light of maximum clarity). $ Geometric mean,

T A B L E 2. F r e q u e n c y of B a c k g r o u n d and I n d u c e d I m p u l s a t i o n of D i f f e r e n t G r o u p s of N e u r o n s in the R e t i n a and L G B (Mean v a l u e s ) |

Cell groups ~etinal ganglion ceils f (1)

Backgr.ound activity LGB neurons principal (I)

t

inhibitory (2)

i

Impulsatiou at ~ retinal ganglion ceils (1)

stimulation* LGB neurons . . . . principal (1) inhibitory (2)

Channel "a"

30,44_.+_6,03

17,06:1:2,91

21,6± 4,4

208,46 ± 20,84

181,161 18,47

147,19:£ 17,54

Channel "b"

22,33 i 4,87

28.01 :i:3,38

24,05 ± 6,7

191,83± 13,56

250,31 _+15.14

I 18,69+ 17,39

* The "average maximum" frequency was determined in the induced groups of impulses for t0-20 msec in the region of the poststimulus histogram peak.

i t c h a n g e s l i t t l e , w h i l e in l o n g - l a t e n t c e l l s the t h r e s h o l d s a r e c o n s i d e r a b l y i n c r e a s e d (group 3; T a b l e 1). T h u s , in the L G B t h e r e is a n i n c r e a s e in the a v e r a g e l i g h t s e n s i t i v i t y of the n e u r o n p o p u l a t i o n m a i n l y t h r o u g h the s y s t e m of s u b g r o u p l b c e l l s . 2.

Time

Differences

in Responses

A t t h e t h a l a m i e l e v e l , in c o m p a r i s o n w i t h t h e r e t i n a , the t e m p o r a l c h a r a c t e r i s t i c s of the n e u r o n a l r e a c t i o n s c h a n g e s h a r p l y ( T a b l e 1). T h e l a t e n t p e r i o d of the r e s p o n s e to a n o p t i m u m s t i m u l u s (Tlmin) in the f i r s t g r o u p ( s h o r t - l a t e n t r e l a y c e l l s ) i s s h o r t e r in the L G B than in the r e t i n a b y an a v e r a g e of 4.1 m s (P > 0.99). T h i s i s s e e n f r o m F i g . 3 (A and C) in w h i c h the d i s t r i b u t i o n of r e t i n a l and L G B n e u r o n s b y T l m i n i s shown. T h e m i n i m u m l a t e n c y o f t h e r e s p o n s e of c e l l s w i t h l o w t h r e s h o l d s (lb) i s e s p e c i a l l y s h a r p l y r e d u c e d , b y 10.2 m s (P > 0.999). in the LGB. A t t h e s a m e t i m e , in h i g h - t h r e s h o l d s h o r t - l a t e n t n e u r o n s ( l a ) t h e T l m i n , a c c o r d i n g to t h e m e a n d a t a , p r a c t i c a l l y d i d n o t c h a n g e , w h i l e in l o n g - l a t e n t n e u r o n s ( g r o u p 3) t h i s i n d e x i n c r e a s e d b y 32 m s in the L G B ( F i g . 3A, C, r a n g e f r o m 60 to 200 ms) The maximum (near t h r e s h o l d ) l a t e n t p e r i o d s of t h e r e s p o n s e s ( T t m a x ; T a b l e 1) c h a n g e s i m i l a r l y in the L G B and t h e r e t i n a . T h e e a r l i e r (by 42.1 m s , P > 0.999) c e s s a t i o n in t h e L G B of the i n i t i a l g r o u p of i m p u l s e s in c e l l s of s u b g r o u p l b (T 2 ill T a b l e 1) i s p a r t i c u l a r l y i m p o r t a n t . T h i s m e a n s t h a t f r o m o p t i m u m s t i m u l a t i o n l o w t h r e s h o l d n e u r o n s of t h e L G B (lb) r e c e i v e i m p u l s a t i o n f r o m t h e r e t i n a 40 m s a f t e r t h e d i s c h a r g e t h e m s e l v e s h a v e s t o p p e d . The b l o c k i n g in the L G B of a c o n s i d e r a b l e p o r t i o n of the s e n s o r y t r a n s m i s s i o n c o m i n g f r o m t h e r e t i n a a l o n g c h a n n e l "b" m a y i n d i c a t e t h e high r e l a t i v e e f f e c t i v e n e s s of s u c c e s s i v e i n h i b i t i o n in the s y s t e m of h i g h l y s e n s i t i v e L G B r e l a y n e u r o n s ( l b ) .

11

.?

.roll

¢ i

I

I

I

't"l

I

I

I

o'~-

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S V ~

s I

o

Latent period of discharge, msec

o

10o

2.0#

Length of group ot impulses, rnsec

Fig. 3. Distribution of retinal ganglion cells (A and B) and LGB neurons (C and D) by m i n i m u m latency (A and C) and length of f i r s t group of i m p u l s e s (B and D) of r e s p o n s e to optimum light flash. T i m e in m s is indicated on the a b s c i s s a ; the r e l a t i v e n u m b e r of cells (N = 114 in A, 111 in B, 124 in C, and 104 in D) is indicated on the ordinate. Inhibitory LGB neurons a r e excluded. In connection with this the fact that LGB neurons g e n e r a t e considerably s h o r t e r initial groups of i m p u l s e s than retinal ganglion c e l l s at o p t i m u m stimulation s e e m s n a t u r a l (Fig. 3B and D). In the LGB the n u m b e r of neurons with a d i s c h a r g e s h o r t e r than 50 ms was 67% in c o m p a r i s o n with 48% in the r e t i n a (P > 0.99). As s e e n f r o m Table 1 (AT), the a v e r a g e duration of the group as a whole, judging by the f i r s t group of cells, d e c r e a s e d in the LGB by 8.3 m s , and in cells of subgroup l b by 34.3 m s (P > 0.9997, while in cells of subgroup l a , on the other hand, it was somewhat lengthened (by 3.8 ms). Such shortening of the d i s c h a r g e s of neurons of the visual c e n t e r s was noted e a r l i e r [3], however, h e r e it is shown that this phenomenon o c c u r s in channel "b" and does not affect the s y s t e m of high-threshold (la) cells. The r e s u l t s of m e a s u r e m e n t s of the c r i t i c a l s u m m a t i o n time (Tcr) a r e in a c c o r d with these data: in LGB cells of subgroup l a and group 3 it did not change in c o m p a r i s o n with the retinal cells, while in n e u r o n s of subgroup l b it was 2.5 t i m e s s h o r t e r : f r o m 28.8 to 11.5 m s (P > 0.997. On the a v e r a g e in all group 1 LGB cells the T c r d e c r e a s e d by 4.1 m s (Table 1). 3. the

Recovery Aftereffect

of Reactivity of the

of Retinal

and

LGB

Neurons

in

Stimulus

An analysis of c u r v e s of the r e c o v e r y of the magnitude (number of impulses) and latent period of a n e u r o n ' s r e s p o n s e to a t e s t light stimulus p r e s e n t e d at different intervals after an identical conditioning stimulus showed a c o n s i d e r a b l e difference between types of e l e m e n t s in the c o u r s e of this p r o c e s s . It was found that in l o w - t h r e s h o l d s h o r t - l a t e n t neurons (Fig. 4a) the r e c o v e r y of the r e s p o n s e begins e a r l y (after 20-30 ms), but develops r a t h e r sluggishly, ending in the 350th-500th millisecond. In c o n t r a s t to this, in cells with a high threshold and s h o r t latency (la) the r e c o v e r y takes place m o r e sharply: it begins l a t e r than in cells of subgroup l b , but ends sooner ¢Fig. 4b). Evidently, in connection with the fact that in the LGB the phase nature of the d i s c h a r g e s of the r e l a y neurons i n c r e a s e s c o n s i d e r a b l y (see above), h e r e it is often possible to see a c o n s i d e r a b l y s h a r p e r r e c o v e r y of the r e a c t i o n s than in the retina. This is c h a r a c t e r i s t i c of the p o r t i o n o f the l o w - t h r e s h o l d neurons of the LGB (lb), which, judging by other t e m p o r a l c h a r a c t e r i s t i c s of the activity (Table 1), converge somewhat with neurons of subgroup l a . On the a v e r a g e , the r e c o v e r y of the second r e s p o n s e in LGB neurons o c c u r s 170 ms e a r l i e r than in the r e t i n a (P > 0.95).

12

'..I o 0

IO

-z 700

i..

1000

0

10

a

100

I000

b

Fig. 4. R e c o v e r y cycles of r e s p o n s e s of different types of neurons. Graphs f o r LGB l b (a) and subgroup l a (b) cells. I n t e r v a l between conditioning and t e s t f l a s h e s in m s is indicated on the a b s c i s s a ; the n u m b e r of i m p u l s e s in the r e s p o n s e for 80 (a) and 100 m s (b) is indicated on the left o r d i n a t e s (1), the latency of the r e spouse in m s on the right o r d i n a t e s (2). C i r c l e s on the left a r e the initial l e v e l s of N and "T1. 4.

Transformation

Frequency

by

LGB

of Background

and

Induced

Impulsation

Neurons

A c o m p a r i s o n of the s t a t i s t i c a l c h a r a c t e r i s t i c s of the frequency of the input synaptic b o m b a r d m e n t of LGB neurons (output impulsation of retinal ganglion cells) with the frequency of their natural, output irapulsation m a k e s it p o s s i b l e to evaluate the direction and degree of the power t r a n s f o r m a t i o n of the a f f e r ent t r a n s m i s s i o n , i . e . , the t r a n s m i t t i n g functions of the t h a l a m i c e l e m e n t s . I t w a s found that these indices a r e v e r y different in e l e m e n t s of different groups (Table 2). The m e a n f r e q u e n c y of the background impulsation of LGB neurons was d e c r e a s e d by a total of 4.5 i m p / s e c in c o m p a r i s o n with the r e t i n a (P < 0.95). However, in the LGB 29% %ilent" (without background d i s c h a r g e s ) neurons w e r e counted as against 16% in the r e t i n a {P > 0.99). The h i s t o g r a m of the intervals of the background and light-induced impulsation of LGB neurons w e r e multimodal considerably m o r e frequently than for retinal neurons. The data p r e s e n t e d in Table 2 shows that intensification o c c u r s in the l o w - t h r e s h o l d r e l a y cell s y s t e m (subgroup lb), while in the high-threshold neuron s y s t e m (subgroup la) there is a c o n s i d e r a b l e dec r e a s e in the background and induced impulsation of the principal neurons of the LGB in c o m p a r i s o n with the d i s c h a r g e s in the retinal f i b e r s leading to them. Actually, in channel "b" the frequency of the b a c k ground impulsation i n c r e a s e s in the L G B b y an a v e r a g e of 5.7, while the frequency of the induced i m p u l s a tion i n c r e a s e s by 58.5 i m p / s e c (P > 0.99). At the s a m e t i m e , in channel "a" these f r e q u e n c i e s in the LGB d e c r e a s e by 13.4 and 27.3 i m p / s e c , r e s p e c t i v e l y . The c o n t r a d i c t o r y nature of the p o w e r t r a n s f o r m a t i o n of the signal in the two p a r t s of the optic a f f e r ent channel ("a" and "b") at the l e v e l of the s u b c o r t i c a l center which we established indicates t h a t t h e r e a r e c o n s i d e r a b l e d i f f e r e n c e s in the organization of the synaptic links of the c o r r e s p o n d i n g neurons of the r e t i n a and LGB. DISCUSSION A significant change in a number of characteristics of neuronal activity on the pathways from the retina to the visual cortex has been shown in this work. Two questions naturally arise: what are the morphological and physiological mechanisms of these afferentation transformations and what is their functional significance.

In our p r e v i o u s w o r k [20] evidence was p r e s e n t e d that groups of low and high-threshold r e l a y neurons (subgroups l b and la) which w e r e distinguished in the r e t i n a of the eat p o s s e s s such different p r o p e r t i e s as a r e s u l t of the a r r a n g e m e n t of their r e c e p t i v e fields in the p e r i p h e r a l and c e n t r a l p a r t s of the retina, and, consequently, different spatial s u m m a t i o n s and degree of the inhibitory m e c h a n i s m s . T h e r e are grounds [18] for speaking of the fact that the retinal neurons a r e linked with the LGB neurons, which a r e r e l a t e d to them in b a s i c p r o p e r t i e s , making it p o s s i b l e to c o m p a r e the principal c h a r a c t e r i s t i c s of their activity. The opposite d i r e c t i o n of the t e m p o r a l and f r e q u e n c y - p o w e r t r a n s f o r m a t i o n s of the a f f e r e n t t r a n s m i s s i o n in channels "a" (central) and "b" ( p e r i p h e r a l vision) indicates the effect of m e c h a n i s m s leading in

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the f i r s t case to some lengthening and attenuation, and in the second (channel "b") to considerable s h o r t e n ing and intensification of the neural signal during its switching in the thalamic visual center. In addition, the values p r e s e n t e d in Table 1 of the spread of the latent periods and the time of cessation of the impulsation in subgroups l a and l b confirm the p r e s e n c e in the LGB of synchronization of the beginning and end of the discharge in cells of the p e r i p h e r a l channel (nb") and some desynchronization in channel "a." Based on theoretical concepts and model calculations [1, 4], these effects can be connected with the different d e g r e e s of divergence and secondary overlapping leading to convergence of the branching afferents f r o m the retina, that is, with g r e a t e r or l e s s e r density of the links at the inputs of the LGB r e l a y neurons. It is known [10, 13] that in the projection of the retinal center, t r a n s m i s s i o n is c a r r i e d out by the 1 : 1 principle. As a result, the corresponding LGB neurons receive few [13] afferents f r o m the comparatively synchronously operating ganglion cells of subgroup l a [20] and t h e r e f o r e , in spite of the s m a l l e r size [18, 21] which p r o m o t e s an increase in sensitivity [11], desensitize it. The d e c r e a s e in the synchronism of the beginning and cessation of the group of discharges of these cells can be linked p r i m a r i l y with differences in the p r o p e r t i e s of individual cells which have an influence during such afferentation [6], as well as with the different r a t e s of conducting impulses in afferents of different d i a m e t e r s . tn contrast to this, in the p e r i p h e r a l channel ("b"), thanks to the g r e a t density and overlapping of the afferents [2, 7,13, 14], a relatively weak and tonic signal f r o m the retina [20] is intensified and synchronized. Sequential horizontal inhibition [8] plays an important role in the t e m p o r a l t r a n s f o r m a t i o n of the t r a n s m i s sion at the thalamie level. The frequency of the induced discharges of the inhibitory neurons which we distinguished in the LGB [5, 19] (Table 2) on the average is higher in the central channel ("an). Data on the course of reactivity r e c o v e r y shows that e m e r g e n c e f r o m inhibitory depression takes place more vigorously in ' a " r e l a y cells than in channel "b." However, the relative efficiency, i . e . , the capacity to block output impulsation, as seen in a comparison of the duration of the discharges of the principal cells, is higher in the inhibitory system which s e r v e s the "b" r e l a y neurons in the LGB. The functional significance of the intensification and temporal c o m p r e s s i o n of the afferent t r a n s mission detected by the neuronal activity in the p e r i p h e r a l (according to the visual field) channel of the dir e c t optic projection pathway evidently consists in an increase in the reliability of detecting and d i s c r i m i n ating weak, slowly increasing, and moving light signals. The thus-achieved i n c r e a s e in the rapidity and reliability of these p r o c e s s e s can be of considerable importance in the organization of the tracking movements of the eye and guiding t o the target of the central part of the projection system responsible for p e r ception of complex visual images. The author is grateful to V. Yu. Krylov and T.V. Ostryakova for assistance in machine treatment of the data.

LITERATURE I.

2. 3. 4. 5. 6.

7. 8. 9. 10.

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CITED

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11. 12. 13. 14. 15. 16.

17. 18. 19. 20. 21.

E. Henneman, G. Somjen, and D.O. Carpenter, "Functional significance of cell size in spinal motoneurons, n j . Neurophysiol., 28, 560-580 (1965). A. Herz, O. Creutzfeldt, and J. Fuster, "Statistische eingenschaften der neuronaktivitat im ascendierenden visuellen system," Kybernetik, 2, 61-71 (1964). D.H. Hubel, "Integrative processes in the central visual pathways of the cat," J° Optic. Soc. A m e r . , 53, 58-66 (1963). D.H. Hubel and T.N. Wiesel, "Integrative action in the cat's lateral geniculate body," J. Physiol. (London), 155, 385-398 (1961). G.H. Jacobs, "Receptive fields in the visual systems," Brain R e s . , 14, 553-573 (1969). W.M.H. Kozak, B. Harutiunian-Kozak, and B. Dreher, "Transmission of information about brightness and retinal inhibition, Min:Structure and Function of Inhibitory Neuronal Mechanisms, Pergamon P r e s s (1968), pp. 111-116. K. Motokawa and H. Suzuki, "Central mechanism of vision, " Progr. Brain R e s . , 21A, 163-179 ~1966). H. Noda and K. Iwama, "Unitary analysis of retino-geniculate response time in rats," Vis. Res., 9, 475-492 (1969). ~. A. Shevelev, "Neuronal organization of the initial afferent inflow in the cat LGB," Vis. R e s . , 9, 475-492 (1969). I.A. Shevelev and L . E . Akshenseva-Gorelova, "Temporal and strength characteristics of the ganglion cell activity in the cat retina," Vis. R e s . , 10 (1970). J.A. Stone, "A quantitative analysis of the distribution of ganglion cells in the cat's retina," J. Comp. Neurol., 124, 337-352 (1965).

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