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Role of cortex in Pavlovian discrimination learning
Physiology & Behavior, Vol. 15, pp. 315-321. Pergamon Press and Brain Research Publ., 1975. Printed in the U.S.A.
Role of Cortex in Pavlovian Discrimination Learning DAVID A. OAKLEY AND I. STEELE RUSSELL
Medical Research Council Unit on Neural Mechanisms o f Behaviour, 3 Malet Place, London I¢CIE 7JG, England
(Received 10 February 1975) OAKLEY, D. A. AND I. S. RUSSELL. Role of cortex in Pavlovian discrimination learning. PHYSIOL. BEHAV. 15(3) 315-321, 1975. - Two totally neodecorticate rabbits and two groups of normal rabbits were trained on a light-tone differentiation using a Pavlovian nictitating membrane response. The significance of the two stimuli involved in the differentiation was then reversed. Excellent initial differentiations were produced by both decorticates and normals, although there was some evidence of a small retardation of conditional response acquisition in the decorticates in this f'trst stage of the experiment. Under reversal conditions the decorticates extinguished responding to the formerly positive conditional stimulus in fewer trials and produced more complete differentiation performances than normal animals. The decorticates, unlike the normal animals, failed to show a reduction in conditional response onset latencies during either differentiation or reversal training. Pavlovian conditioning Rabbit Decortication Differentiation
Nictitating membrane response Neocortical lesions Reversal Stimulus feature extraction Response latency
anaesthesia (See [4,7]). Eight days were given interoperatively and approximately 50 days were allowed for postoperative recovery. The unoperated control group consisted of 12 male New Zealand White rabbits aged approximately 20 weeks at the time of the experiment. Two days prior to training a silk suture was placed in the leading edge of the nictitating membrane of the right eye of each animal and the hair was removed from the skin immediately caudal to the same eye. Light ether anaesthesia was used during these two procedures. All animals were housed singly and received ad lib food and water throughout.
WE have previously shown that partial neodecortication does not adversely affect the rabbit's ability to form conditional responses (CRs) in the nictitating membrane situation when a tone is used as the conditional stimulus (CS) [6,7]. Also, the lesioned animals were able to differentiate a non-reinforced visual stimulus from the original tone and to then reverse the distribution of their CRs when the significance of the two stimuli was changed [8,9]. In the reversal situation the neodecorticates produced fewer generalised CRs to the non-reinforced CS ( C S - ) and hence gave significantly better differentiation performances as a group than the normal animals. In fact it appeared that the extent of discrimination was directly related to the amount of cortex removed. Totally neodecorticated rabbits should, therefore, show substantially more complete differentiation and reversal performance than either normal or partially decorticated animals. The present study investigates differentiation and reversal conditioning in totally neodecorticate rabbits in an experimental design which, unlike that of the earlier studies , was balanced with respect to the modality of the initial CS+.
Apparatus The apparatus employed in restraining the animals and recording nictitating membrane movement has been described in detail elsewhere [3,7]. It is sufficient to note here that during testing the animal was restrained in a close fitting stock within a double-shelled sound attenuating compartment. Membrane movement was monitored via the previously implanted suture and recorded photographically from the display on an oscilloscope screen. Membrane movements were recorded during each stimulus presentation and during an interval of equal duration between each set of stimuli to provide a measure of spontaneous membrane activity during the training session. Tonal stimuli were presented via a 15 ohm, 12.7 cm dia. loudspeaker placed 10 cm above the animal's head and directed towards it. The illumination of a 2 4 0 - 2 5 0 V, 25 W frosted bulb behind a 6.5 X 14 cm window of 3.2 mm
METHOD
Animals and Surgery Two male albino (New Zealand White) rabbits aged approximately 25 weeks at the time of surgery were used as the experimental group. Neodecortications were carried out in two stages, by sub-pial aspiration laterally and medially and by pial removal dorsally, under pentobarbitone sodium 315
316
OAKLEY AND RUSSELL
thick milky perspex served as the visual CS. This light source was set 6.5 cm in front of the animal's nose. The unconditional stimulus (US) was a shock delivered via 2 miniature crocodile clips attached to the shaved postorbital area of the right eye, 4 mm above and below the lateral canthus and just caudal to it.
Procedure Throughout training the auditory CS was a 1,000 Hz tone which raised the noise level of the conditioning chamber from 78 to 96 dB (SPL, C scale). The visual CS consisted of the illumination of the light panel already described and raised the light level in the region of the animal's head from 18 to 167 lux. All CSs were presented for 500 msec and the intertrial interval was 55 sec. Training commenced with two daily habituation sessions of 15 and 30 min duration respectively. On each of these days the animal was placed fully restrained into the conditioning box and at the beginning of each session 5 light and 5 tone CSs were presented in randomized order. On the following day differential conditioning was introduced and continued for 14 consecutive daily sessions. Each differentiation session consisted of 20 CS+ and 20 C S - presentations, the order of which was randomized on a daily basis. For one of the decorticates (BAR 12) and 6 of the normals light was CS+ and tone C S - . The other decorticate (BAR 13) and the remaining 6 normals had the reverse condition of tone as CS+ and light as C S - . On 15 of the CS+ trials a 3 ma, 200 msec duration postorbital shock (US) was delivered 300 msec after CS onset. On the remaining 5 CS+ trials, again occurring in randomized sequence throughout the session, no US was presented (probe trials). After a three day break 10 daily sessions of reversal training were given under identical conditions except that the significance of the two CSs for each animal was reversed. That is, the former CS+ became C S - and vice versa. The normal groups are identified as Group L÷ - T ÷ and Group T ÷ - L÷ respectively on the basis of the modality of the positive CS in the two stages, differentiation and reversal, of the experiment. Any nictitating membrane response of 1 mm or more in amplitude as measured from the oscilloscope screen and occurring during the first 320 msec of CS presentation was counted as a CR. RESULTS
Histology Sample post-mortem photographs of the two neodecorticate brains are presented in Fig. 1 and surface reconstructions, based on serial frozen sections, are shown in Fig. 2. The pattern of neocortical removal in both brains was very similar. In BAR 12 there was a small lateral sparing of neocortex above the rhinal fissure in the occipital-temporal region of both hemispheres. Neocortical removal was complete on the lateral aspect of the left hemisphere in BAR 13. On the right hemisphere, in addition to a small occipital-temporal sparing, there was in this animal some sparing of frontal neocortex above the rhinal fissure. Undamaged tissue within the midline was common to the two animals. This represents a neocortical sparing only in the frontal regions of the brain anterior to the limits of fissura sagittalis lateralis, however, as caudal midline is occupied by retrosplenial cortex. The estimated extent of neocortical removal expressed as a percentage of total neocortex is 97 percent
FIG. 1. Lateral views of decorticate brains. The right hemisphere of BAR 12 is shown above and left hemisphere of BAR 13 below. Both brains show the extent to which neocorfical degeneration and ventricular expansion has occurred. The intecrity of the hippocampus is apparent for both animals. The background is marked in 1 cm squares. for BAR 12 and 94 percent for BAR 13. In each of the animals the lesions invaded extraneocortical tissue in both hemispheres along the ventral banks of the rhinal fissure and extensively within the retrosplenial areas from the lateral sagittal fissure to the midline. A summary of subneocortical injury is given in Table 1. No direct damage was in fact seen in subneocortical structures though extensive compression due to ventricular expansion was clearly present and reached its most severe expression in the caudateputamen and septal areas in both animals.
Postoperative Recovery Following the second stage of decortication BAR 12 and BAR 13 passed through a period of aphagia and reduced drinking lasting from 4 to 9 days. At the commencement of training both decorticates were in good physical condition, well groomed and showed no clear after effects of the lesion to casual observation in an open field. Both animals were however smaller in size (BAR 12 body weight 1,890 g and BAR 13 1,955 g) in comparison to normal animals of approximately the same age (mean body weight of normal sample was 3,600 g).
CORTEX AND CONDITIONING
317
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Habituation. The percentage o f trials on w h i c h a CR occurred was taken as the basic measure for determining acquisition rate and level o f a s y m p t o t i c performance. The percentage CR scores during habituation, differentiation and reversal are s h o w n for the t w o decorticates and the t w o normal groups in Fig. 3 and for individual animals in the normal Groups L+ - T + and T + - L÷ in Figs. 4 and 5 respectively. The decorticates responded to the light CS during habituation, but in b o t h cases this occurred o n l y one o f the t w o days. The responses were in addition small ( 1 - 3 m m ) and were limited to the first three CS presentations o f the session. The decorticates s h o w e d n o de novo responsiveness to the t o n e and the normals as a group did n o t respond significantly to either CS. 100-
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TABLE 1 S U B N E O C O R T I C A L
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Differentiation. Once differentiation training was introduced b o t h decorticates responded to their o w n CS+ reaching s o m e w h a t unstable a s y m p t o t i c levels o f CR p r o d u c t i o n at around 75 percent ( B A R 12) and 65 percent ( B A R 13). B A R 13 reached this level o f response to tone via a normal
318 l o o k i n g a c q u i s i t i o n curve whereas B A R 12 c o m m e n c e d even o n Day 1 o f d i f f e r e n t i a t i o n w i t h a high level o f CRs to its visual CS+ (Fig. 3). It is possible t h a t , at least d u r i n g t h e early days o f d i f f e r e n t i a t i o n , this a n i m a l ' s CRs were a variable m i x t u r e o f a l p h a r e s p o n s e s ( u n c o n d i t i o n a l responses to t h e CS) a n d true CRs. It is interesting, h o w e v e r , t h a t B A R 13, w h i c h also p r o d u c e d a l p h a responses to light during h a b i t u a t i o n , did n o t r e s p o n d to its visual C S - at all d u r i n g d i f f e r e n t i a t i o n training. B A R 12 similarly was u n r e s p o n s i v e to its C S - ( t o n e ) d u r i n g this t i m e a n d h e n c e d i f f e r e n t i a l t r a i n i n g was successful in p r o d u c i n g clear s t i m u l u s c o n t r o l in b o t h decorticates. T h e n o r m a l animals as a g r o u p acq u i r e d CRs t o CS+ m o r e r a p i d l y t h a n B A R 13 a n d t o a higher a n d m o r e stable a s y m p t o t e t h a n e i t h e r d e c o r t i c a t e (Fig. 3). T h e a c q u i s i t i o n rate o f B A R 12 is i m p o s s i b l e t o determine with any certainty. F o r t h e n o r m a l animals t o n e was the m o r e p o t e n t CS+ giving rise to significantly fewer trials to the t e n t h C R t h a n was the case w i t h the visual CS+ ( M e a n trials to t h e t e n t h CR: T ÷ = 29.0, L÷ = 82.0, M a n n - W h i t n e y U = 2, p = 0 . 0 0 4 w h e n n 1 = n 2 ~ 6) t h o u g h n o d i f f e r e n c e s in t h e levels o f individual a s y m p t o t i c p e r f o r m a n c e are e v i d e n t b e t w e e n t h e t w o groups (Figs. 4 a n d 5). T h e greater p o t e n c y o f t h e t o n a l CS was also r e f l e c t e d in t h e fact t h a t , whereas generalised r e s p o n d i n g t o t h e visual C S - was virtually a b s e n t in the n o r m a l G r o u p T + - L÷ d u r i n g d i f f e r e n t i a t i o n (see Figs. 3 a n d 5), CRs to t h e t o n a l C S - o f G r o u p L÷ - T ÷ were seen in t h e m a j o r i t y o f animals s o o n a f t e r a c q u i s i t i o n to CS+ had c o m m e n c e d giving a g r o u p m e a n of a p p r o x i m a t e l y 10 percent r e s p o n d i n g to C S - (Figs. 3 a n d 4). Mean CR amplit u d e s o n Day 14 o f d i f f e r e n t i a t i o n similarly were larger for t o n e t h a n for light in b o t h d e c o r t i c a t e s a n d n o r m a l s ( B A R 12 L÷ m e a n CR a m p l i t u d e in m m = 4.66, B A R 13 T ÷ = 19.5, N o r m a l L÷ = 10.1, N o r m a l T ÷= 15.02) t h o u g h this difference is n o t significant statistically for the n o r m a l groups. Reversal. W h e n t h e significance o f CS+ a n d CS was reversed t h e d e c o r t i c a t e s r e a c t e d i m m e d i a t e l y by ceasing t o r e s p o n d to t h e f o r m e r CS+, c o n f i r m i n g t h a t t h e r e s p o n s e s of B A R 12 to light were n o t a l p h a r e s p o n s e s at t h a t stage o f training. T h e n o r m a l s o n t h e o t h e r h a n d c o n t i n u e d to r e s p o n d to t h e f o r m e r CS+ at a high level u n t i l Day 4 in G r o u p L÷ - T ÷ a n d u n t i l Day 7 in G r o u p T ÷ - L+, falling t o 20 a n d 45 p e r c e n t levels respectively b y Day l 0 of reversal (Fig. 3). S u b s t a n t i a l CRs to t h e n e w CS+ a p p e a r e d on Day 3 o f reversal in B A R 12 (T ~) a n d o n Day 4 in B A R 13 (L÷) giving rise t h e r e a f t e r t o a c q u i s i t i o n curves w h i c h were b o t h s t e e p e r a n d w i t h higher, m o r e stable a s y m p t o t e s t h a n had b e e n t h e case for CS+ r e s p o n d i n g d u r i n g d i f f e r e n t i a t i o n . CS+ a c q u i s i t i o n in reversal was, in c o n t r a s t , r e t a r d e d for the n o r m a l s in c o m p a r i s o n w i t h t h e i r d i f f e r e n t i a t i o n performance. T h e m e a n n u m b e r o f trials to the t e n t h CR+ w i t h t o n e as CS+ in reversal ( G r o u p L÷ - T ÷) was 33.67 compared w i t h 29.0 w h e n t o n e was CS+ d u r i n g d i f f e r e n t i a t i o n t r a i n i n g ( G r o u p T ÷ - L÷). F o r light as CS+ the m e a n n u m ber o f trials to t h e t e n t h C R was 120.2 u n d e r reversal c o n d i t i o n s ( G r o u p T ÷ - L÷) a n d 82.0 d u r i n g d i f f e r e n t i a t i o n ( G r o u p L÷ - T+). Within g r o u p variances were high h o w e v e r a n d n e i t h e r o f these differences is significant statistically ( M a n n - W h i t n e y U). One i m p o r t a n t source o f v a r i a t i o n in G r o u p L+ - T ÷ was t h a t A n i m a l s 101 a n d 86, w h i c h had s h o w n a s t r o n g t e n d e n c y to r e s p o n d t o t o n e as C S - d u r i n g d i f f e r e n t i a t i o n , were in fact facilitated in acquiring T ÷ responses w h e n t o n e b e c a m e CS+ in reversal (Fig. 4). In b o t h d e c o r t i c a t e s evidence o f r e s p o n s e synergy [9] appeared in the C S - r e s p o n s e curve d u r i n g reversal in t h a t ,
OAKLEY AND RUSSELL once the a n i m a l began r e s p o n d i n g t o CS+, r e s p o n d i n g t o C S - was also briefly revived in p r o p o r t i o n to t h e s t r e n g t h of CS+ p e r f o r m a n c e . T h e n o r m a l A n i m a l s 101 a n d 91 s h o w a similar r e s p o n s e synergy in G r o u p L÷ - T ÷ (Fig. 4) while in G r o u p T + - L÷ t h e r e is an even clearer e x a m p l e in A n i m a l 90 a n d o t h e r , less m a r k e d , i n s t a n c e s f r o m A n i m a l s 82, 88, 92 (Fig. 5). C S - r e s p o n d i n g r e a c h e d a stable zero o n reversal Day 5 in B A R 12 a n d o n reversal Day 8 in B A R 13 e n s u r i n g perfect d i f f e r e n t i a t i o n scores o n reversal Day 10. In t h e n o r m a l G r o u p L÷ - T ÷ the s t r o n g a c q u i s i t i o n of CRs t o t o n e c o m b i n e d w i t h an a d e q u a t e e v e n t u a l s u p p r e s s i o n o f r e s p o n s e s t o light in reversal r e s u l t e d in a Day 10 different i a t i o n score o f 0.82. This score expresses t h e n u m b e r of CS+ r e s p o n s e s as a p r o p o r t i o n o f t h e t o t a l responses to CS+ a n d C S - (1.0 = p e r f e c t d i f f e r e n t i a t i o n 0.5 = n o different i a t i o n ) . Only partial s u p p r e s s i o n o f C S - responses t o t o n e plus p o o r a c q u i s i t i o n o f CS+ r e s p o n s e to light in G r o u p T + - L+ gave a final d i f f e r e n t i a t i o n score in reversal o f 0.61. B o t h o f these scores clearly c o m p a r e u n f a v o u r a b l y w i t h t h o s e of t h e d e c o r t i c a t e s w h i c h were in b o t h cases 1.0. In fact it is w o r t h n o t i n g t h a t o n l y o n e n o r m a l a n i m a l gave CS+ a n d C S - r e s p o n s e curves in reversal w h i c h a p p r o a c h e d t h o s e of t h e t w o decorticates. This was A n i m a l 83 in G r o u p L÷ T ÷ (Fig. 4) w h i c h had failed to acquire CRs to its visual CS+ u n t i l the t e n t h session o f d i f f e r e n t i a t i o n t r a i n i n g a n d c o n s e q u e n t l y e x t i n g u i s h e d rapidly w h e n light b e c a m e C S in reversal.
Conditional Response Onset Latencies F r o m t h e partial d e c o r t i c a t i o n studies it was clear t h a t , while C R f r e q u e n c y was n o t a f f e c t e d b y t h e lesion, C R o n s e t latencies were s h o r t e r in t h e n o r m a l animals. In o r d e r to e x t e n d this o b s e r v a t i o n we m e a s u r e d l a t e n c y o f CR onset o n CS+ trials for all a n i m a l s in the p r e s e n t s t u d y and t h e results o f this analysis are p r e s e n t e d as m e d i a n values for d e c o r t i c a t e s and n o r m a l s in Fig. 6. T h e sampling interval used was 20 msec a n d CS+ o n s e t was t a k e n as t h e zero p o i n t for l a t e n c y m e a s u r e m e n t . It is first of all clear t h a t m e d i a n C R + o n s e t latencies for the d e c o r t i c a t e s did n o t change s y s t e m a t i c a l l y f r o m t h e i r initial values (range 2 0 0 - 2 8 0 msec) d u r i n g e i t h e r d i f f e r e n t i a t i o n or reversal training. B o t h o f t h e n o r m a l g r o u p s d u r i n g d i f f e r e n t i a t i o n had Day 1 C R latencies w i t h i n the d e c o r t i c a t e range b u t t h e y t h e n s h o w e d t h e n o w classic p i c t u r e o f s h o r t e n i n g r e s p o n s e latencies over t h e initial days o f t r a i n i n g [ 7 , 1 5 ] . This was especially m a r k e d in the T ÷ g r o u p where a s t r o n g p r i m a r y l a t e n c y shift was seen b e t w e e n d i f f e r e n t i a t i o n Days 1 a n d 9, followed b y a s e c o n d a r y shift t o w a r d s longer median latencies. A related m e a s u r e s t-test p e r f o r m e d o n m e a n s s h o w s b o t h t h e p r i m a r y a n d s e c o n d a r y l a t e n c y shifts t o be significant (for t h e n o r m a l T ÷ animals d u r i n g different i a t i o n t h e g r o u p m e a n l a t e n c y o n Day 2 = 184.3 msec, Day 9 = 113.3 msec, Day 14 = 150.0 msec. Day 2 v Day 9, t = 5.883. Day 9 v Day 14, t = 4.575. F o r d f = 5, t ( 0 . 0 1 ) = 4.032. T h u s p < 0 . 0 1 in b o t h cases). The n o r m a l g r o u p w i t h L+ s h o w e d a p r i m a r y l a t e n c y shift of relatively small magnit u d e w h i c h was n o t f o l l o w e d b y a s e c o n d a r y shift to longer latencies. Median C R l a t e n c y values o f t h e L+ n o r m a l group, t h o u g h s h o r t e r t h a n t h o s e o f t h e d e c o r t i c a t e group, t h u s r e m a i n e d longer t h a n t h o s e o f t h e T ÷ n o r m a l group. This difference in CR l a t e n c y values m a y be y e t a n o t h e r reflect i o n o f the relative efficacy o f these t w o stimuli as CSs. In reversal the g r o u p for w h i c h light h a d b e c o m e CS+ prod u c e d a CR l a t e n c y profile w h i c h was virtually indistinguishable f r o m t h e d e c o r t i c a t e s ' . The n e w T + n o r m a l group
CORTEX AND CONDITIONING
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h o w e v e r displayed a very s t r o n g p r i m a r y l a t e n c y shift bet w e e n Days 1 and 4 o f reversal. The u n e x p e c t e d l y s h o r t reversal Day 1 l a t e n c y values for T ÷ in this g r o u p ( c o m p a r a b l e t o Day 2 values o f the d i f f e r e n t i a t i o n T ÷ g r o u p ) reflect t h e fact t h a t a small a m o u n t o f l a t e n c y shifting had t a k e n place while t o n e was the C S - in the prevTous differe n t i a t i o n stage and was p r o d u c i n g generalised CRs. A secondary l a t e n c y shift w o u l d be e x p e c t e d to begin a r o u n d Days 9 - 1 0 o f reversal in this g r o u p o n the basis o f t h e different i a t i o n data b u t n o evidence o f such a change was f o u n d .
10
FIG. 5. Individual percentage CRs during habituation, differentiation and reversal in normal Group T + - L÷.
In o r d e r t o investigate t h e possibility t h a t d e c o r t i c a t i o n p r o d u c e s a change in t h e level o f s p o n t a n e o u s n i c t i t a t i n g m e m b r a n e activity an e s t i m a t e o f s p o n t a n e o u s r e s p o n s e s (SRs) over h a b i t u a t i o n , d i f f e r e n t i a t i o n and reversal was m a d e , based on single sweeps o f the oscilloscope b e a m s (spontaneous response probes) recorded halfway between stimulus p r e s e n t a t i o n s . The p e r c e n t a g e o f p r o b e trials (40 per session) o n w h i c h r e s p o n s e s m e e t i n g the 1 m m criterion o c c u r r e d over all 26 e x p e r i m e n t a l sessions were: - BAR 12 - 0.4 p e r c e n t , B A R 13 - 0.9 p e r c e n t , n o r m a l G r o u p L+ T + - 0.6 p e r c e n t , n o r m a l G r o u p T + - L+ - 0.2 p e r c e n t . These data d o n o t suggest any e f f e c t o n SR f r e q u e n c y f r o m d e c o r t i c a t i o n . The d i s t r i b u t i o n o f SRs was u n s y s t e m a t i c in
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OAKLEY AND RUSSELL
all groups and, as the sound attenuating properties of the experimental enclosure had been substantially improved for this study, this may be taken as confirmation of our suspicion that many of the spontaneous responses associated with a phase of generalisation between CS+ and C S - in an earlier study [9] were in fact emitted in response to external programming noise. DISCUSSION Previous studies of partial neodecorticates using single stimulus (tone) training indicated no impairment of conditioning due to neocortical removal [7]. In the present study, however, it seems necessary to conclude that the two decorticates were in fact impaired in the rate of acquisition of CRs to CS+ and in the level and stability of their asymptotic performance during the 14 days of differentiation training. With a sample of only two decorticates it is difficult to make a strong case for this but inspection of Figs. 3 and 5 shows clearly that the performance of BAR 13 during differentiation was outside the normal range for a tonal CS+. The differentiation performance of BAR 12 remains difficult to interpret in view of the presence of nictitating membrane responses from Day 1 onwards but is obviously less stable than that of the normal control group (Fig. 4). Whether neodecortication as such is the cause of these deficits or whether it is the presence of compression effects in subneocortex (Table 1) is impossible to judge. It may still be that an uncomplicated neodecortication would not affect acquisition performance to CS+. It is perhaps worth noting that if there had been substantial acquisition differences between normal and decorticate animals it is possible that differences in reversal learning could derive from the overtraining reversal effect. As only marginal acquisition differences were seen, the possibility that either group was overtrained with respect to the other does not seem likely. Earlier evidence that partial decorticates performed more efficiently in a reversal conditioning situation than normal animals led to the suggestion that this was due to a reduction in the number of features extracted from sensory inputs as neocortex was depleted [9]. While the ability to extract a wide range of features from the environment by sensory analysis may lead ultimately to highly adaptive forms of behaviour, notably those based on transferring common information across modality "boundaries, a more limited analysis might prove the more efficient strategy in the context of simple behavioural tasks. Specifically, in the l i g h t - t o n e differentiation and reversal situation a system analysing as few as two input features could perform efficiently provided one of these features was modality of input. A system extracting more features contains within itself a potential source of confusion as features common to both CSs will inevitably be sampled. It may be, in other words, that the decorticate's capacity for sensory analysis is the one which more closely matches the rather limited requirements of this experimental paradigm. Widely based stimulus feature extraction might be expected to lead in some instances to inappropriate generalisation across modalities. The previous decortication study introduced a differentiation into an already established single stimulus conditioning situation and found no significant generalisation to the novel C S - in either lesioned or normal animals [9]. More generalisation between CS+ and C S - was anticipated from the normals in the d e n o v o differentiation procedure of the present study while the
decorticates with their less widely based stimulus analysis should extract fewer features c o m m o n to the two CSs and thus show less cross-modality generalisation. The decorticates should, in other words, treat the visual and auditory inputs as independent events. In fact the decorticates show virtually zero C S - responding during differentiation. The same was true for the normals of Group T ÷ - L÷ for which tone was the positive CS; whereas the normal Group L÷ T ÷ did respond to C S - , though in only two animals was the behaviour strong or persistent. Normal animals may thus be more likely to show generalised responding during the acquisition of a differentiation than decorticates but the effect is not strong and appears to require the use of a potent C S - . When the significance of visual and auditory stimuli is reversed the separate modality analysis of the decorticate should allow the extinction of CRs to the old CS+ and acquisition of CRs to the new CS+ to proceed independently. The decorticates did in fact extinguish to the former CS+ rapidly and completely and acquired responses to the new CS+ as well, or better, than they did d e n o v o in the differentiation situation. Nevertheless it should be noted that a certain low-level interaction did occur between CS+ and C S - responding during reversal in the decorticates (See Fig. 3). The abruptness with which responses to the former CS+ extinguished in these two animals was surprising, however, as decortication produced no effect on CR extinction rates after single stimulus training [2] and seems thus to be a product of the reversal procedure itself. In the normal animals multiple feature extraction was expected to lead to difficulty in reversal because of the increased probability of interaction between the two CSs. This indeed seemed to be the case. The normals were slow to eliminate C S - responding from their behaviour during the 10 reversal sessions, especially when tone was the C S - (Group T + - L÷ Fig. 3), suggesting generalised excitation from the newly established CS+. The latter interpretation is supported by various examples in Figs. 4 and 5 of revival in C S - responding to high levels on the commencement of acquisition to CS+ (response synergy). The normals were in addition impaired in acquiring CRs to the former C S - especially when light was the stimulus involved (Group T ÷ - L÷). This latter effect in normals has been reviewed by Rescorla [12] who concluded that the C S - in a differentiation situation develops inhibitory properties with respect to the response system being conditioned. While the present data confirm the inhibitory properties of a former C S - in normals the same does not occur in the decorticates. Indeed a prior differentiation in decorticates would appear if anything to facilitate the acquisition of CRs to the former C S - . It was found in the normal animals of the present study that acquisition of CRs to a tonal CS+ during differentiation proceeded more rapidly and to a higher level than that to a visual CS+. The greater efficiency of tone in this situation could be a product of the particular tone and light parameters chosen. It is however consistent with several earlier reports of the relatively greater potency of an auditory CS [2, 11, 14]. Analysis of CR+ onset latencies confirmed previous observations made on partial decorticates [7] that the normally observed shift towards shorter response latencies with training was abolished by neocortical ablation. CR onset latencies in the decorticates remained constant within a limited range of values throughout the 24 days of differential and reversal conditioning. Both the L÷ and the T +
CORTEX AND CONDITIONING
321
normal groups showed a primary latency shift (p.l.s.) during differentiation training. Additionally, the much larger p.l.s. of the T ÷ group was followed by a significant secondary shift back towards longer mean latencies. The p.l.s, is well documented in the nictitating membrane conditioning literature [1, 7, 15, 16]. The secondary shift reported here seems, however, to have been overlooked heretofore though a similar significant secondary latency shift has been reported from this laboratory using normal animals with single stimulus (tone) conditioning over a 15 day training period [ 10]. Under reversal conditions the p.l.s, was again found in the normals when tone was CS+. Conditioning to light was very much impaired under reversal conditions and in the normal group receiving light as the new CS+ no latency shift was seen and the median CR latencies remained virtually identical to those of the decorticate group. This latter observation leads to the suspicion that the absence of a latency shift is not a direct product of decortication but simply a manifestation of a wider debility involving poor acquisition of CRs. It is clear that the p.l.s, is best displayed in normal animals which condition strongly. In the present study tone was the more potent CS and it was with tone as CS+ that the most marked p.l.s, occurred. Acquisition of CRs to light was rapid, albeit less rapid than to tone, during differentiation but slow during reversal. The latency shift to light was correspondingly present during differentiation and absent during reversal. However, two lots o f evidence suggest that the correlation between poor conditioning and the reduction or loss of the p.l.s, in normals does not account for the absence of a p.l.s, in decorticates. The first evidence derives from earlier partial decorticate studies which showed that subtotal lesions which did
not impair the acquisition of CRs to a tonal CS nevertheless abolished the p.l.s. [7]. Secondly, while it is true that both decorticates produce below normal asymptotic performances during differentiation this is not true of the reversal situation. Both decorticates in reversal show a steep acquisition curve once acquisition has commenced, accompanied by high asymptotic performances. Given that the p.l.s, is normally completed over the first 4 - 6 days of training it is surprising that the decorticates showed no evidence of such a change in their median latency profiles. Normal animals producing acquisition curves of this sort would be expected to show the p.l.s.. It might be suggested that the p.l.s, reflects the instrumentalisation of the original CR as part of a flinching movement serving to ameliorate the effects of the US. In this case the fact that neodecortication impairs or even prevents the acquisition of instrumental responses [5,13] could account for the absence of a p.l.s, in the decorticates. Whatever the details of performance and the problems of interpretation it is clear from these data that total neodecortication does not prevent the rabbit from forming a differentiation in the nictitating membrane conditioning situation between light and tone or from reversing that differentiation readily and efficiently. The decorticates' reversal performance after 14 days of differentiation training was superior to that of normal animals under similar conditions insofar as the behavioural separation between responses to CS+ and C S - occurred sooner and was more complete in the decorticates than it was in the normal control groups. Neocortex, then, is not essential for the mediation of Pavlovian conditioning at this level of complexity.
REFERENCES 1. Coleman, S. R. and I. Gormezano. Classical conditioning of the rabbit's (Oryctolagus cuniculus) nictitating membrane response under symmetrical SC-US interval shifts, or. comp. physiol. Psychol. 77: 447-455, 1971. 2. Garcia, J., B. K. McGowan and K. F. Green. Biological constraints on conditioning. In: Classical Conditioning 11." Current Research and Theory, edited by A. H. Black and W. F. Prokasy, New York: Appleton-Century-Crofts, 1972, pp. 3-27. 3. Gormezano, I. Classical conditioning. In: Experimental Methods and Instrumentation in Psychology, edited by J. B. Sidowski, New York: McGraw Hill, 1966, pp. 385-420. 4. Meyer, P. M. and D. R. Meyer. Neurosurgical procedures with special reference to aspiration lesions. In: Methods in Psychobiology, Volume 1, edited by R. D. Myers, New York: Academic Press, 1971, pp. 91-130. 5. Oakley, D. A. Instrumental learning in neodecorticate rabbits. Nature, New Biol. 233: 185-187, 1971. 6. Oakley, D. A. and I. S. Russell. Mass action and Pavlovian conditioning. Psychon. Sci. 12: 91-92, 1968. 7. Oakley, D. A. and I. S. Russell. Neocortical lesions and Pavlovian conditioning. Physiol. Behav. 8: 915-926, 1972. 8. Oakley, D. A. and I. S. Russell. Acquisition, differentiation and reversal of conditioning in partially decorticate rabbits. 1RCS J. lnt. Res. Communs Med. Sci. 1: 56, 1973. Ref. (73-3) 46-13-2.
9. Oakley, D. A. and I. S. Russell. Differential and reversal conditioning in partially neodecorticate rabbits. Physiol. Behav. 13: 221-230, 1974. 10. Oaldey, D. A., H. C. Plotkin and A. G. Yeo. Primary and secondary shifts in conditional response latency in rabbit nictitating membrane conditioning. IRCS, Research on Psychology 2: 1655, 1974. 11. Parlor, I. P. Conditioned Reflexes. Translated and edited by G. V. Anrep. London: Oxford University Press, 1927. 12. Rescorla, R. A. Pavlovian conditioned inhibition. Psychol. Bull. 72: 77-94, 1969. 13. Russell, I. S. Neurological basis of complex learning. Br. Med. Bull. 27: 278-285, 1971. 14. Saavedra, M. A., E. Garcia and T. Pinto-Hamuy. Acquisition of auditory conditioned responses in normal and neodecorticate rats. J. comp. physiol. Psychol. 56: 31-35, 1963. 15. Schneiderman, N. and I. Gormezano. Conditioning of the nictitating membrane of the rabbit as a function of CS-US interval. J. comp. physiol. Psychol. 57: 188-195, 1964. 16. Smith, M. C. CS-US interval and US intensity in classical conditioning of the rabbit's nictitating membrane response. 3.. comp. physiol. Psychol. 66: 679-687, 1968.
Role of Cortex in Pavlovian Discrimination Learning DAVID A. OAKLEY AND I. STEELE RUSSELL
Medical Research Council Unit on Neural Mechanisms o f Behaviour, 3 Malet Place, London I¢CIE 7JG, England
(Received 10 February 1975) OAKLEY, D. A. AND I. S. RUSSELL. Role of cortex in Pavlovian discrimination learning. PHYSIOL. BEHAV. 15(3) 315-321, 1975. - Two totally neodecorticate rabbits and two groups of normal rabbits were trained on a light-tone differentiation using a Pavlovian nictitating membrane response. The significance of the two stimuli involved in the differentiation was then reversed. Excellent initial differentiations were produced by both decorticates and normals, although there was some evidence of a small retardation of conditional response acquisition in the decorticates in this f'trst stage of the experiment. Under reversal conditions the decorticates extinguished responding to the formerly positive conditional stimulus in fewer trials and produced more complete differentiation performances than normal animals. The decorticates, unlike the normal animals, failed to show a reduction in conditional response onset latencies during either differentiation or reversal training. Pavlovian conditioning Rabbit Decortication Differentiation
Nictitating membrane response Neocortical lesions Reversal Stimulus feature extraction Response latency
anaesthesia (See [4,7]). Eight days were given interoperatively and approximately 50 days were allowed for postoperative recovery. The unoperated control group consisted of 12 male New Zealand White rabbits aged approximately 20 weeks at the time of the experiment. Two days prior to training a silk suture was placed in the leading edge of the nictitating membrane of the right eye of each animal and the hair was removed from the skin immediately caudal to the same eye. Light ether anaesthesia was used during these two procedures. All animals were housed singly and received ad lib food and water throughout.
WE have previously shown that partial neodecortication does not adversely affect the rabbit's ability to form conditional responses (CRs) in the nictitating membrane situation when a tone is used as the conditional stimulus (CS) [6,7]. Also, the lesioned animals were able to differentiate a non-reinforced visual stimulus from the original tone and to then reverse the distribution of their CRs when the significance of the two stimuli was changed [8,9]. In the reversal situation the neodecorticates produced fewer generalised CRs to the non-reinforced CS ( C S - ) and hence gave significantly better differentiation performances as a group than the normal animals. In fact it appeared that the extent of discrimination was directly related to the amount of cortex removed. Totally neodecorticated rabbits should, therefore, show substantially more complete differentiation and reversal performance than either normal or partially decorticated animals. The present study investigates differentiation and reversal conditioning in totally neodecorticate rabbits in an experimental design which, unlike that of the earlier studies , was balanced with respect to the modality of the initial CS+.
Apparatus The apparatus employed in restraining the animals and recording nictitating membrane movement has been described in detail elsewhere [3,7]. It is sufficient to note here that during testing the animal was restrained in a close fitting stock within a double-shelled sound attenuating compartment. Membrane movement was monitored via the previously implanted suture and recorded photographically from the display on an oscilloscope screen. Membrane movements were recorded during each stimulus presentation and during an interval of equal duration between each set of stimuli to provide a measure of spontaneous membrane activity during the training session. Tonal stimuli were presented via a 15 ohm, 12.7 cm dia. loudspeaker placed 10 cm above the animal's head and directed towards it. The illumination of a 2 4 0 - 2 5 0 V, 25 W frosted bulb behind a 6.5 X 14 cm window of 3.2 mm
METHOD
Animals and Surgery Two male albino (New Zealand White) rabbits aged approximately 25 weeks at the time of surgery were used as the experimental group. Neodecortications were carried out in two stages, by sub-pial aspiration laterally and medially and by pial removal dorsally, under pentobarbitone sodium 315
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OAKLEY AND RUSSELL
thick milky perspex served as the visual CS. This light source was set 6.5 cm in front of the animal's nose. The unconditional stimulus (US) was a shock delivered via 2 miniature crocodile clips attached to the shaved postorbital area of the right eye, 4 mm above and below the lateral canthus and just caudal to it.
Procedure Throughout training the auditory CS was a 1,000 Hz tone which raised the noise level of the conditioning chamber from 78 to 96 dB (SPL, C scale). The visual CS consisted of the illumination of the light panel already described and raised the light level in the region of the animal's head from 18 to 167 lux. All CSs were presented for 500 msec and the intertrial interval was 55 sec. Training commenced with two daily habituation sessions of 15 and 30 min duration respectively. On each of these days the animal was placed fully restrained into the conditioning box and at the beginning of each session 5 light and 5 tone CSs were presented in randomized order. On the following day differential conditioning was introduced and continued for 14 consecutive daily sessions. Each differentiation session consisted of 20 CS+ and 20 C S - presentations, the order of which was randomized on a daily basis. For one of the decorticates (BAR 12) and 6 of the normals light was CS+ and tone C S - . The other decorticate (BAR 13) and the remaining 6 normals had the reverse condition of tone as CS+ and light as C S - . On 15 of the CS+ trials a 3 ma, 200 msec duration postorbital shock (US) was delivered 300 msec after CS onset. On the remaining 5 CS+ trials, again occurring in randomized sequence throughout the session, no US was presented (probe trials). After a three day break 10 daily sessions of reversal training were given under identical conditions except that the significance of the two CSs for each animal was reversed. That is, the former CS+ became C S - and vice versa. The normal groups are identified as Group L÷ - T ÷ and Group T ÷ - L÷ respectively on the basis of the modality of the positive CS in the two stages, differentiation and reversal, of the experiment. Any nictitating membrane response of 1 mm or more in amplitude as measured from the oscilloscope screen and occurring during the first 320 msec of CS presentation was counted as a CR. RESULTS
Histology Sample post-mortem photographs of the two neodecorticate brains are presented in Fig. 1 and surface reconstructions, based on serial frozen sections, are shown in Fig. 2. The pattern of neocortical removal in both brains was very similar. In BAR 12 there was a small lateral sparing of neocortex above the rhinal fissure in the occipital-temporal region of both hemispheres. Neocortical removal was complete on the lateral aspect of the left hemisphere in BAR 13. On the right hemisphere, in addition to a small occipital-temporal sparing, there was in this animal some sparing of frontal neocortex above the rhinal fissure. Undamaged tissue within the midline was common to the two animals. This represents a neocortical sparing only in the frontal regions of the brain anterior to the limits of fissura sagittalis lateralis, however, as caudal midline is occupied by retrosplenial cortex. The estimated extent of neocortical removal expressed as a percentage of total neocortex is 97 percent
FIG. 1. Lateral views of decorticate brains. The right hemisphere of BAR 12 is shown above and left hemisphere of BAR 13 below. Both brains show the extent to which neocorfical degeneration and ventricular expansion has occurred. The intecrity of the hippocampus is apparent for both animals. The background is marked in 1 cm squares. for BAR 12 and 94 percent for BAR 13. In each of the animals the lesions invaded extraneocortical tissue in both hemispheres along the ventral banks of the rhinal fissure and extensively within the retrosplenial areas from the lateral sagittal fissure to the midline. A summary of subneocortical injury is given in Table 1. No direct damage was in fact seen in subneocortical structures though extensive compression due to ventricular expansion was clearly present and reached its most severe expression in the caudateputamen and septal areas in both animals.
Postoperative Recovery Following the second stage of decortication BAR 12 and BAR 13 passed through a period of aphagia and reduced drinking lasting from 4 to 9 days. At the commencement of training both decorticates were in good physical condition, well groomed and showed no clear after effects of the lesion to casual observation in an open field. Both animals were however smaller in size (BAR 12 body weight 1,890 g and BAR 13 1,955 g) in comparison to normal animals of approximately the same age (mean body weight of normal sample was 3,600 g).
CORTEX AND CONDITIONING
317
Conditional R esponses
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Habituation. The percentage o f trials on w h i c h a CR occurred was taken as the basic measure for determining acquisition rate and level o f a s y m p t o t i c performance. The percentage CR scores during habituation, differentiation and reversal are s h o w n for the t w o decorticates and the t w o normal groups in Fig. 3 and for individual animals in the normal Groups L+ - T + and T + - L÷ in Figs. 4 and 5 respectively. The decorticates responded to the light CS during habituation, but in b o t h cases this occurred o n l y one o f the t w o days. The responses were in addition small ( 1 - 3 m m ) and were limited to the first three CS presentations o f the session. The decorticates s h o w e d n o de novo responsiveness to the t o n e and the normals as a group did n o t respond significantly to either CS. 100-
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TABLE 1 S U B N E O C O R T I C A L
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Differentiation. Once differentiation training was introduced b o t h decorticates responded to their o w n CS+ reaching s o m e w h a t unstable a s y m p t o t i c levels o f CR p r o d u c t i o n at around 75 percent ( B A R 12) and 65 percent ( B A R 13). B A R 13 reached this level o f response to tone via a normal
318 l o o k i n g a c q u i s i t i o n curve whereas B A R 12 c o m m e n c e d even o n Day 1 o f d i f f e r e n t i a t i o n w i t h a high level o f CRs to its visual CS+ (Fig. 3). It is possible t h a t , at least d u r i n g t h e early days o f d i f f e r e n t i a t i o n , this a n i m a l ' s CRs were a variable m i x t u r e o f a l p h a r e s p o n s e s ( u n c o n d i t i o n a l responses to t h e CS) a n d true CRs. It is interesting, h o w e v e r , t h a t B A R 13, w h i c h also p r o d u c e d a l p h a responses to light during h a b i t u a t i o n , did n o t r e s p o n d to its visual C S - at all d u r i n g d i f f e r e n t i a t i o n training. B A R 12 similarly was u n r e s p o n s i v e to its C S - ( t o n e ) d u r i n g this t i m e a n d h e n c e d i f f e r e n t i a l t r a i n i n g was successful in p r o d u c i n g clear s t i m u l u s c o n t r o l in b o t h decorticates. T h e n o r m a l animals as a g r o u p acq u i r e d CRs t o CS+ m o r e r a p i d l y t h a n B A R 13 a n d t o a higher a n d m o r e stable a s y m p t o t e t h a n e i t h e r d e c o r t i c a t e (Fig. 3). T h e a c q u i s i t i o n rate o f B A R 12 is i m p o s s i b l e t o determine with any certainty. F o r t h e n o r m a l animals t o n e was the m o r e p o t e n t CS+ giving rise to significantly fewer trials to the t e n t h C R t h a n was the case w i t h the visual CS+ ( M e a n trials to t h e t e n t h CR: T ÷ = 29.0, L÷ = 82.0, M a n n - W h i t n e y U = 2, p = 0 . 0 0 4 w h e n n 1 = n 2 ~ 6) t h o u g h n o d i f f e r e n c e s in t h e levels o f individual a s y m p t o t i c p e r f o r m a n c e are e v i d e n t b e t w e e n t h e t w o groups (Figs. 4 a n d 5). T h e greater p o t e n c y o f t h e t o n a l CS was also r e f l e c t e d in t h e fact t h a t , whereas generalised r e s p o n d i n g t o t h e visual C S - was virtually a b s e n t in the n o r m a l G r o u p T + - L÷ d u r i n g d i f f e r e n t i a t i o n (see Figs. 3 a n d 5), CRs to t h e t o n a l C S - o f G r o u p L÷ - T ÷ were seen in t h e m a j o r i t y o f animals s o o n a f t e r a c q u i s i t i o n to CS+ had c o m m e n c e d giving a g r o u p m e a n of a p p r o x i m a t e l y 10 percent r e s p o n d i n g to C S - (Figs. 3 a n d 4). Mean CR amplit u d e s o n Day 14 o f d i f f e r e n t i a t i o n similarly were larger for t o n e t h a n for light in b o t h d e c o r t i c a t e s a n d n o r m a l s ( B A R 12 L÷ m e a n CR a m p l i t u d e in m m = 4.66, B A R 13 T ÷ = 19.5, N o r m a l L÷ = 10.1, N o r m a l T ÷= 15.02) t h o u g h this difference is n o t significant statistically for the n o r m a l groups. Reversal. W h e n t h e significance o f CS+ a n d CS was reversed t h e d e c o r t i c a t e s r e a c t e d i m m e d i a t e l y by ceasing t o r e s p o n d to t h e f o r m e r CS+, c o n f i r m i n g t h a t t h e r e s p o n s e s of B A R 12 to light were n o t a l p h a r e s p o n s e s at t h a t stage o f training. T h e n o r m a l s o n t h e o t h e r h a n d c o n t i n u e d to r e s p o n d to t h e f o r m e r CS+ at a high level u n t i l Day 4 in G r o u p L÷ - T ÷ a n d u n t i l Day 7 in G r o u p T ÷ - L+, falling t o 20 a n d 45 p e r c e n t levels respectively b y Day l 0 of reversal (Fig. 3). S u b s t a n t i a l CRs to t h e n e w CS+ a p p e a r e d on Day 3 o f reversal in B A R 12 (T ~) a n d o n Day 4 in B A R 13 (L÷) giving rise t h e r e a f t e r t o a c q u i s i t i o n curves w h i c h were b o t h s t e e p e r a n d w i t h higher, m o r e stable a s y m p t o t e s t h a n had b e e n t h e case for CS+ r e s p o n d i n g d u r i n g d i f f e r e n t i a t i o n . CS+ a c q u i s i t i o n in reversal was, in c o n t r a s t , r e t a r d e d for the n o r m a l s in c o m p a r i s o n w i t h t h e i r d i f f e r e n t i a t i o n performance. T h e m e a n n u m b e r o f trials to the t e n t h CR+ w i t h t o n e as CS+ in reversal ( G r o u p L÷ - T ÷) was 33.67 compared w i t h 29.0 w h e n t o n e was CS+ d u r i n g d i f f e r e n t i a t i o n t r a i n i n g ( G r o u p T ÷ - L÷). F o r light as CS+ the m e a n n u m ber o f trials to t h e t e n t h C R was 120.2 u n d e r reversal c o n d i t i o n s ( G r o u p T ÷ - L÷) a n d 82.0 d u r i n g d i f f e r e n t i a t i o n ( G r o u p L÷ - T+). Within g r o u p variances were high h o w e v e r a n d n e i t h e r o f these differences is significant statistically ( M a n n - W h i t n e y U). One i m p o r t a n t source o f v a r i a t i o n in G r o u p L+ - T ÷ was t h a t A n i m a l s 101 a n d 86, w h i c h had s h o w n a s t r o n g t e n d e n c y to r e s p o n d t o t o n e as C S - d u r i n g d i f f e r e n t i a t i o n , were in fact facilitated in acquiring T ÷ responses w h e n t o n e b e c a m e CS+ in reversal (Fig. 4). In b o t h d e c o r t i c a t e s evidence o f r e s p o n s e synergy [9] appeared in the C S - r e s p o n s e curve d u r i n g reversal in t h a t ,
OAKLEY AND RUSSELL once the a n i m a l began r e s p o n d i n g t o CS+, r e s p o n d i n g t o C S - was also briefly revived in p r o p o r t i o n to t h e s t r e n g t h of CS+ p e r f o r m a n c e . T h e n o r m a l A n i m a l s 101 a n d 91 s h o w a similar r e s p o n s e synergy in G r o u p L÷ - T ÷ (Fig. 4) while in G r o u p T + - L÷ t h e r e is an even clearer e x a m p l e in A n i m a l 90 a n d o t h e r , less m a r k e d , i n s t a n c e s f r o m A n i m a l s 82, 88, 92 (Fig. 5). C S - r e s p o n d i n g r e a c h e d a stable zero o n reversal Day 5 in B A R 12 a n d o n reversal Day 8 in B A R 13 e n s u r i n g perfect d i f f e r e n t i a t i o n scores o n reversal Day 10. In t h e n o r m a l G r o u p L÷ - T ÷ the s t r o n g a c q u i s i t i o n of CRs t o t o n e c o m b i n e d w i t h an a d e q u a t e e v e n t u a l s u p p r e s s i o n o f r e s p o n s e s t o light in reversal r e s u l t e d in a Day 10 different i a t i o n score o f 0.82. This score expresses t h e n u m b e r of CS+ r e s p o n s e s as a p r o p o r t i o n o f t h e t o t a l responses to CS+ a n d C S - (1.0 = p e r f e c t d i f f e r e n t i a t i o n 0.5 = n o different i a t i o n ) . Only partial s u p p r e s s i o n o f C S - responses t o t o n e plus p o o r a c q u i s i t i o n o f CS+ r e s p o n s e to light in G r o u p T + - L+ gave a final d i f f e r e n t i a t i o n score in reversal o f 0.61. B o t h o f these scores clearly c o m p a r e u n f a v o u r a b l y w i t h t h o s e of t h e d e c o r t i c a t e s w h i c h were in b o t h cases 1.0. In fact it is w o r t h n o t i n g t h a t o n l y o n e n o r m a l a n i m a l gave CS+ a n d C S - r e s p o n s e curves in reversal w h i c h a p p r o a c h e d t h o s e of t h e t w o decorticates. This was A n i m a l 83 in G r o u p L÷ T ÷ (Fig. 4) w h i c h had failed to acquire CRs to its visual CS+ u n t i l the t e n t h session o f d i f f e r e n t i a t i o n t r a i n i n g a n d c o n s e q u e n t l y e x t i n g u i s h e d rapidly w h e n light b e c a m e C S in reversal.
Conditional Response Onset Latencies F r o m t h e partial d e c o r t i c a t i o n studies it was clear t h a t , while C R f r e q u e n c y was n o t a f f e c t e d b y t h e lesion, C R o n s e t latencies were s h o r t e r in t h e n o r m a l animals. In o r d e r to e x t e n d this o b s e r v a t i o n we m e a s u r e d l a t e n c y o f CR onset o n CS+ trials for all a n i m a l s in the p r e s e n t s t u d y and t h e results o f this analysis are p r e s e n t e d as m e d i a n values for d e c o r t i c a t e s and n o r m a l s in Fig. 6. T h e sampling interval used was 20 msec a n d CS+ o n s e t was t a k e n as t h e zero p o i n t for l a t e n c y m e a s u r e m e n t . It is first of all clear t h a t m e d i a n C R + o n s e t latencies for the d e c o r t i c a t e s did n o t change s y s t e m a t i c a l l y f r o m t h e i r initial values (range 2 0 0 - 2 8 0 msec) d u r i n g e i t h e r d i f f e r e n t i a t i o n or reversal training. B o t h o f t h e n o r m a l g r o u p s d u r i n g d i f f e r e n t i a t i o n had Day 1 C R latencies w i t h i n the d e c o r t i c a t e range b u t t h e y t h e n s h o w e d t h e n o w classic p i c t u r e o f s h o r t e n i n g r e s p o n s e latencies over t h e initial days o f t r a i n i n g [ 7 , 1 5 ] . This was especially m a r k e d in the T ÷ g r o u p where a s t r o n g p r i m a r y l a t e n c y shift was seen b e t w e e n d i f f e r e n t i a t i o n Days 1 a n d 9, followed b y a s e c o n d a r y shift t o w a r d s longer median latencies. A related m e a s u r e s t-test p e r f o r m e d o n m e a n s s h o w s b o t h t h e p r i m a r y a n d s e c o n d a r y l a t e n c y shifts t o be significant (for t h e n o r m a l T ÷ animals d u r i n g different i a t i o n t h e g r o u p m e a n l a t e n c y o n Day 2 = 184.3 msec, Day 9 = 113.3 msec, Day 14 = 150.0 msec. Day 2 v Day 9, t = 5.883. Day 9 v Day 14, t = 4.575. F o r d f = 5, t ( 0 . 0 1 ) = 4.032. T h u s p < 0 . 0 1 in b o t h cases). The n o r m a l g r o u p w i t h L+ s h o w e d a p r i m a r y l a t e n c y shift of relatively small magnit u d e w h i c h was n o t f o l l o w e d b y a s e c o n d a r y shift to longer latencies. Median C R l a t e n c y values o f t h e L+ n o r m a l group, t h o u g h s h o r t e r t h a n t h o s e o f t h e d e c o r t i c a t e group, t h u s r e m a i n e d longer t h a n t h o s e o f t h e T ÷ n o r m a l group. This difference in CR l a t e n c y values m a y be y e t a n o t h e r reflect i o n o f the relative efficacy o f these t w o stimuli as CSs. In reversal the g r o u p for w h i c h light h a d b e c o m e CS+ prod u c e d a CR l a t e n c y profile w h i c h was virtually indistinguishable f r o m t h e d e c o r t i c a t e s ' . The n e w T + n o r m a l group
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h o w e v e r displayed a very s t r o n g p r i m a r y l a t e n c y shift bet w e e n Days 1 and 4 o f reversal. The u n e x p e c t e d l y s h o r t reversal Day 1 l a t e n c y values for T ÷ in this g r o u p ( c o m p a r a b l e t o Day 2 values o f the d i f f e r e n t i a t i o n T ÷ g r o u p ) reflect t h e fact t h a t a small a m o u n t o f l a t e n c y shifting had t a k e n place while t o n e was the C S - in the prevTous differe n t i a t i o n stage and was p r o d u c i n g generalised CRs. A secondary l a t e n c y shift w o u l d be e x p e c t e d to begin a r o u n d Days 9 - 1 0 o f reversal in this g r o u p o n the basis o f t h e different i a t i o n data b u t n o evidence o f such a change was f o u n d .
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In o r d e r t o investigate t h e possibility t h a t d e c o r t i c a t i o n p r o d u c e s a change in t h e level o f s p o n t a n e o u s n i c t i t a t i n g m e m b r a n e activity an e s t i m a t e o f s p o n t a n e o u s r e s p o n s e s (SRs) over h a b i t u a t i o n , d i f f e r e n t i a t i o n and reversal was m a d e , based on single sweeps o f the oscilloscope b e a m s (spontaneous response probes) recorded halfway between stimulus p r e s e n t a t i o n s . The p e r c e n t a g e o f p r o b e trials (40 per session) o n w h i c h r e s p o n s e s m e e t i n g the 1 m m criterion o c c u r r e d over all 26 e x p e r i m e n t a l sessions were: - BAR 12 - 0.4 p e r c e n t , B A R 13 - 0.9 p e r c e n t , n o r m a l G r o u p L+ T + - 0.6 p e r c e n t , n o r m a l G r o u p T + - L+ - 0.2 p e r c e n t . These data d o n o t suggest any e f f e c t o n SR f r e q u e n c y f r o m d e c o r t i c a t i o n . The d i s t r i b u t i o n o f SRs was u n s y s t e m a t i c in
320
OAKLEY AND RUSSELL
all groups and, as the sound attenuating properties of the experimental enclosure had been substantially improved for this study, this may be taken as confirmation of our suspicion that many of the spontaneous responses associated with a phase of generalisation between CS+ and C S - in an earlier study [9] were in fact emitted in response to external programming noise. DISCUSSION Previous studies of partial neodecorticates using single stimulus (tone) training indicated no impairment of conditioning due to neocortical removal [7]. In the present study, however, it seems necessary to conclude that the two decorticates were in fact impaired in the rate of acquisition of CRs to CS+ and in the level and stability of their asymptotic performance during the 14 days of differentiation training. With a sample of only two decorticates it is difficult to make a strong case for this but inspection of Figs. 3 and 5 shows clearly that the performance of BAR 13 during differentiation was outside the normal range for a tonal CS+. The differentiation performance of BAR 12 remains difficult to interpret in view of the presence of nictitating membrane responses from Day 1 onwards but is obviously less stable than that of the normal control group (Fig. 4). Whether neodecortication as such is the cause of these deficits or whether it is the presence of compression effects in subneocortex (Table 1) is impossible to judge. It may still be that an uncomplicated neodecortication would not affect acquisition performance to CS+. It is perhaps worth noting that if there had been substantial acquisition differences between normal and decorticate animals it is possible that differences in reversal learning could derive from the overtraining reversal effect. As only marginal acquisition differences were seen, the possibility that either group was overtrained with respect to the other does not seem likely. Earlier evidence that partial decorticates performed more efficiently in a reversal conditioning situation than normal animals led to the suggestion that this was due to a reduction in the number of features extracted from sensory inputs as neocortex was depleted [9]. While the ability to extract a wide range of features from the environment by sensory analysis may lead ultimately to highly adaptive forms of behaviour, notably those based on transferring common information across modality "boundaries, a more limited analysis might prove the more efficient strategy in the context of simple behavioural tasks. Specifically, in the l i g h t - t o n e differentiation and reversal situation a system analysing as few as two input features could perform efficiently provided one of these features was modality of input. A system extracting more features contains within itself a potential source of confusion as features common to both CSs will inevitably be sampled. It may be, in other words, that the decorticate's capacity for sensory analysis is the one which more closely matches the rather limited requirements of this experimental paradigm. Widely based stimulus feature extraction might be expected to lead in some instances to inappropriate generalisation across modalities. The previous decortication study introduced a differentiation into an already established single stimulus conditioning situation and found no significant generalisation to the novel C S - in either lesioned or normal animals [9]. More generalisation between CS+ and C S - was anticipated from the normals in the d e n o v o differentiation procedure of the present study while the
decorticates with their less widely based stimulus analysis should extract fewer features c o m m o n to the two CSs and thus show less cross-modality generalisation. The decorticates should, in other words, treat the visual and auditory inputs as independent events. In fact the decorticates show virtually zero C S - responding during differentiation. The same was true for the normals of Group T ÷ - L÷ for which tone was the positive CS; whereas the normal Group L÷ T ÷ did respond to C S - , though in only two animals was the behaviour strong or persistent. Normal animals may thus be more likely to show generalised responding during the acquisition of a differentiation than decorticates but the effect is not strong and appears to require the use of a potent C S - . When the significance of visual and auditory stimuli is reversed the separate modality analysis of the decorticate should allow the extinction of CRs to the old CS+ and acquisition of CRs to the new CS+ to proceed independently. The decorticates did in fact extinguish to the former CS+ rapidly and completely and acquired responses to the new CS+ as well, or better, than they did d e n o v o in the differentiation situation. Nevertheless it should be noted that a certain low-level interaction did occur between CS+ and C S - responding during reversal in the decorticates (See Fig. 3). The abruptness with which responses to the former CS+ extinguished in these two animals was surprising, however, as decortication produced no effect on CR extinction rates after single stimulus training [2] and seems thus to be a product of the reversal procedure itself. In the normal animals multiple feature extraction was expected to lead to difficulty in reversal because of the increased probability of interaction between the two CSs. This indeed seemed to be the case. The normals were slow to eliminate C S - responding from their behaviour during the 10 reversal sessions, especially when tone was the C S - (Group T + - L÷ Fig. 3), suggesting generalised excitation from the newly established CS+. The latter interpretation is supported by various examples in Figs. 4 and 5 of revival in C S - responding to high levels on the commencement of acquisition to CS+ (response synergy). The normals were in addition impaired in acquiring CRs to the former C S - especially when light was the stimulus involved (Group T ÷ - L÷). This latter effect in normals has been reviewed by Rescorla [12] who concluded that the C S - in a differentiation situation develops inhibitory properties with respect to the response system being conditioned. While the present data confirm the inhibitory properties of a former C S - in normals the same does not occur in the decorticates. Indeed a prior differentiation in decorticates would appear if anything to facilitate the acquisition of CRs to the former C S - . It was found in the normal animals of the present study that acquisition of CRs to a tonal CS+ during differentiation proceeded more rapidly and to a higher level than that to a visual CS+. The greater efficiency of tone in this situation could be a product of the particular tone and light parameters chosen. It is however consistent with several earlier reports of the relatively greater potency of an auditory CS [2, 11, 14]. Analysis of CR+ onset latencies confirmed previous observations made on partial decorticates [7] that the normally observed shift towards shorter response latencies with training was abolished by neocortical ablation. CR onset latencies in the decorticates remained constant within a limited range of values throughout the 24 days of differential and reversal conditioning. Both the L÷ and the T +
CORTEX AND CONDITIONING
321
normal groups showed a primary latency shift (p.l.s.) during differentiation training. Additionally, the much larger p.l.s. of the T ÷ group was followed by a significant secondary shift back towards longer mean latencies. The p.l.s, is well documented in the nictitating membrane conditioning literature [1, 7, 15, 16]. The secondary shift reported here seems, however, to have been overlooked heretofore though a similar significant secondary latency shift has been reported from this laboratory using normal animals with single stimulus (tone) conditioning over a 15 day training period [ 10]. Under reversal conditions the p.l.s, was again found in the normals when tone was CS+. Conditioning to light was very much impaired under reversal conditions and in the normal group receiving light as the new CS+ no latency shift was seen and the median CR latencies remained virtually identical to those of the decorticate group. This latter observation leads to the suspicion that the absence of a latency shift is not a direct product of decortication but simply a manifestation of a wider debility involving poor acquisition of CRs. It is clear that the p.l.s, is best displayed in normal animals which condition strongly. In the present study tone was the more potent CS and it was with tone as CS+ that the most marked p.l.s, occurred. Acquisition of CRs to light was rapid, albeit less rapid than to tone, during differentiation but slow during reversal. The latency shift to light was correspondingly present during differentiation and absent during reversal. However, two lots o f evidence suggest that the correlation between poor conditioning and the reduction or loss of the p.l.s, in normals does not account for the absence of a p.l.s, in decorticates. The first evidence derives from earlier partial decorticate studies which showed that subtotal lesions which did
not impair the acquisition of CRs to a tonal CS nevertheless abolished the p.l.s. [7]. Secondly, while it is true that both decorticates produce below normal asymptotic performances during differentiation this is not true of the reversal situation. Both decorticates in reversal show a steep acquisition curve once acquisition has commenced, accompanied by high asymptotic performances. Given that the p.l.s, is normally completed over the first 4 - 6 days of training it is surprising that the decorticates showed no evidence of such a change in their median latency profiles. Normal animals producing acquisition curves of this sort would be expected to show the p.l.s.. It might be suggested that the p.l.s, reflects the instrumentalisation of the original CR as part of a flinching movement serving to ameliorate the effects of the US. In this case the fact that neodecortication impairs or even prevents the acquisition of instrumental responses [5,13] could account for the absence of a p.l.s, in the decorticates. Whatever the details of performance and the problems of interpretation it is clear from these data that total neodecortication does not prevent the rabbit from forming a differentiation in the nictitating membrane conditioning situation between light and tone or from reversing that differentiation readily and efficiently. The decorticates' reversal performance after 14 days of differentiation training was superior to that of normal animals under similar conditions insofar as the behavioural separation between responses to CS+ and C S - occurred sooner and was more complete in the decorticates than it was in the normal control groups. Neocortex, then, is not essential for the mediation of Pavlovian conditioning at this level of complexity.
REFERENCES 1. Coleman, S. R. and I. Gormezano. Classical conditioning of the rabbit's (Oryctolagus cuniculus) nictitating membrane response under symmetrical SC-US interval shifts, or. comp. physiol. Psychol. 77: 447-455, 1971. 2. Garcia, J., B. K. McGowan and K. F. Green. Biological constraints on conditioning. In: Classical Conditioning 11." Current Research and Theory, edited by A. H. Black and W. F. Prokasy, New York: Appleton-Century-Crofts, 1972, pp. 3-27. 3. Gormezano, I. Classical conditioning. In: Experimental Methods and Instrumentation in Psychology, edited by J. B. Sidowski, New York: McGraw Hill, 1966, pp. 385-420. 4. Meyer, P. M. and D. R. Meyer. Neurosurgical procedures with special reference to aspiration lesions. In: Methods in Psychobiology, Volume 1, edited by R. D. Myers, New York: Academic Press, 1971, pp. 91-130. 5. Oakley, D. A. Instrumental learning in neodecorticate rabbits. Nature, New Biol. 233: 185-187, 1971. 6. Oakley, D. A. and I. S. Russell. Mass action and Pavlovian conditioning. Psychon. Sci. 12: 91-92, 1968. 7. Oakley, D. A. and I. S. Russell. Neocortical lesions and Pavlovian conditioning. Physiol. Behav. 8: 915-926, 1972. 8. Oakley, D. A. and I. S. Russell. Acquisition, differentiation and reversal of conditioning in partially decorticate rabbits. 1RCS J. lnt. Res. Communs Med. Sci. 1: 56, 1973. Ref. (73-3) 46-13-2.
9. Oakley, D. A. and I. S. Russell. Differential and reversal conditioning in partially neodecorticate rabbits. Physiol. Behav. 13: 221-230, 1974. 10. Oaldey, D. A., H. C. Plotkin and A. G. Yeo. Primary and secondary shifts in conditional response latency in rabbit nictitating membrane conditioning. IRCS, Research on Psychology 2: 1655, 1974. 11. Parlor, I. P. Conditioned Reflexes. Translated and edited by G. V. Anrep. London: Oxford University Press, 1927. 12. Rescorla, R. A. Pavlovian conditioned inhibition. Psychol. Bull. 72: 77-94, 1969. 13. Russell, I. S. Neurological basis of complex learning. Br. Med. Bull. 27: 278-285, 1971. 14. Saavedra, M. A., E. Garcia and T. Pinto-Hamuy. Acquisition of auditory conditioned responses in normal and neodecorticate rats. J. comp. physiol. Psychol. 56: 31-35, 1963. 15. Schneiderman, N. and I. Gormezano. Conditioning of the nictitating membrane of the rabbit as a function of CS-US interval. J. comp. physiol. Psychol. 57: 188-195, 1964. 16. Smith, M. C. CS-US interval and US intensity in classical conditioning of the rabbit's nictitating membrane response. 3.. comp. physiol. Psychol. 66: 679-687, 1968.