Exp. Brain Res. 9, 346--356 (1969)

Impaired Discrimination Following Polarisation of the Striate Cortex*

ROGER WARD and LAWRENCE WEISKRA:NTZ

Psychological Laboratory, Downing Street, Cambridge (England)

Received April 15, 1969

Summary. Two rhesus monkeys were t r a ined to solve a p rob lem involving the recognit ion of tachis toscopica l ly presented objects. Per formance a t this t a sk was impai red by the passage of surface-posi t ive po]arising currents th rough the s t r ia te cor tex : the impa i rmen t pers is ted af ter current was tu rned off, decaying in a reason- ab ly exponent ia l manner wi th a t ime cons tan t of abou t 20 rain. The magn i tude of the impa i rmen t var ied bo th wi th the in tens i ty of the appl ied current and with the dura t ion of appl icat ion.

Key Words : Monkeys - - Bra in s t imula t ion - - Discr imina t ion

Introduction

R~sI~ov (I 953) and MORRELL (196 I) showed that the application of polarising currents to the surface of the cerebral cortex could have an effect upon a simple piece of behaviour: application of surface-positive current, by itself insufficient to elicit any overt movement, to the "limb area" of the motor cortex of a rabbit, was accompanied by twitching of the limb when the animal was subjected to a simple stimulus such as a flash of light. MORRELL and NAITOH (1962) subsequently found that the acquisition by a rabbit of leg flexion conditional upon the presenta- tion of a flashing light was hindered by surface-negative polarisation of the striate cortex. This experiment was eriticised by KUPrER~AN~ (1965) on the grounds that the behaviour studied had not been widely investigated; that the response was extremely unstable; and that it was difficult to distinguish "learning vari- ables" from "performance variables". KUPFERMA~ accordingly investigated the effects of cortical polarisation on visual discrimination learning in the rat. He found that the application of polarising currents to striate or motor areas of the rat's cerebral cortex had neither a significant effect upon the acquisition of the solution to a brightness discrimination nor upon the acquisition of the solution to a pattern discrimination.

ALBERT (1966a, b), on the other hand, claimed to have demonstrated detect- able interactions between polarising currents and cortical spreading depression. ALBERT trained rats to perform a one-way shuttle box avoidance task under uni- lateral cortical depression and examined the effect of applying polarising current

* This paper is based on a dissertation submitted by ROGER WA~D in partial fulfilment of the requirements for the degree of Doctor of Philosophy at the University of Cambridge, and was supported by a Medical Research Council Scholarship for Training in Research Methods.

Cortical Polarisation 347

shortly after the animal executed a single tr ial wi thout being subjected to de-

pression. I t appeared to ALBERT tha t re tent ion of the habi t was impaired by the applicat ion of surface-negative current short ly after the single "undepressed" trial.

I n t e rp re t a t ion of the results of ALBERT'S observations is made difficult : both by the fact tha t ALBERT applied a "pulsa t ing" current, in contrast to MORI~ELL and NAITO~ (1962) and KUPFE~AlWV (1965), who subjected their animals to a s teady flow of current ; and by a series of objections raised by SCHNEIDER (1967) to the generally accepted rat ionale of the use of cortical depression as an experi- menta l t rea tment .

KU~'FEI~MAlV_X'S procedure may have failed to produce any detectable effect

because the task he set his animals was too easy, or because a ny al terat ion of cortical funct ion brought about by the polarising current was too slight to impede the ra t in its progress towards a solution to the problem confronting i t : LASHLEY (1931), after all, showed tha t it was possible for the ra t to solve a visual discrimi- na t ion with an almost vanishingly small a m o u n t of visual cortex. I t might be bet ter to confront the an imal with a task in which the indicat ion of performance is con- t inuously variable, and in principle at any rate capable of showing either facilita- t ion or impairment . FUSTER and UYEDA (1962) devised such a task, involving the recognition of tachistoscopically presented objects by rhesus monkeys, and our ex- per iments invest igate the effects of polarisat ion of the striate cortex of rhesus monkeys upon their performance at a similar task. Electrophysiologica] evidence (1V[ORRELL 1961) suggests t ha t any change in nerve cell ac t iv i ty brought about by the passage of polarising current may persist for some t ime bu t will in general be reversible. The animals tak ing par t in our experiments therefore act as their own controls; the object of each exper iment is to see whether the performance of a single animal changes as the result of the applicat ion of a po]arising current.

Procedure

The animals taking part in this experiment were two sexually immature, male rhesus monkeys. One (Fit) had taken part in a study investigating the effects of separation of cue position, response position and site of delivery of reward (CowEY and WEISK~ANTZ 1968) ; the other (Baggy) was experimentally naive at the beginning of training. During the period of the experiment each animal was placed on a restricted diet supplemented with pieces of fruit and the rewards obtained during testing; if testing was interrupted for more than a few days (for example, during a period of recovery from surgery) the animal was given unrestricted access to food. Water was continuously available in the animal's home cage.

All testing was carried out in a modified version of the Wisconsin General Testing Appara- tus. The animal found itself in ~ compartment 1.4 metres high, 0.7 m wide and 0.9 m deep, and faced an opaque screen which when raised gave access to a second compartment 0.5 m deep, 0.7 m wide and 0.7 m high, which was illuminated by a pair of 200 V/60 W ribbon fila- ment lamps mounted in the angle made by the side walls and ceiling. The larger compartment, containing the animal, was illuminated by an identical pair of lamps supplied with 80 V from a Variac toroidM transformer. All surfaces of the apparatus were painted with fiat black paint. The form board placed in the smaller compartment w~s made of wood and was 35 em wide, 7 em from front to back and contained two food wells, each 2.5 cm in diameter, about 1 cm deep and 22.5 cm apart. While the rest of the form board was painted black, the food wells were unpainted, and were covered by unpainted aluminium panels 7 cm square. The form board was attached to a handle about 40 cm long by means of which it could be advanced to- wards the animal.

The first stage of training consisted of teaching the animals to distinguish the objects used as cues. These objects were a polished aluminium cone 4 em high and 2 cm in diameter at the

348 1%. WARD and L. WEISKRA~TZ:

base, and an irregularly 13-sided pyramid of the same overall dimensions. Each animal was given an initial period of adaptat ion to the apparatus, and was not formally tested until it had overcome its initial reluctance to cooperate. Formal testing took place as a rule in the early afternoon each day except Sunday; each testing session consisted of fifty trials. During this period the lights in the apparatus were on continuously and the animal was not subjected to any physical restraints except those imposed by the dimensions of its t ranspor t cage.

For one animal the pyramid was baited by placing a half peanut in the appropriate food well, and for the other animal the cone was baited. The position of the baited object was deter- mined by inspection of a semi-random schedule (GELLE~MAN 1933). Training was continued until the animal picked up the baited object 45 times in 50 trials; one animal reached this criterion in 300 trials and the other in 250 trials. In subsequent sessions, the objects were pre- sented tachistoscopicMly and the animal 's response latency was recorded.

The light source of the tachistoscope was a Phillips TL 8 W 34/L fluorescent lamp, mounted on the ceiling of the smaller compar tment just inside the opaque screen, parallel to the long axis of the form board and supplied with stabilized direct current (250 V, 50 mA) from an Ediswan high-tension power supply. One end of the lamp was heated by applying 6.3 V ~ , and one of the terminals supplying this voltage, together with the negative terminals of the high- tension supply, was connected to ear th ; the other end of the lamp was connected to the posi- t i r e terminal of the high-tension supply through a pair of relay contacts. This relay was in tu rn driven by a modified version of the t imer described by DONALDSON (1965).

In order to show t h a t the animal was not using tactile cues in making the discrimination, a device designed by S.H. SALTER was used which caused a lamp to l ight if the animal touched one of the objects. This device is basically a high-gain amplifier whose output switches a relay. Two such devices were used, each associated with a coloured warning light, t f the animal made an incorrect choice the trial was terminated; if the choice was correct the animal was allowed to retrieve the reward.

The interval between the tachistoscope lamp striking and the animal touching either ob- ject was measured on a Venner t imer and was recorded to the nearest 10 msec.

A conventional eleetromechanical programming circuit, mounted outside the testing room, enabled the sequence of events involved in a single tachistoscopic presentat ion to be carried out automatically. The testing room itself was in darkness and maintained, in common with the rest of the animal section of the laboratory, a t a temperature of 22 ~ C: noises outside the testing room were masked by white noise produced by a Grason-Stadler 9OlA noise gene- rator.

No a t t empt was made to control the intertrial interval rigorously; the routing operations carried out at the end of each trial (recording response latency, scoring the response, baiting the food well, altering the sett ing of the t imer and so on) took a reasonable constant length of of time. The length of a session of fifty tachistoscopic presentations was about half an hour, and sporadic checks with a stopwatch showed the intertrial interval to be about 30 sec.

The measurement of performance used in these experiments was an estimate of the animal 's " threshold", which in tu rn was arrived at by means of a t i t ra t ion procedure which may be summarised as follows. The exposure durat ion was set to a particular value and the animal subjected to two trials a t this exposure. I f both responses were correct, the sett ing of the t imer was reduced; throughout the experiment the values used were 10, 9, 8 . . . 1 sec, 900, 800 . . . 100, 90, 8 0 . . , l0 msec. I f on the other hand, the aninlal made an incorrect response, the durat ion of exposure of the objects was increased and a correction trial given. I f the responses on the correction trial and on two subsequent trials were correct, the setting of the t imer was reduced again, and was fur ther reduced after every other trial until the animal made another mistake. The animal 's " threshold" was estimated by considering those exposure durations a t which the animal made two correct responses, only to make a mistake if the setting was reduced ("des- cending" threshold points), and those durations a t which the animal made two correct respon- ses, having made a mistake at the preceding, shorter durat ion ("ascending" threshold points). At the end of each testing session, the average of the various threshold points, together with its s tandard error, was calculated. This procedure follows closely t ha t used by BUFFE~Y (1965) in estimating the delays which can be tolerated in a matching task by normal and brain- damaged monkeys. Since the difficulty of the task is determined by the animal itself and not by the experimenter, the possibility of encountering the experimental "neuroses" mentioned

Cortical Potarisation 349

by F~STER and UYEDA (1962) is lessened, and no such reactions were exhibited by any of the animals taking part in our experiment. Once the animals learned to perform satisfactorily when the objects were presented tachistoscopically, their electrodes were implanted.

The electrodes were made of sheets of gold foil, 0.0003 inches thick (Johnson, Matthey & Co., Ltd.), bonded on one surface to 50-gauge Melinex (I.C.I.) film by means of an epoxy resin (CIBA-A.I~.L.). The sheet of gold was attached, before being covered with plastic film, to a length of stainless steel wire, either with solder or with FSP 49 conducting cement (Johnson, Matthey), and this wire passed through the lumen of a length of polypropylene tubing (Port- land Plastics, Ltd.). Two cortical electrodes were soldered to one pin of a miniature connector (Cannon Electric [G.B.], Ltd.) and an identical indifferent electrode to a second pin.

Im plantation of the electrodes was carried out under the usual aseptic precautions, while the animal was under Nembutal anaesthesia. A hole was made in the exposed skull, and the electrode, cut to size, was placed under the dura on the lateral surface of the striate cortex. After repeating this procedure with the other cortical electrode the connector was attached to the skull by the procedure recommended by DELGADO (1961) and the indifferent electrode inserted into a pocket created under the skin by blunt dissection. The animal was given an intramuscular injection of penicillin and allowed to recover from the anaesthetic in its home cage.

After recovering from the anaesthetic, each animal was allowed to rest in its home cage for 14 days before being tested again. Once post-operative testing began, each animal was housed in a Forringer monkey chair; in order to avoid the development of pressure sores at those points where the animal comes into contact with the chair (T~o~rsoN, SEAL and BLOOS~[ 1966) the animal was removed from the chair every 2 or 3 days, and with its arms firmly pinned behind its back was permitted to exercise itself for about 10 rain. During this exercise period the chair was cleaned with antiseptic and detergent and rinsed thoroughly. I f at any time one of the animals appeared to be developing pressure sores, it was allowed to rest in its home cage for two days before testing continued.

Polarising cm'rent was drawn from a 120 V dry battery and voltage divider, and controlled by means of a large fixed series resistance of 1--6 Mr? and a 1 Mt~ variable resistance. The intensity of current flowing monitored on a l)ye Scalamp mirror galvanometer with a nominal resistance of 420 f2 and a maximum sensitivity of 8 cm deflexion for a current flow of i /iA.

After the 14-day recovery period, the animal was caught and placed under restraint, and took part in the first experiment as soon as it had overcome its initial distress at being restrain- ed. t)ost-operative testing was carried out at least twice a day, with a minimum interval of 41/2 hours between testing sessions.

Experiments T h e first e x p e r i m e n t was c o n c e r n e d w i t h an i n v e s t i g a t i o n of t h e effect of pass-

ing a w e a k po la r i s ing cu r r en t du r ing t h e t e s t i ng session. T h e a n i m a l was p l aced

in t h e a p p a r a t u s a n d c o n n e c t e d to t h e c u r r e n t source. I n s p e c t i o n of a schedule

d r a w n up in a d v a n c e d e t e r m i n e d w h e t h e r c u r r e n t was to be passed a n d in w h a t

d i rec t ion . T h e a c t u a l p o l a r i t y was u n k n o w n a t t h e t i m e of t h e e x p e r i m e n t , b u t was

r e fe r red to in a r b i t r a r y t e r m s a n d was d e t e r m i n e d b y d i s m a n t l i n g t h e i m p l a n t e d

e lec t rode a s s e m b l y a f t e r t h e a n i m a l h a d b e e n ki l led. A t t h e beg inn ing of a session

du r ing wh ich c u r r e n t was to be passed, t h e source was t u r n e d on and a d j u s t e d to

g ive a cu r r en t o f 20 /~A before t h e first t r i a l of t h e session was he ld : cu r r en t f lowed

t h r o u g h o u t t h e 3 0 - m i n u t e t e s t i ng session a n d was a d j u s t e d i f necessa ry b e t w e e n

tr ials . A t t h e end of t h e t e s t i ng session t h e a n i m a l was r e m o v e d f r o m t h e a p p a r a t u s

a n d d i s c o n n e c t e d f r o m t h e cu r r en t source, a n d t h e m e a n a n d s t a n d a r d e r ror of t h e

v a r i o u s t h r e s h o l d v a l u e s o b s e r v e d du r ing t h a t session was ca lcu la ted . T h e effect u p o n th is a v e r a g e t h r e s h o l d o f t h e passage of t h e po la r i s ing cu r r en t is seen in

Fig. 1 : surface-positive polarisation produces a definite impairment of perform- anee.

25 Exp. Brain l~es. Vol. 9

350 R. WaRD and L. W~ISK~A~TZ:

At the end of the experiment the response lateneies were subjected to a non- parametric analysis (Freidman's "analysis of variance by ranks", SIEGEL 1956). The only effect to be seen in both analyses was an association between response la tency and exposure time. One animal provided da ta indicating tha t latency might be affected by the hand used, but the other did not : nor did either animal give any indication tha t latency and polarisation were associated.

rrlseo

400

300

{ L- -c:

200

100

-t FIT 25o

msec

A surface positive u surface n e g a t i v e

J- oontrol 200

t loll Ill 5

A I I I A I A I A I I I I A

resting sessions

BA55u �9 surface positive u surface negative J_ control

A I I I

fest'ing sessions

A I A I F

Fig. 1. The e#ect, upon an animal's threshold, of passing current during the testing session. Current 20 /~A. Each point represents the mean of from 10--15 separate ascending and descending threshold points; the vertical bars indicate the standard error of this mean. Fig. 1A: Fit.

Fig. 1B: Baggy

A subsequent experiment sought to investigate the way in which the effect of polarisation declined as a function of time, and was carried out as follows. The animal was placed in the apparatus and connected to the current source. Inspec- tion of a schedule determined whether current, of tha t polari ty producing a rise in threshold in the first experiment, was to be passed : if so, 20 / IA were passed for 30 min. At the end of this period of polarisation, an interval elapsed before the animal was subjected to a testing session of 50 trials. The interval between polari- sation and testing was determined by the schedule just mentioned; in this sche- dule each of the intervals 0, 10, 20, 40, 80, and 160 rain appeared once in a random order, to be followed by a control session and a second random series of intervals. The interval "0 min" indicated tha t the testing session began as soon as the current source had been turned off, and during a control session the animal sat in the appa- ratus for 160 min before being tested. The results of this experiment are presented as Fig. 2, in which each point represents the average threshold and its s tandard error for two testing sessions. I t can be seen tha t those points corresponding to short intervals between polarisation and testing tend to fall on a straight line:

Cortical Polarisation 351

since the intervals arc plotted along the x-axis on a logarithmic scale, this suggests that the effect decays reasonably exponentially.

At the end of this experiment the electrode assembly implanted in one animal (Fit) became unserviceable, and the remaining data are based on observations derived from a single animal.

200 - -

msec

150

100

50

I

I0

mset; f l

I --~ 100~

t _

I I [ I ,.,/ I L I I I I I ,;/ I 20 40 80 160 mlns co 0 10 20 40 80 160 co

delay delay

Fig. 2. The decay of the impairment produced by polarisation as the interval between polarisation and testing increases. Current 20 pA. Each point represents the mean of the thresholds observed at two testing sessions, the vertical bars indicating standard error. Fig. 1A: Fit. Fig. 1 B: Baggy

The effect of varying the intensity of current passed for a constant period of time, and of varying the duration of flow of a constant current were also investi- gated. In one series of observations, the animal was exposed to the polarising cur- rent during the testing session, as in the first experiment, and the intensity of current of that polarity producing an impairment in the first experiment was determined by inspection of a schedule. The intensities used were 5, 10, 20, 40 and 80 etA, and the schedule was drawn up in the same way as that used in the second experiment: one random series of intensities followed by a control session and a second random series of intensities concluded by a final control session. The results of this observation are presented as Fig. 3.

Figure 4 shows the effect of varying the duration of application of a constant current of 20/iA. The design of the experiment follows that of the preceding ex- periments: the various durations were set out in a random order, separated from a second random order by a control session and the whole sequence was terminated by a final control session. The durations used were 5, 10, 20, 40 and 80 min. As in Figs. 2 and 3 each point represents the average threshold obtained at two testing sessions.

A final series of observations sought to answer two questions: can the effects of successive, short periods of polarisation summate, and does polarisation in one direction fail to have an effect because it brings about no change at all, or because some process not affected by the polarising current limit the animal's performance ?

In this experiment, therefore, the animal was placed in the apparatus and sub- jected to a series of periods of polarisation. 20/~A were passed for 5 min; the source

25*

352 R. WA~]) ~nd L. WEISKRA~TZ:

was turned off for 5 min and then turned on again for 5 min; then turned off, and so on. After six 5-minute periods of polarisation each separated from its neighbours by a 5 rain interval, the animal was tested in the usual way. Figure 5 shows tha t this procedure did no t lead to a rise in threshold (as in the preceding figures, each point represents the average of two testing sessions). The interval between periods

msec I 100--

50 1

BA G 5Y

] I I I I 5 10 2[2 4-0 p.A 8O

currenf

Fig. 3. The egect of varying the intensity of current. Control threshold 70 + 5 msee. The slope of the regression line is 0.274

ITISSC

";50

100 _c

50

BA56Y

I / / r F ] I I 0 5 10 20 /+0 rnins 80

duration

Fig. 4. The egeet of varying the length of time for which current is passed

of polarisation was therefore reduced from 5 min to 1 rain, and a rise in threshold was observed. After this procedure had been repeated the animal was subjected to two further testing sessions in which the polari ty was reversed during the 1-min interval instead of the source being turned off; reference to Fig. 5 shows tha t this leads to a smaller rise in threshold.

At the end of this experiment the second animal was killed, and the implanted assemblies of both animals were dismantled and the animals ' brains removed and

Cortical Polarisation 353

fixed in formal-saline. Each cortical electrode had become invested with a thick fibrous capsule over its external surface, and the indifferent electrodes were buried in a similar capsule. The cortical electrodes sat in a depression in the cortical sur- face, the floor of which showed patches of yellowish diseolouration. DELGADO (1961) regards the deformation of the cortex as an inevitable result of the presence

19o msec

1,5o

x =

lOO

50--

t

t t 50% 80% duly duly cycle cucle

Fig. 5. Summation and occlusion

t 80% red ZO% blctck

of an electrode on the cortical surface : the yellow diseolonration appears to be due to an iron-containing pigment which remains after damaged neurones have been removed by phagoeytosis ( G ~ S ~ I ~ L D and RVSSELL 1963). I t is of course ira- possible to say whether the damage took place at the time of implantation, or was brought about by movement of the electrode after the operation.

Discussion

Electrophysiological evidence (ALA~IS 1953; FvoRT~S 1954; MOR~]~LL 1961) suggests tha t depolarisation of single nerve cells increases their excitability, and tha t such increases in excitability may be brought about by the application of surface-positive polarising currents (CALvET and S C n E R ~ 1961; BINDMANN, LIPPOLD and I~EDFEAI~ 1964a, b). The experiments just described indicate tha t the application of surface-positive polarising currents lead to an impairment of performance at a tachistoscopic recognition task, and it seems paradoxical to sup- pose that an increase in cortical excitability is associated with impaired perform- ance. I t is also somewhat embarrassing to recall tha t previous demonstrations in- dicate that impaired performance is brought about by the application of surface- negative currents and tha t surface-positive currents are on the whole ineffective (MoI~RELL and NAITOtI 1962; ALBEI~T 1966a, b). This latter discrepancy may be due in part to the fact that MOI~t~ELL and NAITOH and ALBERT used much higher current densities : The densities they describe are of the order of 10/tA per square millimetre. In our experiments a total current of 10 zA flowed through a cortical electrode about one square centimetre in area, and the current density was thus

354 R. WARD and L. WEISKRANTZ:

two orders of magnitude lower than tha t used by these authors. I t is also quite possible that the different species used in the three experiments (rabbits, rats and monkeys) may contribute to the discrepancy.

The apparent paradox posed by an increase in excitability bringing about an impairment of performance calls for further discussion. From an argument ori- ginally put forward by LASHL~u (1951), and from the suppositions made by the theory of signal detection (GR~]n~ and Sw~Ts 1966) it appears that the discrimi- nation made by the nervous system is not that between "signal" and silence, but between two patterns of activity: one corresponding to the presence of a "signal" and the other corresponding to the absence of a "signal". BARLOW (1961) has sug- gested tha t an anatomically discrete sensory pathway may contain a number of discrete physiological channels, each sensitive to a different feature of the pat tern of stimulation of the peripheral receptors. The manner in which the activity of one such channel in the presence of a signal can be distinguished from that in the absence of a signal has been illustrated by FITZI-IuGH (1957, 1958) and by W E ~ E R and MOU~TCASTLE (1963). In the models developed by these authors, the statistical properties of the resting, spontaneous or "no signal" activity of a single nerve cell are of importance.

A suggestion originally put forward by ItORN and BLUND~LL (1959), and developed by Hou~ (1965), has it that the diseriminability of "signal" from "no signal" may be manipulated as a function of attention. One fairly obvious way in which this diseriminability may be altered is by changing the variances of the "signal" and "no signal" activity, so that small alterations in activity brought about by the presentation of a signal may be made more or less discernible. Chan- ges in the statistical properties of spontaneous activity of single units not brought about by any experimental manipulation, and apparently unrelated to any respi- atory or vascular effects have been described (WER~wR and MOU~TCASTLg 1963; GRIFFITI~ and I-IoR~ 1966). WWUNER and MOC~TCASTJm (1963) show that the main effect of this nonstationarity is to inject a large error into the estimate of the mean inter-impulse interval; they conclude their analysis by contrasting the precision with which human observers can detect small differences between stimuli with the relatively large variability of the nervous activity recorded, and they assume either that the variances of the activity of a population of units are eventually cancelled out, or that the variability is of some unidentified importance. I t is tempting to suppose tha t it is this variability which brings about the alterations in ease of discrimination discussed by H o ~ (1966).

A number of investigations (ALANIS 1954; FUORTES 1954; BrSDMA.< and others 1964a, b; C~LV~T and SCHEUR~U 1961) have shown that the effect of depolarisation of a nerve cell is not only to increase the excitability of the cell, but also to increase the level of spontaneous activity exhibited by that cell. While no investigations of the statistical properties of this increased rate of firing have been made, it appears to be possible to put forward a number of testable suppositions : the activ- i ty exhibited by a polarised cell may resemble that of a normal cell in the presence of a signal, or the variability of the activity may be increased so that small changes in activity are less discernible. Either of these changes would probably lead to an impairment of the ability to distinguish small signals from random variation in activity.

Cortical Polarisation 355

R e f e r e n c e s

ALANIS, J. : Effects of direct currents on motor neurones. J . Physiol. (Lond.) 120, 569--578 (1953).

ALBERT, D.J . : The effect of spreading depression on the consolidation of learning. Neuro- psychologia 4, 49--64 (1966a).

- - The effects of cortical polarisation on the consolidation of learning. Neuropsychologia 4, 65--77 (1966b).

BARLOW, i . B . : Possible principles underlying the transformation of sensory messages. In: Sensory communication, pp. 217--234. Ed. by W.A. ROSENBLITH. New York: John Wiley 1961.

BINDNAN, L.J. , O. C.J. LIPPOLD and J .W.T . REDFEAR~ : Relation between the size and form of potentials evoked by sensory stimulation and the background electrical activity in the cerebral cortex of the rat. J. Physiol. (Lond.) 171, 1--25 (1964a).

- - - - - - The action of brief polarising currents on the cerebral cortex of the rat (1) during current flow and (2) in the production of long-lasting aftereffects. J. Physiol. (Lond.) 172, 369--382 (1964b),

BUFFERY, A.W.H. : The effects of frontal and temporal lobe lesions on the behaviour of baboons. Unpublished Ph.D. thesis, University of Cambridge (1964).

CALVET, J., et J. SCHERRER: Relation des decharges unitaires avec les ondes cerebrales et la polarisation corticale. C.R. Acad. Sci. (Paris) 252, 2297--2299 (1961).

Cow:sY, A., and L. WEISKRANTZ: Varying spatial separation of cues, response and reward in visual discrimination learning in monkeys. J. comp. physiol. Psyehoh 66, 220--224 (1968).

DELGADO, J. M. 1%. : Chronic implantation of intracerebrM electrodes in animals. In: Electrical stimulation of the brain, pp. 25--36. Ed. by D.E. SttE~R. Austin: University of Texas Press I961.

DONALDSOn', P. E. K. : Technical note. An improved delay unit for an electro-physiological unit. Med. Electron. Biol. Engng. 3, 433--434 (1965).

FITzHuG~, R. : The statistical detection of threshold signals in the retina. J. gen. Physiol. 40, 925--948 (1957).

- - A statistical analyser for optic nerve messages. J. gen. Physiol. 41, 675--692 (1958). FUORTES, M.G.F.: Direct current stimulation of motoneurones. J. Physiol. (Lond.) 126,

494--506 (1954). FESTER, J.M., and A.A. UyEDA: Facilitation of taehistoseopie performance by stimulation of

midbrain tegmental points in the monkey. Exp. Neurol. 6, 384--406 (1962). G]~LLER~AN, L.W. : Chance orders of alternating stimuli in visual discrimination experiments.

J. genet. Psyehol. 42, 206--208 (1933). GREE~, D.M., and J, A. SWETS : Signal detection theory and psyehophysies. New York: John

Wiley 1966. GREENFIELD, J. G., and D. S. RUSSELL: Traumatic lesions of the central and peripheral nervous

systems. In: Neuropathology, pp. 441--474; second edition revised by W.R. BLACKWOOD and others. London: Edward Arnold 1963.

GmFFITg, J .S. , and G. HoRn: An analysis of spontaneous impulse activity of units in the striate cortex of unrestrained eats. J. Physiol. (Lond.) 186, 516--534 (1966).

I{oa~, G. : Physiological and psychological aspects of selective perception. In: Advances in the study of behaviour, pp. 155--215. Ed. by D.S. LEHRMAN and others. New York: Academic Press 1965.

- - , and J. BLUNDELL: Evoked potentials in visual cortex of unanaesthetised cat. Nature (Lond.) 184, 173--174 (1959).

KUI'FERNANN, I. : Effects of cortical polarisation on visual discrimination. Exp. Neurol. 12, 179--189 (1965).

LAS~L~Y, K.S. : The mechanism of vision. XVI. The functioning of small remnants of the visual cortex. J . eomp. Neurol. 76, 45--67 (1931).

- - The problem of serial order in behavior. In: Cerebral mechanisms of behavior, pp. 112--146. Ed. by L.A. J~FF~SS. New York: John Wiley 1951.

MORRELL, F. : Effect of anodal polarisation on the firing pattern of single cortical cells. Ann. N.Y. Acad, Sci. 92, 860--876 (1961).

356 1~. WARD and L. WEISKRANTZ: Cortical Polarisation

MORR]~LL, F., and P. NAITOH: Effects of cortical polarisation on a conditioned avoidance response. Exp. Neurol. 15, 437--454 (1962).

RusINov, V.S. : An eleetrophysiological analysis of the connecting function in the cerebral cortex in the presence of a dominant area. Commun. X I X Internat. Physiol. Congr., Montreal. 719--720 (abstract) (1953).

SCENEIDER, A . M . : Control of memory by spreading cortical depression: a case for stimulus control. Psychol. Rev. 74, 201--215 (1967).

SIv.G~L, S. : Nonparametric statistics for the behavioral sciences. New York: McGraw Hill 1956. THo~IPSON, T., D. SEAL and W. BLoo~: A suspended platform for use in chronic restraint of

monkeys. J. exp. Anal. Behav. 9, 146 (1966). WERNER, G., and V.B. MOUNTCASTLE: The variability of central neural activity in a sensory

system and its implications for the central reflection of sensory events. J . Neurophysiol. 26, 958---977 (1963).

Dr. R. WA~D The Jackson Laboratory Bar Harbor, Maine 04609 (USA)

Prof. Dr. L. WEIS~:l~A~TZ Institute of Experimental Psychology South Parks Road Oxford (England)