Pretectum and superior colliculus in object vs pattern discrimination in the monkey

Neuropsychoiollm,Vol. 18, pp. 559 to 568 Pergamon Pmu Ltd., 1980. Printed in Great Britain

0028-3932/80/1001-0559 $02.00/0

PRETECTUM AND SUPERIOR COLLICULUS PATTERN DISCRIMINATION IN THE

I N O B J E C T VS MONKEY

BRUNO CARDU,* MAURICE PTITO and MARCEL DUMONT Laboratoire de Psychobiologic, Universith de Montreal, C.P. 6128 Montr~,.P. Qu6bec, Canada, and Laboratoire de Neuropsychoiogie e x p ~ m ~ t a l e Universit6 du Qu~hec, C.P. 500 Trois-Rivi~res, P. Qu6bec, Canada

(Received 27 February 1980) Almtraet--The present study was designed to investigate the role of the collicular-pretectal area in the acquisition of initial learning of pattern vs object discriminations. Naive monkeys with massive bilateral lesions of the pretectum and superior colliculi were trained in object and pattern d'mtion tasks with subsequent generaliTation. Results showed that lesioned monkeys (1) perform as well as normals on an object discriminationproblem and on its subsequent generalization; (2) show great impairment in acquiring an initial pattern discrimination as well as generalizing this task; (3) demonstrate $rater diffgulty than normals in shifting from tridimemional objects to paRerm. These results lead to the conclnsion that massive midbrain lesions affect discrimination of patterns leaving intact the capacity of discriminating objects.

INTRODUCTION FOR MA~ years researchers have attempted to circumscribe the role of the superior colliculi in vision. Data so far have shown that in most of the species these structures contribute to a number of visual functions such as reflex eye movements [1, 2], localization of objects in space [3-5] and foveation [6, 7]. Moreover, recent electrophysiological studies have underscored the role of the superior colliculi in directing attention to peripheral visual stimuli [8-10]. These findings are supported by behavioral results which showed that colliculectomized animals are impaired in tasks requiring shifts of attention and orientation in cats [11]; in the hamster ['12]; and in the monkey [5, 13]. However, these findings are opposite to the ones obtained in the cat [14]. TUNrd. and Be~rd.~'Y [14] have demonstrated that colliculectomized cats tested in tasks where stimulus localization requirements were minimal, acquisition of a new visual discrimination task was impaired as compared to norrnals. Their data are supported by studies in the rat [15] and in the cat [16] which failed to show localization deficits. Another line of research dealt mainly with the role of the superior colliculus in retention and initial learning (new acquisition) of visual pattern discrimination tasks. Indeed, animals trained preoperatively in a wide variety of discriminations show little or no deficits when retested following colliculectomy [17]. Retention and learning of pattern discriminations in colliculectomi:,ed cats [17-21] and monkeys [22-27] are usually good. However, subtotal collicular and pretectal lesions in the cat [14, 17] and the tree shrew [28] prevent or greatly retard [29] the acquisition of a new pattern discrimination (initial * Requests for reprints should be sent to Dr Bruno Cardu, Laboratoire de Psychobiologie, Universit6 de Montr6al, C. P. 6128, Montreal, P.Q., Canada. 559

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learning). It is concluded that the learning impairment currently reported is interpreted in terms of motor impairment which affects processing of spatial information [14]. In fact, this learning deficit might be explained as BERLUCCmet al. [17] proposed by Hebb's theory on pattern perception [30]. Indeed, according to Hebb, when confronted for the first time with a pattern a subject uses eye movements to scan it and the repeated exposures to the pattern lead to the formation of cell assemblies. Hebb insists that learning a pattern is greatly dependant upon eye movements. However, once the cell assemblies for a particular pattern are developed, eye movements are no longer necessary to recognize this pattern (or any kind of related pattern). However, eye movements seem to be necessary for the initial learning of a pattern but not for a relearning. This theory thus finds good support from the results obtained in cats with massive lesion of the colliculus-pretectal area, in which the authors reported a severe deficit in acquiring an initial pattern discrimination and a normal performance when confronted with a previously learned task [14, 17]. However, most of the studies reported above used only patterns as discriminanda. GmSON [31] had already critized the use of patterns in perception experimental designs because they lack the cues necessary for object recognition such as texture and luminosity gradients and motion and binocular parallax. Moreover, BowER [32] has shown that at an early age, size and form constancies axe possible only for tridimensional objects. Human neuropsychological studies currently show that agnosia has more profound effects on pictures than on objects [33-35]. It thus seems that patterns should be more likely affected by CNS lesions than objects. The aim of the present experiment is to determine the nature of the contribution of the superior colliculi to object and form vision. More specifically it asks to verify whether massive midbrain lesions would affect more the initial learning of pattern discrimination (as currently reported in the cat) as compared to initial learning of an object discrimination.

METHOD Sub)ects Eight rhesus monkeys (Macaca mulatta), all males, were used in this experiment. Half the subjects, upon arrival at the laboratory, underwent bilateral lesions of the tcetal-pretectal area. The rest of the animals formed the control group. All subjects were experimentally naive and were housed individually in cages (76 cm on each side) which also served as the testing cages, one side of which was made up of bars spaced by 6 cm which allowed easy access to the discriminanda and to the reward.

Surgery and histoiooy The surgical approach is described elsewhere [36]. Prior to surgery the animals were anaesthetized with

Nembutal(30 mg/kg~The resectionof the superiorcoiliculi-pretectalarea was pm'rorn~ under conditionsof complete asepsis. The monkey's head was rigidly immob/lized in a stereotaxic apparatus and it was shavedand washed with a disinfectant iodine solution. A cardiac as well as a respiratory monitor was installed to control the physiological parameters of the subject. Atropine (0-2 mg/kg) was also given to prevent exce~ fluid secretion by the innp. The skin was incised taking the parieto-occipital suture and the medial line as points of reference. The different muscular planes were dissected and the skull was liberated from its periosteum. The cistcma magna was opened and the cerebrospinal fluid was drained. This was done to diminish the hydrostatic pressure of the fossa posterioris which could cause trauma to the cerebellum during the craniotomy. Using a bone rongeur, the opening of the iossa posterioris was enlarged starting at the foramen magnum rostrally toward the tranverse sinuses and laterally towards the sigmoid sinuses. The dura was then incised. The paraverm/an veins were coagulated and the cerebellum was depressed with a brain retractor. This allowed a perfect vision of the lamina quadrigemina. The resection of the superior colliculi was done de visu and by suction using a dissecting microscope (10 X). After securing good hemostasis, the cerebellum was replaced in the fossa posterioris, the dura and skin were sewn back into place with silk suture. This surgical approach was devised to avoid damage to the genicuio-striate pathway which could occur when the occipital lobes are retracted for a long period.

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Histology Four monkeys which formal the experimental group were anaesthetised and perfus~l through the aorta with 10% formalin. The brains were removed and the blocks were cut and fix~l in paraffin. In all cases, coronal sections, I0 pm in thi~Bem, were taken every 200 #m. The sections were stained using the Kluver-Barrera method [37]. The brains were then reconstructed and the extcnt of the lesions was carefully analysed.

Histological results All lesions were centered in the tcctum and mesodiencephalon and their extent was quite similar for all subjects. In all cases, the SUl~rior colliculi were completely removed as seen in Fig. 1 (the extent of the damage is indicated by the darkened surfaces~ These figures also show the other structures touched by the lesions. The inferior coUiculi were extensively destroyed in animal CAR 3 and slightly damaged in the other animals. Moreover, the ventro-rtu~dial extension of the l~ion touched upon the p~iaquaducal gray matter in all monkeys, but more marl~dly so in CAR 3. The aquedu~us ¢ ~ b r i was also perforated posteriorly in the latter subject. Rostrally the lesions invaded the pr~t~-tal area and ~ markadly so in monkey CAR 3 in which the d~truction was total, including complete lesions of the m of the optic tract (NOT), the PableBtiform nucleus (NSL) and the pret~tal nuclei. The damage was less extensive in CAR I and CAR 4 and, in CAR 2 the lesions ~ r e d the nucleus sublentfform and the nucleus of the optic tract. Mor©over, the rostral pole of the lesions extent~xt to the ventrorr~dial thalamus and more markedly so in CAR 3 in which the destruction included the o~ntromedial and para-fascicular nuclei on both sides. A number of important structures however were spared by the lesion: the oculomotor nuclei, the interstitial nucleus of Cajal and the nucleus of Darkswiteh. Moreov©r, striate cortex and lateral geniculate nuclei appear to be intact.

Apparatus Monkeys were t~t~d in a traditional Wiscousin General Test Apparatus (WGTA) locat~l in a sound-proof, lightfight room. The monkey was sq~u'at~l from the e ~ n t e r and the testing board by a opaque door which serv~l as a on~-way mirror. During testing, the raising of this door, allowed the monkey fr¢~-acc~s to the testing board placed slightly above the floor of the monkey's cage. The stimuli were of two sorts; some were forms made out of wood v~tically mounted on a base 7.5 cm ~ which cov~n~d the food wells; o t ~ t s were patterns affixed on a 7.5 cm 2 plywood P i ~ l ~ tlumm~ves vmically mounted on a 7.5 c m ' base which co,-red the food wells. Five sorts of stimuli were used; some were s i ~ p ~ made out of wood and haviag the san~ surface (22.5 cm=) and color (black) (Fig. 2); others were filled ~ and patterns made out of dashed lines (23.5 cm ~) or dotted lines (22.5 cm ~)(Fig. 2). All the patterns were drawn in black ink on white cardboard which fitted exactly the plaqucs on which they were affixed and were replicates of the initial objects.

Procedure All monkeys underwent a familiarisatinn period in the WGTA for 2 days. They were then tested on the five twochoi¢~ visual di~'imination problems, using first the two objects and then the patterns. The criterion for learning was flx¢~ at 20 consecutive correct responxs and each session conmted of 40 trials. Monkeys were trained 6 days a week. The position of the stimuli were randomly varied according to GELr0mMAN'Stable [38]. Corr¢~ r~ponscs were rt~forccd with diced (1 cm 3) fresh banana; when the animal chose the wrong stimulus, reinforcement w ~ withheld, and the s c r ~ n was lowered to p r ~ a r e for the next trial (non-correction procedure). After the initial Ieau'ning of the o b j ~ t discrimination (1, Fig. 2), the animal was given a sori~ of gcneralixation tests which termimLted in the prc~mtl~ion of an inverted triangle vs a circle of same dimensions as the objects used in the initial discrimination ( d i ~ r i ~ n a t i n n 5 in Fig. 2~ This final phase was preceded by a preparatory period in which the animal was confrout~d with the same stimuli as in test I but whose dimtmsious ~ greatly varied (discrimination 2, 3 and 4 in Fig. 2). Monkeys w ~ thus t ¢ ~ i for their ability to learn an object discrimination and for their generalization capacities. Moreover, the B*ture of the stimuli would also allow a t a t of the monkey's ability to shift from t h r ~ d D u s to two dimensions (object di~Paninatiom always pr~,B,~__~pattern di~rimination). On the other hand, the m g complexity, of one of the diserimi~tions of the same pattern in which pcrc~*ptual information (filled, dashed or dotted) was grndually removed (filled, dash~l or dotted patterns) to the next Jerved not only as a test of elemlmtary generulization but also as a mean to assess the monkeys ability to shift from one configuration to the other (discriminations 6-7-8; Fig. 2).

RESULTS Neurobehavioral results

A battery of neurological tests for optically guided behavior, acoustically guided behavior, locomotion and effective behavior were administered to the monkeys at one week and three months post-operatively. Our results confirmed previous ones obtained in our laboratory e36] and by others [24]. Briefly, the most c~mmon deficits were: fixed gaze, exaggerated

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Fro. 1. Coronal sections of the Brainstem of monkeys CAR 1, CAR 2, CAR 3 and CAR 4 showing the extent of the collicular lesions. (Ncgl: lateral geniculate nucleus; BC: brachium conjunctivium; O1: inferior olive; CI: inferior colliculus; AC: aqueductus cerebri; BP: brachium pontis).

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eyelid retraction (Dalrymple's sign), pupil dilation, absence of coordination of ocular movements, and absence of pupillary reflex. After 3 months, most of these deficits resorbed except for the Dalrymple sign which persisted for 2½ yr (until the monkeys were sacrificed). Though seemingly behaviorally normal after 3 months, ocular movements manifested some abnormalities. Indeed, we noticed spontaneous saccadic movements of the eyes when the monkey fixed a target which seemed uncontrolled by the animals and which resembled the "flutter-like oscillations" described by A~,.d~ and BI~NDEg f39]. Moreover, acoustically guided behavior, locomotion and effective behavior remained unaffected by the lesions.

Discrimination problems The monkeys with the massive collicular-pretectal lesions acquired both the object and pattern discriminations. From Fig. 3, it can be seen that naive lesioned monkeys can achieve initial learning of an object discrimination as well as normals. The mean number of trials required to reach the learning criterion was about the same for both groups, the experimental group (CAR) being slightly better. Statistical analysis (see below) however showed this effect to be not significant. In the preparatory phase, lesioned animals performed as well as normals. The generalization task (discrimination 5, fig,. 2) was acquired easily and equally well by both groups of subjects. In the first pattern discrimination problem (6, fig. 2), the experimental group (CAR) showed a great impairment (Fig. 3). Indeed, the mean number of trials required to reach learning criterion for the lesioned group as about twice the control object discriminations

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564

BRUNOCARDU,MAURICEPT1TOand MARCELDUMONT

group. An analysis of variance showed this effect to be significant (FI.~ = 6, 62; P <0.05). In the second pattern discrimination (dashed lines patterns) the experimental group still demonstrated a poor performance, the mean number of trials required to reach criterion being two times as great as the control group (Fig. 3). However, it appears from Fig. 3 that the CAR group required fewer trials to learn the second discrimination than the first. This increase in the rate of learning could be attributed to some amount of generalization still functioning after the lesion. However, generalization was not so effective as in the control group. In the third pattern discrimination (dotted lines patterns), the lesioned monkeys performed far worse than normals the mean number of trials required to reach criterion being four times greater (Fig. 3). The shift from the second discrimination to the third was slightly worse in the lesioned monkeys but the difference was not statistically significant. These results thus showed that (1) lesions of the collicular-pretectal area do not impair initial learning of an object discrimination nor do they affect the generalization ability of monkeys as compared to normal controls; (2) these same lesions however profoundly affect initial learning of a pattern discrimination in monkeys as well as their generalization capacities when confronted with patterns in which perceptual information is gradually removed; (3) coUicular-pretectal lesions greatly retard learning when monkeys are shifted from tridimensional stimuli to bidimensional ones. DISCUSSION Our results showed that naive monkeys with massive tectal-pretectal lesions acquired both object and pattern discriminations. The number of trials required on the object discrimination problems was similar for both groups of subjects. However, there was a striking difference between the normal and experimental group in the pattern discrimination task. Tectal-pretectal monkeys showed great difficulty in acquiring the initial pattern discrimination as well as the generalization tasks. An explanation of why midbrain lesions retards only the acquisition of pattern discrimination leaving intact object discrimination is not readily available but two possible interpretations can be offered. The first relates to gaze impairment while the second emphasizes the perceptual differences between hi- and tridimensional stimuli. 1. Altered gaze hypothesis The deficit observed in the pattern discrimination study confirms previous results obtained by several investigators [14, 17, 29]. These authors reported that naive coUiculectomized cats are unable to learn or are greatly retarded in acquiring pattern discrimination. They attributed their results to the fact that lesions of the tectal-pretectal area impaired some neurological elements (e.g. eye and head movements) necessary for learning a pattern (inspection of the figure). This type of impairment could also account for the deficits observed on the second and third pattern discriminations. In our study, it seems as if monkeys have become impaired in their generalization ability in transferring from filled patterns to dashed patterns and from the latter to dotted patterns. This incapacity to shift from one pattern discrimination to the next in which visual information was gradually removed could be accounted for by the perturbation of ocular movements produced by the midbrain lesions. Indeed, fixed gaze followed by abnormal eye movements [36, 39] could have prevented the lesioned monkeys from filling in the dashes or the dots thus greatly retarding their learning [24]. Thus, superior collicular damage could impair pattern

PRETEC'I'UM AND SUPERIOR COLLICULUS IN VISION

565

perception by reducing the number of scanning eye movements and consequently reducing the number of stimulus features acquired during each observation interval [14]. An alternative explanation concerns a deficit in stimulus localization in space as reponsable for the pattern discrimination impairment [3-5]. Recent data however seem to rule out this hypothesis. Indeed, ffcolliculectomizedanimals are subjected to a task where the importance of stimulus localization is reduced to a minimum, they still demonstrate the acquisition impairment [14]. Moreover, spatial localization ability directly tested in a colliculectomized cat proved to be comparable to a normal animal [14]. These results are supported by studies done in the rat [15] and in the tree shrew [28]. It thus seems that stimulus localization does not account for the deficit in pattern perception. More recently, Btn-n~ [13] has shown that in tasks requiring shifts of attention, colliculectomized monkeys are greatly impaired. If our collicular monkeys presented problems in shifting attention in the pattern discrimination tasks, how is it that they behaved as well as normal in the object discrimination tasks? Recall that patterns were drawn replicates of the objects and that objects discriminations always preceded pattern discriminations! As far as patterns are concerned, the hypothesis of a gaze impairment could thus find some support. However, the results obtained on object discriminations show that some other explanation is needed to account for the dissociation between object and pattern perception. 2. Perceptual nature of the stimuli

As previously mentioned, o u r lesioned monkeys had difficulty shifting from object discriminations to the initial pattern discrimination. This could be accounted for by the fact that in object discrimination, tridimensional cues such as texture and luminosity gradients as well as motion and binocular parallax are present whereas in patterns these cues are neutralised. This loss of information makes patterns much more difficult to "perceive" and it takes lesioned monkeys a greater number of trials to achieve the learning criterion on the first pair of patterns. This impairment, moreover, is still present on the second and third pattern discriminations. The differences in perception between solid and drawn forms [31] could provide an interesting explanation for this type of deficit. GIbsoN[31] considers drawings (patterns) a human production never encountered in a natural environment; solid (object) vision is thus primary whereas plane (pattern) vision is secondary. This view is actually supported by a considerable number of studies in human infants and adults. Indeed, children are more responsive to simple objects at a very early age and to a represented object (drawn or photographed) at a later age [31]. In adults, SEGALLet al. [40] have reported that a female "Bush negro" (member of a primitive tribe) had great difficulty recognizing her son on a photograph. Her first reaction was to consider the picture as an object and her attention was attracted by its rectangular limits, then by the white lines constituting the frame and finally by the contrasts inside the picture itself. It was only after a great amount of examination that she finally extracted the necessary perceptual information which lead her to recognize her son. The behavior displayed by our monkeys in the first pattern discrimination is similar to the one reported by SEG^L~. et aL [40]. Indeed, lesioned monkeys probably considered the patterns as objects (recall that the patterns are replicates of the objects previously learnt by the monkeys). Monkeys could have been attracted by the frame on which the pattern was drawn or by the base on which the drawn pattern was affixed, thus delaying considerably the learning process. It is only after a while that lesioned monkeys took into consideration the other perceptual cues (e.g. the lines which formed the patterns) which lead them to learn. Moreover, results obtained by human neuropsychologists on brain damaged patients always

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B~UNO CARDU, MAURICE PTITO and MARCEL D U M O N T

show that lesions in the central nervous system interfere with plane vision and not with solid vision ['34, 35]. This evidence seems to support our initial hypothesis that midbrain lesions were more likely to affect secondary vision (patterns) than primary vision (objects). The latter type of perception is more natural and innate, the former consists in substitutes or representations of objects and is acquired, therefore making pattern discrimination more vulnerable to CNS damage. Acknowledoements--The authors are indebted to Drs M. C. ~ n d e , A. Deiorme, F. Lepore and R. Ward for helpful criticmm. This study was supported by a grant from the Conseil national de la recherche en Sciences Naturelles et en G6nie Canada, APA, No. 271 to the first author.

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Cortex 12, 88-89, 1976. 37. KLOVER,H. and BARItEltA,E. A. A method for the combined staining of cells and fibers in the nervous system. J. NeuroputhoL exp. Neurol. 12, 400-403, 1953. 38. GELLBaMAN,L. W. Chance order of alteruating stimuli in visual discrimination experiments. J. Genet. Psychol. 42, 207-208, 1933. 39. ATKIN,A. and BENDEr, M. B. Lightning eye movements (ocular myoclonns). J. NeuroL Sci. 1, 2-12, 1964. 40. SEGALL,M. H., C.O~PBELL,D. T. and HERSKOWTS,M. G. The Influence of Culture on Visual Perception. BobbsMerrill, 1966.

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BRUNO CARDU, MXURICE PTrro and MARCEL DUMONT Zusammenfassun~: Die Untersuehun E hatte das Ziel, die Bedeutung der~ollikullr-pr~ek~s/en Region beim Lernen von Muster- und Objektunterscheidungen zu studleren. Naive Allen mit ausgedehnten bilateralen Lilsionen des Prf~ektum und der oberen VierhfiEeL wurden mit Musterunterscheidungo

AufEaben zur Objektunterscheidung und

mit nachfolgender Generalisierung,

trainiert.

E s z e i g t e s i c h , daft A l l e n m i t s o i c h e n H i r n l ~ I s i o n e n so gut wie C-esunde e i n e O b j e k t u n t e r s c h e i d u n g s a u f g a b e und i h r e n a c h f o l g e n d e G e n e r a l i s i e r u n g a u s f t t h r e n . Sie s t n d s t a r k d a r i n b e e i n t r ~ c h t i g t , e i n e M u s t e r u n t e r s c h e i d u n g s a u f g a b e und d e r e n G e n e r a l i s i e r u n g zu e r l e r n e n .

Sie h a b e n g r 6 f l e r e S c h w i e -

r i g k e i t e n a l s n i c h t o p e r i e r t e A f f e n b e i m W e c h s e l yon d r e i d i m e n s i o n a l e n O b j e k t e n zu M u s t e r n . D i e s e E r g e b n i s s e f a h r e n zu d e r S c h l u f l f o l g e r u n g , da/~ m a s s i v e M i t t e l h i x n l ~ l s i o n e n die U n t e r s c h e i d u n g yon M u s t e r n b e e i n t r J t c h t i g e n , a b e r die F l h i g k e i t z u r U n t e r s c h e i d u n g v o n O b j e k t e n i n t a k t l a s s e n .