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Age and serial ablations of somatosensory cortex in the rat
Physiology & Behavior, Vol. 17, pp. 13-17. Pergamon Press and Brain Research Publ., 1976. Printed in the U.S.A.
Age and Serial Ablations of Somatosensory Cortex in the Rat B O N N I E B. W A L B R A N 1"2
Department o f Psychology, Washington University, St. Louis, MO 63130 (Received 25 S e p t e m b e r 1975) WALBRAN, B. B. Age and serial ablations of somatosensory cortex in the rat. PHYSIOL. BEHAV. 17(1) 13-17, 1976. The present study investigated the effects of one-stage and two-stage ablations of somatosensory cortex at 30 days, 270 days and 570 days of age on the acquisition of tactile discriminations in rats. Deficits in the ability to acquire tactile discriminations following such cortical lesions appeared to be similar regardless of the age at which the damage was sustained. Serial placement of the lesions did not attenuate the deficits in 30-day or 570-day-old rats. Some mitigation of the impairment was present in animals sustaining serial surgery at 270 days of age. In contrast, rats sustaining one-stage lesions at 570 days of age performed somewhat better than their age-mates sustaining two-stage lesions. Factors possibly relevant to the serial lesion effect and to recovery of function in general are suggested. Age
Serial lesions
Somatosensory cortex
Recovery of function
Discrimination
METHOD
B I L A T E R A L lesions of somatosensory cortex in 60- to 90-day-old rats have been shown to result in deficits in the acquisition of tactile discriminations [3, 6, 251. However, when cortical tissue was r e m o v e d in 2 stages, sparing of such somatosensory functioning was observed [9}. Stein and Firl [20] reported similar results on a spatial alternation task following serial removal of frontal cortex f r o m young adult rats. In contrast, rats sustaining serial removal of frontal cortex at about 2 years of age showed a severe impairment on the spatial alternation task when compared with age-mates sustaining simultaneous damage. The only investigation of a g e - l e s i o n interaction effects on tactile discriminations that has been reported is a study by Benjamin and T h o m p s o n [1]. T h e y evaluated the effects of bilateral ablations of somatosensory cortex in n e w b o r n and adult cats. Severe deficits in the acquisition of tactile discriminations were observed when the adult cats were tested six m o n t h s after surgery. In contrast, after a similar recovery period, the animals operated in infancy were almost indistinguishable from normals. To date, no studies investigating possible interactions between age and the temporal parameters of surgery in the somatosensory system o f the rat have been reported. The present study posed 2 questions: 1) are the deficits observed in the acquisition of tactile discriminations following ablation of somatosensory cortical areas I and II in the rat similar regardless of the age at which the damage is sustained? and 2) can these deficits be mitigated by serial surgery?
Animals Data were collected for 3 groups of naive, male albino rats derived f r o m the Sprague-Dawley strain (Thorp Industries, Inc., White Bear Lake, Minnesota). The 30-day-old group included 35 animals, weighing an average of 125.7 g. There were 30 rats in the 270-day-old group, with an average weight of 618.9 g. The 570-day-old group was made up of 33 animals, weighing an average of 663.2 g. T h r o u g h o u t the experiment, the animals were housed by pairs of the same age in 30.5 cm x 36.6 cm × 17.7 cm plastic cages under constant illumination. All animals were habituated to the laboratory for at least 1 week prior to being assigned at random to 1 of 4 surgical groups.
Surgery Within each age group, there were 4 surgical conditions. Group 1 sustained aspirative ablations of somatosensory cortical areas I and II of the right hemisphere. Following surgery, they were returned to their h o m e cages for a 17 day recovery period. On Day 18, they received lesions of somatosensory cortical areas I and II of the left hemisphere. Group 2 rats received simultaneous bilateral lesions of the same somatosensory areas at the time that Group 1 u n d e r w e n t removal of the right somatosensory areas. Groups 3 and 4 had sham operations paralleling the one-stage and two-stage ablations. At the time of the initial
1This report is based on a dissertation submitted to Washington University in partial fulfillment of the requirements for the Ph.D. degree. The author wishes to thank her dissertation advisor, Stanley Finger, for making the facilities of his laboratory available and for his support and encouragement throughout the couse of this investigation. The research was supported in part by NIMH Grant MH-25397; NINDS Grant NS-11002; Biomedical Sciences Support Grant Fund 54993; GM 1900; and NICH & ND Training Grant HD-00047. Present address: Department of Psychiatry, Washington University School of Medicine, St. Louis, MO 63130. 13
14 or o n l y surgery, all a n i m a l s were e n u c l e a t e d to e l i m i n a t e visual cues d u r i n g testing. No a t t e m p t was m a d e to p r e v e n t the possible use of a u d i t o r y cues d u r i n g testing. Previous e x p e r i e n c e w i t h t h e same t e s t i n g a p p a r a t u s [3, 6. 9] i n d i c a t e d t h a t if s u c h cues were available, t h e y were n o t of a p p r e c i a b l e value in m a k i n g t h e d i s c r i m i n a t i o n s . Prior t o surgery, all a n i m a l s were deprived of food a n d w a t e r for 12 hr. Surgery on the 2 older g r o u p s was c o n d u c t e d u n d e r p e n t o b a r b i t a l s o d i u m a n e s t h e s i a (Diab u t a l , 4 0 50 m g / k g IP). E t h e r was used for t h e 30-day-old a n i m a l s b e c a u s e o f excessive m o r t a l i t y associated w i t h a trial use of p e n t o b a r b i t a l sodium. T h e areas to b e a b l a t e d were d e t e r m i n e d b y c o n s u l t i n g e l e c t r o p h y s i o l o g i c a l m a p s such as t h o s e o f Woolsey [ 2 4 ] , Welker [22] a n d Welker and S i n h a [ 2 3 ] . A b l a t i o n s o f cortical tissue were m a d e b y a s p i r a t i o n w i t h a glass p i p e t t e . A m i d l i n e i n c i s i o n c o n s t i t u t e d t h e s h a m o p e r a t i o n . All w o u n d s were s u t u r e d w i t h silk t h r e a d a n d t r e a t e d with Bacitracin. E a c h rat was i n j e c t e d w i t h 1 0 0 , 0 0 0 u penicillin (Crysticillin 6 0 0 AS) b e f o r e b e i n g r e t u r n e d to its h o m e cage. It s h o u l d b e n o t e d t h a t all r e c o v e r y periods were calculated f r o m t h e t i m e o f t h e initial surgery [ 7 ] .
Testing O n Day 29 of t h e r e c o v e r y period, all f o o d was r e m o v e d f r o m t h e cages. T h e a n i m a l s were segregated b y age into g r o u p s of 10 t o 15. E a c h g r o u p was allowed a 15 rain access to w e t m a s h ( P u r i n a rat c h o w m i x e d w i t h w a t e r ) e a c h d a y for t h e n e x t 5 days. D u r i n g t h e feeding periods, t h e animals were h a n d l e d to p r e p a r e t h e m for t h e t e s t i n g s i t u a t i o n . In t h e h o m e cages, w a t e r was available at all times. T e s t i n g for t h e ability to d i s c r i m i n a t e b e t w e e n d i f f e r e n t tactile surfaces b e g a n o n D a y 34. All t e s t i n g was d o n e in a T - m a z e previously d e s c r i b e d b y Finger. C o h e n a n d Alongi [ 6 ] . T h e floors of the wings of t h e maze were covered b y c h a n g e a b l e a l u m i n u m d i s c r i m i n a n d a . 6.35 m m t h i c k and 8 8 . 9 0 cm long. In e a c h o f t h e 2 tasks, a plate bearing milled cuts was paired against a s m o o t h , u n c u t plate. F o r the first d i s c r i m i n a t i o n , t h e milled plate had parallel grooves 0.7~ m m deep a n d 1.57 m m wide s e p a r a t e d b y ridges 4 . 6 4 m m wide r u n n i n g f r o m t h e c h o i c e p o i n t to the f o o d cup. T h e s e c o n d d i s c r i m i n a t i o n , a m o r e difficull one. was b e t w e e n t h e s m o o t h p l a t e a n d a plate m a r k e d w i t h fine triangular scorings 0.04 m m wide at t h e surface and 3.93 m m apart. A n i m a l s of e a c h surgical g r o u p were assigned at r a n d o m to 1 o f 2 test c o n d i t i o n s , c o r r e s p o n d i n g to w h e t h e r t h e s m o o t h plate or t h e r o u g h plate was t h e positive stimulus. This was c o u n t e r b a l a n c e d in o r d e r to c o n t r o l for possible surface p r e f e r e n c e effects. E a c h a n i m a l was given 5 trials per day. T h e left-right p o s i t i o n o f t h e positive s t i m u l u s also was d e t e r m i n e d b y a r a n d o m p r o c e d u r e , w i t h t h e res t r i c t i o n t h a t t h e s t i m u l i could n o t a p p e a r in t h e s a m e wings of t h e maze m o r e t h a n 3 t i m e s in a row for a n y a n i m a l o n a given day. T o facilitate testing, a n i m a l s were assigned to s q u a d s on t h e basis o f age and the n u m b e r of t i m e s t h e y h a d b e e n a n e s t h e t i z e d . W i t h i n each s q u a d t h e o r d e r in w h i c h t h e a n i m a l s were t e s t e d was r a n d o m i z e d at the s t a r t of e a c h trial. Wet m a s h was placed in a f o o d cup al t h e e n d of e a c h d i s c r i m i n a n d u m to b a l a n c e o l f a c t o r y cues: b u t a s h e e t m e t a l b a r r i e r b l o c k e d access to t h e f o o d cup in t h e wing c o n t a i n i n g t h e negative stimulus. R e t r a c i n g was p r e v e n t e d b y l o w e r i n g t h e guillotine d o o r as s o o n as t h e a n i m a l left t h e c h o i c e p o i n t . A n i m a l s m a k i n g a c h o i c e were allowed to r e m a i n in t h e m a z e for a p p r o x i m a t e l y 2 0 sec, regardless of w h e t h e r t h e c h o i c e was correct or i n c o r r e c t .
WALBRAN A f t e r an e n t i r e s q u a d h a d gone t h r o u g h t h e maze. t h e a l u m i n u m plates were w a s h e d w i t h a I.ysol s o l u t i o n and dried b e f o r e t h e n e x t trial in an a t t e m p t t o m i n i m i z e o d o r trails. T h e r e was a m i n i m u m interval of 5 rain b e t w e e n trials w i t h t h e same animal. T h e first d i s c r i m i n a t i o n was t e r m i n a t e d w h e n an a n i m a l m e t an a priori c r i t e r i o n of a p e r f e c t score on a n y day, p r e c e d e d by n o t m o r e t h a n 1 e r r o r on t h e previous 2 days. W h e n a rat m a s t e r e d t h e first d i s c r i m i n a t i o n , the less difficult grooved surface was replaced by t h e more difficult d i s c r i m i n a n d u m . A n i m a l s originally r e w a r d e d for c h o o s i n g the s m o o t h s t i m u l u s c o n t i n u e d to be so r e w a r d e d , and vice versa. "resting was t e r m i n a t e d for an a n i m a l if a p r o b l e m was not m a s t e r e d w i t h i n 72 days. This was a p p r o x i m a t e l y 3 times t h e m e a n n u m b e r of days r e q u i r e d by all groups of c o n t r o l a n i m a l s to m a s t e r t h e first p r o b l e m . An e s t i m a t e d score o f 75 days was assigned to a n i m a l s n o t learning a problem.
Histology A t the end of t h e testing, each a n i m a l was a n e s t h e t i z e d deeply w i t h b a r b i t u r a t e anesthesia. P e r f u s i o n was accomplished with 0.9 p e r c e n t saline f o l l o w e d b y l O percenl F o r m a l i n injected i n t o t h e aorta. Brains were p o s t f i x e d in 10 percent F o r m a l i n for at least 1 m o n t h before slicing. Dorsal and lateral views of each ablated b r a i n were p h o t o g r a p h e d . T h e p h o t o g r a p h s were p r o j e c t e d o n t o p a p e r and d r a w n t o t h e same scale t o facilitate t h e d e t e r m i n a t i o n of t h e e x t e n t o f t h e surface lesions. T w o i n d e p e n d e n t .judges t h e n used the d e l i n e a t i o n of cortical s o m a t o s e n s o r y areas r e p o r t e d b y W e l k e r ' s l a b o r a t o r y [ 2 2 , 2 3 ] as a s t a n d a r d for r a n k i n g t h e d r a w i n g s in t e r m s o f size of s o m a t o s e n s o r y lesions. C o r o n a l s e c t i o n s were cut a~ 50 ~ o n a freezing m i c r o t o m e . S e c t i o n s were stained w i t h cresyl violet for e x a m i n a t i o n w i t h a low p o w e r m i c r o s c o p e to d e t e r m i n e possible s u b c o r t i c a l d a m a g e and t h a l a m i c d e g e n e r a t i o n in the v e n t r o b a s a l c o m p l e x . RESULTS
Behavioral Testing N o n - p a r a m e t r i c statistical tests were used for all data anlayses because t h e a s s u m p t i o n s u n d e r l y i n g p a r a m e t r i c tests, e.g., h o m o g e n e i t y of variance, could n o t be m e t [ 1 9 ] . Table I presents t h e m e d i a n ( M d n ) and s e m i - i n t e r q u a r t i l e range (Q) for the n u m b e r of days to c r i t e r i o n for each of the 12 s u b g r o u p s in this e x p e r i m e n t . A n a l y s i s of the e r r o r s - t o - c r i t e r i o n m e a s u r e p r o v i d e d a similar p i c t u r e a n d is n o t r e p o r t e d . All c o n t r o l a n i m a l s learned t h e d i s c r i m i n a t i o n rapidly. D i f f e r e n c e s b e t w e e n these 6 s u b g r o u p s were n o t significant (Kruskal-Wallis l - w a y A N O V A ; H = 4.6, dJ = 5, p > 0 . 0 5 ) . T h e 6 e x p e r i m e n t a l g r o u p s all s h o w e d a deficil in tactile a c q u i s i t i o n ability w h e n c o m p a r e d to t h e i r respective c o n t r o l s ( M a n n - W h i t n e y U tests, all p ' s < 0 . 0 5 ) . H o w e v e r . there were n o significant d i f f e r e n c e s in p e r f o r m a n c e a m o n g the rats s u s t a i n i n g one-stage a b l a t i o n s at d i f f e r e n t ages (Kruskal-WaUis 1-way A N O V A : H = 2.61 : d f = 2, p > 0.05). In c o n t r a s t , rats s u s t a i n i n g t w o - s t a g e a b l a t i o n s did s h o w an age-related d i f f e r e n c e in p e r f o r m a n c e ( H = 12.29: d f = 2. p<0.01). Serial p l a c e m e n t a t t e n u a t e d t h e effects of the s o m a t i c cortical lesions w h e n m a d e at 2 7 0 d a y s o f age, b u t n o t at 30 or 5 7 0 days. Indeed, at 570 days o f age, t h e d i f f e r e n c e
AGE AND S E R I A L A B L A T I O N S
15
TABLE 1 MEDIAN NUMBER OF DAYS AND PERCENTAGES TO CRITERION ON DISCRIMINATION 1 SURGICAL CONDITION Ablated Rats Control Rats Age at Surgery 30 days
270 days
570 days
l-Stage
2-Stage
l-Stage
2-Stage
74.7 18.5 11 45.5 62.0* 14.0 8 62.5~ 51.0 10.5 11 81.8t
74.7 5.5 10 40.0 36.0 9.8 10 90.0 74.8 5.0 11 36.5
19.0 6.7 7 100.0 21.0 6.7 5 100.0 25.0 4.3 5 100.0
21.0 19.6 7 100.0 23.0 7.0 7 100.0 24.5 7.0 6 100.0
Mdn Q N % to Criterion Mdn Q N % to Criterion Mdn Q N % to Criterion
Note: All ablated groups differed significantly from their respective controls (Mann-Whitney U tests, p ' s < 0.05). * = p <0.025. -t = p < o.001.
was in the o p p o s i t e d i r e c t i o n , a l t h o u g h it was not statistically significant. The u n e x p e c t e d superior p e r f o r m a n c e o f the animals sustaining one-stage ablations at this age was statistically reliable on the p e r f o r m a n c e measure of percentage of animals reaching criterion (X2 = 40.60, p < 0 . 0 0 1 ; see Table 1). In an e f f o r t to d e t e r m i n e w h e t h e r the d i f f e r e n c e s in learning scores m i g h t be a t t r i b u t a b l e to a r e l u c t a n c e to run
the maze, t h e n u m b e r o f trials on w h i c h each animal refused to leave the start b o x was tallied for D i s c r i m i n a t i o n 1. There was no evidence t h a t any o n e age or surgical group was m o r e or less likely t h a n any o t h e r to c o n t a i n animals which did n o t run (all p ' s > 0 . 0 5 ) . The reason, perhaps, was that the animals w i t h the greatest deficits were eliminated w h e n t h e y failed to m a s t e r the first d i s c r i m i n a t i o n . Overall, results f r o m D i s c r i m i n a t i o n 2 were valuable only insofar as
j
~.
r
j
I
FIG. 1. Composite drawings of surface lesions. Two lines under an animal's study number indicate that both problems were mastered. One line indicates that only the first problem was mastered. An absence of underlining indicates failure to master the first problem.
16
WALBRAN
t h e y d e m o n s t r a t e d t h a t t h e a n i m a l s s u s t a i n i n g serial lesions at 2 7 0 days of age were not able to m a i n t a i n t h e i r initial a d v a n t a g e o n a very difficult task.
Lesion Size and Placement In general, w h e n t h e scaled r e p r o d u c t i o n s of t h e brains of a n i m a l s s u s t a i n i n g lesions were c o m p a r e d w i t h t h e Welker [ 2 2 , 2 3 ] d e l i n e a t i o n o f s o m a t o s e n s o r y cortical areas. the lesions were f o u n d to be placed as i n t e n d e d . In every case, t h e r e was d a m a g e to s o m a t o s e n s o r y c o r t e x bilaterally. H o w e v e r , t h e c o m p l e t e n e s s o f t h e d a m a g e varied. D r a w i n g s of t h e surface lesions are s h o w n in Fig. 1. In t h e 3 0 - d a y - o l d and 5 7 0 - d a y - o l d g r o u p s , lesions placed serially were larger in surface area d e s t r o y e d t h a n t h o s e placed s i m u l t a n e o u s l y ( M a n n - W h i t n e y U tests, p ' s < 0 . 0 5 ) . In no case, h o w e v e r , was t h e r e d a m a g e to s u b c o r t i c a l s t r u c t u r e s s u c h as t h e s e p t u m , h i p p o c a m p u s , or dorsal t h a l a m u s . T h e r e was f r e q u e n t d a m a g e to t h e c o r p u s callosum in all groups, e x c e p t in t h e 5 7 0 - d a y - o l d animals sustaining one-stage lesions. This g r o u p had smaller lesions b o t h in t e r m s of surface size a n d d e p t h . R e t r o g r a d e d e g e n e r a t i o n in t h e v e n t r o b a s a l c o m p l e x was assessed b y c o m p a r i n g m o u n t e d s e c t i o n s f r o m ablated brains with c o m p a r a b l e s e c t i o n s f r o m several r e p r e s e n t a t i v e c o n t r o l brains. T h e d e g e n e r a t i o n p a t t e r n s were similar. regardless of w h e t h e r a surface lesion was relatively large or small. T h e r e also were n o d i f f e r e n c e s in d e g e n e r a t i o n p a t t e r n s b e t w e e n lesions m a d e s i m u l t a n e o u s l y or serially. In a d d i t i o n , t h a l a m i c d e g e n e r a t i o n p a t t e r n s in t h e 570-day-old a n i m a l s s u s t a i n i n g one-stage lesions were i n d i s t i n g u i s h a b l e f r o m t h o s e o f t h e o t h e r groups. Overall, d i f f e r e n c e s in lesion size did n o t have a c o n s i s t e n t effect o n b e h a v i o r a l p e r f o r m a n c e (see Fig. 1 t. W i t h i n t h e 30-day-old animals, t h o s e rats s u s t a i n i n g serial surgery had t h e larger lesions, b u t t h e i r p e r f o r m a n c e was similar to t h a t of a g e - m a t c h e d a n i m a l s s u s t a i n i n g simult a n e o u s damage. A l t h o u g h t h e lesions m the 270-day-old a n i m a l s t e n d e d to be e q u i v a l e n t i n d e p e n d e n t of t h e t e m p o r a l s e q u e n c e of t h e surgery, t h e a n i m a l s s u s t a i n i n g serial d a m a g e p e r f o r m e d b e t t e r . In t h e 5 7 0 - d a y - o l d animals, the rats s u s t a i n i n g s i m u l t a n e o u s d a m a g e h a d smaller lesions t h a n t h o s e s u s t a i n i n g serial d a m a g e and s h o w e d b e t t e r p e r f o r m a n c e . T h e s e d a t a m i g h t suggest t h a t the serial lesion effect was n e g a t e d b y relatively large lesions. H o w e v e r . t h e r e were n o significant c o r r e l a t i o n s b e t w e e n size of the surface lesion and b e h a v i o r a l p e r f o r m a n c e , regardless of w h e t h e r c o r r e l a t i o n c o e f f i c i e n t s were calculated f o r each of the 6 s u b g r o u p s , f o r t h e 3 age g r o u p s , or for t h e t o t a l g r o u p of a n i m a l s s u s t a i n i n g lesions. A f u r t h e r a t t e m p t was m a d e to c o n t r o l f o r t h e e f f e c t of lesion stze in t h e y o u n g a n d old groups, since t h e s e were t h e 2 g r o u p s s h o w i n g s i g n i f i c a n t lesion size d i f f e r e n c e s b u t n o serial lesion effect. In each g r o u p , a s u b - g r o u p c o m p o s e d of o n l y t h o s e lesions w h i c h could b e m a t c h e d for surface size a n d s u b c o r t i c a l d e p t h was assembled. T h e r e were n o significant d i f f e r e n c e s m perf o r m a n c e s o f t h e a n i m a l s in e i t h e r g r o u p ( M a n n - W h i t n e y tests: b o t h p ' s > 0 . 0 5 ) . In fact, in several instances, a n i m a l s w i t h c o m p a r a b l e lesions e x h i b i t e d widely divergent perf o r m a n c e s . This a p p e a r e d to b e a m a t t e r o f individual differences. T h u s , even w h e n lesion size was c o n t r o l l e d , the serial lesion e f f e c t a p p e a r e d o n l y in t h e 2 7 0 - d a y - o l d group. DISCUSSION
T h e d a t a collected in this s t u d y c o n f i r m e d t h e findings
t h a t one-stage lesions of t h e s o m a t o s e n s o r y cortex result m severe deficits in tactile h a b i t a c q u i s i t i o n [3, 6, 2 5 ] These deficits a p p e a r e d to be i n d e p e n d e n t of the age at w h i c h tile d a m a g e was sustained. Similar deficits also a p p e a r e d m animals s u s t a i n i n g serial d a m a g e to s o m a t o s e n s o r y c o r t e x at 30 days and 5 7 0 days of age. T h e deficits seen in animals sustaining serial d a m a g e at 270 days of age, h o w e v e r , were less severe t h a n t h o s e e x h i b i t e d b y animals sustaining s i m u l t a n e o u s d a m a g e at the same age. A m o n g t h o s e a n i m a l s u n d e r g o m ~ surgery at 570 days ot age, t h o s e s u s t a i n i n g s i m u l t a n e o u s d a m a g e t e n d e d to b c more likely to m a s t e r t h e first task t h a n those sustaining serial damage. This result agrees with those of Stein and Firl [201. It is possible t h a t w i t h age the ability to t o l e r a t e general a n e s t h e s i a a n d ' o r t r a u m a declines~ The p o o r e r p e r f o r m a n c e of t h e aged animats s u s t a i n i n g two-stage lesions m a y relate to s u c h a decline. On t h e o t h e r h a n d . ~t may be t h a t lesion size played a role at this age since the old animals w i t h s i m u l t a n e o u , lesions t e n d e d to have e x p e r i e n c e d less extensive CNS damage. T h e serial lesiot3 effect has been r e p o r t e d in a d o l e s c e n t m o n k e y s f o l l o w i n g multi-stage a b l a t i o n s of p r e f r o n t a l cortex in spite of larger lesions t h a n t h o s e sustained b y single-stage p r e p a r a t i o n s [ 16]. In the p r e s e n t s t u d y , b o t h 30-day-old surgical groups p e r f o r m e d similarly, a l t h o u g h the serial animats again had the larger lesions. T h e lack o f a reliable c o r r e l a t i o n b e t w e e n lesion size and behavioral p e r f o r m a n c e t e n d s to argue against a c o n s i s t e n t c o n t r i b u t i o n of lesion size ro recovery of f u n c t i o n . However. the possibility ~xists t h a t lesion size may i n t e r a c t d i f f e r e n t i a l l y w i t h the age, species and 'or area sustaining damage. The a c q u i s i t i o n deficits o b s e r v e d m t h e p r e s e n t s t u d y appeared to be q u a n t i t a t i v e r a t h e r t h a n q u a l i t a t i v e since m a n y rats with lesions did learn the task a l t h o u g h t h e y required a m u c h longer t i m e to do so. l n a s m u c h as t h e analysis of t h e n u m b e r of trials on which animals refused ~o leave the start b o x did n o t reveal any t r e n d s , these deficits p r o b a b l y were n o t m o t i v a t i o n a l . In a d d i t i o n , all animals ate eagerly in the maze It also seems unlikely t h a t t h e deficits were related to a general r e t a r d a t i o n in learning. Rats sustaining lesions o f s o m a t o s e n s o r y c o r t e x at 90 to 120 days old have been s h o w n to he capable ol a c q m r i n g t e m p e r a t u r e d i s c r i m i n a t i o n s m t h e same a m o u n t o f time as c o n t r o l a n i m a l s [ 4 , 8 ] . T h e fact t h a t some animals sustaining lesions e v e n t u a l l y learned H~e first d i s c r i m i n a t i o n suggests thai t h e deficits m a y relate ~o a ~Irategy failure or to a p e r c e p t u a l difficulty. P e r h a p s either or b o t h of these m a y "sugges! an e l e v a t i o n of r o u g h n e s s t h r e s h o l d s . Dis.c r i m i n a t i o n t h r e s h o l d s are k n o w n to ~mprove w i t h practice [ 171. It is i n t e r e s t i n g to n o t e in this regard t h a t the c o n t r o l animals s h o w e d no age-related changes m t h e ability to m a k e tactile d i s c r i m i n a t i o n s , l'his ~s c o n s i s t e n t with recent findings c o n c e r n i n g tactile t h r e s h o l d s m h u m a n s [1 a] T h e failure to observe an age-related d i f f e r e n c e m tactile f u n c t i o n i n g following d a m a g e to s o m a t o s e n s o r y c o r t e x does n o t necessarily c o n f l i c t w i t h previous findings of b e t t e r r e c o v e r y of f u n c t i o n f o l l o w i n g surgery very early m life b e c a u s e the y o u n g rats in the present s t u d y were 30 days old at the t i m e o f surgery. B e n j a m i n and T h o m p s o n ' s [11 i n f a n t cats. for i n s t a n c e , u n d e r w e n t surgery o n t h e sixth p o s t n a t a l day. A 30-day-old rat, a l t h o u g h n o t an i n f a n t , is still an i m m a t u r e animal. M y e l i n i z a t i o n of t h e ra! cortex, for i n s t a n c e , does n o t reach its adult s t a t e u n t i l a b o u t 60 days p o s t n a t a l l y [ 11 ]. It is possible t h a t in o r d e r to achieve sparing of tactile f u n c t i o n i n g following d a m a g e
AGE AND SERIAL ABLATIONS
17
to s o m a t o s e n s o r y cortex early in life, the d a m a g e m u s t o c c u r in the n e o n a t a l period r a t h e r t h a n in an i m m a t u r e period. In this s t u d y , serial surgery did n o t m i t i g a t e t h e tactile acquisition deficit in t h e y o u n g and old animals. T h e r e was some a t t e n u a t i o n of t h e deficit in t h e 270-day-old animals. T h e d e n e r v a t i o n s u p e r s e n s i t i v i t y p o s t u l a t i o n of a l t e r n a t e p a t h w a y s [2[ or Norrsell's [15] suggestion of parallel circuitry b o t h provide t h e basis for a possible e x p l a n a t i o n . It is at least plausible t h a t in t h e y o u n g animals, t h e p o s t u l a t e d a l t e r n a t e p a t h w a y s were n o t yet f u n c t i o n a l , while in t h e old animals, t h e possibility of reactivating t h e s e p a t h w a y s had d i m i n i s h e d as a result of p r o l o n g e d disuse or general d e t e r i o r a t i o n . H y p o t h e t i c a l l y , it m i g h t b e assumed t h a t the m a t u r e animals had a l t e r n a t e f u n c t i o n a l p a t h w a y s capable of carrying tactile i n f o r m a t i o n . T h e r e f o r e , t h e y e x h i b i t e d the m o r e control-like p e r f o r m a n c e s . This t e n d s to agree w i t h LeVere's [12] r e c e n t suggestion t h a t behavioral recovery results f r o m spared neural m e c h a n i s m s . One variable long k n o w n to be of i m p o r t a n c e to the serial lesion effect is the l e n g t h of t h e i n t e r l e s i o n interval. Studies r e p o r t i n g the m i n i m u m i n t e r o p e r a t i v e period necessary for sparing of sensory f u n c t i o n i n g to be less t h a n 2 weeks have e m p l o y e d r e t e n t i o n designs [ 1 4 , 2 1 ] . It has been suggested t h a t a c q u i s i t i o n f u n c t i o n s m a y require a longer i n t e r l e s i o n interval t h a n r e t e n t i o n f u n c t i o n s to achieve m a x i m a l p o s t o p e r a t i v e r e c o v e r y [ 7 ] . T h e one s t u d y
[9] d e m o n s t r a t i n g near-zero deficits in tactile h a b i t acquisition following serial surgery utilized an interlesion interval nearly twice as long as t h a t used in the present s t u d y a n d a highly generalizable series o f tasks. T h e m a t t e r of task difficulty has b e e n discussed in several papers [5, 10, 1 8 ] . Isaacson and S c h m a l t z [10] suggested t h a t it m i g h t be impossible to o b t a i n t h e serial lesion effect w h e n the task was o n e designed to reveal " c a p a c i t y difficulties", such as D R L - 2 0 tasks. T h e tasks used in t h e p r e s e n t s t u d y were t h e 2 m o s t difficult tasks in t h e series used in the original investigation [ 9 ] . Simons, P u r e t z and Finger [18] were u n a b l e to o b t a i n the serial lesion effect in a r e t e n t i o n design w i t h the same task as the o n e used in the present study. However, Finger and S i m o n s [5] d e m o n s t r a t e d t h e serial lesion effect in a r e t e n t i o n design identical to the earlier s t u d y [18] e x c e p t for the use o f easier tasks. These data e m p h a s i z e the need to define precisely the tasks used to assess b e h a v i o r a l f u n c t i o n s . It would a p p e a r t h a t the relevant p a r a m e t e r s will include n o t only the i n h e r e n t difficulty of the task and t h e experim e n t a l paradigm b u t also the previous e x p e r i e n t i a l h i s t o r y of the subjects. F u r t h e r , t h e possibility t h a t a n y particular variable m a y exert d i f f e r e n t i a l effects at various t i m e s in the life of the organism m a k e s it i m p o r t a n t to collect a b r o a d range of p e r f o r m a n c e data for n o r m a l animals against w h i c h the b e h a v i o r of animals with lesions can be compared.
REFERENCES 1.
2. 3. 4. 5. 6. 7. 8.
9.
10. ll. 12. 13.
Benjamin, R. M. and R. F. Thompson. Differential effects of cortical lesions in infant and adult cats on roughness discrimination. Expl Neurol. 1:305 321, 1959. Cannon, W. F. and A. Rosenblueth, The Supersensitivity of Denervated Structures. New York: Macmillan, 1949, pp. 1-237. Finger, S. and G. P. Frommer. Effects of somatosensory thalamic and cortical lesions on roughness discrimination in the albino rat. Physiol. Behav. 3: 8 3 - 8 9 , 1968. Finger, S. and G. P. Frommer. Effects of cortical and thalamic lesions on temperature discrimination and responsiveness to foot shock in the albino rat. Brain Res. 2 4 : 6 9 89, 1970. Finger, S. and D. Simons. Effects of serial lesions of somatosensory cortex and further neodecortication on retention of a rough-smooth discrimination in rats. Expl Brain Res. (in press). Finger, S., M. Cohen and R. Alongi. Roles of somatosensory cortical areas 1 and 2 in tactile discrimination in the rat. hrt. J. Psychobiol. 2: 9 3 - 1 0 1 , 1972. Finger, S., B. Walbran and D. G. Stein. Brain damage and behavioral recovery: serial lesion phenomena. Brain Res. 63: 1 18, 1973. I~inger, S., S. Scheff, I. Warshaw and M. Cohen. Retention and acquisition of fine temperature discriminations following somatosensory cortical lesions in the rat. Expl Brain Res. 10: 340 346, 1970. Finger, S., R. A. Marshak, M. Cohen, S. Scheff, R. Trace and D. Niemand. Effects of successive and simultaneous lesions of somatosensory cortex on tactile discrimination in the rat. J. comp. physiol. Psychol. 77: 221-227, 1971. lsaacson, R. L. and L. W. Schmaltz. Failure to find savings f r o m spaced, two-stage destruction of hippocampus. Communs. Behav. Biol. 1:353 359, 1968. Jacobson. S. Sequence of myelinization in the brain of the albino rat. A. Cerebral cortex, thalamus and related structures. J. comp. Neurol. 121: 5 - 3 1 , 1963. keVere, T. E. Neural stability, sparing and behavioral recovery following brain damage. Psychol. Rev. 82: 344-358, 1975. Levin, H. S. and A. L. Benton. Age and susceptibility to tactile masking effects. Geront. Clin. 15: 1-9, 1973.
14. Meyer, D. R., W. Isaac and B. Maher. The role of stimulation in spontaneous reorganization of visual habits. J. comp. physiol. Psychol. 5 1 : 5 4 6 548, 1958. 15. Norrsell, U. A conditioned reflex study of sensory defects caused by cortical somatosensory ablations. Physiol. Behav. 2: 73-81,1967. 16. Rosen, J., D. Stein and N. Butters. Recovery of function after serial ablation of prefrontal cortex in the rhesus monkey. Science 173:353 356, 1971. 17. Semmes, J. and M. Mishkin. Somatosensory loss in monkeys after ipsilateral cortical ablation. J. Neurophysiol. 28: 4 7 3 - 4 8 6 , 1965. 18. Simons, D. J., J. Puretz and S. Finger. Effects of serial lesions of somatosensory cortex and further neodecortication on tactile retention in rats. Expl. Brain Res. 23: 353-365, 1975. 19. Siegel, S. Nonparametric Statistics' for the Behavioral Sciences. New York : McGraw-Hill, 1956. 20. Stein, D. G. and A. Firl. Failure to find recovery of function after 2-stage frontal lesions in aged rats. Paper presented at the Fourth Annual Meeting of the Society for Neurosciences, St. Louis, Mo., October 2 0 - 2 4 , 1974. 21. Stewart, J. W. and H. W. Ades. The time factor in reintegration of a learned habit lost after temporal lesions in the monkey (Macaca mulatta). J. comp. physiol. Prychol. 44: 4 7 9 - 4 8 6 , 1951. 22. Welker, C. Microelectrode delineation of fine grain somatotopic organization of Sm I cerebral neocortex in albino rat. BrainRes. 26: 259-275, 1971. 23. Welker, C. and M. M. Sinha. Somatotopic organization of Sm II cerebral neocortex in albino rat. Brain Res. 3 7 : 1 3 2 136, 1972. 24. Woolsey, C. M. Patterns of sensory representation in the cerebral cortex. Fedn. Proe. 6 : 4 3 7 441,1947. 25. Zubek, J. P. Studies in somesthesis. Ill. Role of somatic areas 1 and II in roughness discrimination in the rat. Can. J. Psychol. 6: 183-193, 1952.
Age and Serial Ablations of Somatosensory Cortex in the Rat B O N N I E B. W A L B R A N 1"2
Department o f Psychology, Washington University, St. Louis, MO 63130 (Received 25 S e p t e m b e r 1975) WALBRAN, B. B. Age and serial ablations of somatosensory cortex in the rat. PHYSIOL. BEHAV. 17(1) 13-17, 1976. The present study investigated the effects of one-stage and two-stage ablations of somatosensory cortex at 30 days, 270 days and 570 days of age on the acquisition of tactile discriminations in rats. Deficits in the ability to acquire tactile discriminations following such cortical lesions appeared to be similar regardless of the age at which the damage was sustained. Serial placement of the lesions did not attenuate the deficits in 30-day or 570-day-old rats. Some mitigation of the impairment was present in animals sustaining serial surgery at 270 days of age. In contrast, rats sustaining one-stage lesions at 570 days of age performed somewhat better than their age-mates sustaining two-stage lesions. Factors possibly relevant to the serial lesion effect and to recovery of function in general are suggested. Age
Serial lesions
Somatosensory cortex
Recovery of function
Discrimination
METHOD
B I L A T E R A L lesions of somatosensory cortex in 60- to 90-day-old rats have been shown to result in deficits in the acquisition of tactile discriminations [3, 6, 251. However, when cortical tissue was r e m o v e d in 2 stages, sparing of such somatosensory functioning was observed [9}. Stein and Firl [20] reported similar results on a spatial alternation task following serial removal of frontal cortex f r o m young adult rats. In contrast, rats sustaining serial removal of frontal cortex at about 2 years of age showed a severe impairment on the spatial alternation task when compared with age-mates sustaining simultaneous damage. The only investigation of a g e - l e s i o n interaction effects on tactile discriminations that has been reported is a study by Benjamin and T h o m p s o n [1]. T h e y evaluated the effects of bilateral ablations of somatosensory cortex in n e w b o r n and adult cats. Severe deficits in the acquisition of tactile discriminations were observed when the adult cats were tested six m o n t h s after surgery. In contrast, after a similar recovery period, the animals operated in infancy were almost indistinguishable from normals. To date, no studies investigating possible interactions between age and the temporal parameters of surgery in the somatosensory system o f the rat have been reported. The present study posed 2 questions: 1) are the deficits observed in the acquisition of tactile discriminations following ablation of somatosensory cortical areas I and II in the rat similar regardless of the age at which the damage is sustained? and 2) can these deficits be mitigated by serial surgery?
Animals Data were collected for 3 groups of naive, male albino rats derived f r o m the Sprague-Dawley strain (Thorp Industries, Inc., White Bear Lake, Minnesota). The 30-day-old group included 35 animals, weighing an average of 125.7 g. There were 30 rats in the 270-day-old group, with an average weight of 618.9 g. The 570-day-old group was made up of 33 animals, weighing an average of 663.2 g. T h r o u g h o u t the experiment, the animals were housed by pairs of the same age in 30.5 cm x 36.6 cm × 17.7 cm plastic cages under constant illumination. All animals were habituated to the laboratory for at least 1 week prior to being assigned at random to 1 of 4 surgical groups.
Surgery Within each age group, there were 4 surgical conditions. Group 1 sustained aspirative ablations of somatosensory cortical areas I and II of the right hemisphere. Following surgery, they were returned to their h o m e cages for a 17 day recovery period. On Day 18, they received lesions of somatosensory cortical areas I and II of the left hemisphere. Group 2 rats received simultaneous bilateral lesions of the same somatosensory areas at the time that Group 1 u n d e r w e n t removal of the right somatosensory areas. Groups 3 and 4 had sham operations paralleling the one-stage and two-stage ablations. At the time of the initial
1This report is based on a dissertation submitted to Washington University in partial fulfillment of the requirements for the Ph.D. degree. The author wishes to thank her dissertation advisor, Stanley Finger, for making the facilities of his laboratory available and for his support and encouragement throughout the couse of this investigation. The research was supported in part by NIMH Grant MH-25397; NINDS Grant NS-11002; Biomedical Sciences Support Grant Fund 54993; GM 1900; and NICH & ND Training Grant HD-00047. Present address: Department of Psychiatry, Washington University School of Medicine, St. Louis, MO 63130. 13
14 or o n l y surgery, all a n i m a l s were e n u c l e a t e d to e l i m i n a t e visual cues d u r i n g testing. No a t t e m p t was m a d e to p r e v e n t the possible use of a u d i t o r y cues d u r i n g testing. Previous e x p e r i e n c e w i t h t h e same t e s t i n g a p p a r a t u s [3, 6. 9] i n d i c a t e d t h a t if s u c h cues were available, t h e y were n o t of a p p r e c i a b l e value in m a k i n g t h e d i s c r i m i n a t i o n s . Prior t o surgery, all a n i m a l s were deprived of food a n d w a t e r for 12 hr. Surgery on the 2 older g r o u p s was c o n d u c t e d u n d e r p e n t o b a r b i t a l s o d i u m a n e s t h e s i a (Diab u t a l , 4 0 50 m g / k g IP). E t h e r was used for t h e 30-day-old a n i m a l s b e c a u s e o f excessive m o r t a l i t y associated w i t h a trial use of p e n t o b a r b i t a l sodium. T h e areas to b e a b l a t e d were d e t e r m i n e d b y c o n s u l t i n g e l e c t r o p h y s i o l o g i c a l m a p s such as t h o s e o f Woolsey [ 2 4 ] , Welker [22] a n d Welker and S i n h a [ 2 3 ] . A b l a t i o n s o f cortical tissue were m a d e b y a s p i r a t i o n w i t h a glass p i p e t t e . A m i d l i n e i n c i s i o n c o n s t i t u t e d t h e s h a m o p e r a t i o n . All w o u n d s were s u t u r e d w i t h silk t h r e a d a n d t r e a t e d with Bacitracin. E a c h rat was i n j e c t e d w i t h 1 0 0 , 0 0 0 u penicillin (Crysticillin 6 0 0 AS) b e f o r e b e i n g r e t u r n e d to its h o m e cage. It s h o u l d b e n o t e d t h a t all r e c o v e r y periods were calculated f r o m t h e t i m e o f t h e initial surgery [ 7 ] .
Testing O n Day 29 of t h e r e c o v e r y period, all f o o d was r e m o v e d f r o m t h e cages. T h e a n i m a l s were segregated b y age into g r o u p s of 10 t o 15. E a c h g r o u p was allowed a 15 rain access to w e t m a s h ( P u r i n a rat c h o w m i x e d w i t h w a t e r ) e a c h d a y for t h e n e x t 5 days. D u r i n g t h e feeding periods, t h e animals were h a n d l e d to p r e p a r e t h e m for t h e t e s t i n g s i t u a t i o n . In t h e h o m e cages, w a t e r was available at all times. T e s t i n g for t h e ability to d i s c r i m i n a t e b e t w e e n d i f f e r e n t tactile surfaces b e g a n o n D a y 34. All t e s t i n g was d o n e in a T - m a z e previously d e s c r i b e d b y Finger. C o h e n a n d Alongi [ 6 ] . T h e floors of the wings of t h e maze were covered b y c h a n g e a b l e a l u m i n u m d i s c r i m i n a n d a . 6.35 m m t h i c k and 8 8 . 9 0 cm long. In e a c h o f t h e 2 tasks, a plate bearing milled cuts was paired against a s m o o t h , u n c u t plate. F o r the first d i s c r i m i n a t i o n , t h e milled plate had parallel grooves 0.7~ m m deep a n d 1.57 m m wide s e p a r a t e d b y ridges 4 . 6 4 m m wide r u n n i n g f r o m t h e c h o i c e p o i n t to the f o o d cup. T h e s e c o n d d i s c r i m i n a t i o n , a m o r e difficull one. was b e t w e e n t h e s m o o t h p l a t e a n d a plate m a r k e d w i t h fine triangular scorings 0.04 m m wide at t h e surface and 3.93 m m apart. A n i m a l s of e a c h surgical g r o u p were assigned at r a n d o m to 1 o f 2 test c o n d i t i o n s , c o r r e s p o n d i n g to w h e t h e r t h e s m o o t h plate or t h e r o u g h plate was t h e positive stimulus. This was c o u n t e r b a l a n c e d in o r d e r to c o n t r o l for possible surface p r e f e r e n c e effects. E a c h a n i m a l was given 5 trials per day. T h e left-right p o s i t i o n o f t h e positive s t i m u l u s also was d e t e r m i n e d b y a r a n d o m p r o c e d u r e , w i t h t h e res t r i c t i o n t h a t t h e s t i m u l i could n o t a p p e a r in t h e s a m e wings of t h e maze m o r e t h a n 3 t i m e s in a row for a n y a n i m a l o n a given day. T o facilitate testing, a n i m a l s were assigned to s q u a d s on t h e basis o f age and the n u m b e r of t i m e s t h e y h a d b e e n a n e s t h e t i z e d . W i t h i n each s q u a d t h e o r d e r in w h i c h t h e a n i m a l s were t e s t e d was r a n d o m i z e d at the s t a r t of e a c h trial. Wet m a s h was placed in a f o o d cup al t h e e n d of e a c h d i s c r i m i n a n d u m to b a l a n c e o l f a c t o r y cues: b u t a s h e e t m e t a l b a r r i e r b l o c k e d access to t h e f o o d cup in t h e wing c o n t a i n i n g t h e negative stimulus. R e t r a c i n g was p r e v e n t e d b y l o w e r i n g t h e guillotine d o o r as s o o n as t h e a n i m a l left t h e c h o i c e p o i n t . A n i m a l s m a k i n g a c h o i c e were allowed to r e m a i n in t h e m a z e for a p p r o x i m a t e l y 2 0 sec, regardless of w h e t h e r t h e c h o i c e was correct or i n c o r r e c t .
WALBRAN A f t e r an e n t i r e s q u a d h a d gone t h r o u g h t h e maze. t h e a l u m i n u m plates were w a s h e d w i t h a I.ysol s o l u t i o n and dried b e f o r e t h e n e x t trial in an a t t e m p t t o m i n i m i z e o d o r trails. T h e r e was a m i n i m u m interval of 5 rain b e t w e e n trials w i t h t h e same animal. T h e first d i s c r i m i n a t i o n was t e r m i n a t e d w h e n an a n i m a l m e t an a priori c r i t e r i o n of a p e r f e c t score on a n y day, p r e c e d e d by n o t m o r e t h a n 1 e r r o r on t h e previous 2 days. W h e n a rat m a s t e r e d t h e first d i s c r i m i n a t i o n , the less difficult grooved surface was replaced by t h e more difficult d i s c r i m i n a n d u m . A n i m a l s originally r e w a r d e d for c h o o s i n g the s m o o t h s t i m u l u s c o n t i n u e d to be so r e w a r d e d , and vice versa. "resting was t e r m i n a t e d for an a n i m a l if a p r o b l e m was not m a s t e r e d w i t h i n 72 days. This was a p p r o x i m a t e l y 3 times t h e m e a n n u m b e r of days r e q u i r e d by all groups of c o n t r o l a n i m a l s to m a s t e r t h e first p r o b l e m . An e s t i m a t e d score o f 75 days was assigned to a n i m a l s n o t learning a problem.
Histology A t the end of t h e testing, each a n i m a l was a n e s t h e t i z e d deeply w i t h b a r b i t u r a t e anesthesia. P e r f u s i o n was accomplished with 0.9 p e r c e n t saline f o l l o w e d b y l O percenl F o r m a l i n injected i n t o t h e aorta. Brains were p o s t f i x e d in 10 percent F o r m a l i n for at least 1 m o n t h before slicing. Dorsal and lateral views of each ablated b r a i n were p h o t o g r a p h e d . T h e p h o t o g r a p h s were p r o j e c t e d o n t o p a p e r and d r a w n t o t h e same scale t o facilitate t h e d e t e r m i n a t i o n of t h e e x t e n t o f t h e surface lesions. T w o i n d e p e n d e n t .judges t h e n used the d e l i n e a t i o n of cortical s o m a t o s e n s o r y areas r e p o r t e d b y W e l k e r ' s l a b o r a t o r y [ 2 2 , 2 3 ] as a s t a n d a r d for r a n k i n g t h e d r a w i n g s in t e r m s o f size of s o m a t o s e n s o r y lesions. C o r o n a l s e c t i o n s were cut a~ 50 ~ o n a freezing m i c r o t o m e . S e c t i o n s were stained w i t h cresyl violet for e x a m i n a t i o n w i t h a low p o w e r m i c r o s c o p e to d e t e r m i n e possible s u b c o r t i c a l d a m a g e and t h a l a m i c d e g e n e r a t i o n in the v e n t r o b a s a l c o m p l e x . RESULTS
Behavioral Testing N o n - p a r a m e t r i c statistical tests were used for all data anlayses because t h e a s s u m p t i o n s u n d e r l y i n g p a r a m e t r i c tests, e.g., h o m o g e n e i t y of variance, could n o t be m e t [ 1 9 ] . Table I presents t h e m e d i a n ( M d n ) and s e m i - i n t e r q u a r t i l e range (Q) for the n u m b e r of days to c r i t e r i o n for each of the 12 s u b g r o u p s in this e x p e r i m e n t . A n a l y s i s of the e r r o r s - t o - c r i t e r i o n m e a s u r e p r o v i d e d a similar p i c t u r e a n d is n o t r e p o r t e d . All c o n t r o l a n i m a l s learned t h e d i s c r i m i n a t i o n rapidly. D i f f e r e n c e s b e t w e e n these 6 s u b g r o u p s were n o t significant (Kruskal-Wallis l - w a y A N O V A ; H = 4.6, dJ = 5, p > 0 . 0 5 ) . T h e 6 e x p e r i m e n t a l g r o u p s all s h o w e d a deficil in tactile a c q u i s i t i o n ability w h e n c o m p a r e d to t h e i r respective c o n t r o l s ( M a n n - W h i t n e y U tests, all p ' s < 0 . 0 5 ) . H o w e v e r . there were n o significant d i f f e r e n c e s in p e r f o r m a n c e a m o n g the rats s u s t a i n i n g one-stage a b l a t i o n s at d i f f e r e n t ages (Kruskal-WaUis 1-way A N O V A : H = 2.61 : d f = 2, p > 0.05). In c o n t r a s t , rats s u s t a i n i n g t w o - s t a g e a b l a t i o n s did s h o w an age-related d i f f e r e n c e in p e r f o r m a n c e ( H = 12.29: d f = 2. p<0.01). Serial p l a c e m e n t a t t e n u a t e d t h e effects of the s o m a t i c cortical lesions w h e n m a d e at 2 7 0 d a y s o f age, b u t n o t at 30 or 5 7 0 days. Indeed, at 570 days o f age, t h e d i f f e r e n c e
AGE AND S E R I A L A B L A T I O N S
15
TABLE 1 MEDIAN NUMBER OF DAYS AND PERCENTAGES TO CRITERION ON DISCRIMINATION 1 SURGICAL CONDITION Ablated Rats Control Rats Age at Surgery 30 days
270 days
570 days
l-Stage
2-Stage
l-Stage
2-Stage
74.7 18.5 11 45.5 62.0* 14.0 8 62.5~ 51.0 10.5 11 81.8t
74.7 5.5 10 40.0 36.0 9.8 10 90.0 74.8 5.0 11 36.5
19.0 6.7 7 100.0 21.0 6.7 5 100.0 25.0 4.3 5 100.0
21.0 19.6 7 100.0 23.0 7.0 7 100.0 24.5 7.0 6 100.0
Mdn Q N % to Criterion Mdn Q N % to Criterion Mdn Q N % to Criterion
Note: All ablated groups differed significantly from their respective controls (Mann-Whitney U tests, p ' s < 0.05). * = p <0.025. -t = p < o.001.
was in the o p p o s i t e d i r e c t i o n , a l t h o u g h it was not statistically significant. The u n e x p e c t e d superior p e r f o r m a n c e o f the animals sustaining one-stage ablations at this age was statistically reliable on the p e r f o r m a n c e measure of percentage of animals reaching criterion (X2 = 40.60, p < 0 . 0 0 1 ; see Table 1). In an e f f o r t to d e t e r m i n e w h e t h e r the d i f f e r e n c e s in learning scores m i g h t be a t t r i b u t a b l e to a r e l u c t a n c e to run
the maze, t h e n u m b e r o f trials on w h i c h each animal refused to leave the start b o x was tallied for D i s c r i m i n a t i o n 1. There was no evidence t h a t any o n e age or surgical group was m o r e or less likely t h a n any o t h e r to c o n t a i n animals which did n o t run (all p ' s > 0 . 0 5 ) . The reason, perhaps, was that the animals w i t h the greatest deficits were eliminated w h e n t h e y failed to m a s t e r the first d i s c r i m i n a t i o n . Overall, results f r o m D i s c r i m i n a t i o n 2 were valuable only insofar as
j
~.
r
j
I
FIG. 1. Composite drawings of surface lesions. Two lines under an animal's study number indicate that both problems were mastered. One line indicates that only the first problem was mastered. An absence of underlining indicates failure to master the first problem.
16
WALBRAN
t h e y d e m o n s t r a t e d t h a t t h e a n i m a l s s u s t a i n i n g serial lesions at 2 7 0 days of age were not able to m a i n t a i n t h e i r initial a d v a n t a g e o n a very difficult task.
Lesion Size and Placement In general, w h e n t h e scaled r e p r o d u c t i o n s of t h e brains of a n i m a l s s u s t a i n i n g lesions were c o m p a r e d w i t h t h e Welker [ 2 2 , 2 3 ] d e l i n e a t i o n o f s o m a t o s e n s o r y cortical areas. the lesions were f o u n d to be placed as i n t e n d e d . In every case, t h e r e was d a m a g e to s o m a t o s e n s o r y c o r t e x bilaterally. H o w e v e r , t h e c o m p l e t e n e s s o f t h e d a m a g e varied. D r a w i n g s of t h e surface lesions are s h o w n in Fig. 1. In t h e 3 0 - d a y - o l d and 5 7 0 - d a y - o l d g r o u p s , lesions placed serially were larger in surface area d e s t r o y e d t h a n t h o s e placed s i m u l t a n e o u s l y ( M a n n - W h i t n e y U tests, p ' s < 0 . 0 5 ) . In no case, h o w e v e r , was t h e r e d a m a g e to s u b c o r t i c a l s t r u c t u r e s s u c h as t h e s e p t u m , h i p p o c a m p u s , or dorsal t h a l a m u s . T h e r e was f r e q u e n t d a m a g e to t h e c o r p u s callosum in all groups, e x c e p t in t h e 5 7 0 - d a y - o l d animals sustaining one-stage lesions. This g r o u p had smaller lesions b o t h in t e r m s of surface size a n d d e p t h . R e t r o g r a d e d e g e n e r a t i o n in t h e v e n t r o b a s a l c o m p l e x was assessed b y c o m p a r i n g m o u n t e d s e c t i o n s f r o m ablated brains with c o m p a r a b l e s e c t i o n s f r o m several r e p r e s e n t a t i v e c o n t r o l brains. T h e d e g e n e r a t i o n p a t t e r n s were similar. regardless of w h e t h e r a surface lesion was relatively large or small. T h e r e also were n o d i f f e r e n c e s in d e g e n e r a t i o n p a t t e r n s b e t w e e n lesions m a d e s i m u l t a n e o u s l y or serially. In a d d i t i o n , t h a l a m i c d e g e n e r a t i o n p a t t e r n s in t h e 570-day-old a n i m a l s s u s t a i n i n g one-stage lesions were i n d i s t i n g u i s h a b l e f r o m t h o s e o f t h e o t h e r groups. Overall, d i f f e r e n c e s in lesion size did n o t have a c o n s i s t e n t effect o n b e h a v i o r a l p e r f o r m a n c e (see Fig. 1 t. W i t h i n t h e 30-day-old animals, t h o s e rats s u s t a i n i n g serial surgery had t h e larger lesions, b u t t h e i r p e r f o r m a n c e was similar to t h a t of a g e - m a t c h e d a n i m a l s s u s t a i n i n g simult a n e o u s damage. A l t h o u g h t h e lesions m the 270-day-old a n i m a l s t e n d e d to be e q u i v a l e n t i n d e p e n d e n t of t h e t e m p o r a l s e q u e n c e of t h e surgery, t h e a n i m a l s s u s t a i n i n g serial d a m a g e p e r f o r m e d b e t t e r . In t h e 5 7 0 - d a y - o l d animals, the rats s u s t a i n i n g s i m u l t a n e o u s d a m a g e h a d smaller lesions t h a n t h o s e s u s t a i n i n g serial d a m a g e and s h o w e d b e t t e r p e r f o r m a n c e . T h e s e d a t a m i g h t suggest t h a t the serial lesion effect was n e g a t e d b y relatively large lesions. H o w e v e r . t h e r e were n o significant c o r r e l a t i o n s b e t w e e n size of the surface lesion and b e h a v i o r a l p e r f o r m a n c e , regardless of w h e t h e r c o r r e l a t i o n c o e f f i c i e n t s were calculated f o r each of the 6 s u b g r o u p s , f o r t h e 3 age g r o u p s , or for t h e t o t a l g r o u p of a n i m a l s s u s t a i n i n g lesions. A f u r t h e r a t t e m p t was m a d e to c o n t r o l f o r t h e e f f e c t of lesion stze in t h e y o u n g a n d old groups, since t h e s e were t h e 2 g r o u p s s h o w i n g s i g n i f i c a n t lesion size d i f f e r e n c e s b u t n o serial lesion effect. In each g r o u p , a s u b - g r o u p c o m p o s e d of o n l y t h o s e lesions w h i c h could b e m a t c h e d for surface size a n d s u b c o r t i c a l d e p t h was assembled. T h e r e were n o significant d i f f e r e n c e s m perf o r m a n c e s o f t h e a n i m a l s in e i t h e r g r o u p ( M a n n - W h i t n e y tests: b o t h p ' s > 0 . 0 5 ) . In fact, in several instances, a n i m a l s w i t h c o m p a r a b l e lesions e x h i b i t e d widely divergent perf o r m a n c e s . This a p p e a r e d to b e a m a t t e r o f individual differences. T h u s , even w h e n lesion size was c o n t r o l l e d , the serial lesion e f f e c t a p p e a r e d o n l y in t h e 2 7 0 - d a y - o l d group. DISCUSSION
T h e d a t a collected in this s t u d y c o n f i r m e d t h e findings
t h a t one-stage lesions of t h e s o m a t o s e n s o r y cortex result m severe deficits in tactile h a b i t a c q u i s i t i o n [3, 6, 2 5 ] These deficits a p p e a r e d to be i n d e p e n d e n t of the age at w h i c h tile d a m a g e was sustained. Similar deficits also a p p e a r e d m animals s u s t a i n i n g serial d a m a g e to s o m a t o s e n s o r y c o r t e x at 30 days and 5 7 0 days of age. T h e deficits seen in animals sustaining serial d a m a g e at 270 days of age, h o w e v e r , were less severe t h a n t h o s e e x h i b i t e d b y animals sustaining s i m u l t a n e o u s d a m a g e at the same age. A m o n g t h o s e a n i m a l s u n d e r g o m ~ surgery at 570 days ot age, t h o s e s u s t a i n i n g s i m u l t a n e o u s d a m a g e t e n d e d to b c more likely to m a s t e r t h e first task t h a n those sustaining serial damage. This result agrees with those of Stein and Firl [201. It is possible t h a t w i t h age the ability to t o l e r a t e general a n e s t h e s i a a n d ' o r t r a u m a declines~ The p o o r e r p e r f o r m a n c e of t h e aged animats s u s t a i n i n g two-stage lesions m a y relate to s u c h a decline. On t h e o t h e r h a n d . ~t may be t h a t lesion size played a role at this age since the old animals w i t h s i m u l t a n e o u , lesions t e n d e d to have e x p e r i e n c e d less extensive CNS damage. T h e serial lesiot3 effect has been r e p o r t e d in a d o l e s c e n t m o n k e y s f o l l o w i n g multi-stage a b l a t i o n s of p r e f r o n t a l cortex in spite of larger lesions t h a n t h o s e sustained b y single-stage p r e p a r a t i o n s [ 16]. In the p r e s e n t s t u d y , b o t h 30-day-old surgical groups p e r f o r m e d similarly, a l t h o u g h the serial animats again had the larger lesions. T h e lack o f a reliable c o r r e l a t i o n b e t w e e n lesion size and behavioral p e r f o r m a n c e t e n d s to argue against a c o n s i s t e n t c o n t r i b u t i o n of lesion size ro recovery of f u n c t i o n . However. the possibility ~xists t h a t lesion size may i n t e r a c t d i f f e r e n t i a l l y w i t h the age, species and 'or area sustaining damage. The a c q u i s i t i o n deficits o b s e r v e d m t h e p r e s e n t s t u d y appeared to be q u a n t i t a t i v e r a t h e r t h a n q u a l i t a t i v e since m a n y rats with lesions did learn the task a l t h o u g h t h e y required a m u c h longer t i m e to do so. l n a s m u c h as t h e analysis of t h e n u m b e r of trials on which animals refused ~o leave the start b o x did n o t reveal any t r e n d s , these deficits p r o b a b l y were n o t m o t i v a t i o n a l . In a d d i t i o n , all animals ate eagerly in the maze It also seems unlikely t h a t t h e deficits were related to a general r e t a r d a t i o n in learning. Rats sustaining lesions o f s o m a t o s e n s o r y c o r t e x at 90 to 120 days old have been s h o w n to he capable ol a c q m r i n g t e m p e r a t u r e d i s c r i m i n a t i o n s m t h e same a m o u n t o f time as c o n t r o l a n i m a l s [ 4 , 8 ] . T h e fact t h a t some animals sustaining lesions e v e n t u a l l y learned H~e first d i s c r i m i n a t i o n suggests thai t h e deficits m a y relate ~o a ~Irategy failure or to a p e r c e p t u a l difficulty. P e r h a p s either or b o t h of these m a y "sugges! an e l e v a t i o n of r o u g h n e s s t h r e s h o l d s . Dis.c r i m i n a t i o n t h r e s h o l d s are k n o w n to ~mprove w i t h practice [ 171. It is i n t e r e s t i n g to n o t e in this regard t h a t the c o n t r o l animals s h o w e d no age-related changes m t h e ability to m a k e tactile d i s c r i m i n a t i o n s , l'his ~s c o n s i s t e n t with recent findings c o n c e r n i n g tactile t h r e s h o l d s m h u m a n s [1 a] T h e failure to observe an age-related d i f f e r e n c e m tactile f u n c t i o n i n g following d a m a g e to s o m a t o s e n s o r y c o r t e x does n o t necessarily c o n f l i c t w i t h previous findings of b e t t e r r e c o v e r y of f u n c t i o n f o l l o w i n g surgery very early m life b e c a u s e the y o u n g rats in the present s t u d y were 30 days old at the t i m e o f surgery. B e n j a m i n and T h o m p s o n ' s [11 i n f a n t cats. for i n s t a n c e , u n d e r w e n t surgery o n t h e sixth p o s t n a t a l day. A 30-day-old rat, a l t h o u g h n o t an i n f a n t , is still an i m m a t u r e animal. M y e l i n i z a t i o n of t h e ra! cortex, for i n s t a n c e , does n o t reach its adult s t a t e u n t i l a b o u t 60 days p o s t n a t a l l y [ 11 ]. It is possible t h a t in o r d e r to achieve sparing of tactile f u n c t i o n i n g following d a m a g e
AGE AND SERIAL ABLATIONS
17
to s o m a t o s e n s o r y cortex early in life, the d a m a g e m u s t o c c u r in the n e o n a t a l period r a t h e r t h a n in an i m m a t u r e period. In this s t u d y , serial surgery did n o t m i t i g a t e t h e tactile acquisition deficit in t h e y o u n g and old animals. T h e r e was some a t t e n u a t i o n of t h e deficit in t h e 270-day-old animals. T h e d e n e r v a t i o n s u p e r s e n s i t i v i t y p o s t u l a t i o n of a l t e r n a t e p a t h w a y s [2[ or Norrsell's [15] suggestion of parallel circuitry b o t h provide t h e basis for a possible e x p l a n a t i o n . It is at least plausible t h a t in t h e y o u n g animals, t h e p o s t u l a t e d a l t e r n a t e p a t h w a y s were n o t yet f u n c t i o n a l , while in t h e old animals, t h e possibility of reactivating t h e s e p a t h w a y s had d i m i n i s h e d as a result of p r o l o n g e d disuse or general d e t e r i o r a t i o n . H y p o t h e t i c a l l y , it m i g h t b e assumed t h a t the m a t u r e animals had a l t e r n a t e f u n c t i o n a l p a t h w a y s capable of carrying tactile i n f o r m a t i o n . T h e r e f o r e , t h e y e x h i b i t e d the m o r e control-like p e r f o r m a n c e s . This t e n d s to agree w i t h LeVere's [12] r e c e n t suggestion t h a t behavioral recovery results f r o m spared neural m e c h a n i s m s . One variable long k n o w n to be of i m p o r t a n c e to the serial lesion effect is the l e n g t h of t h e i n t e r l e s i o n interval. Studies r e p o r t i n g the m i n i m u m i n t e r o p e r a t i v e period necessary for sparing of sensory f u n c t i o n i n g to be less t h a n 2 weeks have e m p l o y e d r e t e n t i o n designs [ 1 4 , 2 1 ] . It has been suggested t h a t a c q u i s i t i o n f u n c t i o n s m a y require a longer i n t e r l e s i o n interval t h a n r e t e n t i o n f u n c t i o n s to achieve m a x i m a l p o s t o p e r a t i v e r e c o v e r y [ 7 ] . T h e one s t u d y
[9] d e m o n s t r a t i n g near-zero deficits in tactile h a b i t acquisition following serial surgery utilized an interlesion interval nearly twice as long as t h a t used in the present s t u d y a n d a highly generalizable series o f tasks. T h e m a t t e r of task difficulty has b e e n discussed in several papers [5, 10, 1 8 ] . Isaacson and S c h m a l t z [10] suggested t h a t it m i g h t be impossible to o b t a i n t h e serial lesion effect w h e n the task was o n e designed to reveal " c a p a c i t y difficulties", such as D R L - 2 0 tasks. T h e tasks used in t h e p r e s e n t s t u d y were t h e 2 m o s t difficult tasks in t h e series used in the original investigation [ 9 ] . Simons, P u r e t z and Finger [18] were u n a b l e to o b t a i n the serial lesion effect in a r e t e n t i o n design w i t h the same task as the o n e used in the present study. However, Finger and S i m o n s [5] d e m o n s t r a t e d t h e serial lesion effect in a r e t e n t i o n design identical to the earlier s t u d y [18] e x c e p t for the use o f easier tasks. These data e m p h a s i z e the need to define precisely the tasks used to assess b e h a v i o r a l f u n c t i o n s . It would a p p e a r t h a t the relevant p a r a m e t e r s will include n o t only the i n h e r e n t difficulty of the task and t h e experim e n t a l paradigm b u t also the previous e x p e r i e n t i a l h i s t o r y of the subjects. F u r t h e r , t h e possibility t h a t a n y particular variable m a y exert d i f f e r e n t i a l effects at various t i m e s in the life of the organism m a k e s it i m p o r t a n t to collect a b r o a d range of p e r f o r m a n c e data for n o r m a l animals against w h i c h the b e h a v i o r of animals with lesions can be compared.
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