Memory after frontal/temporal disconnection in monkeys: conditional and non-conditional tasks, unilateral and bilateral frontal lesions

~ Pergamon PII: S0028-3932(97)00112- 7

Neuropsycholooia, Vol. 36, No. 3, pp. 259-271, 1998 © 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain 0028-3932/98 $19.00+0.00

Memory after frontal/temporal disconnection in monkeys: conditional and non-conditional tasks,

unilateral and bilateral frontal lesions

AMANDA PARKER* and DAVID GAFFAN

Department of Experimental Psychology, Oxford University, South Parks Road, Oxford OX1 3UD, U.K.

(Received23 May 1997; accepted 14 July 1997)

Abstract--Seven Cynomolgus monkeys (Macacafascicularis) learned a series of reward-visual conditional discrimination problems, in which the arrival or non-arrival of a food pellet at the beginning of each trial acted as an instruction cue, signalling which of two visually distinct stimulus objects the animal should choose on that trial in order to obtain a further food pellet reward. Following surgical removal of the ventrolateral prefrontal cortex in one hemisphere and the inferior temporal cortex in the contralateral hemisphere, combined with forebrain commissurotomy, the four operated animals were severely impaired at relearning this task. They were not impaired, however, in non-conditional visual discrimination learning. Extending the unilateral frontal lesion to include the ventromedial prefrontal cortex had no detrimental effect, nor did complete unilateral removal of the frontal cortex. In a third experiment, the operated animals underwent a further surgery to remove either ventrolateral, ventral or complete frontal cortex similar to that in the opposite hemisphere. Compared to their previous level of performance, the animals with bilateral ventrolateral prefrontal lesions were now mildly impaired and the animals with the bilateral lesion extended to the ventromedial cortex more severely impaired on the non-conditional visual discrimination task. The bilaterally lobectomized animals were unable to relearn the task. We suggest that behaviour in visual learning tasks is controlled by cortical convergence upon subcortical structures, possibly by striatal efferents from both the visual cortex and frontal cortex, and that intrahemispheric convergence of these two efferents within the corpus striatum of one hemisphere could allow detailed control of visual choices by non-visual information, while subcortical interhemispheric transfer allows only less detailed, more general control. © 1998 Elsevier Science Ltd. All rights reserved

Key Words: conditional tasks; habit learning; primates; ventrolateral prefrontal cortex; inferior temporal cortex; disconnection.

Introduction

Current evidence suggests that the frontal lobes are involved in memory processing in a variety of complex ways which modulate according to both the demands of the task and the other brain structures which are involved in meeting those demands. Conditional memory tasks are presently thought to require frontal involvement to a greater degree than simple associative memory tasks [20], and specific sites within the frontal cortex, such as the inferior convexity or the periarcuate region, have been implicated in conditional task performance [20-23]. However, small discrete bilateral lesions of the ventromedial prefrontal cortex disrupt non-conditional associative learning [7], suggesting that different frontal

* Address for correspondence: Department of Experimental Psychology, Oxford University, South Parks Road, Oxford OX1 3UD, U.K.: tel.: +44-1865-271413; fax: +44-1865- 310447; e-mail: [email protected].

areas may be involved in conditional and non-conditional memory.

The temporal and frontal lobes interact in memory- guided performance by a variety of different routes. There is a cortico-cortical connection, the uncinate fascicle, through which the inferior temporal cortex projects to the ventral frontal cortex [27, 29]. Experiments which have investigated the effects of transection of the uncinate fascicle on memory have pointed to a very restricted role for this projection, specifically in the association of a visual conditional cue with one of two visual choice stimuli [1, 11], or with one of two motor responses [1]. Unci- hate fascicle transection had no effect on delayed matching to sample with either small or large sets [3], nor on visual learning for an auditory secondary reinforcer [4], nor on visual learning when reward is delayed by 1 sec [12]. A second projection from the temporal lobe to the prefrontal cortex is via the amygdala and mediodorsal nucleus of the thalamus. Although aspiration lesions of the amygdala produce memory deficits on some tasks [7],

259

260 A. Parker and D. Gaffan/Frontal-temporal disconnection

recent studies using excitotoxic lesions did not show any impairment in delayed non-matching-to-sample performance [17], or in associative learning [14]. The importance of this projection in memory is therefore uncertain at present.

A third possibility for temporal-frontal interaction in memory is via a subcortical route such as the ventroanterior nucleus of the thalamus, which receives a projection from the inferior temporal cortex and projects to most regions of the prefrontal cortex, or via their converging projections onto subcortical structures, for example the striatum. Investigation of these cortico-striatal projections using retrograde and anterograde tracers showed partially overlapping parasagitally extended strips projecting to the caudate from the temporal visual cortex, as opposed to very widespread projections from frontal areas to the caudate [26]. This pattern of partially overlapping projections to the striatum could allow temporal and frontal cortical influences to interact together both in learning about visual objects and their connection with reward, and in conditional learning in which choices between two previously equally rewarded items could be made conditional on another event. This possibility is explored in the present experiments, using a reward- visual conditional task known to be unaffected by uncinate fascicle transection [1] and a two-choice visual associative learning task with lists of 10 pairs of objects. The monkeys had crossed unilateral lesions of the inferior temporal cortex and either the ventral or ventrolateral prefrontal cortex, or a frontal lobectomy (sparing primary motor cortex), combined with forebrain commissurotomy in Experiments 1 and 2. These crossed unilateral lesions prevent intrahemispheric temporal- frontal interaction via convergent projections onto the corpus striatum. We then considered the further impairment in visual association learning caused by a bilateral frontal lesion in Experiment 3. A summary of the lesions each monkey had at each stage of each experiment is given in Table 1.

Experiment 1

Materials and methods

Subjects. Seven male Cynomolgus monkeys (Macaca fas- cicularis) served as subjects. At the start of testing they were experimentally naive and weighed between 2.2 and 2.7 kg.

There were two groups of subjects, an operated group which comprised four animals, and an unoperated control group of three animals. Each animal in the operated group had two surgeries• All four of these animals had a two-stage disconnection of the inferior temporal cortex in one hemisphere and the ventrolateral prefrontal cortex in the opposite hemisphere, combined with forebrain commissurotomy.

Surgery. For each of the ablations described in this paper, operations were carried out under aseptic conditions, and the monkeys were anaesthetized throughout surgery with bar- biturate (thiopentone sodium) administered through an intra- venous cannula. The ablations were made by aspiration under

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visual guidance with the aid of an operating microscope. The tissue was closed in layers and a prophylactic dose of antibiotic was administered at the end of the surgery. After each operation the animals rested for 10-14days before beginning post- operative behavioural testing.

Each experimental animal had a two-stage disconnection surgery. In the first operation, OPI and OP2 received left inferior temporal ablation and forebrain commissurotomy, while OP3 and OP4 received left ventrolateral prefrontal ablation and forebrain commissurotomy. In the second operation, OPI and OP2 had a right ventrolateral prefrontal ablation, while OP3 and OP4 had a right inferior temporal ablation. Thus, all animals had both cortical lesions and forebrain commissurotomy after the second operation, while the first operation acted as a control procedure to show that the animals were able to perform the behavioural tasks with either cortical lesion and forebrain commissurotomy. The details for each procedure were as follows.

For the ventrolateral prefrontal lesion, a bone opening was made, the dura was cut and the frontal lobe was retracted from the orbit in order to expose its inferior surface. The pia mater was cauterized in a line along the lateral bank of the medial orbital sulcus. This line ofcautery was then extended posteriorly to the lateral sulcus and anteriorly to the lip of the inferior convexity and then further continued at the anterior tip to extend along a line 2 mm below the lip of the principal sulcus. From an imaginary line drawn inferiorly from the tip of the ascending branch of the arcuate sulcus the line of cautery was extended inferiorly to meet the point of the line of cautery which extended to the lateral sulcus. The cortex and pia mater bounded by this continuous line of cautery were then removed by aspiration. The intended lesion is shown in Fig. 1.

For the inferior temporal lesion, the arch of the zygoma was removed and the temporal muscle retracted. A bone flap was made and extended with a rongeur and the dura cut. A line of cautery was extended along the anterior limit of the lesion, which was a line drawn at a right angle to, and beginning at the anterior tip of, the superior temporal sulcus. The line ofcautery was extended along the superior limit of the lesion, which was

the lower bank of the superior temporal sulcus, which the lesion included. The posterior extent of the lesion was a line parallel to the anterior extent, drawn 5ram anterior to the inferior occipital sulcus. The inferior extent of the lesion was parallel to the superior extent and approximately 2 mm lateral to the lateral bank of the rhinal sulcus, meeting the anterior limit described. The cortex and pia mater bounded by this continuous line ofcautery was then removed by aspiration. Figure 1 shows the intended lesion.

For forebrain commissurotomy, a bone flap was raised in the cranium over the area of operation, the dura mater was cut and one hemisphere was exposed up to the midline. Veins draining into the sagittal sinus were cauterized and cut. The hemisphere was retracted from the falx to enable access to the interhemispheric fissure. A glass aspirator was used to section the corpus callosum in the midline from the posterior limit of the splenium to the genu. At the level of the interventricular foramen, anterior to the thalamus, one descending column of the fornix was retracted in order to gain access to the foramen. The anlerior commissure was visualized and sectioned in the midline by electrocautery and aspiration with a 23-gauge metal aspirator, which was insulated to the tip. [Figs 2 and 3]

Histology. At the conclusion of the behavioural experiments, the operated animals were deeply anaesthetized, then perfused through the heart with saline followed by formol-saline solution. The brains were blocked in the coronal stereotaxic plane posterior to the lunate sulcus, removed from the skull, photo- graphed and allowed to sink in sucrose-formalin solution. The brains were sectioned in the coronal plane on a freezing micro- tome. Sections (50/tin) wej'e retained throughout the block at the rate of one section each 0.5mm and stained with Cresyl Violet. In all four subjects the transection of the corpus callosum and the anterior commissure was complete. The inferior temporal lesions were largely similar to each other and as intended, excepting that OP2 had less sparing of the temporal pole than intended. The ventrolateral prefrontal lesions were also similar to each other and as intended.

Apparatus. The monkey was brought to the training apparatus in a wheeled transport cage, which was then fixed to the

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Fig. 1. On the left the intended inferior temporal lesion is shown by lined hatching. On the right the ventrolateral lesion (shown as diamond hatching), the complete ventral prefrontal cortex lesion (illustrated by the combination of diamond and lined hatching)

and the frontal lobectomy (illustrated by the combination of diamond and lined hatching and stippling) are shown.


Fig. 2. A series of sections from animal OP2, from anterior at the top to posterior at the bottom, showing the ventrolateral prefrontal cortex ablation. Note that this lesion is bilateral due

to the animal's third surgery for Experiment 3.

front of the apparatus. Once the transport cage had been fixed in place, the room lighting was turned off and the room door was closed. The monkey could reach out through bars at the front of the transport cage to touch a touch-sensitive monitor screen which was 150 mm from the front of the cage. The screen was 380 mm wide and 280 mm high. A closed-circuit infrared television system allowed the experimenter in another room to watch the monkey. Small food rewards (pellets, 190rag) were delivered into a hopper placed centrally underneath the monitor screen. A single large food reward was delivered at the end of each training session by opening a box which was set to one side of the centrally placed hopper. The box contained peanuts, raisins, proprietary monkey food, fruit and seeds. The amount of this large reward was adjusted for individual animals in order to avoid obesity. The small and large rewards dispensed in the training apparatus provided the whole daily diet of the monkeys on days with a training session. Opening of the box with the large food reward, like all other aspects of the stimulus display and the experimental contingencies during any session of training, was under computer control.

Pretraining. Each of the animals underwent a process of shaping in the automatic equipment to train them to respond to visual patterns appearing on the computer screen (this procedure is described in [8]). The animals then learned a set of 20 two-choice visual discriminations using compound stimuli, each of which consisted of two superimposed alpha-numeric characters, 180 mm apart, of randomly selected specifications (from 127 available characters and 255 available colours), one of which was rewarded and the other unrewarded. All animals were trained daily until they reached a criterion of >80% correct in two consecutive sessions of the task in which the animal earned 100 rewards.

Experimental task. The experimental task used here, reward- visual associative learning, requires the animal to respond to a cue, then make a choice (for a longer description of this task, see [1]). The cue phase on each trial began when a small white square (10ram x 10 mm) appeared in the centre of the screen. The animal touched the square to initiate the trial. On half the trials, randomly selected, a 190-mg food pellet would be delivered into the hopper in front of the animal. On the other half of the trials, no food pellet was delivered. Four seconds later, two visual stimuli of the type described above appeared on the screen, one to the right and one to the left, 150 mm apart. Each animal had a different randomly selected pair of stimuli for each reward-visual association they learned. One of the

Fig. 3. A series of sections from animal OP 1, from anterior on the left to posterior on the right, showing the inferior temporal cortex ablation.


stimuli was correct if a reward pellet had been dispensed as a cue, the other was correct if no pellet had been delivered. If the correct stimulus was chosen, it stayed on the screen for 1.7 sec whilst a reward pellet was delivered. Following successful completion of the trial, an intertrial interval of at least 20 sec began. If the monkey responded incorrectly no pellet was delivered and the screen remained blank for 50 sec. If the animal touched the screen during the interval, the counter was reset to zero and the intertrial interval began again. The relatively long intertrial interval used here was intended to separate the events of a food pellet being earned as a reward from a pellet being delivered as a cue. Each session continued until the animal earned a total of 50 rewards at choice trials. The last reward in the session also opened the box which contained the large food reward (see Apparatus). Each animal completed one session per day until a criterion of 21 or less errors occurred in 200 rewarded trials (i.e. 200 correct choices in 221 trials, over 90% correct). Pre- operatively, each animal learned two of these reward-visual associations, each with different stimuli. Following surgery and a 2-week period for recovery, or a matched period of rest for normal controls, each animal was tested for retention of each of the problems he had learned, and then learned a third problem. A further surgery and recovery, or period of rest, was followed by testing on previously learned problems, then learning of a final new problem. In all, therefore, each monkey learned four reward-visual associations.

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Fig. 4. Results from Experiment 1. Errors to criterion on old (pre surgery learning), on the left, and new learning, on the right, for each of the groups post disconnection. Symbols represent: control subjects, open symbols; operated subjects, filled symbols; C, control group; Op, operated group; F, failed; OP1, filled triangle; OP2, filled square; OP3, filled inverted triangle; OP4, filled circle; NCI , open triangle; NC2, open square; NC3,

open inverted triangle.

Results

Preoperative and post control surgery learning. Table 2 shows the errors made by each individual monkey in each phase of the experiment. There were no significant differences in errors to criterion between the two groups of animals before the first surgery [t(5)= 0.63]. An analysis of variance (ANOVA) of the errors made following the first surgery and period of recovery or equivalent period of rest showed no significant difference between the operated and control groups [F(1,5)=0.03, P=0.96]. There was a significant effect of learning type, indi- cating that all the animals made more errors with new learning than with old [F(1,5) = 8.28, P < 0.05]. There was no interaction between these effects [F(1,5)=0.43, P=0.54]. A t-test com- paring old learning after their first operation showed that the operated animals made more errors [t(5)= 3.36, P < 0.05].

Post disconnection surgery learning. As shown in Table 2 and

Fig. 4, there was a large difference between the groups after their second operation or corresponding period of rest both in old and new learning [F(1,5)=39.771, P<0.01]. Two of the four operated animals failed to reach criterion in 1000 trials, and the other two made 254 and 397 errors before reaching criterion. The unoperated controls, by comparison, made an average of 75 errors before attaining criterion (range 57-102). There were no other significant effects.

Discussion

In the first stage o f surgery the vent ro la te ra l p ref ronta l cor tex was ab la t ed uni la tera l ly in two monkeys , while in the o ther two the infer ior t empora l cor tex was ab la ted uni lateral ly, in all cases in combina t i on with fo rebra in commissu ro tomy . The opera ted group were mi ld ly imp-

Monkey

Table 2. Errors to criterion in Experiment 1

Postop. 1 Preop. 1

Mean new Mean old New

Postop. 2

Old New

NC1 111 21 174 16 66 NC2 73 14 403 37 102 NC3 66 18 201 29 57 Mean NC 83 18 259 27 57

OP1 55 74 530 295 254 OP2 225 35 74 421F 397 OP3 90 77 175 356F 492F OP4 81 48 62 583F 412F Mean OP 113 59 210 413 289


aired at relearning the previously learned set o f stimuli after their first surgery, whereas there was no difference between the groups in postoperat ive learning of a new problem. It is difficult to assess the reason for this mild post control surgery deficit in relearning, but unilateral inferior temporal lesions have also been shown to cause mild deficits in visual association learning in a previous experiment [10]. Similar data for unilateral ventrolateral frontal lesions in monkeys are not available in the litera- ture. However, the deficit in condit ional learning after the control surgery was much smaller in magni tude compared to that observed after the second surgery, when all the operated group had unilateral inferior temporal cortex ablation in one hemisphere and unilateral ventrolateral prefrontal ablat ion in the opposite hemisphere, combined with forebrain commissurotomy. All the operated animals now showed a severe impairment in conditional learning, both for previously learned and new problems.

These results show that the frontal cortex and temporal cortex need to be able to interact with each other in the same hemisphere for normal learning in the reward- visual condit ional task, suggesting that, for normal learning to occur in the reward-visual condit ional task, visual information f rom the temporal lobe must interact with the ventrolateral prefrontal cortex in the same hemisphere (intrahemispherically). Further, Eacot t and Gaffan 's [1] results in the same task showed that learning was unaffected by disconnection o f cortico-cortical interaction between the temporal and frontal lobes, by unci- hate fascicle section. Therefore, it appears that the necessary interaction of frontal and temporal lobes in this task is by a subcortical convergence, which could be onto the corpus str iatum as suggested in the Introduct ion, or possibly via the ventroanter ior nucleus o f the thalamus.

Experiment 2

The results o f Experiment 1 are a clear illustration o f the importance o f the ventrolateral prefrontal cortex in condit ional learning. However, in the In t roduct ion we suggested that condit ional and non-condi t ional learning might involve the frontal cortex differentially. The present experiment therefore examines whether non-conditional learning is also affected by disconnection o f the inferior temporal cortex f rom the ventrolateral prefrontal cortex.

Since the results in fact showed no effect o f this ventrolateral frontal disconnection, we also examined in the same task the effects o f a more widespread disconnection of the frontal cortex. Two of the animals had the additional ablation of the ventromedial frontal cortex in the same hemisphere as their existing ventrolateral lesion. An additional two animals had unilateral frontal lobectomy (sparing the pr imary moto r cortex) combined with

an inferior temporal ablation in the opposite hemisphere and forebrain commissurotomy.

Materials and methods

Subjects. Nine male Cynomolgus monkeys (Macaca .fi~s- cicularis) served as subjects. Seven had been subjects in the previous experiment, the other two had taken part in tasks in an automated apparatus similar to that used in the present experiment.

Surgery. There were three groups of subjects, two operated groups, one of four animals from the previous experiment, a new group of two animals and one group of three normal controls. As part of the previous experiment subjects OPI, OP2, OP3 and OP4 had undergone section of the forebrain commissures and crossed unilateral lesions of the left ventrolateral prefrontal cortex and right inferior temporal cortex. For the present experiment, subjects OP5 and OP6 had a first surgery to section the forebrain commissures and ablate the cortex of the left frontal lobe, and a second surgery to remove the inferior temporal cortex of the contralateral hemisphere. The surgeries for forebrain commissurotomy and inferior temporal ablation were as in the Materials and Methods section of Experiment I. These three groups of subjects constituted phase 1 of the experiment. In phase 2, two of the animals (OP3 and OP4) had additional ventromedial frontal cortex lesions in the same hemispheres as their existing ventrolateral lesion.

For the frontal lobectomy, a large bone flap was raised over the area of the intended ablation and the dura was cut. The frontal lobe was removed in three parts, and in each case the cortex and pia mater bounded by each continuous line of cautery was then removed by aspiration. The first area of cortex to be removed was the lateral surface. The continuous line of cautery extended firstly through the centre of the precentral dimple from the most dorsal point on the lateral surface of the frontal lobe to the ventral surface, following a line approximately parallel to the central sulcus and ventrally 1-2 mm posterior to the descending limb of the arcuate sulcus. The line then extended around the boundary of the visible lateral surface of the frontal lobe, along the margin of the orbital surface and the lip of the interhemispheric fissure, until it rejoined the point at which it started. The second line of cautery began from the ventralmost point of the previous line, and extended along the lip of the lateral sulcus on the orbital surface of the frontal lobe to the base of the olfactory tract and beyond it, to the boundary of the medial surface. It then extended along the orbital surface anteriorly to the frontal pole, joining the line of cautery from the first area of cortex which was removed. The final line of cautery extended from the origin of the first line of cautery ventrally to the corpus callosum, then followed the line of the corpus callosum anteriorly and ventrally, then followed the boundary of the cortical surface until meeting the line ofcautery made on the orbital surface. In all cases, the gray matter within the sulci was removed, and the white matter surrounding the striatum and the striatum itself were left intact. The intended lesion is shown in Fig. 1.

For the ventromedial prefrontal lesion, the bone opening which had been made for the previous ventrolateral prefrontal ablation was enlarged with rongeurs and the dura was cut. The frontal lobe was retracted from the orbit in order to expose its inferior surface. The line of cauterized pia mater along the medial bank of the medial orbital sulcus which extended posteriorly to the lateral sulcus and anteriorly to the lip of the inferior convexity was further continued, first along the lateral sulcus in a medial direction, then along the cortex adjacent to the olfactory tract at the most medial part of the inferior convexity, and finally along the lip of the inferior convexity to


meet the original cautery along the line of the lateral orbital sulcus. The cortex and pia mater bounded by this continuous line of cautery were then removed by aspiration. Finally, the lesion was extended by aspiration into the cortex in the inferior part of the medial wall of the hemisphere, the intention being to extend the lesion to the level of the rostral sulcus. The intended combined ventrolateral and ventromedial lesion is shown in Fig. I.

Histology. The unilateral frontal lobectomy received by subjects OP5 and OP6 is illustrated in Fig. 5 (on the right). In both animals the lesion was as intended, with a complete removal of the frontal cortex, sparing the primary motor area. As can be

seen in Fig. 5, the striatum and internal capsule are partially preserved. In both of these animals the striatal damage was due to degeneration, rather than unintended damage during surgery. The additional ablation of the ventromedial prefrontal cortex in the ipsilateral hemisphere to the existing ventrolateral prefrontal lesion was as intended in both OP3 and OP4. This ablation is also shown in Fig. 5 (on the left).

Experimental task. For the seven animals from the previous experiment, on completion of the reward-visual task for the previous experiment, each monkey began learning new problems of the concurrent two-choice visual discrimination task that they had learned preoperatively. Following retraining on

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Fig. 5. A series of sections from animal OP3, in the left panel, showing the combined ventromedial and ventrolateral prefrontal cortex ablation. On the right, a series of sections from OP5 are shown, illustrating the frontal lobectomy. Both are from anterior at

the top to posterior at the bottom. Note that these ablations are bilateral due to the animal's later surgery.


10 sets of one pair of stimuli per session to a criterion of >90% in a session of 100 rewards, each animal learned five sets of 10 new pairs of stimuli to a criterion of > 90% correct in a session of 100 rewards.

The two monkeys who did not take part in Experiment 1 had previously learned the 20 two-choice visual discriminations using compound stimuli which the other animals had learned. After taking part as unoperated control animals in an experiment using the same equipment as the present study, they then spent five sessions shaping to the present experimental task, and then completed five sets of new stimuli to the above criterion. Following their first surgery and a two-week period of recovery, each animal learned a further five sets of 10 problems. They then had a further surgery and period of recovery, followed by learning a further five new sets of problems.

Following completion of the first phase of the experiment, animals OP3 and OP4 underwent a third surgery to extend their ventrolateral frontal lesion to include the ventromedial cortex. Following a period of rest, they recommenced training, and both monkeys learned new problems of the concurrent two- choice visual discrimination task to the same criterion as in the previous experiment, each learning another five sets of 10 problems.

Results

For the first phase o f the experiment, Table 3 and Fig. 6 show the average number o f errors made before reaching criterion in five sets o f 10 problems o f the two- choice visual discrimination task. A compar ison of the two disconnection groups and their controls f rom the previous experiment revealed that there was no significant difference in their scores [F(2,6)= 1.09, P = 0.39].

In the second phase, animals OP3 and OP4 learned another five sets o f 10 problems. Table 3 and Fig. 6

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Fig. 6. Results from the two phases of Experiment 2 (A-D, left) and Experiment 3 (E-G, right). Column A shows performance of unoperated controls, B shows performance of OP 1-OP4 post second surgery, C shows performance of OP5 and OP6 post second surgery, D shows performance of OP3 and OP4 post third surgery, E shows performance of OP1 and OP2 post third surgery, F shows performance of OP3 and OP4 post fourth surgery, and G shows performance of OP5 and OP6 post third surgery. Symbols represent: OP1, filled square; OP2, filled circle; OP3, filled triangle; OP4, filled inverted triangle; OP5, filled diamond; OP6, filled star; NC1, open triangle; NC2, open

square; NC3, open inverted triangle.

show the average number of errors made before reaching criterion in five sets of 10 problems of the two-choice visual discrimination task. OP3 made an average of 42 errors in learning five sets of 10 problems to criterion;


Set

Monkey 1 2 3 4 5 Mean

Phase 1 NC1 NC2 NC3 Mean NC

OP1 OP2 OP3 OP4 Mean

OP5 OP6 Mean

Mean OP

Phase 2 OP3 OP4 Mean OP

42 37 55 23 56 66 38 60 41 143 89 50 58 35 31 66 41 58 33 77

49 94 22 33 56 64 22 40 48 77

127 46 25 65 50 113 62 15 59 52 88 56 26 51 59

36 58 43 27 42 60 36 40 56 70 48 47 42 42 56

79 53 31 48 58

41 48 49 44 29 49 40 43 29 31 51 42 44 50 50

43 69 54 55

51 57 64 60 58

41 52 47

54

42 38 46


OP4 made an average of 38 errors. A comparison of the two animals with the ventrolateral prefrontal lesion extended to include the ventromedial cortex in their third surgery (OP3 and OP4) with the scores of animals OPl, OP2, OP5 and OP6 from phase 1 of Experiment 2 did not show any impairment [F(3,5) = 0,66, P = 0.61].

Discussion

One interpretation of the finding of a lack of impair- merit in two-choice visual discrimination in the operated groups is that interaction between the frontal and temporal lobes is not required for two-choice visual discrimination learning. This is a surprising finding when considering both the animals with ventromedial lesions and those with a complete unilateral frontal lobectomy, as bilateral ventromedial prefrontal lesions have been shown to impair this type of learning [7]. As the ipsilateral inferior temporal cortex and ventral frontal cortex can no longer interact in these animals, the results of the present experiment cast doubts upon explanations of stimulus reinforcement learning which posit an interaction between visual, olfactory and taste information in creating a rapid association between the visual properties of the stimulus and the gustatory qualities of the reward (for review see [24]). Another explanation for their unimpaired performance must therefore be sought. One possibility is that interaction between the ventromedial prefrontal cortex and hypothalamus about the reward value of the stimuli is important for this task, for making judgements about stimulus choice. It could be the case that one intact ventromedial prefrontal cortex is enough to enable this decision-making procedure.

Experiment 3

One possible explanation of the results of Experiment 2 is that the frontal lobes have nothing to do with non- conditional learning. This explanation follows from the finding that the animals in the previous experiment in whom interaction between frontal and temporal lobes is no longer possible are not impaired at two-choice discrimination learning. In the animals with frontal lobectomy, the remaining frontal lobe has no access to the visual information in the inferior temporal cortex, yet their level of performance was still very high.

Another possible explanation of Experiment 2 is that the frontal lobes are involved in non-conditional learning, but that they do not need to interact with the inferior temporal cortex. Their role in this case could be modu- latory, and related to aspects of behaviour other than the selection of the hand movement towards the selected stimulus. In this view, this role could be served by either frontal lobe, which would explain the lack of impairment caused by the crossed unilateral lesions the animals received. Bilateral lesions of the frontal lobes, however,

should reveal the extent to which these structures are important in this form of learning.

To examine the above possibility, we looked at the effects of bilateral frontal lesions on two-choice visual discrimination in the six operated animals from the previous two experiments. The first comparison was the two animals with unilateral ventrolateral lesions, the second comparison was the two animals with ventral frontal lesions, and the third comparison was the two animals with unilateral frontal lobectomy, all of whom received a similar contralateral lesion.

Materh~ls and methods"

Subjects. These were six Cynomolgous monkeys (Macaca jascicularis), all of whom were operated animals who had taken part in Experiment 2.

Surgery. There were three operated groups of subjects, each of two animals. As part of the previous experiment, subjects OP1. OP2, OP3 and OP4 had undergone section of the forebrain commissures and crossed unilateral lesions of the left ventrolateral prefrontal cortex and right inferior temporal cortex. Following the training described in Experiment 2, OPI and OP2 had a third surgery to remove the ventrolateral prefrontal cortex of the right hemisphere, and subjects OP3 and OP4 had a fourth surgery to remove the ventrolateral and ventromedial cortex from the right hemisphere. Subjects OP5 and OP6 had previously had left frontal lobectomy, contralateral inferior temporal ablation and forebrain commissurotomy. They then underwent ablation of the cortex of the right frontal lobe. The surgical procedure was the same as that described in Experiment 2. The extent of the frontal ablation for these groups after this surgery therefore was: OP1 and OP2, bilateral ventrolateral removal; OP3 and OP4, bilateral ventral frontal removal; OP5 and OP6, bilateral frontal lobectomy.

Histology. Figures 2 and 5 present sections from animals OP2, OP3 and OP5 showing, in each monkey, the contralateral frontal lesion which was added to the prior unilateral lesion. The ventral prefrontal lesion in monkeys OP3 and OP4 was as intended in both animals. The unilateral ventrolateral ablations in animals OP1 and OP2 were as intended, as were the unilateral frontal lobectomies performed on monkeys OP5 and OP6. In both of these animals the striatal damage was due to degeneration, rather than unintended damage during surgery.

Experimental task. Each monkey began learning new problems of the concurrent two-choice visual discrimination task to the same criterion as in the previous experiment.

Results

Table 4 and Fig. 6 show the average number of errors made before reaching criterion in five sets of 10 problems of the two-choice visual discrimination task. The two animals with bilateral ventrolateral prefrontal lesions (OPI and OP2) were mildly impaired in their performance relative to their first five sets of 10 problems-- they made on average 31% more errors after their third surgery [t(9) = 1.85, P<0.05]. The two animals with ventral prefrontal lesions were more severely impaired, making on average 88% more errors than on their previous five sets of 10 problems [t(9)=4.30, P<0.001]. The two



Set

Monkey 6 7 8 9 10 Mean

OPl 87 58 72 45 69 66 OP2 86 94 87 57 49 75 Mean 87 76 80 51 59 71

OP3 78 99 105 59 68 82 OP4 136 50 43 58 54 68 Mean 107 75 74 59 61 75

OP5 F OP6 F

animals with bilateral frontal lobectomies (OP5 and OP6) were unable to relearn the task. OP5 responded to the visual stimuli only intermittently, and OP6 showed little evidence of learning which stimulus was rewarded in 200 trials with one pair of stimuli (last 75 trials, 25 correct). However, both the animals maintained their body weight after their third surgery, and completed some of the experimental sessions. We can therefore conclude that their motivation to eat and their ability to work for reward were preserved to some extent.

Discussion

The effect of adding a contralateral frontal lesion in all the animals in this experiment was clear--a decrement in performance was observed in all subjects. The degree of decrement varied with the lesion. In the animals with bilateral ventrolateral ablations, a mild impairment in visual discrimination performance was observed. The effects of bilateral complete ventral lesions were more severe. In the case of the animals with bilateral frontal lobectomies, a profound deficit was observed. Although these animals would respond to the stimuli and work intermittently, they could not learn lists of discriminations, and showed little evidence of learning with a single pair of stimuli to choose between.

General discussion

The results of this series of experiments show a complex pattern of dissociations between types of learning and between unilateral and bilateral frontal lesions. In Experi- ment 1 we investigated the learning of reward-visual conditional discriminations, in which the presence or absence of a free food reward at the beginning of a trial signalled which of two visual objects the monkey should choose on that trial in order to obtain a further food reward. This type of learning was severely impaired (Fig. 4) by the disconnection of the visual cortex in the inferior temporal lobe from the ventrolateral prefrontal cortex,

i.e. by the combination of a unilateral ablation of the visual temporal cortex in one hemisphere with a unilateral ablation of the ventrolateral prefrontal cortex in the opposite hemisphere. Neither of these two lesions alone had a strong effect on learning. This result indicates that, for normal learning rate in the reward-visual conditional task, visual information from the temporal lobe must interact with the ventrolateral prefrontal cortex in the same hemisphere (intrahemispherically). These two areas of cortex, the visual inferior temporal cortex and ventrolateral prefrontal cortex, are connected by a direct reciprocal cortico-cortical pathway, the uncinate fascicle [27]. In principle, therefore, the results of Experiment 1 could be explained by supposing that learning in the reward-visual conditional task depends on cortico-cortical temporofrontal interaction through the uncinate fascicle. However, Eacott and Gaffan [1] found that section of the uncinate fascicle had no effect on learning rate in the reward-visual conditional task. Thus, the reward- visual conditional task depends on intrahemispheric temporofrontal interaction, and this interaction proceeds through a subcortical route rather than by direct cortico- cortical interaction. This interaction could take place in the cortico-striatal-nigral-thalamic projections, which are one route by which the visual cortex can control behaviour in memory tasks [2], or via a subcortical route such as the ventroanterior nucleus of the thalamus and its afferents from the inferior temporal cortex and projection to the prefrontal cortex. Furthermore, as discussed further below, Experiment 2 showed that the crossed unilateral lesions of the frontal and temporal cortex, which were effective in Experiment 1, had no effect on non- conditional visual learning. Therefore, the learning deficit which we observed in Experiment 1 cannot be attributed to an impairment of visual learning in general. Thus, temporo-frontal interaction through an intrahemispheric and subcortical route, presumably involving the basal ganglia, is of specific importance in conditional, as opposed to non-conditional, visual learning.

Experiment 2 investigated learning of non-conditional visual discriminations. These were two-choice discriminations in which one object of a pair was the correct one to choose whenever that pair appeared, the same correct choice on every trial with that pair. This kind of learning was tested in a difficult version of the task, with 10 such pairs to learn concurrently in each set of two- choice problems. Nevertheless, as shown in Fig. 4, temporal-frontal disconnection by crossed unilateral lesions of the frontal and inferior temporal cortex had no effect on learning rate in this task, even when the frontal lesion included the whole of the frontal cortex unilaterally, sparing only the primary motor cortex (unilateral frontal lobectomy). This non-conditional visual learning task is known to depend on the temporal visual cortex, since bilateral inferior temporal cortex ablation in a task similar in detail to the present Experiment 2 produced a severe impairment [6, 9]. Furthermore, Experiment 3 showed that bilateral lesions of the ventral part of the prefrontal


cortex retarded learning rate in the non-conditional visual learning task, while bilateral frontal lobectomy prevented the learning of even one single problem in this task (Fig. 6). Thus, the results of Experiments 2 and 3 taken together showed that non-conditional visual learning depends on the frontal lobe (and presumably its interaction with the caudate nucleus), but does not require the frontal cortex to interact with the visual temporal cortex in the same hemisphere.

Putting these results from conditional and non-conditional visual learning together, we conclude that the cortex in the frontal lobe plays an important role in both of these types of learning, but in a different way in the two types. Conditional visual learning requires intrahemispheric subcortical interaction between the frontal and temporal cortex if it is to proceed at a normal rate; non-conditional visual learning, by contrast, though it requires both the frontal and temporal cortex, can proceed at a normal rate even when the frontal cortex and the temporal cortex are in opposite hemispheres, sep- arated by forebrain commissurotomy (Experiment 2).

How are these two apparently different roles for the frontal cortex, in conditional and in non-conditional learning, related to each other? One possible answer derives from considering the role of non-visual information in visual learning tasks, since the frontal cortex receives multiple polysensory inputs, both cortico-cort- ically and by relay through striatal-thalamic loops, and is therefore able to contribute to the control of behaviour by non-visual information. In the reward-visual conditional learning task, non-visual information exercises detailed control of visual choices, changing from trial to trial, since two non-visual cues (the presence and the absence of free food reward) signal that two different visual choices are correct. The non-conditional visual learning task, however, is also controlled to some extent by non-visual cues, even though these are not explicit in the experimental design in this task. For example, the monkey performs the task only because hunger is the monkey's main current motivation; the monkey would not work at the non-conditional visual learning task if he were not hungry or if some over-riding motivation were engaged, for example by the presence of a frightening stimulus in the test cubicle. However, these non-visual cues are general and do not change from trial to trial; the general aim of obtaining food reward applies throughout the test session. Therefore, it is possible that the effect of temporo-frontal disconnection in these two tasks reflects the amount of information which must be exchanged between visual and non-visual control stimuli. In these disconnected animals, non-conditional learning can proceed normally when exchange of information between the frontal and temporal cortex is only by a subcortical interhemispheric route, but this route is not sufficient for conditional learning; instead, if conditional learning is to proceed at a normal rate there must be temporal-frontal information exchange within one hemisphere (intrahemispheric). It is plausible to suppose that the within-

hemisphere route of information exchange has more capacity than the between-hemisphere route. Both the frontal and temporal cortex send dense projections into the corpus striatum [26], and these allow the temporal and frontal cortex to interact subcortically within the same hemisphere. The route of interaction between the hemispheres, after forebrain commissurotomy and crossed unilateral cortical ablations, by contrast can only rely on the sparse projections which pass between subcortical structures independently of the forebrain commissures, such as the crossed projection from the corpus striatum to the contralateral substantia nigra [[28], p. 41].

Another possible explanation which cannot presently be ruled out is that task difficulty was a critical factor in the results seen in the two disconnection experiments. The observation that conditional learning was impaired, whereas non-conditional learning was not, might reflect that the association of visual objects with reward is simply easier than learning to apply a conditional rule to a pair of stimuli based on the appearance or non-appearance of a reward pellet. It may be the case that a spectrum of task performance could be observed in animals with frontal/temporal disconnection, with more 'elementary' learning unimpaired, tasks such as conditional learning possible under certain circumstances, after many more trials than normal animals, and performance on tasks such as object-strategy association very severely impaired [19]. This is an important topic for future research, but it may not be possible to separate the concept of task difficulty from the possibility that certain brain areas are more involved in complex tasks than simple ones.

These results address a number of issues relating to the frontal lobes and memory. The first of these issues we consider here is the importance of the ventral frontal cortex in non-conditional, stimulus reinforcement learning. This area has been proposed as a multimodal (i.e. visual, olfactory and taste) pattern associator for recent reinforcement associations [24]. We have shown [18] that association of visual and olfactory information depends upon intrahemispheric interaction of information from these two sensory modalities. It is also known that at least 70% of information about the flavour of food is derived from olfactory information [15, 16]. On this basis, we should expect that preventing interaction of visual and olfactory information will result in a deficit in learning which is based upon the association of a visual stimulus with the rewarding flavour of the banana pellet which the monkey receives. However, the disconnection of the inferior temporal cortex from the ventral frontal cortex in Experiment 2 did not impair the association of the visual stimuli from their flavour reward properties, imply- ing that this interpretation of ventral frontal cortex function is problematic.

The second issue which we consider here is the dis- tinction between working and reference memory. Although it has been proposed that ventral frontal lesions differentially affect recognition performance over delays [13], it has also been argued [5] that the frontal lobes are


involved in reference memory to the same extent that they are involved in working memory. In that experiment, it was shown that an impairment in a conditional task resulted from bilateral principal sulcus lesions. No delay was used in that task. Similarly, our present results suggest that ventral frontal cortex lesions impair conditional task performance without a delay. These results are in accord with the findings of Rushworth et al. [25], who also found that inferior convexity lesions did not produce delay-dependent deficits. Other possible accounts of frontal lobe function should therefore be explored.

In conclusion, we suggest that behaviour in visual learning tasks is controlled by the striatal efferents from both the visual cortex and frontal cortex, and that intrahemispheric convergence of these two efferents within the corpus striatum of one hemisphere allows detailed control of visual choices by non-visual information, while interhemispheric transfer allows only less detailed, more general control.

Acknowledgements--This research was supported by the U.K. Medical Research Council. We thank Judi Wakeley for help in training the monkeys.

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Memory after frontal/temporal disconnection in monkeys: conditional and non-conditional tasks, unilateral and bilateral frontal lesions