Electroencephalography and Clinical Neurophysiology, 1977, 42:341--354

341

© Elsevier/North-Holland Scientific Publishers Ltd.

A CORTICAL EVOKED POTENTIAL THAT REFLECTS THE CONDITIONED, POSITIVE INCENTIVE VALUE OF THE STIMULUS. I. THE EVOKED POTENTIAL *

E.S. BOYD, E.H. BOYD and L.E. BROWN Department o f Pharmacology and Toxicology, University o f Rochester School o f Medicine and Dentistry, Rochester, N.Y. 14642 (U.S.A.)

(Accepted for publication : June 7, 1976)

Evoked potentials in man, with m a x i m u m amplitude at the vertex, have been linked to many psychological variables. For instance, the contingent negative variation (CNV) has been related to such psychological constructs as expectancy (Walter 1964; McAdam and Rubin 1971), attention (McAdam and Rubin 1971} and motivation (Knott et al. 1970; McAdam and Rubin 1971). The P3 or P300 wave is highly influenced by the attention of the subject (Donald and Goff 1971) and the meaningfulness of the stimulus (Sutton et al. 1967; Ford et al. 1973} and it has been related to the resolution of uncertainty (Sutton et al. 1965}, recognition of the stimulus (Sutton et al. 1967}, and decision making (Debecker and Desmedt 1971; Rohrbaugh et al. 1974). The area of cortex generating vertex potentials is polysensory, in that it exhibits stimulus-related electrical activity in response to visual, auditory and somatosensory input (Sutton et al. 1965; Hay and Davis 1971}. There are indications that activity evoked at frontal or vertex recording sites, but n o t at primary sensory sites, reflects the meaning of the stimulus to the subject (Begleiter et al. 1973; Johnston and Chesney 1974}. In appropriate paradigms, the CNV, or steady potential changes like it, have been demonstrated in macaque monkeys (Low et al. 1966; Borda 1970; Donchin et al. 1971; Rebert 1972; Hab* This investigation was supported by U.S. Public Health Service General Research Support Grant to the University of Rochester.

litz 1973). Although all of these potentials can be recorded from frontal, central and parietal sites, it is becoming apparent that the potentials recorded at different sites differ in regard to the paradigm generating them and/or the influence of psychological variables upon them (see Poon et al. 1974, Weinberg and Papakostopoulos 1975, for differing human CVNs; Poon et al. 1974; Courchesne et al. 1975, for differing human P300 waves; and Borda 1970, Donchin et al. 1971, Hablitz 1973, for differing macaque CNVs). We describe here, and in two papers to follow (Boyd et al. 1977a,b), a surface negative potential in the squirrel m o n k e y which is recorded from the same area of cortex as the potentials discussed above. However, it will be shown that this potential differs in a significant number of respects from any previously reported potential. In particular, it differs from a typical CNV in that: (a) it is not a steady potential shift, but resembles more an N200 wave (which also distinguishes it from the P3 or P300 wave); (b) it can be generated by a single cue, whereas the CNV typically requires a pair of cues; and (c) it appears to reflect mainly one psychological variable, i.e., incentive. It also differs from the readiness (Deecke et al. 1969) and m o t o r (Gliden et al. 1966) potentials in that it requires no m o t o r response, other than picking up and consuming a food pellet, and it can be present, at reduced amplitude, when the well-trained animal makes no m o t o r response at all.

312 This first p a p e r will describe the e v o k e d p o t e n t i a l , including its shape and site o f generation, and b r a c k e t its p o s i t i o n in the s t i m u l u s r e s p o n s e s e q u e n c e . A s e c o n d p a p e r ( B o y d et al. 1977a) will describe the influence of drive m o t i v a t i o n (hunger), and a third ( B o y d et al. 1 9 7 7 b ) , the influence o f m a n i p u l a t i v e m o t i v a tion and the value of t h e reward. We use the term, manipulative motivation, to refer to a " d r i v e " which m o t i v a t e s the animal to o p e r a t e on his e n v i r o n m e n t with no a p p a r e n t r e w a r d o t h e r t h a n the change t h a t he p r o d u c e s . This is p r o b a b l y identical with w h a t H a r l o w et al. ( 1 9 5 6 ) called m a n i p u l a t o r y m o t i v a t i o n . It will b e c o m e a p p a r e n t t h a t the a b o v e t h r e e variables, all c o n t r i b u t i n g to the c o n d i t i o n e d incentive value o f the cue, can a c c o u n t for m o s t o f the e v o k e d w a v e f o r m and the associated b e h a v i o r of the animal.

Methods

Animals and electrodes T r a n s c o r t i c a l e l e c t r o d e s were c h r o n i c a l l y i m p l a n t e d in m a l e squirrel m o n k e y s (Saimiri sciureus) u n d e r p e n t o b a r b i t a l anesthesia. T h e s e usually consisted of a silver ball on the pial or dural surface and an a d j a c e n t stainless-steel wire (0.18 m m d i a m e t e r ) , insulated e x c e p t at the cut end, which p r o j e c t e d 3 m m b e l o w the cortical surface. T h e e x p e r i m e n t a l l y determ i n e d fall t i m e c o n s t a n t o f t w o o f these elect r o d e pairs was a b o u t 100 m s e c with a phase shift of a b o u t 35 ° . T h e t i m e c o n s t a n t of imp l a n t e d e l e c t r o d e s always a p p e a r e d to be longer t h a n this (see Fidelity o f r e c o r d i n g in Results), p e r h a p s because o f the f o r m a t i o n o f s o m e silver chloride, with t i m e . T r a n s c o r t i c a l sintered silver--silver chloride e l e c t r o d e s were also i m p l a n t e d in t w o animals. E l e c t r o d e s were i m p l a n t e d in p r e - a r c u a t e , p o s t - a r c u a t e , p o s t - c e n t r a l , m i d - p a r i e t a l and occipital c o r t e x . In five animals t h a t have died all e l e c t r o d e s were w h e r e t h e y were s u p p o s e d t o be. T h e rem a i n i n g seven animals are still u n d e r s t u d y so histological c o n f i r m a t i o n o f their e l e c t r o d e p l a c e m e n t s is lacking.

E.S. BOYD ET AL,

Behavioral equipment and training During testing sessions the animals were restrained in a c o m m e r c i a l m o n k e y chair inside a sound-attenuating chamber containing a pellet f e e d e r b y the a n i m a l ' s left h a n d , a manip u l a n d u m by the a n i m a l ' s right h a n d , and a w h i t e light a b o u t 4 in. in f r o n t of the a n i m a l ' s face. A s p e a k e r to deliver a u d i t o r y stimuli (cues) was m o u n t e d directly a b o v e the anim a l ' s head. Animals were fasted for 24 h before test sessions (which o c c u r r e d 3 t i m e s a w e e k ) . T h e y were t a u g h t to bar-press for banana pellets (75 mg). A c u m u l a t i v e r e c o r d e r registered: (a) the a n i m a l ' s r e s p o n s e s (bar presses); (b) r e i n f o r c e m e n t s ; and (c) changes in schedule. Most o f the animals were t r a i n e d initially on a m o d i f i e d c o n t i n u o u s r e i n f o r c e m e n t schedule ( C R F ) . A delay o f 0.5 sec was introd u c e d b e t w e e n the a n i m a l ' s r e s p o n s e and activation of the feeder. When the animal had learned to bar-press, find the f o o d pellet, etc., the schedule was gradually c h a n g e d to one where the p r e s e n t a t i o n o f 10 m s e c bursts o f 4 kc/sec t o n e (78 dB r e f e r r e d to 0 . 0 0 0 2 d y n e s / c m 2) signalled the availability o f a f o o d pellet if the animal bar-pressed within 2 sec. Testing sessions consisted of t h r e e or f o u r periods, each o f which included 50 t o n e cues p r e s e n t e d at variable intervals (range 5 - - 4 0 sec) with a m e a n o f 22 sec, f o l l o w e d b y 1 1/8 m i n o f C R F , signalled b y a w h i t e light. T h e periods of C R F were i m m a t e r i a l to the results p r e s e n t e d here.

Recording of evoked activity Electrical activity was led f r o m the animal t h r o u g h p r e a m p l i f i e r s to a c o m p u t e r . T h e preamplifiers w e r e used with a high f r e q u e n c y t i m e c o n s t a n t of 0.1 msec. Until an a n i m a l was r e a s o n a b l y well trained it was usually necessary to use a l o w f r e q u e n c y t i m e cons t a n t of 100 msec in o r d e r to avoid overloading the p r e a m p l i f i e r s with m o v e m e n t artifacts. As training progressed it was possible to shift this to 6 0 0 m s e c . T h e c o m p u t e r ran the schedule, c o u n t e d responses and rewards, calculated the m e a n

EVOKED POTENTIAL AND INCENTIVE. I. reaction time, and collected evoked activity f r o m up to four recording electrodes. At the end of the session, it calculated average evoked potentials and standard deviations for each of the recording sites according to w het her the animal had, or had not , responded behaviorally within 2 sec and thus been rewarded. In recordings from bipolar electrodes, cortical surface negativity, relative to underlying white matter, was plotted upward. Where recordings were made between di f f er e nt sites, negativity at the first-named site, relative to the second, was p lo tted upward.

Extinction and pseudoconditioning The effects of behavioral extinction were studied with the regularly scheduled program but with the o u t p u t from the c o m p u t e r to the feeder disconnected. The effects of pseudoconditioning were studied by switching the animal from the regular schedule to a program in which tone bursts were still presented at variable intervals (mean, 22 sec) but t he y no longer signalled the availability of reward. Instead, the animal was rewarded on a simultaneously running standard fixed interval schedule (FI 22). The two schedules were independen t ex cep t t hat tone bursts were n o t presented during periods when the FI schedule had made reward available but the animal had n o t y et bar-pressed. No periods of CRF were presented.

Learning In order to det er m i ne when an animal had learned the meaning of the t one cues, two statistical estimates were used, as well as direct observation o f the animal's behavior. Both estimates were based on the binominal distribution {Edwards 1960). The first ( Z l ) was based on the animal's correct responses {G), total responses (R}, and the fraction of time during the session when a response would have been correct (P). The standard score ( Z l ) was calculated from : Z1

= G

-- (RP) -- 0.5 vT-QR

343 where Q = 1 - P . The second (Z2) was based on the n u m b e r of correct responses (G) compared to the n u m b e r of cues presented (C) and: Z2 = G -- (CP) -- 0.5

-QC where P = Q = 0.5. The c o m p u t e r calculated the statistics at the end of each testing session. The two statistics were used together to avoid the problem of high rates o f extraneous behavioral responding, since such rates could make Z2 significant but would simultaneously make Z1 non-significant. Thus the statistics furnished an indication that the animal was deliberately responding to the cues only when both were significantly positive.

Results

Twelve animals were trained to know that the tone cue signalled the availability of a food pellet. Learning time ranged from 4 to 37 sessions. The devel opm ent of cortical activity evoked by the tone cues followed the same general pattern in all animals. There was often initially some short latency evoked activity, probably due to the novelty of the stimulus. This was usually greater when the animal did n o t respond behaviorally and it decreased or disappeared in succeeding sessions, presum ably due to habituation. Then, as the animal learned the meaning of the stimulus, a surface negative waveform developed at at least one, and usually several, recording sites. This peaked 100-200 msec after the stimulus and ranged, among animals, from about 20 to 200 pV in average amplitude. For convenience in referring to it, we have called this waveform an M-wave. We chose the letter M when we realized t hat the waveform was reflecting some aspect of the Meaning of the cue to the animal. The M-wave was oft en preceded by small evoked waves, both negative and positive, or, at some recording sites, by a large positive wave. An example of the devel opm ent of the M-wave as the animal learned the meaning of the cue is shown

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Fig. 1. Averaged activity in left post-arcuate c o r t e x in Phil evoked by 10 msec bursts o f 4 kc/see t o n e used as a cue to the availability o f reward. The evoked activity was averaged separately according to w h e t h e r he did or did n o t r e s p o n d behaviorally to the cue. The large n u m b e r at the beginning o f each line is the n u m b e r o f the session. The smaller n u m b e r at the end o f each average, in this and s u b s e q u e n t figures, is the n u m b e r of responses in the average. Z l and Z2 are statistical estimates o f the animal's being r e w a r d e d by chance r e s p o n d i n g (see text). By the n i n t h session it was a p p a r e n t , f r o m observing the animal, t h a t he was learning the m e a n i n g o f the cue. Z1 and Z2 were s i m u l t a n e o u s l y significant for the first time in the t w e l f t h session. T h e y t h e n r e m a i n e d so for the following 30 m o n t h s . (Large negative values o f Z1 were caused by high rates o f behavioral r e s p o n d i n g which t e n d e d to be suppressed by the t o n e cues in the u n t r a i n e d animal.) The bars on the b o t t o m lines o f each c o l u m n indicate the time o f p r e s e n t a t i o n o f the t o n e cues. The calibrations r e p r e s e n t 100 msec and 100 pV. Preamplifier t i m e c o n s t a n t , 100 msec. In this and s u c c e e d i n g figures, cortical surface negativity, relative to white m a t t e r , is p l o t t e d upward.

E V O K E D P O T E N T I A L A N D I N C E N T I V E . I.

345

in Fig. 1. Once developed, the M-wave was usually stable over long periods of time, although at some recording sites it decreased slowly over a period of months•

Fidelity of recording As a check on the fidelity of recording of the M-wave, simultaneous recordings were I00

made from the same site through two preamplifiers with fall time constants of 100 and 600 msec respectively. Examples are shown in Fig. 2. Recordings at 600 msec showed an increase in the amplitude of the M-wave and usually some increase in late slow activity, but very little change in the shape or latency to peak of the M-wave• Thus the system time constant appeared to be 100 msec when the

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346

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p r e a m p l i f i e r s were set for this value, and s o m e greater, i n d e t e r m i n a t e value w h e n the preamplifiers were set for 6 0 0 m s e c . In t w o animals M-wave activity, p i c k e d up b y s i l v e r silver chloride e l e c t r o d e s , was r e c o r d e d simult a n e o u s l y t h r o u g h a DC p r e a m p l i f i e r and an AC p r e a m p l i f i e r with the t i m e c o n s t a n t at 6 0 0 msec. An e x a m p l e is s h o w n in Fig. 3. T h e r e was no significant d i f f e r e n c e in the re-

c o r d e d M-wave. It was c o n c l u d e d t h a t the Mwave was being r e c o r d e d w i t h r e a s o n a b l e fidelity.

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Fig. 3. M-wave activity r e c o r d e d with transcortical silver--silver chloride e l e c t r o d e s in R y a n ' s right post-central c o r t e x . Activity picked up by the e l e c t r o d e pair was led s i m u l t a n e o u s l y t h r o u g h b o t h an AC p r e a m p l i f i e r with the time c o n s t a n t at 600 msec and a DC preamplifier. Activity evoked by the first 25 cues r e s p o n d e d to, and by the total 100 r e s p o n d e d to, in the session is s h o w n . Fig. 4. I n d i c a t i o n s o f the location o f the generator of the M-wave in AI. Average evoked p o t e n t i a l s within each o f the six vertical sets o f data were m a d e f r o m s i m u l t a n e o u s l y r e c o r d e d activity. The first five s h o w recordings f r o m first the bipolar e l e c t r o d e , then the surface and d e p t h parts o f the e l e c t r o d e , each r e c o r d e d first with the lateral o r b i t and t h e n with occipital c o r t e x as reference. The sixth s h o w s first the bipolar and t h e n the surface and d e p t h , r e c o r d e d first with the lateral o r b i t and t h e n with the nasion as reference. The right pre-arcuate, right and left post-arcuate, and right parietal recordings s h o w a negative M-wave at the surface and a positive wave in the d e p t h , w h e t h e r the recordings were r e f e r r e d rostrally to the lateral o r b i t or caudally to occipital c o r t e x . Thus the g e n e r a t o r appeared to be located in the c o r t e x b e t w e e n the parts o f each o f these bipolar electrodes. There was no a p p a r e n t M-wave in the recordings f r o m occipital c o r t e x (sixth set o f data). The vertical bars s h o w the time of delivery o f the t o n e cues. The calibrations r e p r e s e n t 100 msec and 50 pV. Time c o n s t a n t , 600 msec.

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the M-wave was generated in c o r t e x and neither elsewhere in the brain n o r by eye m o v e m e n t or muscle c o n t r a c t i o n s . The g e n e r a t o r a p p e a r e d to be diffuse, with maxima] activity at postarcuate or post-central sites, less activity at some pre-arcuate and mid-parietal, and n o n e at occipital, sites. The e x a m p l e s h o w n in Fig. 4 indicates that, in this animal, the right prearcuate, right and left p o s t - a r c u a t e and right mid-parietal, b u t n o t the left pre-arcuate or right occipital, bipolar electrodes were spanning the generator.

Extinction F o u r well-trained animals were subjected to a behavioral e x t i n c t i o n p r o c e d u r e , in r e p e a t e d sessions, until the animal r e s p o n d e d behaviorally to less than 10 o f the 150 cues presented. The n u m b e r of sessions n e e d e d to reach this criterion ranged f r o m 3 to 7. The results are s u m m a r i z e d in Table I. In all cases the amplitude of the total average M-wave e v o k e d b y the cues decreased as the n u m b e r of responses to the cue decreased. In three o f the animals this was due to a decrease in M-wave e v o k e d b o t h by cues r e s p o n d e d to and by cues n o t res p o n d e d to. With the f o u r t h animal (Phil) there was some decrease in the M-wave e v o k e d by

cues r e s p o n d e d to (as well as by cues n o t res p o n d e d to) in the first t w o e x t i n c t i o n sessions, b u t in the third, and final, session the M-wave e v o k e d b y the 6 cues r e s p o n d e d to behaviorally was b a c k within the range o f pre-extinction values, even t h o u g h there was essentially no M-wave w h e n the animal failed t o respond. This is illustrated in Fig. 5.

Pseudoconditioning F o u r well-trained animals were subjected to p s e u d o c o n d i t i o n i n g . When the animals h a d a d a p t e d to the change in p r o g r a m , t h e y received a p p r o x i m a t e l y equal n u m b e r s o f t o n e bursts and rewards per session. In all cases p s e u d o c o n d i t i o n i n g resulted in a m a r k e d decrease in the a m p l i t u d e o f the M-wave. Pseudoc o n d i t i o n i n g sessions were c o n t i n u e d until it appeared t h a t no f u r t h e r r e d u c t i o n in amplitude was likely to o c c u r ( 7 - - 1 6 sessions). The results for the f o u r animals are p r e s e n t e d in Table II, and an e x a m p l e is s h o w n in Fig. 6. Three animals were successfully retrained, with c o n c o m i t a n t recovery o f the M-wave to control a m p l i t u d e . In all cases retraining after p s e u d o c o n d i t i o n i n g was m o r e difficult than after e x t i n c t i o n . The f o u r t h animal died during retraining.

TABLE I Effects of behavioral extinction on performance and M-wave amplitude in post-arcuatc cortex, both for cues responded to and for total cues. Animal and hemisphere Jack, Jack, Phil, Phil, Stu, AI, AI,

right left right left right right left

Extinction session number 5 5 3 3 7 4 4

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M-wave amplitude (% of control) Responded to cue

Total cues

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Fig. 5. E f f e c t o f e x t i n c t i o n p r o c e d u r e , in Phil, on behavioral r e s p o n d i n g t o t o n e cues and o n the activity e v o k e d by the t o n e cues in right and left p o s t - a r c u a t e c o r t e x . At the left is a p l o t o f t h e average a m p l i t u d e o f M-waves e v o k e d by the 150 cues per session against the n u m b e r o f cues r e s p o n d e d to behaviorally. The p o i n t s at the right o f this plot are f r o m the p r e - e x t i n c t i o n session. Straight lines were fitted by the m e t h o d o f least squares. T h e y are n o t m e a n t to i m p l y any linearity o f f u n c t i o n . A t the right are the averaged e v o k e d w a v e f o r m s for the p r e - e x t i n c t i o n session and the third (last) e x t i n c t i o n session. In each case, the evoked activ!ty f r o m right and left p o s t - a r c u a t e c o r t e x is divided into t w o averages a c c o r d i n g to w h e t h e r the animal r e s p o n d e d behaviorally t o the cue. The calibrations r e p r e s e n t 100 msec and 50 p V . Time c o n s t a n t , 600 msec. T A B L E II E f f e c t s o f p s e u d o c o n d i t i o n i n g on cues r e s p o n d e d to and average a m p l i t u d e o f M-wave in post-arcuate c o r t e x evoked b y t h e cues. Animal and hemisphere

Pseudoconditioning session n u m b e r

Cues r e s p o n d e d to, o f t o t a l presented *

P2

M-wave a m p l i t u d e (% o f c o n t r o l )

Phil, Phil, Stu, Jack, Jack, Al, AI,

7 7 16 13 13 7 7

69/172 69/172 70/169 68/171 68/171 67/160 67/160

ns ns ns ns ns ns ns

13 4 19 26 17 22 23

Mean

right left right right left right left

** ** ** ** ** ** **

18

* This is the n u m b e r o f cues following w h i c h t h e animal bar-pressed w i t h i n 2 sec. Since it was impossible to tell w h e t h e r t h e animal was r e s p o n d i n g t o t h e cues or r e s p o n d i n g o n t h e FI s c h e d u l e , P2 is an e s t i m a t e o f the maxim u m p r o b a b i l i t y t h a t t h e animal r e s p o n d e d to t h e cue. ** P < 0.001• ns, n o t significant; P > 0.05.

350

E.S. BOYD ET AL. f'vx

,% ,

"x.._

Effects o f stimulus intensity

.,,f-v,~ )o

A

32 E

A,

.=

25-

RETRAINING

PSEUDOCONDITIONING

-.-~-

T -~

; --~7-7

....7 - ~

5

SESSIONS

Fig. 6. E f f e c t o f p s e u d o c o n d i t i o n i n g on activity evoked in ,4,1's right post-arcuate c o r t e x by t o n e bursts. The average a m p l i t u d e o f the M-wave (+ S.E.M. ) evoked by all t o n e bursts for the session is p l o t t e d for the last c o n t r o l session, 7 sessions of p s e u d o c o n d i t i o n i n g , and the first 4 sessions o f retraining. At the top is s h o w n the average evoked response for the last c o n t r o l session (n = 150), the last p s e u d o c o n d i t i o n i n g session (n = 160), ~nd the last retraining session (n = 150). Time c o n s t a n t , 600 reset.

T o n e cues were p r e s e n t e d to five animals at b o t h 73 and 78 dB (re: 0 . 0 0 0 2 d y n e s / c m 2 ) . F o r these tests, one t o n e intensity was used for a b o u t the first half o f a session; t h e n the intensity was changed and new averages of e v o k e d activity were begun. T o eliminate the e f f e c t o f changes in e v o k e d activity as the session progressed, the first intensity used was alternately high and low in succeeding sessions. The increase in intensity f r o m 73 to 78 dB p r o d u c e d m a r k e d increases in the a m p l i t u d e s o f m o s t c o m p o n e n t s o f the e v o k e d response occurring earlier than the M-wave, b u t had n o significant e f f e c t on the a m p l i t u d e o f the Mwave. T h e results are s u m m a r i z e d in Table III. Averaging s y n c h r o n i z e d behavioral response

with

the

animal's

In four animals the same sets of e v o k e d activity were averaged: (a) with the t o n e cue as r e f e r e n c e p o i n t ; and (b) with the animal's barpress as r e f e r e n c e p o i n t . In these e x p e r i m e n t s o n l y activity related to cues to which the ani-

T A B L E III Average p e r c e n t changes in a m p l i t u d e o f evoked activity in post-arcuate c o r t e x on changing t o n e cue i n t e n s i t y f r o m 73 t o 78 dB. Animal and h e m i s p h e r e

Early activity (%) *

P

A cortical evoked potential that reflects the conditioned, positive incentive value of the stimulus. I. The evoked potential.

Electroencephalography and Clinical Neurophysiology, 1977, 42:341--354 341 © Elsevier/North-Holland Scientific Publishers Ltd. A CORTICAL EVOKED PO...
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