Principal component and cluster analysis of layer V pyramidal cells in visual and non-visual cortical areas projecting to the primary visual cortex of the mouse

The long-distance corticocortical connections between visual and nonvisual sensory areas that arise from pyramidal neurons located within layer V can be considered as a subpopulation of feedback connections. The purpose of the present study is to determine if layer V pyramidal neurons from visual and nonvisual sensory cortical areas that project onto the visual cortex (V1) constitute a homogeneous population of cells. Additionally, we ask whether dendritic arborization relates to the target, the sensory modality, the hierarchical level, or laterality of the source cortical area. Complete 3D reconstructions of dendritic arbors of retrogradely labeled layer V pyramidal neurons were performed for neurons of the primary auditory (A1) and somatosensory (S1) cortices and from the lateral (V2L) and medial (V2M) parts of the secondary visual cortices of both hemispheres. The morphological parameters extracted from these reconstructions were subjected to principal component analysis (PCA) and cluster analysis. The PCA showed that neurons are distributed within a continuous range of morphologies and do not form discrete groups. Nevertheless, the cluster analysis defines neuronal groups that share similar features. Each cortical area includes neurons belonging to several clusters. We suggest that layer V feedback connections within a single cortical area comprise several cell types.

Indirect pathway between the primary auditory and visual cortices through layer V pyramidal neurons in V2L in mouse and the effects of bilateral enucleation

Visual cortical areas are activated by auditory stimuli in blind mice. Direct heteromodal cortical connections have been shown between the primary auditory cortex (A1) and primary visual cortex (V1), and between A1 and secondary visual cortex (V2). Auditory afferents to V2 terminate in close proximity to neurons that project to V1, and potentially constitute an effective indirect pathway between A1 and V1. In this study, we injected a retrograde adenoviral vector that expresses enhanced green fluorescent protein under a synapsin promotor in V1 and biotinylated dextran amine as an anterograde tracer in A1 to determine: (i) whether A1 axon terminals establish synaptic contacts onto the lateral part of V2 (V2L) neurons that project to V1; and (ii) if this indirect cortical pathway is altered by a neonatal enucleation in mice. Complete dendritic arbors of layer V pyramidal neurons were reconstructed in 3D, and putative contacts between pre-synaptic auditory inputs and postsynaptic visual neurons were analysed using a laser-scanning confocal microscope. Putative synaptic contacts were classified as high-confidence and low-confidence contacts, and charted onto dendritic trees. As all reconstructed layer V pyramidal neurons received auditory inputs by these criteria, we conclude that V2L acts as an important relay between A1 and V1. Auditory inputs are preferentially located onto lower branch order dendrites in enucleated mice. Also, V2L neurons are subject to morphological reorganizations in both apical and basal dendrites after the loss of vision. The A1-V2L-V1 pathway could be involved in multisensory processing and contribute to the auditory activation of the occipital cortex in the blind rodent.

Distribution and progression of amyloid-beta deposits in the amygdala of the aged macaque monkey, and parallels with zinc distribution

In this study, we have mapped amyloid beta (Abeta) deposition in the amygdala of five aged Japanese monkeys (from 23 to 30 years old). In brief, the aged monkey amygdala shows a topographic distribution of Abeta deposits that is subnucleus specific and exhibits a distinct temporal progression. The pattern is similar to the distribution of Abeta deposits in the human amygdala of Alzheimer's patients and of high plaque nondemented cases. The spatial distribution and temporal progression were correlated with the distribution of free zinc (Zn), which is known to mediate Abeta aggregation. For the basolateral group of subnuclei in particular, there is a clear dorsoventral gradient in the progressive distribution of Abeta. Abeta depositions first appear in the ventral division of the lateral nucleus and parvicellular division of the accessory basal nucleus, and then extend into the ventral part of the basal and paralaminar nuclei. All these nuclei are also Zn-dense. Conversely, Zn-weak nuclei, which are more dorsally situated (i.e. dorsal division of lateral nucleus and magnocellular division of basal nucleus) showed only a low level of Abeta deposits, even in brains with the greatest Abeta burden. In contrast to the basolateral group, the central and medial nuclei and cortical group had Abeta deposits only at later stages. In the central and medial nuclei, we identified a lateromedial gradient of Abeta deposits, again similar to the gradient of Zn-distribution. In the cortical group, Abeta deposits are densest in the deep layer, where Zn is also densest. Thus, we suggest the macaque amygdala, with its clear topographic distribution of Abeta deposits, may be an effective model for examining the complex mechanisms of vulnerability to Abeta deposits. A primate model would be advantageous for experimental interventions geared toward therapeutic protection from Alzheimer's disease, including by microarray analysis and genetic manipulation.

Zinc-rich transient vertical modules in the rat retrosplenial cortex during postnatal development

The rat retrosplenial cortex is part of a heavily interconnected limbic circuit, considered to have an important role in spatial memory. Interestingly, the granular retrosplenial cortex has an exceptionally distinct system of dendritic bundles, originating from callosally projecting pyramidal neurons in layer II. These can be detected as early as postnatal day 5; and, although their functional significance remains to be elucidated, the existence of these bundles makes the granular retrosplenial cortex an attractive model system for a wide range of development and functional investigations. Here, we report four results concerning the development of modularity in the granular retrosplenial cortex in rats as investigated by neurochemical markers associated to cortico-cortical and thalamo-cortical connections. Emphasis is placed on zinc, an activity-related substance associated with glutamatergic, non-thalamic terminations. 1) Zinc shows a transient strong expression during early postnatal development, but later than the appearance of the upper layer bundles (at postnatal day 5). By postnatal day 11 to postnatal day 15 staining for zinc achieved its most complex pattern; such that layer I had an elaborate organization both in the tangential and radial dimensions. Three sublaminae were distinguished (layers Ia-c): a superficial, thin tier (Ia) with patchy, moderate staining which periodically intruded into the underlying layer Ib ("funnel" modules), a middle band of variable width and light staining (Ib), and a deep, thin band with heavy and patchy staining (Ic) which, at rostral levels, spread upward into layer Ib (as "dome-like" modules). 2) At postnatal day 15, immunohistochemical methods showed that layers Ia, b zinc-funnels were co-localized with glutamate receptor subunits 2/3, GABA receptor type A alpha1 subunit and the thalamo-cortical marker, vesicular glutamate transporter 2. Layer Ic and the zinc dome-like modules were co-labeled for the cortico-cortical marker, vesicular glutamate transporter 1 and calretinin. 3) The spatial coincidence between zinc funnels in layers Ia, b and vesicular glutamate transporter 2 was further investigated by electron microscopy, which demonstrated co-localization of zinc and vesicular glutamate transporter 2 in synaptic boutons. The unusual co-localization of zinc and thalamo-cortical terminations was confirmed by retrograde transport of zinc to neurones in the anterodorsal thalamic nucleus at postnatal day 9 and postnatal day 13, and can thus be considered a transient zinc expression in thalamo-cortical boutons. This was not observed at postnatal day 28 or later. 4) After postnatal day 18, zinc staining started to fade in all layers. Before postnatal day 21, the heavy staining for zinc in the domes had completely disappeared. Zinc staining in layer Ia and the funnels virtually disappeared after postnatal day 28. A transient expression of zinc is reported in at least one other cortical area (layer IV of barrel cortex from postnatal day 5 to postnatal day 14, maximal at postnatal days 9-11). We conclude that the transient expression of zinc can occur in both limbic and sensory areas, and that down-regulation of zinc in cortical modules might be related to synaptic plasticity and remodeling during development.

Strong expression of NETRIN-G2 in the monkey claustrum

The claustrum is a phylogenetically conserved structure, with extensive reciprocal connections with cortical regions, and has thus been considered important for sensory, motor, emotional, and mnemonic coordination or integration. Here, we show by in situ hybridization that the adult monkey claustrum is strongly positive for NETRIN-G2, a gene encoding a glycosyl phosphatidyl-inositol-linked membrane protein, which constitutes a subfamily with NETRIN-G1 within the netrin/UNC6 family. There is a conspicuous dorsal/ventral differentiation, where the label is stronger in the ventral claustrum. NETRIN-G2 positive neurons are not GABAergic, but rather correspond to claustrocortical projection neurons, as demonstrated by retrograde transport of Fast Blue from cortical injections and by double in situ hybridization for NETRIN-G2 and GAD67. Since NETRIN-G2 is known to be preferentially expressed in cortex, in contrast with the thalamically expressed NETRIN-G1, these results raise the possibility of some functional similarity in regulation of excitatory neural transmission in the claustrum and cortex.

A voltage-gated potassium channel, Kv3.1b, is expressed by a subpopulation of large pyramidal neurons in layer 5 of the macaque monkey cortex

In the cerebral cortex, the voltage-gated potassium channel, Kv3.1b, a splicing variant of Kv3.1, has been associated with fast-firing interneurons. Here, we report strong expression of Kv3.1b-protein and mRNA in both Betz and Meynert pyramidal cells of the monkey, as shown by immunohistochemistry and in situ hybridization. Strong expression also occurs in large pyramidal neurons in layer 5 of several cortical areas. In addition, most of these Betz and layer 5 pyramids, and about 10% of the labeled Meynert cells weakly co-expressed the calcium binding protein parvalbumin. Electron microscopy shows that the expression of Kv3.1b is localized to the somal and proximal dendritic cytoplasmic membrane, as expected for a channel protein. These results suggest that some large pyramidal neurons may constitute a functional subpopulation, with a distinctive distribution of voltage-gated potassium channels capable of influencing their repetitive firing properties.

Intrinsic collaterals of layer 6 Meynert cells and functional columns in primate V1

Meynert cells are a distinct type of large neuron which project to area MT/V5 and to subcortical targets, including the superior colliculus. They have recently been shown to have extensive intrinsic collaterals spreading up to 8.0 mm within layer 6 of area V1 [J Comp Neurol 441 (2001) 134]. Using intrinsic signal imaging combined with tracer injections, this study investigates how Meynert cell collaterals are mapped in relation to the functional architecture of area V1 in macaque monkeys. In particular, we examined whether terminations of individual axon segments are selective for same-eye or opposite-eye domains. Analysis of 39 anterogradely labeled axon segments (from six injection sites in four hemispheres) showed that terminal segments cross over several pairs of ocular dominance columns (ODCs) and contact both left- and right-eye ODCs, with a slight bias for the contralateral eye. This contrasts with the same-eye bias previously reported for intrinsic collaterals of pyramidal neurons in layer 3. The suggestion is that the system of Meynert intrinsic collaterals is involved with binocular interactions over wide sectors of the visual field. This might be related to processes such as optic flow or, especially given the wide-field spread, even contour completion or interpolation.

Axon collaterals of Meynert cells diverge over large portions of area V1 in the macaque monkey

Patchy intrinsic connections, originating mainly from horizontal collaterals of pyramidal neurons, have been demonstrated in area V1 and many other cortical areas. In this article, we identify a network of intrinsic connections concentrated in layer 6 of area V1. These are visualized by extracellular injections of anterograde tracers in V1, which label small clusters of large terminal boutons in layer 6, in conjunction with thick axon segments. These segments can be traced back to infragranular Meynert cells (n = 10), which are retrogradely labeled from the injections. By using serial section analysis, we identified the following features of this distinctive system of Meynert cell collaterals: (1) terminal clusters are relatively small (<100 microm); (2) each cluster has a small number of rather large boutons (up to 3.0 microm); (3) there is typically a termination-free zone in the immediate vicinity (0.5-2.0 mm) of the cell body; (4) a single neuron has multiple branches that can extend up to 8.0 mm from the soma; and (5) the collaterals are concentrated in layer 6. These features are different from those of horizontal intrinsic connections in the supragranular layers of area V1. They are consistent with fast dynamics and a possible role in wide-field motion processing, such as has been associated with Meynert cells from other studies.

Modular organization of the monkey presubiculum

There have been previous reports of somatostatin- and acetylcholinesterase (AChE)-positive patches in the superficial layers of the presubiculum in monkeys. In this study, we show additional instances of patches in the presubiculum, as demonstrated by cytochrome oxidase (CO), by myelin and Nissl stains, and by the calcium-binding proteins calbindin (CB), calretinin (CR), and parvalbumin (PV). Markers are differentially expressed along the lateral and longitudinal axes. Comparisons of adjacent sections reacted for different markers suggest that the CB+ and CR+ patches, and CO+ and AChE+ patches generally correspond, but not the CB+ and CO+ patches. In cross section, patches are about 100-300 microm in width. Sections cut tangentially through the superficial layers indicate that the pattern of CO and AChE labeling is in fact patchlike in layer I, but that the apparent patches in layer II (CB and CR) form a reticular or lattice-like network. As patches are restricted to the presubiculum, these labeling patterns provide a convenient marker for the boundary between the presubiculum and the adjoining posteroventral retrosplenial cortex. More work is necessary to determine how this modularity may relate to the functional organization of the presubiculum.

A transient deficit of motion perception in human

We studied the motion perception abilities in a young adult, SF, who had her right occipito-temporal cortices resected to treat epilepsy. Following resection, SF showed transient deficits of both first- and second-order motion perception that recovered to normal within weeks. Previous human studies have shown either first- or second n order motion deficits that have lasted months or years after cerebral damage. SF also showed a transient defect in processing of shape-from-motion with normal perception of shape from non-motion cues. Furthermore, she showed greatly increased reaction times for a mental rotation task, but not for a lexical decision task. The nature and quick recovery of the deficits in SF resembles the transient motion perception deficit observed in monkey following ibotenic acid lesions, and provides additional evidence that humans possess specialized cortical areas subserving similar motion perception functions.

Inferior parietal lobule projections to the presubiculum and neighboring ventromedial temporal cortical areas

The entorhinal and perirhinal cortices have long been accorded a special role in the communications between neocortical areas and the hippocampal formation. Less attention has been paid to the presubiculum, which, however, is also a component of the parahippocampal gyrus, receives dense inputs from several cortical areas, and itself is a major source of connections to the entorhinal cortex (EC). In part of a closer investigation of corticohippocampal systems, the authors applied single-axon analysis to the connections from the inferior parietal lobule (IPL) to the presubiculum. One major result from this approach was the finding that many of these axons (at least 10 of 14) branch beyond the presubiculum. For 4 axons, branches were followed to area TF and to the border between the perirhinal and entorhinal cortices, raising the suggestion that these areas, which sometimes are viewed as serial stages, are tightly interconnected. In addition, the current data identify several features of presubicular organization that may be relevant to its functional role in visuospatial or memory processes: 1) Terminations from the IPL, as previously reported for prefrontal connections (Goldman-Rakic et al. [1984] Neuroscience 12:719-743), form two to four patches in the superficial layers. These align in stripes, but only for short distances ( approximately 1.5 mm). This pattern suggests a strong compartmentalization in layers I and II that is also indicated by cytochrome oxidase and other markers. 2) Connections tend to be bistratified, terminating in layers I-II and deeper in layer III. 3) Single axons terminate in layer I alone or in different combinations of layers. This may imply some heterogeneity of subtypes. 4) Individual axons, both ipsilateral projecting (n = 14 axons) and contralateral projecting (n = 6 axons), tend to have large arbors (0.3-0.8 mm across). Finally, the authors observe that projections from the IPL, except for its anteriormost portion, converge at the perirhinal-entorhinal border around the posterior tip of the rhinal sulcus. These projections partially overlap with projections from ventromedial areas TE and TF, and this convergence may contribute to the severe deficits in visual recognition memory resulting from ablations of rhinal cortex.

Feedback connections from area MT of the squirrel monkey to areas V1 and V2

Area MT/V5 is reciprocally connected with both V1 and V2; but, despite extensive anatomical and physiological investigations, detailed information on the feedback component of these connections is still not available. The present report uses serial section reconstruction of single axons, labeled by anterograde tracers injected in area MT of squirrel monkeys, to characterize these connections further. As with other feedback systems, MT axons terminating in both areas V1 (n = 9) and V2 (n = 6) are widely divergent. In area V1, MT fields are larger than those from V2 and are about comparable to those from V4 or TEO. Terminations in V1, unlike other feedback connections described so far, terminate in several laminar combinations: only layer 1 (n = 2); only layer 4B (n = 3); layers 1 and 4B (n = 1); and layers 1, 4B, and 6 (n = 3). In V2, they occur mainly in layers 1 and 5 or 6. Terminations have two patterns even within a single axon: strung along collateral segments and grouped within small clusters. There are no apparent differences in the size, shape, or density of terminal specializations in V1 or V2, and, consistently with previous double-labeling experiments (Kennedy and Bullier [1985] J Neurosci 5:2815-2830), some axons can branch to both areas. This result, along with the laminar evidence for subtypes of feedback connections, argues against an exclusively hierarchical organization based on "pairwise" connectivity. For V1 and MT, there may be directly reciprocal loops between feedforward and feedback projecting neurons, but this is less likely to be so for V2 and MT.

Parvalbumin immunoreactive Cajal-Retzius and non-Cajal-Retzius neurons in layer I of different cortical regions of human newborn

Neurons of layer I play an important role in the development of the basic structural and functional organization of the mammalian cerebral cortex. Basic data, however, concerning the spatial and temporal distribution of the neuron populations in layer I are still limited, especially for human material. The present study investigates the distribution of Cajal-Retzius (CR) and non Cajal-Retzius (NCR) neurons in thirteen cortical areas in the newborn human in terms of their relative density and possible subtypes. Neuronal populations were identified by immunohistochemistry for parvalbumin. Three main results are reported. First, parvalbumin-immunoreactive (Parv-ir) CR cells were observed in all of the neocortical areas examined. These areas also had a Parv-ir horizontal fiber plexus in deep layer I, confirming to the horizontal plexus classically associated with CR neurons. Second, many Parv-ir CR cells showed clear signs of degeneration. Third, in addition to the large CR cells, smaller Parv-ir NCR neurons occurred in many of the neocortical areas examined. These were morphologically heterogeneous and may represent several subtypes. By sampling across several areas, we were able to establish that these NCR cells occurred at higher density in primary sensory areas 3, 1, 17, and 41. Because of this variability in density of Parv-ir NCR cells, the ratio of Parv-ir CR to Parv-ir NCR cells is selectively lower in primary sensory areas. Recent investigations in somatosensory cortex of early postnatal rat report complex spatiotemporal patterns of correlated spontaneous activity among neurons in layer I (Schwartz et al. 1998). An interesting possibility is that regional variability in this activity may play a major role in the organization of cortical circuitry in different areas.

The distribution of neuronal nitric oxide synthase in the nucleus tractus solitarii of the squirrel monkey

The distribution of neuronal nitric oxide synthase (nNOS) containing neurons and fibers in subnuclei of the nucleus tractus solitarii (NTS) in the squirrel monkey, Saimuri sciureus, was investigated by nNOS immunohistochemistry and nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry. Generally, the staining pattern of nNOS and NADPH-diaphorase in the NTS was similar. A high density of neurons and fibers exhibiting both nNOS immunoreactivity and NADPH-diaphorase reactivity was present in the central, medial, intermediate, and dorsolateral subnuclei of the NTS. A moderate density of neurons and fibers that stained for both nNOS and NADPH-diaphorase was noted in the interstitial and ventromedial subnuclei. The gelatinosus and commissural subnuclei contained a low density of neurons and fibers exhibiting nNOS immunoreactivity and NADPH-diaphorase staining. The dorsal motor nucleus of vagus contained a high density of nNOS immunopositive and NADPH-diaphorase containing neurons and fibers at the rostral level, but contained a moderate density of positive fibers and very few positive neurons at the intermediate, subpostremal and commissural NTS levels. Incongruence was noted, however, between nNOS immunostaining and NADPH-diaphorase staining in blood vessels in the brainstem. Capillaries and small vessels exhibited strong staining for NADPH-diaphorase but no nNOS immunoreactivity. In summary, this work substantiates the presence of nNOS in subnuclei of the monkey NTS and is consistent with a role for NO(.) in neurotransmission in primate NTS.

Localization of area prostriata and its projection to the cingulate motor cortex in the rhesus monkey

Area prostriata is a poorly understood cortical area located in the anterior portion of the calcarine sulcus. It has attracted interest as a separate visual area and progenitor for the cortex of this modality. In this report we describe a direct projection from area prostriata to the rostral cingulate motor cortex (M3) that forms the fundus and lower bank of the anterior part of the cingulate sulcus. Injections of retrograde tracers in M3 resulted in labeled neurons in layers III, V and VI of prostriate cortex. However, injections of anterograde tracers in M3 did not demonstrate axon terminals in area prostriata. This connection was organized topographically such that the rostral part of M3 received input from the dorsal region of prostriate cortex, whereas middle and caudal levels of M3 received input from more ventral locations. Injections of retrograde and anterograde tracers in the caudal cingulate motor cortex (M4) did not produce labeling in prostriate cortex. Cytoarchitectural analysis confirmed the identity of area prostriata and further clarified its extent and borders with the parasubiculum of the hippocampal formation rostrally, and V1 of the visual cortex caudally. This linkage between cortex bordering V1 and cortex giving rise to a component of the corticofacial and corticospinal pathways demonstrates a more direct visuomotor route than visual association projections coursing laterally.

Single axon analysis of pulvinocortical connections to several visual areas in the macaque

The pulvinar nucleus is a major source of input to visual cortical areas, but many important facts are still unknown concerning the organization of pulvinocortical (PC) connections and their possible interactions with other connectional systems. In order to address some of these questions, we labeled PC connections by extracellular injections of biotinylated dextran amine into the lateral pulvinar of two monkeys, and analyzed 25 individual axons in several extrastriate areas by serial section reconstruction. This approach yielded four results: (1) in all extrastriate areas examined (V2, V3, V4, and middle temporal area [MT]/V5), PC axons consistently have 2-6 multiple, spatially distributed arbors; (2) in each area, there is a small number of larger caliber axons, possibly originating from a subpopulation of calbindin-positive giant projection neurons in the pulvinar; (3) as previously reported by others, most terminations in extrastriate areas are concentrated in layer 3, but they can occur in other layers (layers 4,5,6, and, occasionally, layer 1) as collaterals of a single axon; in addition, (4) the size of individual arbors and of the terminal field as a whole varies with cortical area. In areas V2 and V3, there is typically a single principal arbor (0.25-0.50 mm in diameter) and several smaller arbors. In area V4, the principal arbor is larger (2.0- to 2.5-mm-wide), but in area MT/V5, the arbors tend to be smaller (0.15 mm in diameter). Size differences might result from specializations of the target areas, or may be more related to the particular injection site and how this projects to individual cortical areas. Feedforward cortical axons, except in area V2, have multiple arbors, but these do not show any obvious size progression. Thus, in areas V2, V3, and especially V4, PC fields are larger than those of cortical axons, but in MT/V5 they are smaller. Terminal specializations of PC connections tend to be larger than those of corticocortical, but the projection foci are less dense. Further work is necessary to determine the differential interactions within and between systems, and how these might result in the complex patterns of suppression and enhancement, postulated as gating mechanisms in cortical attentional effects, or in different states of arousal.

Some temporal and parietal cortical connections converge in CA1 of the primate hippocampus

Large sectors of polymodal cortex project to the hippocampal formation via convergent input to the entorhinal cortex. The present study reports an additional access route, whereby several cortical areas project directly to CA1. These are parietal areas 7a and 7b, area TF medial to the occipitotemporal sulcus (OTS), and a restricted area in the lateral bank of the OTS that may be part of ventromedial area TE. These particular cortical areas are implicated in visuospatial processes; and their projection to and convergence within CA1 may be significant for the elaboration of 'view fields', for the postulated role of the hippocampal formation in topographic learning and memory, or for the snapshot identification of objects in the setting of complex visuospatial relationships. Convergence of vestibular and visual inputs (from areas 7b and 7a respectively) would support previous physiological findings that hippocampal neurons respond to combinations of whole-body motion and a view of the environment. The direct corticohippocampal connections are widely divergent, especially those from the temporal areas, which extend over much of the anteroposterior axis of the hippocampal main body. Divergent connections potentially influence large populations of CA1 pyramidal neurons, consistent with the suggestion that these neurons are involved in conjunctive coding. The same region of ventromedial TE, besides the direct connections to CA1, also gives rise to direct projections to area V1, and may correspond to a functionally specialized subdivision, perhaps part of VTF.

The connection from cortical area V1 to V5: a light and electron microscopic study

Area V5 (middle temporal) in the superior temporal sulcus of macaque receives a direct projection from the primary visual cortex (V1). By injecting anterograde tracers (biotinylated dextran and Phaseolus vulgaris lectin) into V1, we have examined the synaptic boutons that they form in V5 in the electron microscope. Nearly 80% of the target cells in V5 were spiny (excitatory). The boutons formed asymmetric (Gray's type 1) synapses with spines (54%), dendrites (33%), and somata (13%). All somatic targets and some (26%) of the target dendritic shafts showed features characteristic of smooth (inhibitory) cells. Each bouton formed, on average, 1.7 synapses. The larger boutons formed multiple synapses with the same neuron and completely enveloped the entire spine head. On most dendritic shafts and all somata the postsynaptic density en face was disk-shaped but in about half the cases the reconstructed postsynaptic densities of synapses on spines appeared as complete or partial annuli. Even in the zones of densest innervation only 3% of the asymmetric synapses were formed by the labeled boutons. Although the V1 projection forms only a small minority of synapses in V5, its affect could be considerably amplified by local circuits in V5, in a way analogous to the amplification of the small thalamic input to area V1.

Complex microstructures of sensory cortical connections

The neocortex has a distinctive laminar and modular organization. Although important questions remain regarding structure and function at this level of organization, recent studies are addressing a finer scale of synaptic and network microstructure. New findings concerning network properties are rapidly emerging from approaches in which dual or triple intracellular recordings in vitro are combined with analyses of cell and synaptic morphology, as well as from experiments designed to label multiple cell populations.

Convergence and branching patterns of round, type 2 corticopulvinar axons

Corticopulvinar connections consist of at least two morphologically distinct subpopulations. In one subgroup (E, type 1), axons have an "elongated" terminal field and thin, spinous terminations; in the other (R, type 2), axons have a small, round arbor and large, beaded terminations. Previous work (Rockland, 1996) indicates that E-type axons from several occipitotemporal areas branch extensively within and sometimes between pulvinar subdivisions, but that R-type axons tend to have spatially delimited arbors. The present report is a further investigation of R-type axons from areas V1 and MT and was initiated to test the generality of the previous findings. There are four main results: 1) By serial section reconstruction of anterogradely labeled axons, 10 of 25 axons originating in area V1 had two or three spatially separate arbors (8 and 2 axons, respectively). Sixteen axons analyzed from area MT, however, all had single arbors, although the arbors were often formed by the convergence of widely separate branches. 2) Multiple (at least 2-5) R-type corticopulvinar axons, from V1 or from MT, can converge in a single focus. 3) R-type axons originating from both areas V1 and MT can branch to other structures; namely, the superior colliculus, the pretectal area, and/or the reticular nucleus of the thalamus. 4) Finally, corticopulvinar terminations from area V1 are predominantly R-type, whereas those from MT are more predominantly E-type. These results thus provide additional evidence of the special relationship of area V1 to the pulvinar. They also emphasize that the idea of corticopulvinocortical "feedback loops," although convenient as a shorthand nomenclature, does not adequately convey the full complexity of the system.

Divergent cortical connections to entorhinal cortex from area TF in the macaque

The entorhinal cortex (EC) is an important component of the medial temporal lobe memory system in the primate and is often viewed as a "gatekeeper" area that passes on highly convergent cortical inputs toward the hippocampus. Further analysis of these connections at a microcircuitry level regarding the actual size and shape of arbors and terminations is not yet available, but may contribute to understanding the role of the EC in memory or other functions. The main emphasis of this report was on serial section analysis of anterogradely labeled axons that project from area TF (lateral parahippocampal cortex; Bonin and Bailey, 1947) to the EC (n = 12). By way of evaluating network organization, other projections from area TF--to TH (in the medial parahippocampal gyrus; n = 5) and to posterior visual areas (n = 3)--were also investigated. All three systems were found to terminate heavily in layer 1, as expected from previous investigations, but some terminations were verified in layer 6 of the EC as well. This technique further demonstrated that terminal fields are widely divergent and elongated. In the EC, terminal fields extended over 6-11 mm and spanned multiple cell islands and interislands. These axons resemble "feedback" cortical connections by virtue of their layer 1 terminations and their markedly divergent geometry, but not by their origin from layer 3. Spatially extended terminal fields recall the nontopographic, distributed character of olfactory connections and raise questions of how these features might be related to the memory functions attributed to medial temporal regions.

Collateralized divergent feedback connections that target multiple cortical areas

Nonreciprocal feedback connections from ventromedial areas TE and TF have previously been reported to visual areas V1 and V2 (Kennedy and Bullier, 1985; Rockland and Van Hoesen, 1994). The present report confirms these earlier observations by utilizing anterograde label in conjunction with serial section analysis. Furthermore, it directly demonstrates the divergent configuration and range of these terminal fields. Thirteen axons were analyzed from ventromedial TE (4) or area TF (9) to occipitotemporal areas, and two from area TF to the upper bank of the intraparietal sulcus (IPS). All these axons have narrow, elongated fields that range from 4.0-21.0 mm. Terminations are distributed linearly along the axon or, in some cases, concentrated in irregularly spaced clusters. Most of these axons have terminations concentrated in layer 1. The two axons in the IPS have a bistratified terminal distribution (in layers 1-3 and 6) in their anterior field, but a distinctly different laminar pattern (with terminations concentrated in layer 1) in their distal 2.0 mm. These fields probably correspond to different areas, most likely MIP and PO. Axons projecting from higher order to early visual areas may contribute to extraperceptual, complex processes within area V1, such as activation in response to visual imagery, and are a possible substrate for synchronous linkage of spatially discrete assemblies of neurons. In summary, these results demonstrate 1) that some neurons in ventromedial TE and TF are in direct communication with early visual areas, including V1 and V2, and 2) that some feedback axons target several areas, sometimes with different laminar termination patterns. These results emphasize that cortical areas are interrelated by multiple direct and indirect pathways, not all of which are strictly hierarchical.

Two types of corticopulvinar terminations: round (type 2) and elongate (type 1)

Corticopulvinar axons were anterogradely labeled by Phaseolus vulgaris-leucoagglutinin injections in the occipitotemporal cortex of the macaque to determine quantitative parameters of divergence and convergence, arbor size and shape, and distribution of terminal specializations. Forty individual axons were analyzed by serial section reconstruction and divided into two major groups. The majority of axons have numerous (typically 500-1,000) small, spinous endings (boutons terminaux). These axons have terminal fields that are beam-like or elongated (E, corresponding to classical type 1) and highly divergent (1.0-3.0 mm). These frequently innervate several of the traditionally designated pulvinar subdivisions; namely inferior pulvinar (PI) and the ventral part of interal pulvinar (PL); medial pulvinar (PM) and dorsal PL, and (one axon) PM, dorsal PL, and PI. Some axons, however (R or round, corresponding to classical type 2), have a small number (typically 70-160) of primarily large, beaded endings (boutons en passant), which concentrate in sharply delimited, round arbors (diameters 100-125 microns). R axons appear to be larger caliber than E axons (1.0-1.5 microns vs. 0.5-1.0 micron, respectively). These differences in phenotype are probably associated with distinct types of projection neurons. In visual areas, corticopulvinar terminations are reported to originate from pyramidal cell subpopulations in layer 5. Indirect evidence, presented here, suggests that the more numerous medium-sized neurons give rise to E axons, and the sparser giant pyramids give rise to R corticopulvinar axons. If this is correct, corticopulvinar connectivity may be involved in multiple transformations. Spatially, axons of giant neurons (with basal dendrites that collect intracortically from a disc-like area, about 1.0 mm in diameter) converge onto a small number of pulvinar neurons. Axons of medium neurons (with basal dendrites that occupy a small intracortical disc, about 0.3 mm in diameter) diverge over 1.0-3.0 mm in the pulvinar and may form many contacts. Giant neurons, although numerically few in relation to medium pyramids (1 or 2: 50?), are likely to have distinctive membrane properties (functionally equivalent to bursting neurons?). Their larger boutons and axon caliber may be associated with a faster transmission that compensates for their small numbers. In primates, the E and R duality does not characterize cortical projections to the caudate, lateral geniculate nucleus, pons, or superior colliculus and thus may be essentially linked to pulvinar-specific processes.

Morphology of individual axons projecting from area V2 to MT in the macaque

Efferent axons from area V2 to the middle temporal area (MT) were anterogradely labeled by Phaseolus vulgaris-leucoagglutinin (PHA-L) or biocytin and analyzed in serial reconstructions. Five of seven reconstructed axons had three arbors (each < or = 200 microns in diameter) in layers 3-4, separated by 200-600 microns. Two axons terminated in what was apparently a single focus in layers 3-4. Of 15 additional single arbors analyzed, 12 were concentrated in layers 3-4, and measured 200-250 microns across at their widest point. Three of these arbors were more columnar in shape (about 400 microns in diameter), and extended from layer 4 toward layer 1. This system differs in several features from MT-projecting axons originating from V1. Namely, V2 axons terminating in MT are thinner (approximately 1.0 microns vs. 3.0 microns), their terminal specializations are more delicate, and their arbors are concentrated in layer 4 and overlying layer 3, with no collaterals to layer 6. These differences may reflect the distinctive neuronal populations giving rise to these two connectional systems (different sizes of pyramidal neurons in layer 3 of V2, and a mix of pyramidal and spiny stellate cells in area V1). Differences may have implications for timing factors; that is, impulses from V1, subserved by large-caliber axons, may arrive in MT coincidentally with indirect connections via V2 to MT. Another consideration may be the functional architecture of MT. Regularly spaced clusters of neurons have been described in MT which have similar directionality preferences. The interarbor spacing of cortical efferents is consistent with a columnar organization, but the laminar specificity may indicate recruitment of different combinations of postsynaptic populations by V1 or V2 terminations.

Further evidence for two types of corticopulvinar neurons

Retrograde tracing experiments suggest that corticopulvinar connections originate from at least two subpopulations: medium and giant pyramids in layer 5, each with distinctive dendritic and local axonal arborizations. The present study used extracellular injections of PHA-L to delineate extrinsic axon arbors and to assess the possibility of categories that might correlate with these specific neuronal subpopulations. Two distinct types of terminations were found. Type I have smaller caliber preterminal axons, are generally elongate, and are studded with spinelike terminal specializations. Type II have larger axons (> 1.0 microns), spherical arbors, and mainly beaded specializations, some of which are conspicuously large (approximately 3.0 microns). These two axon types may differ in conduction velocity and membrane properties, and the interactions of these different features may be important for pulvinar function.

Direct temporal-occipital feedback connections to striate cortex (V1) in the macaque monkey

Although there have been reports of sparse projections from temporal areas TE, TF, and even TH to area V1, it is generally believed that cortical afferents to V1 originate exclusively from prestriate areas. Injections of anterograde tracers in anterior occipital and temporal areas, however, consistently produce labeled terminals in area V1. In order to confirm these results and display the full range of foci projecting to V1, we injected V1 in two monkeys with the retrograde tracer fast blue. Feedback connections were found, as expected, from several prestriate areas (V2, V3, V4, and MT). These originate from neurons in layers 3A and 6. Connections were also found from several more distal regions, namely, areas TEO, TE, TF, TH, and from cortex in the occipitotemporal and superior temporal (STS) sulci. Filled neurons occurred in two small foci in the caudal intraparietal sulcus. These more distal feedback connections tend to originate only from layer 6. An additional injection of the retrograde tracer diamidino yellow in area V2 of one animal revealed a similarly widespread network of feedback connections. In some areas (in the STS and in TEO), 10-15% of fluorescent neurons were double-labeled. These results indicate that feedback connections to early visual cortex derive from a widespread network of areas, including limbic-associated cortices. These connectional patterns testify to the massive recursiveness of anatomical pathways. As there are no reports of projections from V1 to anterior temporal cortices, our results also indicate that some cortical feedback connections may not be strictly reciprocal.

Divergent feedback connections from areas V4 and TEO in the macaque

Extrastriate areas TEO and V4 have been associated with form and color vision. Area V4 has also been suggested to participate in processes concerned with attention, stimulus salience, and perceptual learning. In a continuing effort to elucidate the connectional interactions and microcircuitry of these areas, we describe in this report the pattern of feedback connections from TEO and V4. Connections were demonstrated by injections of the high-resolution anterograde tracers PHA-L or biocytin and further analyzed by reconstruction of 25 individual axons through serial sections. This analysis yielded several new results: (1) Both areas TEO and V4 have widespread feedback connections (defined by their preferential termination in layer 1 and avoidance of layer 4). From TEO, there are dense projections to area V4 and moderate ones to V2 and V1. From V4, there are dense projections to V2 and moderate ones to V3 and V1. (2) Terminal fields span large territories in area V1, up to 6.0 mm in the case of axons originating from TEO; up to 5.0 mm in the case of axons originating from V4. In V2, fields tend to be smaller, between 3.0-5.0 mm. (3) Many axons from TEO and some from V4 have terminations in both areas V1 and V2. (4) Because individual terminal clusters and segments are often larger than cytochrome oxidase compartments, especially in V1, we suggest they may not be correlated with this compartmental organization. These results are consistent with the hypothesis that feedback connections may contribute to processes other than perceptual discrimination.

Specific and columnar projection from area TEO to TE in the macaque inferotemporal cortex

The organization of connections from area TEO to TE was studied by the use of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L). Single or, in one case, two small injections of the tracer were made in the lateral part of TEO. Labeled terminals were found in restricted regions of TE within two to five dense foci. In these foci, terminals were distributed beyond the middle layers to form a columnar cluster elongated vertical to the cortical surface. This specificity of connections may derive from an organization that is feature specific rather than retinotopic. Single axons reconstructed from the columnar foci showed heterogeneous features in the laminar distribution. Arbors of some axons were localized in the superficial layers, and those of other were in the middle layers.

Configuration, in serial reconstruction, of individual axons projecting from area V2 to V4 in the macaque monkey

The detailed morphology of long extrinsically projecting axons in the neocortex has been difficult to investigate and is in fact poorly understood. Some data, based on extracellular injections of Phaseolus vulgaris leucoagglutinin (PHA-L), are available for individual axons projecting from area V1 to area V2 or MT. Like geniculocortical projections, axons projecting from area V1 to area MT are readily identifiable (they typically have a bistratified termination pattern and large terminal specializations and are of large caliber), but those projecting from area V1 to V2 are more variable. To provide a broader basis for interpreting constant and variable features of axon morphology, we used high-resolution serial section reconstruction to analyze small populations of PHA-L-labeled axons projecting from area V2 to V4. Reconstruction of 20 axons suggests that this system is variable in terms of overall configuration and laminar distribution. Most terminal arbors are located at the border between layers 3 and 4, but some remain entirely within layer 3 or 4, some target preferentially the superficial layers (1, 2, and 3A), and some have collaterals in layer 5 or, rarely, layer 6. Arbor size is fairly constant among the three visual cortical projections examined so far (typically about 200 microns in diameter). In area V4, however, axons frequently have three or four separate arbors, which branch divergently (in one instance, over 2.6 mm x 3.0 mm). These features may be correlated with aspects of the particular functional organization of area V4, such as coarse topography, large receptive field size, and modularity. Axonal variability may also denote differences, morphological or physiological, among neurons of origin in area V2.

Laminar distribution of neurons projecting from area V1 to V2 in macaque and squirrel monkeys

The present study reevaluates the sublaminar distribution and cellular morphology of neurons projecting from area V1 to V2. Observations are based on retrogradely transported HRP, Phaseolus vulgaris leucoagglutinin (PHA-L), or biocytin after injections made in area V2 of three squirrel monkeys and eight macaques. With material prepared in the coronal or horizontal tissue planes, it is clear that projection neurons in V1, in both species, are concentrated in layer 4B and in a single band (150-250 microns wide) restricted to the upper subdivision of layer 3 (layer 3A). There are also labeled neurons, but fewer in number, in layers 3B and 4A, and occasionally in layers 2 and 5. Golgi-like labeling from PHA-L or biocytin confirmed that most of the projection neurons in layer 3A are pyramidal. As reported for several other corticocortical systems, these pyramidal neurons differ in soma size, soma shape, and dendritic geometry. These results emphasize the complex organization of layer 3, and the distributed nature of efferent projections from area V1. Given the selective connectivity of vertical interlaminar networks, these results specifically suggest that information transmitted to area V2 from neurons in layer 3A reflects more highly processed, convergent input than that originating from either layer 3B or 4B.

Organization of individual cortical axons projecting from area V1 (area 17) to V2 (area 18) in the macaque monkey

The present study uses the anterograde tracer, Phaseolus vulgaris-leucoagglutinin (PHA-L), to investigate the detailed morphology of individual axons projecting from area V1 to prestriate area V2. Observations are derived from serial reconstructions of 45 axons. Axons are found to differ both in laminar distribution and in arbor size. The majority (25/45; 56%) terminate in the upper half of layer 4 and the lower part of layer 3. Terminal clusters typically measure about 200 microns in diameter (dimensions are uncorrected for shrinkage), and are either in one, two, or occasionally three patches. Patches are separated by 200-500 microns. Of these 25 axons, four also have minor collaterals to layer 5. Of the remaining 20 axons in our sample, eight have one or two terminal arbors (about 200 microns in diameter) mainly in layer 3; another eight have terminations, organized as a single field (about 350 microns in diameter), within layer 4; and four axons have much larger terminal fields (1.0-1.2 mm x 0.3 mm), in layers 3 and 4. These morphological differences might constitute a gradient or, alternately, indicate distinct subgroups within the striate efferent population. Large terminal fields are asymmetrical, with their long axis oriented in an anterior-posterior fashion toward the depth of the lunate sulcus. Axons with two terminal arbors have a similar bias. As this arrangement is approximately perpendicular to the border of V1, we suggest that striate axons may be extended preferentially along the length of the stripelike compartments in V2. These compartments are also arrayed perpendicular to the border between areas V1 and V2. Reconstruction of small groups of 2-4 convergent axons demonstrates that axons with different morphology (i.e. large or small terminal fields) can occur within the same projection focus. Terminal arbors belonging to different axons can overlap, but tend not to be superimposed exactly.

Bistratified distribution of terminal arbors of individual axons projecting from area V1 to middle temporal area (MT) in the macaque monkey

In the present study, the anterograde tracer Phaseolus vulgaris-leucoagglutinin was injected into area V1 in order to demonstrate the detailed morphology of individual axons terminating in prestriate area MT. On the basis of 24 axon reconstructions, several representative (but not necessarily comprehensive) characteristics have been identified: (1) Most axons arborize in a patchy manner over a widespread territory, frequently greater than 1.0 mm and often up to 1.5 x 1.8 mm (dimensions uncorrected for shrinkage). (2) Terminal arbors are distributed to layers 3, 4, and 6. Those in layer 6 need not be in register with those in the upper layers. (3) Number and size of terminal arbors are variable. One axon may have 1-4 arbors in the middle layers; typically at least one of these will have a diameter of 200-250 microns, while the others may be less developed. There are from 1-3 arbors in layer 6, usually 50 microns (but sometimes up to 100 microns) in diameter. (4) Terminal boutons are of mixed morphology, but usually beaded and large (up to 3.0 microns). (5) In the white matter, many axons travel in the external sagittal stratum but some are part of the U-fiber system. Axons commonly branch, sometimes at depths up to 0.75-1.0 mm, below the gray matter of MT. In summary, these axons are not stereotyped, but rather vary in the number and size of their terminal arbors, as well as in their branching and overall geometry. Connections from area V1 to MT have been associated with the magnocellular-dominated "processing channel." As widespread arborizations and bistratified terminations are common to both striate axons in MT and to geniculocortical axons in layer 4C alpha of primary visual cortex, these features might be correlated with magnocellular-specific processing requirements.

Terminal arbors of individual "feedback" axons projecting from area V2 to V1 in the macaque monkey: a study using immunohistochemistry of anterogradely transported Phaseolus vulgaris-leucoagglutinin

In the present study, the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected into area V2 in order to demonstrate the precise morphology of individual axons from area V2 to V1. On the basis of 28 complete axon reconstructions, several characteristic features have been identified. 1) Individual axons arborize in multiple layers: 1, 2, 5, and (inconstantly) 3. A single axon may have numerous terminal clusters in layers 1 and 2, but at most one in layer 3. 2) Axons typically ascend to layer 1, turn asymmetrically in one direction, and travel for long distances in this layer (1.10-4.30 mm; dimensions uncorrected for shrinkage). A few axons (three of 28 reconstructed) were found to have a single terminal cluster (0.3-0.5 mm wide) in layers 1 and 2. 3) Collaterals in layer 5 seem to extend over shorter distances (0.60 mm or less). 4) Delicate sprays of boutons (both beads and spines) are clustered along the main trunk. Spacing is variable but usually ranges from 0.35 mm to 0.65 mm. 5) In addition to clustered boutons, there can be linear collaterals, continuously studded with boutons, parallel to the main axon in layer 1. These results indicate that axons from V2 have complex radial and tangential distributions in V1. Terminations in different layers may be directed to different sets of neurons or to different portions of the dendritic tree (for example, distal portions of pyramidal neuron apical dendrites in layers 1 and 2, but more proximal portions in layer 3). Clustered terminations over wide tangential areas may imply a divergent innervation by a single axon of multiple compartmental structures, such as ocular dominance columns or cytochrome oxidase patches.

Topography of occipital lobe commissural connections in the rhesus monkey

The organization of occipital lobe commissural connections is re-examined in the rhesus monkey by the autoradiographic technique. A general topographic order was observed in the splenium. Fibers from area 18 occupy its most caudal and ventral subdivision, while those from different parts of area 19 surround the area 18 zone rostrally and dorsally. Results also indicate, however, divergent trajectories within each compartment, as well as significant overlap at their borders.

Anatomical organization of primary visual cortex (area 17) in the ferret

The present report describes the intrinsic and extrinsic cortical connectivity of striate cortex (area 17) in the ferret. Injections of horseradish peroxidase demonstrate periodic intrinsic connections over an extent of 2.5-3.0 mm, mainly in the supragranular layers but also occurring secondarily in layer 5. These connections have a stripelike configuration, with a center-to-center spacing of 0.5-0.7 mm. Their laminar distribution and stripelike configuration resemble the pattern in the cat (Gilbert and Wiesel, '83), another member of the carnivore family, but not that in monkeys. In both macaque and squirrel monkeys, these connections have a bilaminar distribution in layers 2-3 and 4B, and a more complicated latticelike geometry (Rockland and Lund, '83). Their interperiod spacing, of about 0.5 mm, however, is relatively constant across species. Extrinsic connections in the ferret link striate cortex with territories probably homologous to feline areas 18 and 19, and to the suprasylvian region. Callosal connections extend on the lateral surface about 1.5 mm into area 17 and 4.0 mm into area 18 beyond their common border. There are homotopical connections between striate cortices and heterotopical connections from at least areas 18 and 19 to contralateral area 17. In addition to gray matter connections, intracortical injections also result in labeled interstitial neurons in the subgriseal white matter. These occur both subjacent to an injection site in area 17, and below labeled foci in area 18 projecting back to area 17, as if interstitial neurons shared the connectivity of overlying layer 6.

A reticular pattern of intrinsic connections in primate area V2 (area 18)

A system of periodic intrinsic connections is demonstrated in area V2 (area 18) of squirrel and macaque monkeys by large injections of tritiated amino acids, horseradish peroxidase (HRP), and fluorescent latex beads. These connections originate from pyramidal neurons concentrated in layers 3 and 5. Terminations occur in all cortical layers, largely coextensive with labeled neurons but more restricted in layer 4. This multilaminar distribution contrasts with the mainly supragranular localization of periodic intrinsic connections in V1 (area 17), and may imply a close interaction, in V2, of periodic intrinsic connections with pulvinocortical, as well as with corticocortical terminations (concentrated, respectively, in layers 3 and 5, and in lower 3 and 4). As in V1, the tangential configuration of these connections in V2 is reticular or latticelike, and is detectable for 2.5-3.0 mm from an injection site of HRP, 3H amino acids, or latex beads. Cross-sectional widths of labeled regions vary from 250 to 800 micron in squirrel monkey and from 400 to 1,000 micron in macaque, depending on which portion of the lattice is measured. When periodic intrinsic connections are compared with stripes labeled histochemically by cytochrome oxidase (CO), no clear relationship is obvious between the two systems. This result contrasts with the orderly tangential alignment reported between CO-reactive zones in V2 and certain extrinsic connections; namely, pulvinocortical terminations (Livingstone and Hubel, '82) and clusters of neurons projecting to area V4 (DeYoe and Van Essen, '84). Other extrinsic connections, however, such as backgoing connections from V2 to V1, do not seem to have a periodic distribution. Thus, although some discontinuous cortical connections relate to each other in a precise mosaic fashion, intrinsic and some extrinsic connections may observe different modes of organization.

Laminar distribution of alpha 1- and beta 1-adrenoceptors in ferret visual cortex

The distribution of alpha 1- and beta 1-adrenoceptors has been examined in the ferret visual cortex (area 17), with an autoradiographic procedure using iodine-125 labeled hydroxy-iodophenyl-ethylaminomethyl-tetralone (HEAT) and iodocyanopindolol (ICYP), respectively. The density of ligand binding with ICYP, known to have selective affinity for beta-receptors, was heavy over layers I-III, very low over layer IV, and medium over V and VI. In contrast, binding sites for HEAT, a new ligand selective for alpha-receptors, were diffusely distributed, although preferentially concentrated in layer IV and the upper layers. The distinct laminar distribution of these two receptor types may imply two cortical channels for norepinephrine influence; alpha 1-receptors may be preferentially associated with enhancement of excitatory inputs to layer IV; beta 1-receptors, in contrast, with enhancement of inhibitory responses outside layer IV.

The distribution of cholinesterase and cytochrome oxidase within the dorsal lateral geniculate nucleus of the squirrel monkey

Within the dorsal lateral geniculate nucleus (dLGN) of the squirrel monkey, acetylcholinesterase (AChE) and cytochrome oxidase (CO) are distributed preferentially in the magnocellular layers. Butyrylcholinesterase (BuChE) activity is comparatively weak, but higher in the parvocellular than in the magnocellular layers. The different patterns of distribution of AChE within the dLGN in different primate species signify differences in geniculate organization, which may reflect adaptation to different visual habitats.

Intrinsic laminar lattice connections in primate visual cortex

Intracortical injections of horseradish peroxidase (HRP) reveal a system of periodically organized intrinsic connections in primate striate cortex. In layers 2 and 3 these connections form a reticular or latticelike pattern, extending for about 1.5-2.0 mm around an injection. This connectional lattice is composed of HRP-labeled walls (350-450 microns apart Saimiri and about 500-600 microns in macaque) surrounding unlabeled central lacunae. Within the lattice walls there are regularly arranged punctate loci of particularly dense HRP label, appearing as isolated patches as the lattice wall labeling thins further from the injection site. A periodic organization has also been demonstrated for the intrinsic connections in layer 4B, which are apparently in register with the supragranular periodicities, although separated from these by a thin unlabeled region. The 4B lattice is particularly prominent in squirrel monkey, extending for 2-3 mm from an injection. In both layers, these intrinsic connections are demonstrated by orthogradely and retrogradely transported HRP and seem to reflect a system of neurons with long horizontal axon collaterals, presumably with arborizations at regularly spaced intervals. The intrinsic connectional lattice in layers 2 and 3 resembles the repetitive array of cytochrome oxidase activity in these layers; but despite similarities of dimension and pattern, the two systems do not appear identical. In primate, as previously described in tree shrews (Rockland et al., '82), the HRP-labeled anatomical connections resemble the pattern of 2-deoxyglucose accumulation resulting from stimulation with oriented lines, although the functional importance of these connections remains obscure.

Anatomical binding of intrinsic connections in striate cortex of tree shrews (Tupaia glis)

The intrinsic connectivity of striate cortex was investigated by injecting horseradish peroxidase (HRP) into this area in tree shrews. Such HRP injections demonstrated periodically organized, stripelike connections within area 17. These stripes occur in layers I-IIIA and consist of a small number or retrogradely filled neurons, some clearly pyramidal, together with HRP-labeled axon terminals. HRP-filled axons trunks run between labeled stripes, interconnecting adjacent and distant regions of the stripe pattern. Correlation with Golgi-stained tissue suggests that these stripes are horizontally interconnected by pyramidal neurons with long intracortical axon collaterals (followed for distances over 1 mm from the soma). The HRP-labeled strips measure about 230 micrometers in width, with a center-to-center repeat distance of 450--500 micrometers. They have been mapped over an 8 mm2 area of striate cortex and would thus seem capable of effecting lateral interactions over considerable portions of the retinotopic map. In their dimensions and overall pattern, these anatomical stripes resemble the 2-deoxyglucose (2-DG) bands resulting from visual stimulation of trees shrews with stripes of a single orientation. While the functional role of the HRP-labeled stripes is unclear, their similarities with the 2-DG pattern raise the intriguing possibility that they may be related to orientation selectivity. The striking regularity of these extensive lateral interconnections emphasizes the importance of horizontal intralaminar connections within the cortex.

Widespread periodic intrinsic connections in the tree shrew visual cortex

Intrinsic connections within the tree shrew (Tupaia glis) visual cortex (area 17) are organized in periodic stripelike patterns within layers I, II, and III. This anatomical network resembles the regularly organized stripes of 2-deoxyglucose accumulation seen after stimulation of alert animals with uniformly oriented lines. Such connections imply that widespread lateral interactions are superimposed on the retinotopic organization of area 17 and suggest alternative interpretations of cortical columns.

Cortical connections of the occipital lobe in the rhesus monkey: interconnections between areas 17, 18, 19 and the superior temporal sulcus

Using both anterograde and retrograde tracing techniques, the present report investigates the cortical connections of the lateral, median and ventral portions of areas 17 and 18 in the rhesus monkey. All parts of area 17 are found to send topographically organized connections to a strip of prestriate cortex which closely corresponds to area OB of Bonin and Bailey or area 18 of Vogt and Vogt. Striate-recipient area 18, in turn, is topographically connected with an anterior prestriate zone, whose borders coincide with those of area OA or 19. These efferents are topographically organized, with connections from the medial surface of area 18 directed to lateral parts of area 19. In addition, certain parts of area 18, in the annectent gyrus and the inferior occipital sulcus, send 'crossed', dorsoventral connections to ventral and dorsal parts of area 19, respectively. Both areas 17 and 18 project in a topographic fashion to a distinct region in the caudal part of the superior temporal sulcus. Topographically organized reciprocal connections are also found from area 18 to 17, from area 19 to 18, and from the superior temporal sulcus to both areas 17 and 18.

Laminar origins and terminations of cortical connections of the occipital lobe in the rhesus monkey

Cortical connections within the occipital lobe (areas 17, 18 and 19) of the rhesus monkey are investigated with the autoradiographic and horseradish peroxidase procedures. Two efferent systems, each with a specific laminar organization, are observed. (1) Rostrally directed connections, from area 17 to 18, area 18 to 19 and area 19 to the inferotemporal region (area TE), originate from neurons in layer IIIc (and, in area 19, from a small complement of neurons in layer Va), and terminate in and around layer IV. (2) In contrast, connections in the reverse direction ('caudally directed' connections), from area TE to 19, area 19 to 18, and area 18 to 17, originate from neurons in layers Vb, VI and, to a lesser extent, IIIa, and terminate mainly in layer I. In addition, the laminar organization of several intrinsic and callosal connections are observed. In trinsic connections within areas 18 and 19 originate from neurons in layers IIIc and, to a lesser extent, Va, and terminate in vertical bands in layers I to IV. Callosal connections from areas 18, 19, and the caudal inferotemporal region originate from neurons mainly in layer IIIc. From areas 18 and 19, these callosal connections terminate in vertical bands in layers I through IV. Thus, different cortical projection systems are characterized by specific laminar distributions of efferent terminations as well as of their neurons of origin.