Receptive field properties of somatosensory callosal fibres in the monkey

Brain Research, Elscvier

402 ( 1 9 8 7 )

293-302

293

BRE 1231{)

Receptive field properties of somatosensory callosal fibres in the monkey* •

?

?

Jean-Paul Guillemot ~, Louis Richer-, Louise Prevost ~, M a u r i c e Ptito 3 a n d F r a n c o L e p o r e 2 Qh)mrtemem de Kimmthropologie, UniversitO du C)uObec(i Montreal, Montreal, (,)u~. (Canada), 2DOparternenl de Psychologie, l:niver.~it~~de Montreal, Montreal, Qu~~. (Canada) and 3DOpartement de Psychologie, UniversitO du Qu~;bec ~'~Trois-RiviOres, Trois-RiviOres, QuO. ( ('anada)

(Accepted 17 June 1986) Key words: Corpus callosum; Monkey: Somatosensory system: lnterhemispheric transfer: Receptive field: Midline fusion

The corpus callosum is the principal neocortical commissure which transmits lateralized information between the hemispheres. Thc aim of the present experiment was to study the receptive field properties of somatosensory callosal fibres in rhesus macaque monkeys. The callosum was approached under direct visual control and axonic responses were recorded using tungsten microelectrodes. All sensor~ submodalities which could be examined with the available instruments were found (light touch, medium and deep pressure, joint movement and light pinches). Most fibres had receptive fields concerned with the trunk, followed by the head, with only a few responding to stimulation of the cxtremities. The medial borders of the unilateral receptive fields situated on the trunk and the head extended to the midline. The results are interpreted in terms of the roles of the corpus callosum in midline fusion and interhemispheric transfer.

INTRODUCTION The corpus callosum is the principal neocortical commissure whose major function is to transmit lateralized information between the hemispheres of the brain. In the monkey, the pattern of callosal projections has been elaborated by a n u m b e r of researchers using various methods, including horseradish peroxidase transport, degeneration studies and autoradiographic tracing techniques. In the visual system, callosal projecting and recepient neurons were shown to be mainly concentrated in the border regions of the various cortical areas, although a more irregular and widespread representation was evident in 'higher order' areas ~e~zz~. This pattern of distribution was similar to that found for the cat s'925-ev. In the somatosensory system, the distribution of callosal neurons was also unevenly distributed across the various

areas, in the sense that the axial and para-axial representations of the body were over-represented in comparison to the extremities 3'~'13'H'2~ :3,>. However, contrary to earlier studies le'>, callosal neurons were also found in regions of primary and "higher order" areas where the hands are generally represented 3' 14,>. These data showing a preponderance of callosal neurons situated in regions concerned with midline functions were taken as evidence that one of the principal roles of the callosum was to unite the sensory half-fields to ensure continuity in visual or body space 2,2~. It follows from this organization that mainly cells having receptive fields (RFs) situated near the midlines of the visual and somatosensory fields should be represented in the callosum. This has in fact been confirmed, mainly in the visual cortex of the cat, where the RFs of neurons, both projecting to and re-

• Portions of the results presented in this paper have appeared in summary form elsewhere ~s.>. Correspondetwe: J.-P. Guillemot, Ddpartement de Kinanthropologie, Universite du Qudbec "aMontr6al, (.P. 8888. Montrdal, Oud. H3C 3P8 Canada.

tl006-8993/87 $1)3.51)© 1987 Elsevier Science Publishers 13.V. (Biomedical Division)

294 ceiving from callosum, have RFs situated near the vertical meridian 12'171'~. A similar organization was found for the somatosensory system, where most callosal neurons represent axial and para-axial body regions, as well as the head, and most RFs extended to the midline 1~. However, an analysis of the data obtained in our laboratory ls'19 and by others ~°'2~ indicated that a significant proportion of units represented distal body parts such as the paws and forepaws. This can, however, still be interpreted in terms of midline fusion if this concept is viewed not so much in anatomical, but rather in functional terms. Thus. whenever an animal manipulates an objec~ with its paws, the callosum would play a role in ensuring smooth flow of information between the hands I and the hemispheres) and thereby ensure ease of comparison. Because bimanual coordination when handling an object gives rise to highly similar sensory experience, the information reaching the hemispheres must be quite congruent and susceptible to excite, directly and via the callosum, the same neuron. Midline fusion interpreted in this fashion would imply that an animal such as the monkey, which shows fine manipulative abilities, should also have a large number of callosal units representing the hands. This. of course, is in direct contradiction to the demonstrated patterns of callosal projections, which, as we have indicated, show a sparse representation of the distal extremities in primary somatosensory areas, These apparent contradictions are not. however. irreconcilable. It is possible that this functional fusion pesent during manipulative activity takes place in higher order areas, where the distribution of callosal neurons is more widespread. Recording in the callosal terminal or projecting zones of primary somatosensory areas would thus tend to underestimate the relative importance of the callosal representation of the extremities. Moreover. there appears to be a difference in the number of callosal fibres representing the extremities depending on whether one looks at its afferent or efferent projections 14. A simplified solution to the problem would be to record directly from callosal axons since one would presumably sample more equitably the varied types of neurons sending information through the structure. The aim of the present experiment was therefore two-fold. First, to examine the RF properties of calIosal neurons. It is surprising that despite the anatom-

ical importance of the structure and the complexity of its projections and terminations. Iew studies have looked at the nature of the somatosensorv information it is transmitting. For example, it would be interesting to know whether all submodalities are represented in the callosum. The second purpose of the experiment was to determine whether the extremmes. which are under-represented cattosallv in the primary sensory areas but which arc involved in intermanual (and interhemispheric~ coordination, are also under-represented in the callosum, given that there is greater homogeneity of distribution in higher order areas Callosal activity was evaluated by recording directly from callosal axons and studying their RF properties, especially as regards location with respect to body midline. The recording site in the callosum was determined using the coordinates derived from the study of Pandya and Seltzer 22 on the areal distribution of the axons which cross in the callosum. MATERIALS AND METHODS

Subjects The experiment was carried out on 3 rhesus macaque monkeys weighing approximately 6 - 1 0 kg. Although two of them had served in a behavioral experiment, they had not been submitted to any invasive manipulation prior to recording. They were m good health and had no obvious malformation or pathology.

Materials and procedure The monkey was first injected with a light dose of Ketamine (15 mg/kg) to render it drowsy, after which it was intubated with a tracheal cannula and connected to a respirator. It was then anaesthetized with a gaseous solution of nitrous oxide, oxygen (N20:O 2, 70:30) and Ethrane (3% of total gaseous mixture). The saphenous vein was also cannulated and the animal was administered 5% dextrose in lactated ringer to maintain blood pressure a n d h y d r a t i o n . The scalp was shaved and the skuUexposed. A 5 × 15 mm bone flap was removed from the skull overlying the somatosensory cailosum. The dura was cut parallel to the central sinus so that the hemispheres could be separated from each other. This was done using a Sperry retractor and the callosum exposed for approximate-

295 ly a 5 mm strip. The hemispheres were maintained open with a small retractor having at its base a 1 x 5 mm hole through which the microelectrode could be lowered under visual control using an operating microscope. Once the electrode had been placed on the surface and just touching the callosum, the space between the hemispheres was filled with agar which not only kept them apart but also stabilized the brain against pulsation. Once this preparatory procedure terminated, pressure points and wounds were infused with local anesthetic (Xylocaine 2%) and the Ethrane anaesthesia was lowered to 0.5% of total gas inspired (N20:O2; 70:30 and Ethrane). Flaxedil (5 mg/kg/h) was continuously administered through the saphenous vein cannula to maintain light paralysis of skeletal muscles. Respiratory rate was controlled by the experimenter so as to maintain constant, physiological levels of expired CO 2 (4-4.5%). Temperature was also kept constant with the help of a heating pad thermostatically controlled with a rectal thermoprobe. Heart rate and, occasionally, E E G were also monitored during the experiment. The recording site was situated around A-16 and was based on Pandya and Seitzer's 22 topographical maps of commissural fibres. Axonal activity was recorded with tungsten microelectrodes having a tip diameter of 1 #m and an impedance measured at 1000 Hz of 6-8 Mf~. Spikes were amplified and recorded in the conventional manner and a detailed procedure is presented elsewhere 17. When quantification was required, the spikes were transformed into square pulses and fed to an Apple IIe microcomputer and analyzed. At the end of a penetration, which was easily determined since the electrode tip fell into the third ventricle, the electrode was raised to about half way and a small electrolytic lesion was made in the callosum. Two types of stimuli were used. For qualitative analyses, these consisted of soft touches or strokes of the skin, light pinches, pressure application, joint movement and air puffs. Most of these stimuli could also be quantified using pressure gauges, Von Frey hairs, vibrators and calibrated tuning forks. No nociceptive stimuli other than the light pinches were used and no attempt was made to stimulate thermoreceptors. At the end of the experiment, the monkey was sacrificed by deeply anaesthetizing it with 6% Ethrane,

after which it was perfused through the heart with isotonic saline followed by formalin (10%). Its brain was removed, placed in formalin and later prepared for histology. In order to verify electrode placement, the block containing the complete callosum and the two banks of the interhemispheric fissure was cut in sagittal sections 20 ,urn thick. The slices were then colored using the Kluver-Barrera method 15. By observing the sites of the electrolytic lesions, it was easy to determine the relative position on the anteroposterior plane of each penetration. RESULTS

Histological results The analysis of the Kluver-Barrera stained slices confirmed that all penetrations were in the callosum and that the A-P coordinates were as indicated above. They also showed that the cortex suffered no observable damage due to the retraction, except for small, circumscribed regions which were lesioned during the coagulation of one and at most 3 small blood vessels which cross the midline. None of these regions, however, concerned the somatosensory areas.

Electrophysiological results In all, 89 fibres which could be excited by the somatosensory stimuli were recorded from the monkeys. Of these, RFs could only be mapped for 75 fibres. Moreover, quite a few penetrations yielded fibres which could not be driven with the stimuli at our disposal. This inability to map some of the somatosensory RFs and the lack of responsivity of some of the fibres might be due to one or a combination of factors: inappropriate recording site; effects of the light Ethrane anesthesia; fluctuating state of arousal of the animal; 'higher order' RFs which, though somatosensory, could not be driven by our stimuli; nociceptive or thermal RFs, etc. Only those penetrations which yielded fibres responsive to the various mechanical stimuli are therefore presented. Given the objectives of the study, two parameters of callosal function were of particular concern: the position of the somatosensory RFs with respect to body midline and/or the extremities; and the number and types of submodalities which were represented in the callosum.

29h RF maps from two penetrations which gave mean-

found which responded either to deep pressure stnnulation (fibre 3. Fig. [A: fibres 13--16. Fig. IB) or to

ingful results in the first m o n k e y are presented in Fig. 1. A n u m b e r of points stand out immediately upon

ioint m o v e m e n t {fibres 1 and 5-1{t. Fig 1Al. It was

inspection of these maps. First, most RFs concern the

impossible for us to determine, however, whether

trunk. The only exceptions were fibres 5 and h of ~hc

these responded to stimulation ot receptors in the

which responded to

joints themselves or to the stretching of the muscles

wrist supination, and fibres 9 and 18 of the penetra-

penetration shown in Fig. I A

or tendons during joint rotation. As for the fibres re-

tion presented in Fig. lB, whose RFs were situated

sponding to cutaneous or deep pressure stimulation,

on the arm. Most of the fibres were sensitive to cuta-

4 RFs were bilateral and joined a! the midline lfibres

neous stimulation t fibres 2 . 4 . 1 and 12 of Fig. IA: fibres 1-12, 17 and 18 of Fig. IB). Some fibres were

o, 7 . 1 0 and 1 l of Fig. 1B), while the rest were unilateral. The latter, except those of fibres 9 and 18 of Fig.

A

1- M.RA

2- C.RA

3- D.SA

4- C.SA

5- M.SA

6- M.SA

7- M.RA

8- M.SA

9- M.SA

10- M.SA

11- C.RA

12- C.RA

4- C.RA

5- C.RA

6- C.RA

B

1- C.RA

2- C.RA

7- C.RA

8- C.RA

9-C, RA

10- C.RA

11- C.RA

12- C.RA

13- D,SA

14- D,SA

15- O.SA

16- D.SA

17- C.RA

18- C.RA

I

Fig. 1. Somatosensory receptive fields recorded from the corpus callosum of one monkey. A and B represenl ~wo different penetrations. M, movement: C, cutaneous: D. deep: RA. rapidly adapting; SA, slowly adapting.

297 1B, all touched or abutted the body midline. Fibres

cited by gently stroking the eyelid. However, it only

sensitive to arm movement (fibres 7 - 1 0 of Fig. 1A) might conceivably also conform to this midline 'rule' if they are in fact tendon receptors which attach muscle bundles of the shoulder and arm to midline structures. Results from the only effective penetration in the second monkey are shown in Fig. 2. Except for fibres 8 and 9, which responded to arm (8) and forearm (8 and 9) cutaneous stimulation, they were all concerned with the head. Of these, one (fibre 5) was excited by downward movement of the jaw, whereas all the others were sensitive to light stimulation of the face (2), eyelids (1,3 and 4) or lips (6 and 7). As was the case with the cutaneous fibres described for the first monkey, all RFs, except for those of fibres 8 and 9, extended to or straddled the midline of the face. In 3 cases, response properties were quantified using appropriately designed somatic stimuli. Fibre 2 gave a sharp response to the application or withdrawal of a very light stimulus to the face. However, during the ON-phase, which lasted 5 s, some maintained discharge was evident. Fibre 4 could also only be ex-

responded when the movement was directed toward the center of the face. Fibre 9 gave only an ON response, with no maintained discharge, whenever the dorsal part of the forearm, and especially the little finger, was gently touched. Three penetrations gave meaningful results in the third monkey, and these are shown in Fig. 3. RFs similar to those found in the two other monkeys were also obtained and concerned both the trunk and the head, and the extremities. Fibres 3 and 16, Fig. 3A, are quite unlike any other already described in that they responded to pressure applied anywhere on the tongue and on the upper right canine, respectively. Of particular interest, given one of the objectives of this study, are fibres l l, 13 and especially 8. of Fig. 3A, fibres 5 and 8 of Fig. 3B and fibres 5 and 7 of Fig. 3C. These all have cutaneous or deep RFs, responding to more or less gently applied touch or pressure stimuli, but whose medial borders do not extend to the body midline. In order to demonstrate the special relationship of somalosensory RFs of callosal fibres with the body midline, all the RFs were examined to determine whether their medial borders extended to or straddled the midline. If we exclude the 19 fibres sensitive to shoulder to mandibular movement, which, as we have previously indicated~ are difficult to categorize in terms of midline function, wc find that 42 units have RFs which are either bilateral or touch the body midline. Only 14 fibres can be classified as being genuinely unconcerned with midline function, at least when defined in terms of their having RFs which do not extend to the midline. What is even more surprising, given our working hypothesis, is that onl\ half of these (fibres 5 and 6, Fig. 1A, fibres S and 9, Fig. 2 and fibres 8, Fig. 3A and 5. Fig. 3C} involve the hands to some degree. This proportion is not very different from that found in the cat in our laboratory using identical recording procedures. Given that a number of fibres were recorded during one penetration and that more than one penetration gave a meaningful result in the same monkey, we looked at the possibility that the callosum, as is the case with the sensory areas which it interconnects, was organized somatotopically. As is clear from the data presented in Figs. 1-3, no strict somatotopic representation of the various body regions exists in

1- C.SA

3- C.RA

4- C.SA

9- C.RA

108

Fig. 2. Somatosensory receptive fields recorded from the corpus callosum of one monkey. C. cutaneous: D. deep: RA, rapidly adapting: SA, slowly adapting.

298

3- D.RA

1- C,RA

2- M.SA

6- M.RA

7- C.RA

4- M.RA

5-M.RA

9- M.RA

10- C.RA

A

11- n u

1

~oA

I

I .....

I- M.SA

3- MJIA

2- C~IA

4- C.RA

]

5- C, RA

/

1

6- C . ~

B

a~,

.~I~..~

Io. ~

1- M.SA

2- M.RA

3- C,R A

4- M,IIA

6- C.RA

-

8- C.RA

9- C.RA

C

.RA

(~t~e~.~lA

Fig. 3. Somatosensory receptive fields recorded from the corpus callosum of one monke}. A. B and C represent 3 different penetrations. M, movement: C, cutaneous: D. deep; T. tooth: SA, slowly adapting; RA, rapidly adapting.

the callosum, at least in the transverse penetrations carried out in these cases. In fact. the results of the two penetrations which yielded the most number of excitable fibres (Figs. IB (t8 fibres) and 3A (16 fibres), suggest no orderly displacement across body surface as the electrode is advanced from the surface

of the calLosum to its depth. Some order is nonetheless present, however, in the sense that clusters o f fibres representing one body region were found during most penetrations. We did not study m a systematic manner the callosal organization across its rostrocaudal extent. However, given that single penetrations

299 TABLE I Receptive field properties of fibres recorded in the corpus callosum of the monkey Oral and perioral refers to the teeth and gums, the lips and the tongue; para-axial refers to fields which do not touch the midline but which include the arms up to, but excluding, the wrist. Position on the body surjhce

Submodality Cutaneous

Adaptation Deep

Movement

Slowly adapting

Total Rapidly adapting

Head Face Oral and perioral

6 3

2

10

9

7

16

-

-

5

5

Forelimb Axial Para-axial Extremities

8 5 2

3 -

2 7 2

2 8 2

8 7 2

l(/ 15 4

11 4

1 4

3 4

9 4

12 8

4

4

1

1

47

75

Trunk Ventral Dorsal Hindlimb Axial Para-axial Extremities

4 1

Overall total

44

10

yielded RFs which concerned all body regions, we would predict little or no somatotopic organization along this axis. A n o t h e r finding concerns the callosal representation of the somatotopic system of the head. A total of 21 out of 75 fibres which represented the head region were found. Of these, nearly half (10 out of 21) were sensitive to jaw movement. W h a t is somewhat surprising of the remaining fibres (11 out of 21), given the d e m o n s t r a t e d ipsilateral projection of the trigeminal system 5, is that their RFs were nearly all unilateral (10 out of 11). This p r o p o r t i o n is quite similar to that found for the rest of the body, which is more completely crossed at the midbrain and spinal levels. The results are also presented in s u m m a r y fashion in Table I. In the table, the RFs are g r o u p e d in terms of location on the body surface and the submodality to which they belong, and whether they are rapidly or slowly adapting. Both were found, although the former were p r o p o r t i o n a t e l y more numerous than the latter by a ratio of 3:2.

21

28

DISCUSSION Results obtained in this experiment are in general agreement with the known anatomical callosal organization in this species. As suggested by the results presented in a n u m b e r of anatomical studies, the axial and para-axial body regions have a greater representation in the callosal projecting and receiving zones than the extremities. Nearly four-fifths of the units recorded in the callosum had RFs which concerned the head or the trunk. The results, however, agree more closely with the recent anatomical studies 3'11'14'2°'28 showing some limited callosal representation in cortical regions representing the extremities than with the older reports 12'23 showing no hand representation, at least in SI and SII. These results also agree with the recent electrophysiological study of Manzoni, et al. 2° in the m o n k e y showing that units in SI and SI1, which were retrogradely labeled with H R P injected in the contralateral hemisphere, could also be driven by stimulation of the contralateral hand.

300 The fact that only 6 fibres (8%) were found which concerned almost exclusively the hand, though predictable from the anatomical standpoint, was somewhat surprising from a functional one. The m o n k e y is capable of fine bimanual manipulative abilities. It has, m o r e o v e r , been d e m o n s t r a t e d that unilaterallx learned tactile discriminations involving the hand are readily p e r f o r m e d with the untrained hand. The integrity of the corpus callosum is essential in realizing these transfer tasks a. One would, on these functional considerations, have expected a much more , n p o r tant r e p r e s e n t a t i o n of the hands in the callosum of the monkey. Since this is not the case, it must be assumed that these neurons, although somewhat limited in number, are sufficient to assure transfer There is also the possibility that transfer does not depend exclusively, or even principally, on primarx sensory areas but on 'higher order" areas, where rotegrated information could transfer through the callosum but might not be a m e n a b l e to examination with our technique. The face region is also very sensitive and, if one looks at the magnification factor, well represented at the cortical level. Functionally, no tests of the ability to recognize with the untrained side information presented to the trained side has been carried oul (among other reasons, because it would be difficult to titre out what constitutes interhemispheric transfer and what is due to the ipsilateral projections of the trigeminal system). O n e would nonetheless assume that coordination b e t w e e n lateralized sensory information is extremely i m p o r t a n t for this reDon. Because the trigeminal system is not as lateralized as the spinal system, this coordination could be assured intrahemispherically. H o w e v e r , some callosal contribution might also be involved. That this is in effect the case is shown by the fact that a rather large proportion of the callosal fibres r e c o r d e d represent the head region (21 out of 75 fibrs or 28%). All sensory submodalities tested were r e p r e s e n t e d in the callosum. This is in accordance with the anatomical data showing that the 3a, b, 1 and 2 subdivisions of SI, which a p p e a r to be concerned with specific submodalities 24, and SII all send axons through the callosum. In the m a c a q u e , there appears to be a progression in the r e p r e s e n t a t i o n of callosal neurons as one goes from areas 3b to 1 to 2 (ref. 14). This would suggest a likewise p r o p o r t i o n a l increment in the

number of fibres representing each submodahtv. However. the paucity of the s a m p l e recorded in the present experiment and the facl that these neurons might be originating in other somatosensorv areas. including SII and other parietal regions, precludes a more detailed analysis of this s t r u c t u r e - f u n c t i o n correlation. One important functional collslderation concern,', the role of the callosum in midline fusion. This hx-pothesis, which we have at times t e r m e d the z i p p e r hypothesis, suggests that one of the principal roles of the callosum is to permit continuit,~ of sensation at lhe body midline, given that the projections of the body to the cortex are highly lateralized. 1"his hypothesis finds its support m the folh~wmg observations. The anatomical studies citcd above _oeneratlv indicate that callosal neurons arc most densely found m those parts of the primary somatosensorv area,,, whereto are represented the axial and para-axial body surfaces. This is similar to thc results found for the visual system, where callosal neurons are morc abundanl near the vertical meridian representation and have RFs whose medial borders are likewise situated near the verncal meridiau which lhex sometimes straddle, Since each visual hemifield is also represented m the contralateral hemisphere, the argument is advanced that the function of these neurons is to ensure continuity across the midline by uniting, for the somatosensorv system, the t~vo hemibodies, as they join, for the visual system, the two hemifields, This notion is s u p p o r t e d by experiments with h u m a n collasotomized subjects which show that these subjects have difficulties in identifying discontinuities in lines extending across the meridian of the visual hemifields"-. The results obtained in this e x p e r i m e n t clearly support this hypothesis. In facl. a large p r o p o m o n of neurons concerned the trunk and the h e a d and had RFs which either touched or straddled the body midline. Some were. m fact. clearly bilaterally organized. M o r e o v e r , as previously indicated, if the notion of fusion is u n d e r s t o o d m functional rather than anatomical terms, it becomes possible to include the few hand RFs which we found among those supportmg the 'fusion" hypothesis. T h e s e would ensure smooth flow of information across the midline whenever the animal manipulates an o b i e c t with ~ts two hands {represented in different hemispheres} De-

301 spite these accommodations to the midline fusion hy-

lateral suprasylvian area of the cat receives extensive

pothesis, however, a few units were found which did

projections from the callosum throughout its extent,

not fit this model and it must be assumed that they are

even in parts which have been shown by retinotopic mapping studies to contain the representation of the

subserving some other functions. This argument against the generality of the mJdline

periphery of the visual field s'>'-'7. However, as we

fusion principle was made quite forcibly on anatontical grounds by Killackev and collaborators 14. They

termined RFs have medial borders which extended

suggested that this postulate is due to an incorrect

to or straddled the vertical meridian. The fact that

identification of the callosal projections in the sub-

the retinotopic organization of this area is less precise

have previously shown jl~'- 1~, nearly all callosally de-

areas of S1. In the macaque, they show that the mid-

and that the RFs are so large as to cover a large por-

line is in fact more densely' represented at the 3b/3a

tion of the hemifield explains how a cell can be situ-

and at the 3b/l borders. This, however, is not the

ated in a region apparently far from the vertical meri-

case for areas 1 and 2, where the callosal density of

dian representation and have an RF whose medial

projecting cells is higher and more widespread, in-

border extends to the midlme. Given that only a few

volving axial structures, but also most of the other

fibres recorded in the callosum did not extend to the

body parts.

midline in the present experiment, one may have to

This argument against viewing the callosum as be-

entertain a similar argument for the somatosensory

ing principally involved in midline fusion should not be exaggerated, as one might be tempted to do from

system. Only further experimentation will permit us to arrive to a definitive conclusion of this problem.

the data of Killackey and collaborators ]a. The anatomical data must be confronted to the functional

ACKNOWLEI)(;EMENTS

one. Despite the unbiased sampling procedure used in the present experiment, only a minority of units recorded in the callosum had RFs which appeared to be clearly unrelated to midline fusion (as defined by the fact that their medial borders extended to the midline). Again, if we look at the visual system, thc

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This research was made possible in part by grants to J.P.G., M.P. and F.L. from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Ministere de I'Education du Quebec (Programme FCAR).

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