Morphological and neurochemical comparisons between pulvinar and V1 projections to V2

The flow of visual information is clear at the earliest stages: the retina provides the driving (main signature) activity for the lateral geniculate nucleus (LGN), which in turn drives the primary visual cortex (V1). These driving pathways can be distinguished anatomically from other modulatory pathways that innervate LGN and V1. The path of visual information after V1, however, is less clear. There are two primary feedforward projections to the secondary visual cortex (V2), one from the lateral/inferior pulvinar and the other from V1. Because both lateral/inferior pulvinar and V2 cannot be driven visually following V1 removal, either or both of these inputs to V2 could be drivers. Retinogeniculate and geniculocortical projections are privileged over modulatory projections by their layer of termination, their bouton size, and the presence of vesicular glutamate transporter 2 (Vglut2) or parvalbumin (PV). It has been suggested that such properties might also distinguish drivers from modulators in extrastriate cortex. We tested this hypothesis by comparing lateral pulvinar to V2 and V1 to V2 projections with LGN to V1 projections. We found that V1 and lateral pulvinar projections to V2 are similar in that they target the same layers and lack PV. Projections from pulvinar to V2, however, bear a greater similarity to projections from LGN to V1 because of their larger boutons (measured at the same location in V2) and positive staining for Vglut2. These data lend support to the hypothesis that the pulvinar could act as a driver for V2. J. Comp. Neurol. 521:813–832, 2013. © 2012 Wiley Periodicals, Inc.     

The projections from striate cortex (V1) to areas V2 and V3 in the macaque monkey: asymmetries, areal boundaries, and patchy connections

The organization of projections from V1 to areas V2 and V3 in the macaque monkey was studied with a combination of anatomical techniques, including lesions and tracer injections made in different portions of V1 and V2 in 20 experimental hemispheres. Our results indicate that dorsal V1 (representing the inferior contralateral visual quadrant) consistently projects in topographically organized fashion to V3 in the lunate and parietooccipital sulci as well as to the middle temporal area (MT) and dorsal V2. In contrast, ventral V1 (representing the superior contralateral quadrant) projects only to MT and ventral V2. A corresponding dorsoventral asymmetry in myeloarchitecture supports the idea that V3 is an area that is restricted to dorsal extrastriate cortex and lacks a complete representation of the visual field. The average surface area of myeloarchitectonically identified V3 was 89 mm2. Additional information was obtained concerning the laminar distribution of connections from V1 to V2 and V3, the patchiness of these projections, and the consistency of projections to other extrastriate areas, including V4 and V3A.     

Neuronal activity in monkey visual areas V1, V2, V4, and TEO during fixation task

We analyzed 577 neurons recorded from visual areas V1, V2, V4, and the inferotemporal area (TEO) of macaque monkeys, which performed a visual fixation task and a spot-off-on (blink) test during the fixation period. Among these neurons, 35% were defined as task-related cells, because they gave responses at the task-start, fixation, or task-end periods but were unresponsive to the spot blink, which was physically identical to these stimuli. Blink-responsive cells accounted for 29% and task-unresponsive cells for 30% of the neurons. The task-related response was large and frequent in V4 (34%) and TEO (41%), but small and less frequent in V1 (31%) and V2 (27%). Other observations further demonstrated nonsensory activities in these areas: In some cells, response to the fixation spot was inhibitory, whereas light stimulation on the fovea was excitatory; some V1 and V2 cells had color-irrelevant responses, and some cells responded to the spot-off only when the monkey regarded it as a task-end cue.     

The organization of projections from the amygdala to visual cortical areas TE and V1 in the macaque monkey

We examined the organization of amygdaloid projections to visual cortical areas TE and V1 by injecting anterograde tracers into the amygdaloid complex of Macaca fascicularis monkeys. The magnocellular and intermediate divisions of the basal nucleus of the amygdala gave rise to heavy projections to both superficial layers (border of I/II) and deep layers (V/VI) throughout the rostrocaudal extent of area TE. Although most of the injections led to heavier fiber and terminal labeling in the superficial layers of area TE, the most dorsal injections in the basal nucleus produced denser labeled fibers and terminals in the deep layers of area TE. Area V1 received projections primarily from the magnocellular division of the basal nucleus, and these terminated exclusively in the superficial layers. As in area TE, projections from the amygdala to area V1 were distributed throughout its rostrocaudal and transverse extents. Labeled axons demonstrated 11.67 varicosities/100 microm on average in the superficial layers of area TE and 8.74 varicosities/100 microm in the deep layers. In area V1 we observed 8.24 varicosities/100 microm. Using confocal microscopy, we determined that at least 55% of the tracer-labeled varicosities in areas TE and V1 colocalized synaptophysin, a marker of synaptic vesicles, indicating that they are probably synaptic boutons. Electron microscopic examination of a sample of these varicosities confirmed that labeled boutons formed synapses in areas TE and V1. These feedback-like projections from the amygdala have the potential of modulating key areas of the visual processing system.     

Synaptic organization of projections from the amygdala to visual cortical areas TE and V1 in the macaque monkey

The primate amygdaloid complex projects to a number of visual cortices, including area V1, primary visual cortex, and area TE, a higher-order unimodal visual area involved in object recognition. We investigated the synaptic organization of these projections by injecting anterograde tracers into the amygdaloid complex of Macaca fascicularis monkeys and examining labeled boutons in areas TE and V1 using the electron microscope. The 256 boutons examined in area TE formed 263 synapses. Two hundred twenty-three (84%) of these were asymmetric synapses onto dendritic spines and 40 (15%) were asymmetric synapses onto dendritic shafts. Nine boutons (3.5%) formed double asymmetric synapses, generally on dendritic spines, and 2 (1%) of the boutons did not form a synapse. The 200 boutons examined in area V1 formed 211 synapses. One hundred eighty-nine (90%) were asymmetric synapses onto dendritic spines and 22 (10%) were asymmetric synapses onto dendritic shafts. Eleven boutons (5.5%) formed double synapses, usually with dendritic spines. We conclude from these observations that the amygdaloid complex provides an excitatory input to areas TE and V1 that primarily influences spiny, probably pyramidal, neurons in these cortices.     

Neuronal response properties across cytochrome oxidase stripes in primate V2

Cytochrome oxidase histochemistry reveals large-scale cortical modules in area V2 of primates known as thick, thin, and interstripes. Anatomical, electrophysiological, and tracing studies suggest that V2 cytochrome oxidase stripes participate in functionally distinct streams of visual information processing. However, there is controversy whether the different V2 compartments indeed correlate with specialized neuronal response properties. We used multiple-electrode arrays (16 × 2, 8 × 4 and 4 × 4 matrices) to simultaneously record the spiking activity (N = 190 single units) across distinct V2 stripes in anesthetized and paralyzed capuchin monkeys (N = 3 animals, 6 hemispheres). Visual stimulation consisted of moving bars and full-field gratings with different contrasts, orientations, directions of motion, spatial frequencies, velocities, and color contrasts. Interstripe neurons exhibited the strongest orientation and direction selectivities compared to the thick and thin stripes, with relatively stronger coding for orientation. Additionally, they responded best to higher spatial frequencies and to lower stimulus velocities. Thin stripes showed the highest proportion (80%) of neurons selective to color contrast (compared to 47% and 21% for thick and interstripes, respectively). The great majority of the color selective cells (86%) were also orientation selective. Additionally, thin stripe neurons continued to increase their firing rate for stimulus contrasts above 50%, while thick and interstripe neurons already exhibited some degree of response saturation at this point. Thick stripes best coded for lower spatial frequencies and higher stimulus velocities. In conclusion, V2 CytOx stripes exhibit a mixed degree of segregation and integration of information processing, shedding light into the early mechanisms of vision.     

Neuronal projections to the guinea pig stellate ganglion investigated by retrograde tracing

Previous electrophysiological studies have revealed a peripheral sensory input to the stellate ganglion which does not originate from the dorsal root ganglia. The present retrograde tracing study aimed at evaluating whether the parent cell bodies are located in the periphery, i.e. in mediastinal ganglia. Following injection of Fast blue or wheat germ agglutinin-horseradish peroxidase into the right stellate ganglion of the guinea pig, retrogradely labelled cell bodies were observed in the intermediolateral and intercalated nuclei of the spinal cord as well as in dorsal root ganglia at segmental levels C8 to T6. In another case, the stellate ganglion was resected and replaced by a sponge soaked with 10 μl of Fast blue. Labelling of preganglionic and sensory neurons parallelled that obtained by tracer injections. In neither case, however, were retrogradely labelled neurons found within or around the thoracic viscera (thymus, trachea, bronchi, esophagus, heart, great vessels of upper mediastinum) when these were cut serially en bloc. Controls performed by injection of Fast blue into the inferior mesenteric ganglion and investigation of the distal colon showed that our experimental protocol was able to visualize a peripheral projection towards a sympathetic ganglion — in this case from myenteric ganglia to the inferior mesenteric ganglion. We conclude that, in contrast to the circuitry connecting prevertebral sympathetic ganglia with the gut, the neuronal cell bodies providing peripheral sensory input from thoracic viscera to the right stellate ganglion most likely are not located within the mediastinal ganglia. Instead, they may reside within the stellate ganglion itself.     

Functional implications of the anatomical organization of the callosal projections of visual areas V1 and V2 in the macaque monkey

The efferent and afferent connections of the V1/V2 border with the contralateral hemisphere have been examined using anatomical tracers. The V1/V2 border was found to exchange connections with the contralateral V2 area as well as a restricted strip of V1 lying adjacent to the V1/V2 border. Besides these homotopic projections, two heterotopic projections were found to V3/V3A and V5. Anterograde tracing of callosal connections showed that terminals in these heterotopic sites were focused in layer 4, the recipient layer of projections originating from the ipsilateral V1/V2 border. Bilateral injections of fluorescent dyes showed that these heterotopic targets of the V1/V2 border are connected to the homologous ipsilateral V1/V2 border region. The laminar location of callosal projecting neurons as well as their terminals were characteristic for each cortical region. The laminar pattern of callosal connectivity was found to differ markedly from that of associational visual pathways. Two principal hypotheses are suggested by these results. First, the fact that V1 in part is reciprocally callosally connected in all mammals supports the notion that this interhemispheric pathway completes long-range intrinsic cortical connections. Second, the convergence of inter- and intrahemispheric pathways could provide the anatomical basis for the modulation of the sensory processing within one hemisphere by ongoing activity in the contralateral hemisphere.     

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.     

Synaptic connection from cortical area V4 to V2 in macaque monkey

The major target of the V4 projection in V2 is layer 1, where it forms a tangential spread of asymmetric (excitatory) synapses. This is characteristic of a "feedback" projection. Some axons formed discrete clusters of bouton terminaux between lengths of myelinated axon, while others were unbranched and formed a continuous distribution of en passant boutons with no intercalated myelin. Minor projections were found in layers 2/3 and 6. Dendritic spines were the most frequently encountered targets of the V4 projection (80% in layer 1 and layer 2/3, 94% in layer 6). The remaining targets were dendritic shafts. In layer 1, 69% of target dendrites (12% of all targets) had characteristics identifying them as smooth (GABAergic) cells. In layer 2/3 and layer 6 virtually all the shaft synapses were on smooth dendrites (86% and 100%, respectively). Multisynaptic boutons were rare (mean 1.1 synapses per bouton). Synapses formed in layer 6 were smaller than those of layer 1 (mean area 0.073 microm(2) vs. 0.117 microm(2)). Synapses formed with spines had a more complex postsynaptic density than those formed with dendritic shafts. With respect to targets and synaptic type and size and morphology of synapses, the feedback projection from V4 to V2 resembles those of feedforward projections. The principal difference between the feedforward and feedback projection is in the lamina location of their terminal boutons. The concentration of the V4 projection on layer 1, where it forms asymmetric synapses mainly with spines, suggests that it excites the distal apical dendrites of pyramidal cells.     

Connection from cortical area V2 to V3 A in macaque monkey

The V2 projection to V3 A was labeled by pressure microinjecting biotinylated dextran amine (BDA) and Phaseolus vulgaris lectin (PHA-L) into V2 just posterior to the lunate sulcus. Dense terminal labeling in clusters was found in layer 4, with a weaker terminal projection in layer 3. About 3.5--4.1% of the synapses in the densest bouton clusters in layer 4 were made by labeled boutons. All were asymmetric (Gray's type 1) synapses, made by spiny, excitatory neurons. The most frequently encountered synaptic targets were spines (76% in layer 4, 98% in layer 2/3). The remainder of the synaptic targets were dendritic shafts, of which just less than half (44%) had the characteristic ultrastructure of smooth (inhibitory) cells. Multisynaptic boutons were rare (mean synapses per bouton for layer 4 1.2, for layer 2/3 1.1). The mean size of the postsynaptic densities found on spines (0.11 microm(2)) was not significantly different from that for dendrites (0.09 microm(2)). In terms of their type, laminar location, number, and targets, the synapses that formed the V2 projection to V3 A are typical of a major, excitatory, feedforward projection of macaque visual cortex.     

Corticostriatal projections from layer V cells in rat are collaterals of long-range corticofugal axons

Corticostriatal projections arising from the infragranular layers of the motor and second somatosensory cortices were studied in rats after labeling small pools of neurons with biocytin. Camera lucida reconstruction of 263 fibers arising from laminae V and VI revealed that all corticostriatal projections derive from collaterals of lamina V cells whose main axons descend into the cerebral peduncle. In contrast, lamina VI cells do not branch upon the striatum, but upon the thalamus. Together with the results obtained in previous tracing studies, the present data raise the possibility that no neuron is exclusively corticostriatal. We therefore propose that all corticostriatal projections are collaterals given off by the axons of two types of neurons: layer V cells whose main axon project to the brainstem and/or spinal cord, and layer III cells that project to the contralateral hemisphere.     

Corticothalamic projections from layer V cells in rat are collaterals of long-range corticofugal axons

The vast majority of corticothalamic (CT) axons projecting to sensory-specific thalamic nuclei arise from layer VI cells but intralaminar and associative thalamic nuclei also receive, to various degrees, a cortical input from layer V pyramidal cells. It is also well established that all long-range corticofugal projections reaching the brainstem and spinal cord arise exclusively from layer V neurons. These observations raise the possibility that the CT input from layer V cells may be collaterals of those long-range axons projecting below thalamic level. The thalamic projections of layer V cells were mapped at a single cell level following small microiontophoretic injections of biocytin performed in the motor, somatosensory and visual cortices in rats. Camera lucida reconstruction of these CT axons revealed that they are all collaterals of long-range corticofugal axons. These collaterals do not give off axonal branches within the thalamic reticular nucleus and they arborize exclusively within intralaminar and associative thalamic nuclei where they form small clusters of varicose endings. As layer V cells are involved in motor commands everywhere in the neocortex, these CT projections and their thalamic targets should be directly involved in the central organization of motor programs.     

Projections to the superior temporal sulcus from the central and peripheral field representations of V1 and V2

In a series of three studies, we have begun to explore the sequence of visual information processing along the pathway from striate cortex (V1), through MT, into the parietal lobe. In this first study, we sought to establish the relationships among MT, the heavily myelinated zone of the superior temporal sulcus (STS), and the V1 and V2 projection fields in the STS. Autoradiographic material from seven hemispheres of six macaques injected with tritiated amino acids into either V1 or V2 was analyzed in detail, and the results were plotted onto two-dimensional reconstructions of the STS. Autoradiographic material from eight additional macaques with V2 injections was also examined. The results indicate that the central visual field representations of both V1 and V2 project into the heavily myelinated zone in the lower bank and floor of the STS, confirming prior studies, whereas the far peripheral representations of both V1 and V2 project into the cortex medial to this zone on the upper bank of the sulcus. There is no evidence that this medial cortex is a separate area that receives projections from V1 and V2 in parallel with the projections these areas send to the heavily myelinated zone. Rather, there seems to be a single projection field of V1 and V2 whose central representation lies within the heavily myelinated zone and whose most peripheral representation lies medial to it. Because of the difference in myelination between the central and peripheral field representations as well as visuotopic anomalies between them, we retain the term "MT" for the heavily myelinated zone and apply the term "MTp" to the far peripheral projection zone. Both MT and MTp are required to process the complete outputs of V1 and V2 within the STS and thus should probably be regarded as two distinctive parts of a single visual area. The difference in myelination between MT and MTp suggests that there is a difference in visual processing between the central and peripheral visual fields. The average size of MT is estimated to be 62 mm2, and the average size of MT and MTp combined to be 76 mm2, which is consistent with estimates derived from several other studies.     

V1R and V2R segregated vomeronasal pathways to the hypothalamus

The vomeronasal system is segregated from the epithelium to the bulb. Two classes of receptor neurons are apically and basally placed in the vomeronasal epithelium, express Gi2alpha and Goalpha proteins and V1R and V2R receptors and project to the anterior and posterior portions of the accessory olfactory bulb, respectively. Apart from common vomeronasal recipient structures in the amygdala, only the anterior accessory olfactory bulb projects to the bed nucleus of the stria terminalis and only the posterior accessory olfactory bulb projects to the dorsal anterior amygdala. The efferent projections from these two amygdaloid structures to the hypothalamus were investigated. These two vomeronasal subsystems mediated by V1R and V2R receptors were partially segregated, not only in amygdala, but also in the hypothalamus.     

The brain of Cataglyphis ants: Neuronal organization and visual projections

Cataglyphis ants are known for their outstanding navigational abilities. They return to their inconspicuous nest after far-reaching foraging trips using path integration, and whenever available, learn and memorize visual features of panoramic sceneries. To achieve this, the ants combine directional visual information from celestial cues and panoramic scenes with distance information from an intrinsic odometer. The largely vision-based navigation in Cataglyphis requires sophisticated neuronal networks to process the broad repertoire of visual stimuli. Although Cataglyphis ants have been subjected to many neuroethological studies, little is known about the general neuronal organization of their central brain and the visual pathways beyond major circuits. Here, we provide a comprehensive, three-dimensional neuronal map of synapse-rich neuropils in the brain of Cataglyphis nodus including major connecting fiber systems. In addition, we examined neuronal tracts underlying the processing of visual information in more detail. This study revealed a total of 33 brain neuropils and 30 neuronal fiber tracts including six distinct tracts between the optic lobes and the cerebrum. We also discuss the importance of comparative studies on insect brain architecture for a profound understanding of neuronal networks and their function.     

Collateral axonal projections to limbic structures from ventrolateral medullary A1 noradrenergic neurons

Experiments were done to investigate whether catecholaminergic neurons within the ventrolateral medulla (VLM) send collateral axonal projections to the central nucleus of the amygdala (ACe) and the bed nucleus of the stria terminalis (BST). Unilateral microinjections of the fluorescent retrograde tracers fluorogold (FG) or rhodamine labelled latex micro-beads (Rd) were made into either ACe or BST in the rat. Brainstem sections were then processed immunohistochemically for the identification of cell bodies containing the catecholamine biosynthetic enzymes tyrosine hydroxylase, dopamine β-hydroxylase (DBH) or phenylethanolamine-N-methyltransferase (PNMT). Retrogradely labelled cell bodies projecting to either ACe or BST were found throughout the rostrocaudal extent of VLM, bilaterally. Approximately 44% Of these retrogradely labelled neurons were found to contain both eetrograde tracers. In addition, approximately 91% of the VLM neurons that send collateral axonal projections to ACe and BST were also immunoreactive to DBH. None were found to contain PNMT immunoreactivity. These results demonstrate that noradrenergic neurons of the A1 cell group in VLM innervate ACe and BST via collateral axonal projections and suggest that these VLM neurons may be directly involved in relaying cardiovascular afferent and/or visceral afferent information directly to these limbic structures.