Interhemispheric and subcortical collaterals of single cortical neurons in the adult cat

Brain Research, 276 (1983) 333-338

333

Elsevier

Interhemispheric and subcortical collaterals of single cortical neurons in the adult cat JOSEPH T. WEBER 1, RICHARD W. RIECK l and HARRY J. GOULD, 1II2 ~Department of Anatomy, Tulane University Medical School, New Orleans, LA 70112; and 2Department of Anatorny, Louisiana State University Medical School, New Orleans, LA 70112 (U.S.A.)

(Accepted June 7th, 1983) Key words: superior colliculus - - callosal projections - - axon collaterals - - fluorescent dyes - - cat

The results of this study demonstrate the existence of single neocortical neurons that send axon collaterals into the corpus callosum, to terminate within the contralateral hemisphere, and subcortically, to terminate within the ipsilateral superior colliculus. Although results of previous studies failed to demonstrate subcortical collateralization of callosal neurons6, 47, several lines of reasoning suggested that this issue should be examined in the visual system. It is generally assumed that interhemispheric connections play an important role in visual function including the cortical control of vergence movements of the eyes3, 5.25.46. The control of vergence eye movements has been related to the integration of widely disparate visual images (coarse stereopsis) at a single cortical locus and such coarse stereopsis may be the signal to initiate these movements 44. Traditionally most visual callosal connections have been thought to arise and terminate within cortical regions associated with the representation of the vertical meridian8,31,32, leaving doubt as to a source of neurons that could convey information about widely disparate images. Recently, several studies have shown that the origins of callosal connections are more widely distributed than earlier studies suggested 12,33,39~45 and thus, the existence of callosally projecting neurons that could potentially provide a basis for integrating widely disparate visual images has been established. The actual mechanisms for initiating vergence movements, however, are still unresolved and the implication of subcortical mechanisms involved in these movements is problematical. One subcortical region most likely to be involved in this function is the superior colliculus because of its relation to saccadic eye movements29,36,38, 48 and visuomotor integrationl,21. 0006-8993/83/$03.00 © 1983 Elsevier Science Publishers B.V.

Thus far, however, there has been no basis for relating callosal projections with the superior colliculus, although indirect pathways, including intracortical circuits, might be postulated as linking these two areas. Two recent observations suggest the potential for a more direct relationship. First, it has been known for several years that visual corIical neurons located within layer V project subcortically to the superior colliculus11,14,2°. More recently, collicular afferent neurons also have been demonstrated within layers IV and V113. Second, although callosal neurons have been thought to be restricted largely to layers III and II t6, recent studies have shown that, within the visual cortices, callosally projecting neurons are also found within layers IV, V and V112.33.34. The overlap of these expanded patterns of projecting neurons, particularly in layer V, logically suggests that some of these neurons might project both to the contralateral cortex and to the ipsilateral superior colliculus. In order to test this hypothesis, experiments were conducted in 5 adult cats utilizing the retrograde transport of the two fluorescent dyes, fast blue and nuclear yellow 4. Multiple injections (10-15 injection sites) of 7% fast blue in distilled water were placed unilaterally within the superior coUiculus. Because the transport of fast blue is slower than that for nuclear yellow, a 5-7-day waiting period was required before multiple injections (120-140 injection sites) of 10% nuclear yellow in distilled water were placed

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335 throughout the visual cortices of the contralateral hemisphere. Each injection for either fast blue or nuclear yellow was accomplished by depositing 0.1/A of the dye solution with pressure through a 1/A syringe. Following an additional 18 h the cats were sacrificed via an overdose of pentobarbital and perfused transcardially with 0.1 M cacodylate buffer (pH 7.2) followed by a 10% buffered formalin solution. The brains were removed from the cranium and stored for 3 days at 4 °C in a solution of 30% sucrose in buffered formalin. The brains were subsequently sectioned at 30 ~m on a freezing microtome and every fifth section was rinsed in 0.1 M cacodylate buffer, mounted on subbed slides and air-dried. The sections were viewed on a Nikon Labophot fluorescence microscope using a 320-380 nm excitation filter and a 420 nm barrier filter. Injection sites of fast blue included all layers in the rostral two-thirds of the superior colliculus. Cortical injections of nuclear yellow were placed with cortical visual areas 17, 18, 19, and the lateral suprasylvian areas and area 7. The injec-

tion sites extended approximately 7 mm rostral to the interaural line. After examination on the fluorescence microscope, the sections were counterstained with cresyl violet and coverslipped for the verification of visual cortical areas. The cortical maps of Tusa and colleagues 40-42, Palmer et al. 28 and Updyke 43 were used to identify specific cortical areas. The size, areal distribution, and laminar location of neurons labeled with only one fluorescent dye are in good agreement with that previously reported for callosaP 3-35 and subcorticaln,13,14,20 projecting cells. Double-labeled neurons containing both fast blue and nuclear yellow are found primarily within visual cortical areas 17, 18, 19, the anterior and posterior lateral suprasylvian areas (i.e. AMLS, ALLS, PMLS and PLLS) and the ventral lateral suprasylvian area (Fig. 1). Numerous double-labeled cells are also found within area 7. The double-labeled neurons are always small to medium pyramidal cells and are located within the lower half of layer IV, layer V, and occasionally within layer VI (Fig. 3). In these experi-

Fig. 2. Photomicrographs of labeled layer V pyramidal neurons within PMLS followingplacement of fast blue in the ipsilateral superior colliculus and nuclear yellow in the contralateral visual cortex. A: a neuron that is labeled only with fast blue; note nuclear ghost (arrow). B: a double-labeled neuron; note intense labeling of the nucleus with nuclear yellow. Bar equals 10/~m.

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Fig. 3. Schematic representation of the laminar location of projection neurons in visual cortex. Left: open circles depict the laminar origin of the major (> 1%) caUosal projections from area 17 and the posterior medial lateral suprasylvian area (PMLS). Representation of data as presented by Segraves and Rosenquist 33. Center: shows the location of collicular afferents that arise from neurons (open circles) in deep layer IV, layers V and VI in visual areas as shown by Hartwich-Young et al. 13. Right: closed circles show the laminar distribution of double-labeled neurons in areas 17 and PMLS as presented in the present report. ments, double-labeled neurons were estimated to form only 1-5% of the labeled corticotectai cells, but because of potential shortcomings due to the sensitivity of the method employed2.15, this figure may only be a conservative estimate of the true number of neurons with both commissural and subcortical projections. Thus, the demonstration of double-labeled neurons supports our original hypothesis that individual neurons within the visual cortex of the adult cat can send axon collaterals to both the contralateral cortex and the ipsilateral superior colliculus. These results allow us to argue that the specific population of dual projecting neurons found within layers IV, V and VI of visual cortex is involved in both the integration of widely disparate stimuli and with movement of the eyes. These cells are thought to make heterotopic connections with the contralateral hemisphere 16,33,34 and at least with respect to area 17 might be predicted to terminate within that part of visual cortex that contains a representation of the vertical meridian u,33.34. Neurons within the infragranular layers also are found to have Y-cell characteristics, which include responses to large receptive fields and movements suggesting that cells in layers V and VI are dominated by Y-cell afferents, although these laminae are not the exclusive domain of Y-cell terminations from the dorsal lateral geniculate nucleus 9,23,24. Cortical cells of this type would recognize widely disparate stimuli and could respond to stimuli

that might initiate vergence movements of the eyes. If homotopic connections are concomitantly made with the superior colliculus~0, then the relative disparity with respect to the vertical meridian is registered and intracollicular organization may initiate an appropriate response for repositioning the eyes. Cortical guidance would further reinforce the vergence response since the initial stimulus would sequentially stimulate similar cells closer to the representation of the vertical meridian. The appropriate interhemispheric and subcortical connections of these neurons would thus continue and control the vergence movement by setting up a series of cortically guided servo-responses. It might be further postulated that populations of neurons with similar dual projections exist in regions subserving other sensory modalities, i.e. somesthesia and audition, since it is known that the superior colliculus represents a topographically organized integrator of multimodal sensory stimuli 7,37. These other neurons might therefore be related to directing movements of body parts toward a tactile stimulus or toward an auditory stimulus. Other studies indicate that subcortical collaterals of callosal axons may be found in other systems. For example, it was postulated that cortical neurons which project to the caudate nucleus may have collaterals that are commissuraP 0. Within the auditory system, the same question was asked with respect to neurons in the primary auditory cortex. It was concluded, however, that no cells appeared to send axons both to the medial geniculate nucleus and to the contralateral hemisphere 47. Finally, although some workers 17-19,26 have shown that a large percentage of neurons within the neocortex eliminate their callosal collaterals during early postnatal development, our data indicate that at least a small population of subcortically projecting neurons exhibit a callosal collateral in the adult. Although we have argued that the dual projection neurons are a functional entity of the nervous sytem of the adult cat, the possibility exists that the persistence of these neurons may represent an incomplete elimination of these collateralized projections during development 26. This study was supported by N.I.H. Grants EY03731 and MH36418. We thank Dr. Heinz Loewe

337 for p r o v i d i n g n u c l e a r y e l l o w , D r . G w e n O. Ivy for

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