Trajectories in the spinal cord and the mediolateral spread in the cerebellar cortex of spinocerebellar fibers from the unilateral lumbosacral enlargement in the chicken.

Spinocerebellar (SC) neurons in the lumbosacral enlargement (LSE) give rise mainly to crossed fibers and generally terminate in parasagittal bands in the granular layer of the chicken cerebellar cortex. However, parasagittal bands for mossy fiber terminals have not always been clear in some cerebellar folia. The present study aimed at (1) observing the course in the spinal cord of the spinocerebellar tracts (SCTs), (2) confirming whether SC fibers originating from the unilateral LSE terminate in parasagittal bands, and (3) elucidating the relationship between the ventral and lateral funicular parts of the SCTs in the cervical enlargement (CE) using anterograde and retrograde labeling methods. The SCTs were located in the medial part of the ventral funiculi in spinal segment (SS) 27, the full width of the ventral funiculi in SS 22, the lateral and ventral funiculi in SS 14 and in the lateral funiculi from SS 10 rostralward. Projection areas in the cerebellar cortex of SC fibers were studied following unilateral injections of WGA-HRP into the LSE. As a result, SC fibers from the LSE terminated bilaterally in parasagittal bands of folia II-VI and IXc. Labeled terminals in the injected side were similar in number to those in the other side in folia II-IV and IXc and more than those in the other side in folia V and VI. Following ablation of the left (contralateral) lateral funiculus of the CE, the same tracer was injected into the right (ipsilateral) LSE or into the anterior or posterior cerebellar lobe. As a result, anterogradely labeled SC fibers passing through the ventral funiculus in the CE mainly terminated in the contralateral cerebellar cortex in folia II, III and IV, and in the ipsilateral cerebellar cortex in folia V, VI and IX. Following ablation of the unilateral lateral funiculus, retrogradely labeled neurons in the contralateral LSE were found in all SC neuron groups showing marked reduction in number. Thus, the ventral and lateral funicular parts of the SCTs in the CE were not pathways for specific SC neuron groups but different in projection areas.

Distinct central representations for sensory fibers innervating either the conjunctiva or cornea of the rat.

The laminar sheet of epithelium (e.g., skin and mucous membrane) enclosing our bodies is represented in the dorsal horns of the medulla and spinal cord. The eyeball however indents this laminar sheet and is shrouded by different layers: the cornea/sclera, the conjunctiva, and hairy skin. This involution of the orb confounds defining the central representation of the cornea and its surrounding mucosa and skin. We used herein the transganglionic transport of a cocktail of HRP conjugated to cholera toxin and wheat germ agglutinin to determine the central representation of these epithelia in the dorsal horns of the rat. The HRP cocktail was injected either into the stroma of the cornea, the mucosa of the conjunctiva, or the supraorbital and infraorbital nerves. Injections of the cornea produced dense label in the interstitial islands in the ventral medullary dorsal horn (MDH), probably lamina I, and in neuropil in the ventromedial tip of the MDH, probably lamina II. There sometimes was variable, diffuse label in the C1 dorsal horn after corneal injections but more rostral parts of the trigeminal sensory complex were never labeled. Injections of the conjunctiva densely labeled laminae I-III in the C1 dorsal horn, while laminae IV-V were diffusely labeled. Sparser reaction product also was seen in lamina I in positions similar to the cornea projection. Label was seen ventrally in subnuclei interpolaris and oralis, as well as the principal trigeminal nucleus. Projections of the infraorbital nerve included all laminae in the trigeminocervical complex as well as large portions of the rostral subnuclei in the spinal trigeminal nucleus. The projections of the supraorbital nerve were similar, but were restricted to ventral parts of the trigeminal sensory complex. In other cases the cornea was injected either after cutting the supraorbital and infraorbital nerves or the conjunctiva was injected after enucleating the eyeball. Any reaction product from corneal injections was reduced dramatically in the C1 dorsal horn after transection of the infraorbital and supraorbital nerves. Injecting the conjunctiva after enucleating the eyeball densely labeled the C1 projection to the dorsal horn, a small patch in lamina I in the MDH, as well as the rostral trigeminal complex. We propose that the cornea has but a single representation in the trigeminocervical complex in its ventral part near the caudal end of the medulla. We also propose the palpebral conjunctiva mucosa is represented in the C1 dorsal horn, and speculate that the bulbar conjunctiva overlaps with that of the cornea in lamina I. We discuss these projections in relation to the circuitry for the supraorbital-evoked and corneal-evoked blink reflexes. The relationship of the cornea and conjunctiva is intimate, and investigators must be very careful when attempting to stimulate them in isolation.

Striatal projections from the lateral and posterior thalamic complexes. An anterograde tracer study in the cat.

Striatal projections from the lateral intermediate (LI) and posterior (Po) thalamic complexes were studied with the anterograde tracers wheat germ agglutinin-horseradish peroxidase and Phaseolus vulgaris leucoagglutinin. Projections to the lateral part of the head and body of the caudate nucleus (CN) and to the putamen (Pu) were found to arise from the ventral parts of the caudal subdivision of the LI besides the well established sources in the intralaminar and ventral thalamic nuclei. No projections to the CN and only a few to the Pu were found to arise from the medial division of the Po. The presence of terminal and intercalated varicosities in the thalamostriatal fibers suggests that they form both terminal and en passant synapses. Thalamostriatal fibers from these thalamic sectors were unevenly distributed within the CN, with patches of either low-density innervation or with no projections at all interspersed within irregular, more densely innervated areas. The former coincided with the acetylcholinesterase-poor striosomes and the latter areas of dense projection with the extrastriosomal matrix.

Frontal lobe inputs to the digit representations of the motor areas on the lateral surface of the hemisphere.

We examined the frontal lobe connections of the digit representations in the primary motor cortex (M1), the dorsal premotor area (PMd), and the ventral premotor area (PMv) of cebus monkeys. All of these digit representations lie on the lateral surface of the hemisphere. We used intracortical stimulation to identify the digit representations physiologically, and then we injected different tracers into two of the three cortical areas. This approach enabled us to compare the inputs to two digit representations in the same animal. We found that the densest inputs from the premotor areas to the digit representation in M1 originate from the PMd and the PMv. Both of these premotor areas contain a distinct digit representation, and the two digit representations are densely interconnected. Surprisingly, the projections from the digit representation in the supplementary motor area (SMA) to the PMd and PMv are stronger than the SMA projections to M1. The projections from other premotor areas to M1, the PMd, and the PMv are more modest. Of the three digit areas on the lateral surface, only the PMv receives dense input from the prefrontal cortex. Based on these results, we believe that M1, the PMd, and the PMv form a densely interconnected network of cortical areas that is concerned with the generation and control of hand movements. Overall, the laminar origins of neurons that interconnect the three cortical areas are typical of "lateral" interactions. Thus, from an anatomical perspective, this cortical network lacks a clear hierarchical organization.

Afferent and efferent connections of the nucleus rotundus demonstrated by WGA-HRP in the chick.

Organization of the fibre connections in the chick nucleus rotundus (Rt) was investigated by an axonal tracing method using wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). After an injection of WGA-HRP into the Rt, labelled neurones were observed in the striatum griseum centrale (SGC) in both sides of the tectum (TO) and in the ipsilateral nucleus subpretectalis/nucleus interstito-pretecto-subpretectalis (SP/IPS). Labelled fibres and terminals were also found in the ipsilateral ectostriatum (Ect). These fibre connections were topographically organized rostrocaudally. In the TO-Rt projection, the rostral and the dorsocaudal parts of the Rt received afferents from the superficial part of the SGC, the middle part of the Rt received afferents from the intermediate part of the SGC, and the ventrocaudal part of the Rt received mainly fibres from the deep part of the SGC. These topographic projections were accompanied by a considerable number of diffuse projections to the thalamic regions surrounding the Rt. In addition, the rostral and middle caudal parts of the Rt received afferents from the lateral and medial parts of the SP/IPS, respectively, and respective parts of the Rt sent efferents to the lateral and medial parts of the Ect.

Descending projections to the inferior colliculus from the posterior thalamus and the auditory cortex in rat, cat, and monkey.

Projections from the posterior thalamus and medial geniculate body were labeled retrogradely with wheat germ agglutinin conjugated to horseradish peroxidase injected into the rat, cat, and squirrel monkey inferior colliculus. Neurons were found ipsilaterally in the (1) medial division of the medial geniculate body, (2) central gray, (3) posterior limitans nucleus, and the (4) reticular part of the substantia nigra. Bilateral projections involved the (5) peripeduncular/suprapeduncular nucleus, (6) subparafascicular and posterior intralaminar nuclei, (7) nucleus of the brachium of the inferior colliculus, (8) lateral tegmental/lateral mesencephalic areas, and (9) deep layers of the superior colliculus. The medial geniculate projection was concentrated in the caudal one-third of the thalamus; in contrast, the labeling in the subparafascicular nucleus, substantia nigra, and central gray continued much further rostrally. Robust anterograde labeling corresponded to known patterns of tectothalamic projection. Biotinylated dextran amine deposits in the rat inferior colliculus revealed that (1) many thalamotectal cells were elongated multipolar neurons with long, sparsely branched dendrites, resembling neurons in the posterior intralaminar system, and that other labeled cells were more typical of thalamic relay neurons; (2) some cells have reciprocal projections. Similar results were seen in the cat and squirrel monkey. The widespread origins of descending thalamic influences on the inferior colliculus may represent a phylogenetically ancient feedback system onto the acoustic tectum, one that predates the corticocollicular system and modulates nonauditory centers and brainstem autonomic nuclei. Besides their role in normal hearing such pathways may influence behaviors ranging from the startle reflex to the genesis of sound-induced seizures.

Cardiac afferents to the nucleus of the tractus solitarius: A WGA-HRP study in the rat.

Central distribution of the sensory fibers of the heart was investigated in the rat by the use of transganglionic transport of horseradish peroxidase (HRP). After the left intercostal thoracotomy was done under deep anesthesia and artificial respiration, wheat germ agglutinin-conjugated HRP (WGA-HRP) was injected into the left and right ventricular walls and the apex of the heart. HRP-labeled fibers were observed to be distributed to the dorsomedial portion of the medulla oblongata through the vagal nerve. The labeled fibers were present in various subnuclei of the nucleus of the tractus solitarius (NTS) bilaterally at the level of +0.36 to -1.74 mm to the obex. However, the most conspicuous feature in the present study was that the labeled fibers were exclusively confined to the medial, ventrolateral and commissural NTS with some distribution to the dorsolateral NTS. Although the labeling in the medial and ventrolateral NTS was observed to extend rostrocaudally, it was of interest that the labeling in the medial NTS was divided into the ventral and dorsal parts at the level around the obex. Accumulation of the labeled fibers in the commissural NTS was found at the level caudal to the obex and these fibers were traced to the caudal portion of its subnucleus with a gradual decrease in number. This pattern of distribution of cardiac afferents in the NTS was considered to be peculiar to the rat, because it was quite different from that reported previously in the cat.

Laminar organization of the intrinsic connection neurons and their axon terminals in the lateral suprasylvian cortex of the cat.

The lateral suprasylvian cortex (LS) of the cat has numerous interconnections with other visual cortical areas, as well as with subcortical structures implicated in visually-guided behavior. However, little data are available regarding connections within the LS itself. In order to investigate the intrinsic connections within LS, we have examined the laminar patterns of terminal and cellular labeling following injections of either the anterograde tracer biocytin, or the retrograde tracer, WGA-HRP, into the subregions of the LS (AMLS, PMLS, ALLS, and PLLS). Tracer deposits spanning all cortical layers in a LS subregion basically resulted in labeling of axons and terminals (biocytin cases) or cells of origin (WGA-HRP cases) in all layers, although intensity of the labeling seemed to be different among subregions. The present study demonstrated that the interconnections among LS subregions provide no clear evidence of simple hierarchial relationships between regions.

A visuo-somatomotor pathway through superior parietal cortex in the macaque monkey: cortical connections of areas V6 and V6A.

This report addresses the connectivity of the cortex occupying middle to dorsal levels of the anterior bank of the parieto-occipital sulcus in the macaque monkey. We have previously referred to this territory, whose perimeter is roughly circumscribed by the distribution of interhemispheric callosal fibres, as area V6, or the 'V6 complex'. Following injections of wheatgerm agglutinin conjugated to horseradish peroxidase (WGA-HRP) into this region, we examined the laminar organization of labelled cells and axonal terminals to attain indications of relative hierarchical status among the network of connected areas. A notable transition in the laminar patterns of the local, intrinsic connections prompted a sub-designation of the V6 complex itself into two separate areas, V6 and V6A, with area V6A lying dorsal, or dorsomedial to V6 proper. V6 receives ascending input from V2 and V3, ranks equal to V3A and V5, and provides an ascending input to V6A at the level above. V6A is not connected to area V2 and in general is less heavily linked to the earliest visual areas; in other respects, the two parts of the V6 complex share similar spheres of connectivity. These include regions of peripheral representation in prestriate areas V3, V3A and V5, parietal visual areas V5A/MST and 7a, other regions of visuo-somatosensory association cortex within the intraparietal sulcus and on the medial surface of the hemisphere, and the premotor cortex. Subcortical connections include the medial and lateral pulvinar, caudate nucleus, claustrum, middle and deep layers of the superior colliculus and pontine nuclei. From this pattern of connections, it is clear that the V6 complex is heavily engaged in sensory-motor integration. The specific somatotopic locations within sensorimotor cortex that receive this input suggest a role in controlling the trunk and limbs, and outward reaching arm movements. There is a secondary contribution to the brain's complex oculomotor circuitry. That the medial region of the cortex is devoted to tightly interconnected representations of the sensory periphery, both visual and somatotopic-which are routinely stimulated in concert-would appear to be an aspect of the global organization of the cortex which must facilitate multimodal integration.

Evidence for a GABAergic projection from the central nucleus of the amygdala to the brainstem of the macaque monkey: a combined retrograde tracing and in situ hybridization study.

The central nucleus of the amygdala is interconnected with a variety of visceral and autonomic nuclei of the brainstem. These include the parabrachial nucleus, the nucleus of the solitary tract, the nucleus ambiguus and the dorsal motor nucleus of the vagus. Despite repeated attempts, neurochemical characterization of the major subcortical connections of the central nucleus has not yet been accomplished. Based on earlier immunohistochemical and in situ hybridization evidence indicating the presence of numerous GABAergic neurons in the macaque monkey central nucleus, we predicted that a sizeable portion of the descending projections may be GABAergic. We tested this hypothesis using a novel double labelling method with gold conjugated WGA-apoHRP as a retrograde tracer and in situ hybridization for detecting the mRNA that encodes the enzyme glutamic acid decarboxylase (GAD67) as a marker for GABAergic cells. Following WGA-apoHRP-gold injections into the brainstem, a large number of retrogradely labelled cells was observed in the medial and lateral divisions of the central nucleus. Of the retrogradely labelled cells observed in the medial division of the central nucleus, approximately half were double-labelled for GAD67 mRNA; about 30% double labelling was observed in the lateral division. These data support the view that a sizeable component of the central nucleus projection to the brainstem is GABAergic.

Projections from the caudal spinal trigeminal nucleus to commissural interneurons in the supratrigeminal region: an electron microscope study in the rat.

Electron microscopic double-labeling study in the rat indicated that projection fibers from the caudal spinal trigeminal nucleus (Vc) were distributed ipsilaterally within the supratrigeminal region (STR) capping the trigeminal motor nucleus (Tm) and made synaptic contact with neurons projecting to the contralateral Tm. Nociceptive inputs to the Vc may reflexly control, via interneurons in the STR, the activities of Tm neurons innervating the masticatory, tensor tympani, and/or tensor veli palatine muscles.

Induction of cell division in olfactory basal epithelium following intranasal irrigation with wheat germ agglutinin-horseradish peroxidase.

The lectin, wheatgerm agglutinin (WGA) conjugated to horseradish peroxidase (HRP), previously was shown to be transported into the central nervous system following application by intranasal irrigation. The current study investigated the hypothesis that uptake of molecules, such as the lectin-conjugate, by olfactory receptor cells would mimic internalization of other substances including odorants. This process would result in both premature death of receptor cells and increased turnover of their precursors, globose basal cells. Tetramethylbenzidine histochemical analysis showed the presence of significant amounts of the lectin-conjugate in both the receptor epithelium and olfactory bulb until at least 2 weeks postintranasal application. Neither supporting nor globose basal cells contained WGA-HRP, suggesting that uptake was primarily into olfactory receptor cells. Cell turnover, assessed by tritiated-thymidine (thymidine) autoradiography, increased both 1 and 2 weeks, but not 3 and 4 weeks, following intranasal irrigation with WGA-HRP. Most of the cells containing thymidine labelling appeared to be globose basal cells, although supporting cells also occasionally exhibited labelling. Survival of either mature or immature receptor cells in the epithelium, indicated by epithelial thickness and cell density of the septal epithelium, also declined following treatment. These data suggest that uptake of substances may result in cell loss from the olfactory epithelium and increased mitotic activity of basal cells.

Occlusion of the femoral artery induced fos-like immunoreactive neurons in the lateral habenular nucleus projecting to the midbrain periaqueductal gray in the rat.

Wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) injection into the midbrain periaqueductal gray (PAG) resulted in heavy accumulation of retrogradely labeled neurons in the lateral habenular nucleus (LHb) bilaterally. When the left femoral artery was persistently ligated for 2 h, the expression of Fos-like immunoreactivity (FOS-LI) was also found bilaterally in the LHb. In the present study, by combining the retrograde labeling method of injecting WGA-HRP into the PAG and the immunohistochemical staining of the FOS-LI neurons in the LHb induced by occlusion of the femoral artery, it was demonstrated that there are neurons containing both HRP labeling and FOS-LI. These neurons appeared to be located mainly in the medial part and posterior half of the LHb, and constitute about 24% of the total number of all labeled cells.

Targeted delivery of antigen to hamster nasal lymphoid tissue with M-cell-directed lectins.

The nasal cavity of a rodent is lined by an epithelium organized into distinct regional domains responsible for specific physiological functions. Aggregates of nasal lymphoid tissue (NALT) located at the base of the nasal cavity are believed to be sites of induction of mucosal immune responses to airborne antigens. The epithelium overlying NALT contains M cells which are specialized for the transcytosis of immunogens, as demonstrated in other mucosal tissues. We hypothesized that NALT M cells are characterized by distinct glycoconjugate receptors which influence antigen uptake and immune responses to transcytosed antigens. To identify glycoconjugates that may distinguish NALT M cells from other cells of the respiratory epithelium (RE), we performed lectin histochemistry on sections of the hamster nasal cavity with a panel of lectins. Many classes of glycoconjugates were found on epithelial cells in this region. While most lectins bound to sites on both the RE and M cells, probes capable of recognizing alpha-linked galactose were found to label the follicle-associated epithelium (FAE) almost exclusively. By morphological criteria, the FAE contains >90% M cells. To determine if apical glycoconjugates on M cells were accessible from the nasal cavity, an M-cell-selective lectin and a control lectin in parallel were administered intranasally to hamsters. The M-cell-selective lectin was found to specifically target the FAE, while the control lectin did not. Lectin bound to M cells in vivo was efficiently endocytosed, consistent with the role of M cells in antigen transport. Intranasal immunization with lectin-test antigen conjugates without adjuvant stimulated induction of specific serum immunoglobulin G, whereas antigen alone or admixed with lectin did not. The selective recognition of NALT M cells by a lectin in vivo provides a model for microbial adhesin-host cell receptor interactions on M cells and the targeted delivery of immunogens to NALT following intranasal administration.

Alpha ganglion cells of the rabbit retina lose antagonistic surround responses under dark adaptation.

Alpha ganglion cells from the midperiphery of the rabbit retina were recorded intracellularly under visual control, in a superfused everted eyecup, and labeled with HRP. Their physiology and large somata with broad dendritic arbors identified them as uniform populations of ON- and OFF-center alpha ganglion cells, which typically displayed transient/sustained light-evoked responses. When dark adapted, the light-evoked responses from both ON- and OFF-center alpha ganglion cells were more sustained than those generally seen under light-adapted conditions. During dark-adapted (scotopic) conditions, stimulation with dim full-field illumination and small spots, either positioned over the soma or displaced 450 microns from the soma, all elicited pure center responses. After light adaptation (photopic conditions), the displaced small spots that previously evoked center responses elicited antagonistic surround responses from both ON- and OFF-center cells. Thus, as originally described in cat retina (Barlow et al., 1957), the receptive-field organization of ganglion cells changed between dark and light adaptation, and an absence or presence of surround antagonism was indicative of scotopic versus photopic states.

Glutamate containing neurons in the cat superior colliculus revealed by immunocytochemistry.

Glutamate is the probable neurotransmitter of both retinal and cortical afferents to the cat superior colliculus (SC). The present study shows that glutamate is also contained in many postsynaptic neurons in SC. The distribution, morphology, and ultrastructure of neurons in SC were examined using glutamate antibody immunocytochemistry. Labeled cells were widely distributed throughout, but a specific laminar pattern was evident. Relatively few cells were found in the zonal and upper superficial gray layers (SGL). A dense band of intensely labeled neurons was found within the deep superficial gray and upper optic layers. Many cells were also labeled in the deeper layers. Labeled cells had varied sizes and morphologies. Soma diameters ranged from 9-67 microns, with a mean of 22 microns. Cells with stellate, vertical fusiform, and multipolar morphologies were labeled. Cells in the deep subdivision all had morphologies and sizes typical of projection neurons. To determine if labeled cells in the dense band were also projection neurons, WGA-HRP was injected into the lateral posterior nucleus and these sections were double-labeled with the glutamate antibody. Over one-half of cells in the dense band that were labeled by HRP were also obviously labeled by antibody. At the electron-microscope level, both medium- and large-sized neurons were also labeled by glutamate antibodies. These cells had different but characteristic morphologies.

Retrograde labeling of rat dorsolateral septal nucleus neurons following intraseptal injections of WGA-HRP.

Projection neurons in the rat dorsolateral septal nucleus (DLSN) were retrogradely labeled following intraseptal injection of wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). Injections of WGA-HRP centered in the medial septum (MS) and parts of the intermediate and ventrolateral subdivisions of the lateral septum retrogradely labeled only a few centrally scattered multipolar-shaped neurons. In contrast, injections placed in the nucleus of the diagonal band of Broca (DBB) consistently resulted in labeling of DLSN neurons of all sizes and shapes. Large injections in rostral DBB appeared to retrogradely label every DLSN neuron, while similar injections in caudal DBB only labeled neurons in restricted regions of the nucleus. A collection of small cells forming the ventricular border of caudal DLSN and a group of larger cells situated in the dorsolateral tip of rostral DLSN were consistently labeled following each DBB injection. The pattern of retrogradely labeled neurons in the DLSN appeared in a complementary fashion to that seen in the other lateral septal nuclei. Our findings support the conclusion that the DLSN is a morphologically heterogeneous nucleus consisting almost entirely of projection neurons. The pattern of retrograde labeling in the lateral septum suggests that these projection neurons may be topographically organized since distinct subpopulations of cells were labeled following different injections in the MS/DBB complex.

Cerebellar afferences from the mesencephalic trigeminal nucleus in the rat.

The aim of the present study was to test for and characterize the organization of a direct projection from neurones of the trigeminal mesencephalic nucleus (Vme) to the cerebellum. WGA-HRP was used as a retrograde tracer following injections in the cerebellar cortex. The extent of each injection site within the sagittal zones was determined according to corticonuclear and olivocortical connections. Retrogradely labelled neurons were observed in the caudal part of the ipsilateral Vme only following vermal injection. The Vme projections reached exclusively the ipsilateral sagittal zone X in the anterior lobe, lobule VI and lobule IX. This identification was confirmed by anterograde labelling of mossy fibre terminals following a biocytin injection restricted to the Vme.

Supplementary eye field as defined by intracortical microstimulation: connections in macaques.

In macaques, the frontal eye field and the recently defined supplementary eye field play a role in the production of eye movements. Whereas the structure and function of the frontal eye field are well understood, little is known about the supplementary eye field. The goal of this study was to determine the connections of the physiologically defined supplementary eye field. In each case, the location of the supplementary eye field was determined by using intracortical microstimulation, the borders were marked with small electrolytic lesions, and horseradish peroxidase conjugated to wheat germ agglutinin was injected into the supplementary eye field. After the tissue was incubated with tetramethyl benzidine, it was determined that in three cases the injection site was confined to the physiologically defined supplementary eye field. The present results indicate that the supplementary eye field is reciprocally connected with the claustrum, ventral anterior nucleus, including pars magnocellularis, nucleus X, posterior subdivision of the ventral lateral nucleus, multiform, parvocellular, magnocellular, and densocellular subdivisions of the medial dorsal nucleus, central lateral nucleus, parafascicular nucleus, and suprageniculate-limitans complex. The supplementary eye field projects to the putamen, caudate, reticular nucleus of the thalamus, central densocellular nucleus, zona incerta, subthalamic nucleus, rostral interstitial nucleus of the medial longitudinal fasciculus, parvocellular part of the red nucleus, intermediate and deep layers of the superior colliculus, central gray, cuneiform nucleus, mesencephalic reticular formation, pontine gray, nucleus reticularis tegmenti pontis, and nucleus reticularis pontis oralis. The supplementary eye field is reciprocally and bilaterally connected with periprincipal and inferior prefrontal cortex, with periarcuate cortex, including the frontal eye field, the frontal ventral region, and with postarcuate premotor cortex, and cortex surrounding the supplementary eye field, including the supplementary motor area. The supplementary eye field is also reciprocally connected ipsilaterally with cortex in and around the cingulate sulcus and the intraparietal sulcus, whereas cortex within the superior temporal sulcus projects to the supplementary eye field. The connections of the physiologically defined supplementary eye field are compared to previously demonstrated connections of the supplementary motor region and of the physiologically defined frontal eye field. Comparisons between the connections of the frontal and supplementary eye fields reveal that both regions are connected with structures related to visuomotor functions, but the frontal eye field has more extensive connections with vision-related structures, and the supplementary eye field has more extensive connections with structures related to prefrontal and skeletomotor functions. Such connectional differences suggest functional differences between these two sensorimotor regions of the frontal lobe.