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Effect of specific and non-specific stimuli on the visual and motor cortex of the rat
Brain Research, 170 (1979) 61-69 © Elsevier/North-Holland Biomedical Press
61
E F F E C T OF SPECIFIC A N D NON-SPECIFIC STIMULI ON T H E VISUAL A N D M O T O R C O R T E X OF T H E RAT*
ANTONIO RUIZ-MARCOS, JOSI~ SALA and RAQUEL ALVAREZ Departamento de Biofisica, Centro de Investigaciones Biol6gicas, Velazquez 144, Madrid-6 (Spain)
(Accepted October 26th, 1978)
SUMMARY In order to study the possible simultaneous influences of light and darkness, and mobility and restraint on the visual and motor areas of the cortex, we have studied the visual and motor cortex of 4 groups of rats raised under the following conditions: (1) in normal light conditions and large cages; (2) in normal light conditions and cages small enough to restrain the movement of the animals; (3) in total darkness and large cages and (4) in total darkness and small cages. They were kept in these conditions from weaning until they were 90 days old. At this time they were killed and their brains stained following the rapid Golgi procedure. The spines along the apical shaft of the pyramidal neurons of layer V of the visual and motor cortex (a total of 20-25 cells/group) were counted. The results were processed in a PDP 11/40 computer. While darkness produced a decrease in the mean number of dendritic spines of only the visual cortex, restraining the animal produced a significant decrease in the mean number of spines in both cortices, motor and visual. Furthermore, the influence of restraining was found so strong on the visual cortex that it masked the effect of darkness. There was no significant difference between the mean number of spines per shaft of animals held restrained and in light, versus animals held restrained and in darkness. These results indicate that movement itself is very important for the correct development of dendritic spines in the visual cortex.
INTRODUCTION In 1969 Ruiz-Marcos and Valverde 11 showed that mice raised in darkness from birth develop fewer spines in their visual cortex than their corresponding controls * Part of the present work was presented at the XXVI1International Congress of PhysiologicalSciences. Paris, 1977.
62 raised under normal conditions of light; nevertheless, it is well known that the complexity of the environment in which the animal is raised has a great influence in the development of the visual cortex in particular7,1~ and also on other cortical areas3,4, 6. This plasticity of the visual and other areas of the cortex, which cause them to react to different types of non-specific stimuli, together with the fact that in order to perform the experiment mentioned above the mice had to be confined in small cages, posed the question of whether the environmental conditions in which these animals were raised could have had some influence in the decrease of spines observed and, what we consider more important, if movement itself can have some influence on the development of the visual cortex. The present paper describes an experiment designed with the purpose of answering these two important questions. MATERIALS AND METHODS
Animals and experimental conditions A total of 32 male Wistar rats were used for this study. All litters from which these animals were taken were reduced to 8 pups each the day after birth. They were kept with their mothers until weaning (21 days of age). At this time they were picked up from a total of 10 litters and were randomly divided into 8 groups of 4 rats each which were kept, until they were 90 days of age, under the following experimental conditions: (1) in normal light conditions and large cages (50 >: 50 × 50 cm); (2) in normal light conditions and cages small enough to restrain the movement of the animals; (3) in total darkness and large cages and (4) in total darkness and small cages. The rats raised under normal light conditions were housed in temperaturecontrolled animal quarters with automatic light and darkness cycles of 14 and 10 h, respectively. The rats raised in darkness were housed in a dark room inside cages equal in size to those used for animals raised under normal conditions. Cleaning was carried out with the aid of a very dim red light for approximately 15 min every 2 days. The animals were fed in total darkness. The cages designed to restrain the movement of the animals were 7 cm wide by 5 cm high. An adjustable back wall was moved according to the size of the growing animal so that, although the rats could not make any movement with their bodies, the stress of confinement was reduced to a minimum in order to eliminate, as far as possible, the influence of this factor on the development of the dendritic spines. At the age already specified (90 days) the rats were killed. The portions of their brains containing the visual and motor cortices were rapidly dissected out and kept in buffered 1 0 ~ formaldehyde, pH 7.2, for 2 days, after which they were stained as indicated below.
Staining and quantification of dendritic spines The portions of brain comprising the visual and motor cortices were stained according to the rapid Golgi procedure described by Cajal and de Castro 1. After 2 days in buffered 10% formaldehyde they were transferred for 72 h into a solution
63 containing 2.4 ~o potassium dichromate and 0.2 ~ osmium tetroxide in distilled water. They were then transferred for 24 h into a 0.75 ~ silver nitrate solution, also prepared in distilled water. Slices (200/~m thick) of the visual and motor cortex were obtained with a slicing microtome and mounted. A total of approximately 20 pyramidal neurons of layer V of each cortex (visual and motor) were chosen at random, following a 'blind' procedure, from each group of 4 rats and spines were counted in each 50 # m segment along their pyramidal shaft. In order to balance the possible influence that individual animals could have on the final results, an equal number of neurons was taken from each animal.
Data processing All the data were stored in the permanent disk memory of a D E C G R A P H I C PDP 11/40 computer which, with a program prepared for this purpose, calculated the mean number and standard deviation of spines per 50/~m longitudinal segment of apical shaft of each group of neurons studied. The values obtained are represented in Table I together with the results of the comparison made between different groups by the t-test with the correction introduced by Dunn and Clark 5 for multiple-t confidence intervals. The value of P corresponds to a level of significance of 0.05. RESULTS AND COMMENTS Fig. 1 shows one representative neuron from the visual cortex of the rat raised under normal conditions of light and mobility; the apical shaft is covered with spines, as is normal in this type of neuron. Fig. 2 shows one of the most affected neurons from the visual cortex of a rat kept restrained under normal light conditions. Fig. 3 shows one of the neurons found in the motor cortex of a rat kept restrained under normal light conditions. Fig. 4 shows a neuron from the motor cortex of a rat kept restrained in darkness. A comparison of Figs. 1 and 2 indicates how restraining the animal's movement can affect the development of the visual cortex. Figs. 3 and 4 show how restriction of movement affects the motor cortex neurons of the rats independently of whether they were raised under normal light conditions or in darkness. Clearly, the observation of a single neuron cannot be taken as conclusive evidence of the effects that light and darkness or mobility and restraining could have on the entire population of pyramidal neurons. Side by side with one neuron deprived of spines (Figs. 3 and 4) or with very few spines (Fig. 2) it is possible to find other neurons with different numbers of spines, as was, in fact, the case. Consequently, the appropriate statistical analysis was made; results are shown in Table I. It is clear from this table that the development of the visual cortex is affected not only by light (see part A, first column) but also by restriction of the animal (see part A, first and second rows). Nevertheless, the development of the motor cortex seems to be affected only by restraining the animal (see part B, first and second rows) and not by the light conditions in which the animal is raised (see part B, first column).
64
Fig. 1. Pyramidal neuron of layer V of the visual cortex of a 90-day-old rat raised under normal conditions of light and mobility. Golgi method. × 256.
65
Fig. 2. Pyramidal neuron of layer V of the visual cortex of a 90-day-old rat raised under normal light conditions and restrained. Golgi method, x 256.
66
Fig. 3. Pyramidal neuron of layer V of the motor cortex of a 90-day-old rat raised under normal light conditions and restrained. Golgi method, x 256.
67
Fig. 4. Pyramidal neuron of layer V of the motor cortex of a 90-day-old rat raised in darkness and restrained. Golgi methods, x 256.
68 TABLE I Number o f spines (mean 4- S.E.) per segment o f 50 i~m o f the apical shafts o f pyramidal neurons o f the ( A ) visual and ( B) motor cortex o f rats raised under the conditions specified below
t and P values indicate the results of the statistical comparison made between the two groups located to the left or above these values. N is the number of dendrites considered in each group. Mobility
Immobility
t
P <
~ = 20.76 s = 7.08 N -- 20
5.93
0.01
~ = 23.47 s = 5.79 N = 20
.'K = 18.10 s -- 4.69 N -- 20
2.86
0,05
5.12 0.01
1.09 N.S.
R -- 19.95 s = 5.63 N -- 20
4.21
0.01
~ = 26.57 s -- 4.16 N --20
,X -- 18.72 s -- 8.11 N -- 20
3.85
0.01
0.54 N.S.
0.56 N.S.
(A) Visual cortex Light ~ -- 33.03 s = 6.16 N -- 21
Darkness
t P <
(B) Motor cortex Light R = 27.45 s = 6.31 N = 27
Darkness
t P <
T h e fact t h a t n e i t h e r in t h e visual n o r in t h e m o t o r c o r t e x was a n y statistical difference f o u n d b e t w e e n t h e m e a n n u m b e r o f spines p e r 5 0 / ~ m o f a p i c a l shaft o f r e s t r a i n e d a n i m a l s raised in n o r m a l light a n d r e s t r a i n e d a n i m a l s raised in d a r k c o n d i t i o n s , is due, in o u r o p i n i o n , t o t h e fact t h a t this m e a n n u m b e r r e a c h e d s u c h a l o w level in b o t h g r o u p s t h a t it was i m p o s s i b l e to d i f f e r e n t i a t e b e t w e e n t h e m (see t h e s e c o n d c o l u m n in b o t h p a r t s A a n d B o f T a b l e I). DISCUSSION A s a l r e a d y m e n t i o n e d , different a u t h o r s 7,tz h a v e f o u n d t h a t t h e visual c o r t e x s h o w s g r e a t plasticity t o c h a n g e s in t h e e n v i r o n m e n t a l c o n d i t i o n s u n d e r w h i c h t h e a n i m a l is r a i s e d ; o u r results c o u l d t h u s h a v e b e e n i n t e r p r e t e d as a m e r e n o n - s p e c i f i c r e a c t i o n o f this p a r t o f t h e c o r t e x to t h e r e s t r i c t i o n o f t h e a n i m a l . N e v e r t h e l e s s , s o m e authors~,S-10,14 h a v e s h o w n t h e i m p o r t a n c e o f m o v e m e n t o n t h e d e v e l o p m e n t o f visual c a p a c i t y in t h e a n i m a l . O u r results are in a g r e e m e n t w i t h all t h e s e p r e v i o u s findings a n d s h o w t h a t , while t h e d e v e l o p m e n t o f t h e m o t o r c o r t e x is affected b y t h e different d e g r e e o f m o b i l i t y b u t n o t by t h e light c o n d i t i o n s u n d e r w h i c h t h e a n i m a l has
69 grown, the d e v e l o p m e n t o f the visual c o r t e x can be affected b y light as well as b y the c o n d i t i o n s o f m o b i l i t y u n d e r which t h e rat has been raised. These results do n o t invalidate those previously o b t a i n e d in the e x p e r i m e n t m e n t i o n e d at the beginning o f the p r e s e n t p a p e r 11 but, nevertheless, t h e y indicate t h a t the m o b i l i t y c o n d i t i o n s u n d e r which those mice g r o w n should have been taken into c o n s i d e r a t i o n at the time to evaluate the effect o f d a r k n e s s in the o b s e r v e d decrease o f d e n d r i t i c spines. ACKNOWLEDGEMENTS This research was s u p p o r t e d , in p a r t , b y a g r a n t f r o m the E. R o d r i g u e z Pascual F o u n d a t i o n to Dr. A. R u i z - M a r c o s . W e are grateful to Miss M. C. Vidal a n d Miss P a l o m a L a r g a c h a for their assistance in p r e p a r i n g the histological material, to Miss E. F d e z . de M o l i n a for t y p i n g the m a n u s c r i p t a n d to Miss C. G a b r i e l de R u i z - M a r c o s for her assistance in the c o m p u t e r work.
REFERENCES 1 Cajal, S. R. and de Castro, F., Elementos de Tdcnica Microgrtifica delSistema Nervioso. Salvat Edit S.A. Madrid, 1972, pp. 65-68. 2 Dec, K., Sarna, M., Tarnechi, R. and Zernichi, B., Effect of binocular deprivation of pattern vision on single unit responses in the cat superior colliculus, Acta neurobiol, exp., 36 (1976) 687-692. 3 Diamond, M. C., Extensive cortical depth measurements and neuron size increases in the cortex of environmentally enriched rats, J. comp. NeuroL, 131 (1967) 357-364. 4 Diamond, M. C., Krech, D. and Rosenzweig, M. R., The effect of an enriched environment on the histology of the rat cerebral cortex, Y. comp. NeuroL, 123 (1964) 111-120. 5 Dunn, O. J. and Clark, V. A., Applied Statistics: Analysis of Variance and Regression. John Wiley and Sons, New York, 1974, pp. 80-81. 6 Globus, A., Rosenzweig, M. R., Bennett, E. L. and Diamond, M. C., Effects of differential experience on dendritic spine counts in rat cerebral cortex, J. comp. physioL PsychoL, 82 (1973) 175-181. 7 Greenough, W. T. and Volkmar, F. R., Pattern of dendritic branching in occipital cortex of rats reared in complex environments. Exp. NeuroL, 40 (1973) 491-504. 8 Hein, A. and Diamond, R. M., Locomotory space as a prerequisite for acquiring visually guided reaching in kittens, J. comp. physiol. PsychoL, 81 (1972) 394-398. 9 Held, R. and Bayer, Jr., J. A., Development of sensorially-guided reaching in infant monkeys, Brain Research, 71 (1974) 265-271. 10 Riesen, A. H. and Aarons, L., Visual movement and intensity discrimination in cats after early deprivation of pattern vision, J. eomo. physiol. Psychol., 52 (1959) 142-149. 11 Ruiz-Marcos, A. and Valverde, F., The temporal evolution of the distribution of spines in the visual cortex of normal and dark-reared mice, Exp. Brain Res., 8 (1969) 284-294. 12 Valverde, F. and Ruiz-Marcos, A., Dendritic spines in the visual cortex of the mouse. Introduction to a mathematical model, Exp. Brain Res., 8 (1969) 269-283. 13 Volkmar, F. R. and Grcenough, W. T., Rearing complexity affects branching of dendrites in the visual cortex of the rat, Science, 176 (1972) 1451-1447. 14 Zablocka, T., Zernichi, B. and Kosmal, A., Visual cortex role in object discrimination in cats deprived of pattern vision from birth, Acta neurobiol, exp., 36 (1976) 157-168.
61
E F F E C T OF SPECIFIC A N D NON-SPECIFIC STIMULI ON T H E VISUAL A N D M O T O R C O R T E X OF T H E RAT*
ANTONIO RUIZ-MARCOS, JOSI~ SALA and RAQUEL ALVAREZ Departamento de Biofisica, Centro de Investigaciones Biol6gicas, Velazquez 144, Madrid-6 (Spain)
(Accepted October 26th, 1978)
SUMMARY In order to study the possible simultaneous influences of light and darkness, and mobility and restraint on the visual and motor areas of the cortex, we have studied the visual and motor cortex of 4 groups of rats raised under the following conditions: (1) in normal light conditions and large cages; (2) in normal light conditions and cages small enough to restrain the movement of the animals; (3) in total darkness and large cages and (4) in total darkness and small cages. They were kept in these conditions from weaning until they were 90 days old. At this time they were killed and their brains stained following the rapid Golgi procedure. The spines along the apical shaft of the pyramidal neurons of layer V of the visual and motor cortex (a total of 20-25 cells/group) were counted. The results were processed in a PDP 11/40 computer. While darkness produced a decrease in the mean number of dendritic spines of only the visual cortex, restraining the animal produced a significant decrease in the mean number of spines in both cortices, motor and visual. Furthermore, the influence of restraining was found so strong on the visual cortex that it masked the effect of darkness. There was no significant difference between the mean number of spines per shaft of animals held restrained and in light, versus animals held restrained and in darkness. These results indicate that movement itself is very important for the correct development of dendritic spines in the visual cortex.
INTRODUCTION In 1969 Ruiz-Marcos and Valverde 11 showed that mice raised in darkness from birth develop fewer spines in their visual cortex than their corresponding controls * Part of the present work was presented at the XXVI1International Congress of PhysiologicalSciences. Paris, 1977.
62 raised under normal conditions of light; nevertheless, it is well known that the complexity of the environment in which the animal is raised has a great influence in the development of the visual cortex in particular7,1~ and also on other cortical areas3,4, 6. This plasticity of the visual and other areas of the cortex, which cause them to react to different types of non-specific stimuli, together with the fact that in order to perform the experiment mentioned above the mice had to be confined in small cages, posed the question of whether the environmental conditions in which these animals were raised could have had some influence in the decrease of spines observed and, what we consider more important, if movement itself can have some influence on the development of the visual cortex. The present paper describes an experiment designed with the purpose of answering these two important questions. MATERIALS AND METHODS
Animals and experimental conditions A total of 32 male Wistar rats were used for this study. All litters from which these animals were taken were reduced to 8 pups each the day after birth. They were kept with their mothers until weaning (21 days of age). At this time they were picked up from a total of 10 litters and were randomly divided into 8 groups of 4 rats each which were kept, until they were 90 days of age, under the following experimental conditions: (1) in normal light conditions and large cages (50 >: 50 × 50 cm); (2) in normal light conditions and cages small enough to restrain the movement of the animals; (3) in total darkness and large cages and (4) in total darkness and small cages. The rats raised under normal light conditions were housed in temperaturecontrolled animal quarters with automatic light and darkness cycles of 14 and 10 h, respectively. The rats raised in darkness were housed in a dark room inside cages equal in size to those used for animals raised under normal conditions. Cleaning was carried out with the aid of a very dim red light for approximately 15 min every 2 days. The animals were fed in total darkness. The cages designed to restrain the movement of the animals were 7 cm wide by 5 cm high. An adjustable back wall was moved according to the size of the growing animal so that, although the rats could not make any movement with their bodies, the stress of confinement was reduced to a minimum in order to eliminate, as far as possible, the influence of this factor on the development of the dendritic spines. At the age already specified (90 days) the rats were killed. The portions of their brains containing the visual and motor cortices were rapidly dissected out and kept in buffered 1 0 ~ formaldehyde, pH 7.2, for 2 days, after which they were stained as indicated below.
Staining and quantification of dendritic spines The portions of brain comprising the visual and motor cortices were stained according to the rapid Golgi procedure described by Cajal and de Castro 1. After 2 days in buffered 10% formaldehyde they were transferred for 72 h into a solution
63 containing 2.4 ~o potassium dichromate and 0.2 ~ osmium tetroxide in distilled water. They were then transferred for 24 h into a 0.75 ~ silver nitrate solution, also prepared in distilled water. Slices (200/~m thick) of the visual and motor cortex were obtained with a slicing microtome and mounted. A total of approximately 20 pyramidal neurons of layer V of each cortex (visual and motor) were chosen at random, following a 'blind' procedure, from each group of 4 rats and spines were counted in each 50 # m segment along their pyramidal shaft. In order to balance the possible influence that individual animals could have on the final results, an equal number of neurons was taken from each animal.
Data processing All the data were stored in the permanent disk memory of a D E C G R A P H I C PDP 11/40 computer which, with a program prepared for this purpose, calculated the mean number and standard deviation of spines per 50/~m longitudinal segment of apical shaft of each group of neurons studied. The values obtained are represented in Table I together with the results of the comparison made between different groups by the t-test with the correction introduced by Dunn and Clark 5 for multiple-t confidence intervals. The value of P corresponds to a level of significance of 0.05. RESULTS AND COMMENTS Fig. 1 shows one representative neuron from the visual cortex of the rat raised under normal conditions of light and mobility; the apical shaft is covered with spines, as is normal in this type of neuron. Fig. 2 shows one of the most affected neurons from the visual cortex of a rat kept restrained under normal light conditions. Fig. 3 shows one of the neurons found in the motor cortex of a rat kept restrained under normal light conditions. Fig. 4 shows a neuron from the motor cortex of a rat kept restrained in darkness. A comparison of Figs. 1 and 2 indicates how restraining the animal's movement can affect the development of the visual cortex. Figs. 3 and 4 show how restriction of movement affects the motor cortex neurons of the rats independently of whether they were raised under normal light conditions or in darkness. Clearly, the observation of a single neuron cannot be taken as conclusive evidence of the effects that light and darkness or mobility and restraining could have on the entire population of pyramidal neurons. Side by side with one neuron deprived of spines (Figs. 3 and 4) or with very few spines (Fig. 2) it is possible to find other neurons with different numbers of spines, as was, in fact, the case. Consequently, the appropriate statistical analysis was made; results are shown in Table I. It is clear from this table that the development of the visual cortex is affected not only by light (see part A, first column) but also by restriction of the animal (see part A, first and second rows). Nevertheless, the development of the motor cortex seems to be affected only by restraining the animal (see part B, first and second rows) and not by the light conditions in which the animal is raised (see part B, first column).
64
Fig. 1. Pyramidal neuron of layer V of the visual cortex of a 90-day-old rat raised under normal conditions of light and mobility. Golgi method. × 256.
65
Fig. 2. Pyramidal neuron of layer V of the visual cortex of a 90-day-old rat raised under normal light conditions and restrained. Golgi method, x 256.
66
Fig. 3. Pyramidal neuron of layer V of the motor cortex of a 90-day-old rat raised under normal light conditions and restrained. Golgi method, x 256.
67
Fig. 4. Pyramidal neuron of layer V of the motor cortex of a 90-day-old rat raised in darkness and restrained. Golgi methods, x 256.
68 TABLE I Number o f spines (mean 4- S.E.) per segment o f 50 i~m o f the apical shafts o f pyramidal neurons o f the ( A ) visual and ( B) motor cortex o f rats raised under the conditions specified below
t and P values indicate the results of the statistical comparison made between the two groups located to the left or above these values. N is the number of dendrites considered in each group. Mobility
Immobility
t
P <
~ = 20.76 s = 7.08 N -- 20
5.93
0.01
~ = 23.47 s = 5.79 N = 20
.'K = 18.10 s -- 4.69 N -- 20
2.86
0,05
5.12 0.01
1.09 N.S.
R -- 19.95 s = 5.63 N -- 20
4.21
0.01
~ = 26.57 s -- 4.16 N --20
,X -- 18.72 s -- 8.11 N -- 20
3.85
0.01
0.54 N.S.
0.56 N.S.
(A) Visual cortex Light ~ -- 33.03 s = 6.16 N -- 21
Darkness
t P <
(B) Motor cortex Light R = 27.45 s = 6.31 N = 27
Darkness
t P <
T h e fact t h a t n e i t h e r in t h e visual n o r in t h e m o t o r c o r t e x was a n y statistical difference f o u n d b e t w e e n t h e m e a n n u m b e r o f spines p e r 5 0 / ~ m o f a p i c a l shaft o f r e s t r a i n e d a n i m a l s raised in n o r m a l light a n d r e s t r a i n e d a n i m a l s raised in d a r k c o n d i t i o n s , is due, in o u r o p i n i o n , t o t h e fact t h a t this m e a n n u m b e r r e a c h e d s u c h a l o w level in b o t h g r o u p s t h a t it was i m p o s s i b l e to d i f f e r e n t i a t e b e t w e e n t h e m (see t h e s e c o n d c o l u m n in b o t h p a r t s A a n d B o f T a b l e I). DISCUSSION A s a l r e a d y m e n t i o n e d , different a u t h o r s 7,tz h a v e f o u n d t h a t t h e visual c o r t e x s h o w s g r e a t plasticity t o c h a n g e s in t h e e n v i r o n m e n t a l c o n d i t i o n s u n d e r w h i c h t h e a n i m a l is r a i s e d ; o u r results c o u l d t h u s h a v e b e e n i n t e r p r e t e d as a m e r e n o n - s p e c i f i c r e a c t i o n o f this p a r t o f t h e c o r t e x to t h e r e s t r i c t i o n o f t h e a n i m a l . N e v e r t h e l e s s , s o m e authors~,S-10,14 h a v e s h o w n t h e i m p o r t a n c e o f m o v e m e n t o n t h e d e v e l o p m e n t o f visual c a p a c i t y in t h e a n i m a l . O u r results are in a g r e e m e n t w i t h all t h e s e p r e v i o u s findings a n d s h o w t h a t , while t h e d e v e l o p m e n t o f t h e m o t o r c o r t e x is affected b y t h e different d e g r e e o f m o b i l i t y b u t n o t by t h e light c o n d i t i o n s u n d e r w h i c h t h e a n i m a l has
69 grown, the d e v e l o p m e n t o f the visual c o r t e x can be affected b y light as well as b y the c o n d i t i o n s o f m o b i l i t y u n d e r which t h e rat has been raised. These results do n o t invalidate those previously o b t a i n e d in the e x p e r i m e n t m e n t i o n e d at the beginning o f the p r e s e n t p a p e r 11 but, nevertheless, t h e y indicate t h a t the m o b i l i t y c o n d i t i o n s u n d e r which those mice g r o w n should have been taken into c o n s i d e r a t i o n at the time to evaluate the effect o f d a r k n e s s in the o b s e r v e d decrease o f d e n d r i t i c spines. ACKNOWLEDGEMENTS This research was s u p p o r t e d , in p a r t , b y a g r a n t f r o m the E. R o d r i g u e z Pascual F o u n d a t i o n to Dr. A. R u i z - M a r c o s . W e are grateful to Miss M. C. Vidal a n d Miss P a l o m a L a r g a c h a for their assistance in p r e p a r i n g the histological material, to Miss E. F d e z . de M o l i n a for t y p i n g the m a n u s c r i p t a n d to Miss C. G a b r i e l de R u i z - M a r c o s for her assistance in the c o m p u t e r work.
REFERENCES 1 Cajal, S. R. and de Castro, F., Elementos de Tdcnica Microgrtifica delSistema Nervioso. Salvat Edit S.A. Madrid, 1972, pp. 65-68. 2 Dec, K., Sarna, M., Tarnechi, R. and Zernichi, B., Effect of binocular deprivation of pattern vision on single unit responses in the cat superior colliculus, Acta neurobiol, exp., 36 (1976) 687-692. 3 Diamond, M. C., Extensive cortical depth measurements and neuron size increases in the cortex of environmentally enriched rats, J. comp. NeuroL, 131 (1967) 357-364. 4 Diamond, M. C., Krech, D. and Rosenzweig, M. R., The effect of an enriched environment on the histology of the rat cerebral cortex, Y. comp. NeuroL, 123 (1964) 111-120. 5 Dunn, O. J. and Clark, V. A., Applied Statistics: Analysis of Variance and Regression. John Wiley and Sons, New York, 1974, pp. 80-81. 6 Globus, A., Rosenzweig, M. R., Bennett, E. L. and Diamond, M. C., Effects of differential experience on dendritic spine counts in rat cerebral cortex, J. comp. physioL PsychoL, 82 (1973) 175-181. 7 Greenough, W. T. and Volkmar, F. R., Pattern of dendritic branching in occipital cortex of rats reared in complex environments. Exp. NeuroL, 40 (1973) 491-504. 8 Hein, A. and Diamond, R. M., Locomotory space as a prerequisite for acquiring visually guided reaching in kittens, J. comp. physiol. PsychoL, 81 (1972) 394-398. 9 Held, R. and Bayer, Jr., J. A., Development of sensorially-guided reaching in infant monkeys, Brain Research, 71 (1974) 265-271. 10 Riesen, A. H. and Aarons, L., Visual movement and intensity discrimination in cats after early deprivation of pattern vision, J. eomo. physiol. Psychol., 52 (1959) 142-149. 11 Ruiz-Marcos, A. and Valverde, F., The temporal evolution of the distribution of spines in the visual cortex of normal and dark-reared mice, Exp. Brain Res., 8 (1969) 284-294. 12 Valverde, F. and Ruiz-Marcos, A., Dendritic spines in the visual cortex of the mouse. Introduction to a mathematical model, Exp. Brain Res., 8 (1969) 269-283. 13 Volkmar, F. R. and Grcenough, W. T., Rearing complexity affects branching of dendrites in the visual cortex of the rat, Science, 176 (1972) 1451-1447. 14 Zablocka, T., Zernichi, B. and Kosmal, A., Visual cortex role in object discrimination in cats deprived of pattern vision from birth, Acta neurobiol, exp., 36 (1976) 157-168.