Cortical and thalamic connections of the parietal ventral somatosensory area in marmoset monkeys (Callithrix jacchus)

Corticocuneate projections are altered after spinal cord dorsal column lesions in New World monkeys

Recovery of responses to cutaneous stimuli in the area 3b hand cortex of monkeys after dorsal column lesions (DCLs) in the cervical spinal cord relies on neural rewiring in the cuneate nucleus (Cu) over time. To examine whether the corticocuneate projections are modified during recoveries after the DCL, we injected cholera toxin subunit B into the hand representation in Cu to label the cortical neurons after various recovery times, and related results to the recovery of neural responses in the affected area 3b hand cortex. In normal New World monkeys, labeled neurons were predominately distributed in the hand regions of contralateral areas 3b, 3a, 1 and 2, parietal ventral (PV), secondary somatosensory cortex (S2), and primary motor cortex (M1), with similar distributions in the ipsilateral cortex in significantly smaller numbers. In monkeys with short-term recoveries, the area 3b hand neurons were unresponsive or responded weakly to touch on the hand, while the cortical labeling pattern was largely unchanged. After longer recoveries, the area 3b hand neurons remained unresponsive, or responded to touch on the hand or somatotopically abnormal parts, depending on the lesion extent. The distributions of cortical labeled neurons were much more widespread than the normal pattern in both hemispheres, especially when lesions were incomplete. The proportion of labeled neurons in the contralateral area 3b hand cortex was not correlated with the functional reactivation in the area 3b hand cortex. Overall, our findings indicated that corticocuneate inputs increase during the functional recovery, but their functional role is uncertain.

Neural network of social interaction observation in marmosets

A crucial component of social cognition is to observe and understand the social interactions of other individuals. A promising nonhuman primate model for investigating the neural basis of social interaction observation is the common marmoset (Callithrix jacchus), a small New World primate that shares a rich social repertoire with humans. Here, we used functional magnetic resonance imaging acquired at 9.4 T to map the brain areas activated by social interaction observation in awake marmosets. We discovered a network of subcortical and cortical areas, predominately in the anterior lateral frontal and medial frontal cortex, that was specifically activated by social interaction observation. This network resembled that recently identified in Old World macaque monkeys. Our findings suggest that this network is largely conserved between New and Old World primates and support the use of marmosets for studying the neural basis of social cognition.

Comparative Functional Anatomy of Marmoset Brains

Marmosets and closely related tamarins have become popular models for understanding aspects of human brain organization and function because they are small, reproduce and mature rapidly, and have few cortical fissures so that more cortex is visible and accessible on the surface. They are well suited for studies of development and aging. Because marmosets are highly social primates with extensive vocal communication, marmoset studies can inform theories of the evolution of language in humans. Most importantly, marmosets share basic features of major sensory and motor systems with other primates, including those of macaque monkeys and humans with larger and more complex brains. The early stages of sensory processing, including subcortical nuclei and several cortical levels for the visual, auditory, somatosensory, and motor systems, are highly similar across primates, and thus results from marmosets are relevant for making inferences about how these systems are organized and function in humans. Nevertheless, the structures in these systems are not identical across primate species, and homologous structures are much bigger and therefore function somewhat differently in human brains. In particular, the large human brain has more cortical areas that add to the complexity of information processing and storage, as well as decision-making, while making new abilities possible, such as language. Thus, inferences about human brains based on studies on marmoset brains alone should be made with a bit of caution.

Whole brain mapping of somatosensory responses in awake marmosets investigated with ultra-high-field fMRI

The common marmoset (Callithrix jacchus) is a small-bodied New World primate that is becoming an important model to study brain functions. Despite several studies exploring the somatosensory system of marmosets, all results have come from anesthetized animals using invasive techniques and postmortem analyses. Here, we demonstrate the feasibility for getting high-quality and reproducible somatosensory mapping in awake marmosets with functional magnetic resonance imaging (fMRI). We acquired fMRI sequences in four animals, while they received tactile stimulation (via air-puffs), delivered to the face, arm, or leg. We found a topographic body representation with the leg representation in the most medial part, the face representation in the most lateral part, and the arm representation between leg and face representation within areas 3a, 3b, and 1/2. A similar sequence from leg to face from caudal to rostral sites was identified in areas S2 and PV. By generating functional connectivity maps of seeds defined in the primary and second somatosensory regions, we identified two clusters of tactile representation within the posterior and midcingulate cortex. However, unlike humans and macaques, no clear somatotopic maps were observed. At the subcortical level, we found a somatotopic body representation in the thalamus and, for the first time in marmosets, in the putamen. These maps have similar organizations, as those previously found in Old World macaque monkeys and humans, suggesting that these subcortical somatotopic organizations were already established before Old and New World primates diverged. Our results show the first whole brain mapping of somatosensory responses acquired in a noninvasive way in awake marmosets.NEW & NOTEWORTHY We used somatosensory stimulation combined with functional MRI (fMRI) in awake marmosets to reveal the topographic body representation in areas S1, S2, thalamus, and putamen. We showed the existence of a body representation organization within the thalamus and the cingulate cortex by computing functional connectivity maps from seeds defined in S1/S2, using resting-state fMRI data. This noninvasive approach will be essential for chronic studies by guiding invasive recording and manipulation techniques.

Reorganization of Higher-Order Somatosensory Cortex After Sensory Loss from Hand in Squirrel Monkeys

Unilateral dorsal column lesions (DCL) at the cervical spinal cord deprive the hand regions of somatosensory cortex of tactile activation. However, considerable cortical reactivation occurs over weeks to months of recovery. While most studies focused on the reactivation of primary somatosensory area 3b, here, for the first time, we address how the higher-order somatosensory cortex reactivates in the same monkeys after DCL that vary across cases in completeness, post-lesion recovery times, and types of treatments. We recorded neural responses to tactile stimulation in areas 3a, 3b, 1, secondary somatosensory cortex (S2), parietal ventral (PV), and occasionally areas 2/5. Our analysis emphasized comparisons of the responsiveness, somatotopy, and receptive field size between areas 3b, 1, and S2/PV across DCL conditions and recovery times. The results indicate that the extents of the reactivation in higher-order somatosensory areas 1 and S2/PV closely reflect the reactivation in primary somatosensory cortex. Responses in higher-order areas S2 and PV can be stronger than those in area 3b, thus suggesting converging or alternative sources of inputs. The results also provide evidence that both primary and higher-order fields are effectively activated after long recovery times as well as after behavioral and electrocutaneous stimulation interventions.

Neural Basis of Touch and Proprioception in Primate Cortex

The sense of proprioception allows us to keep track of our limb posture and movements and the sense of touch provides us with information about objects with which we come into contact. In both senses, mechanoreceptors convert the deformation of tissues-skin, muscles, tendons, ligaments, or joints-into neural signals. Tactile and proprioceptive signals are then relayed by the peripheral nerves to the central nervous system, where they are processed to give rise to percepts of objects and of the state of our body. In this review, we first examine briefly the receptors that mediate touch and proprioception, their associated nerve fibers, and pathways they follow to the cerebral cortex. We then provide an overview of the different cortical areas that process tactile and proprioceptive information. Next, we discuss how various features of objects-their shape, motion, and texture, for example-are encoded in the various cortical fields, and the susceptibility of these neural codes to attention and other forms of higher-order modulation. Finally, we summarize recent efforts to restore the senses of touch and proprioception by electrically stimulating somatosensory cortex. © 2018 American Physiological Society. Compr Physiol 8:1575-1602, 2018.

Corticocortical projections to area 1 in squirrel monkeys (Saimiri sciureus)

Cortical area 1 is a non-primary somatosensory area in the primate anterior parietal cortex that is critical to tactile discrimination. The corticocortical projections to area 1 in squirrel monkeys were determined by placing multiple injections of anatomical tracers into separate body part representations defined by multiunit microelectrode mapping in area 1. The pattern of labeled cells in the cortex indicated that area 1 has strong intrinsic connections within each body part representation and has inputs from somatotopically matched regions of areas 3b, 3a, 2 and 5. Somatosensory areas in the lateral sulcus, including the second somatosensory area (S2), the parietal ventral area (PV), and the presumptive parietal rostral (PR) and ventral somatosensory (VS) areas, also project to area 1. Topographically organized projections to area 1 also came from the primary motor cortex (M1), the dorsal and ventral premotor areas (PMd and PMv), and the supplementary motor area (SMA). Labeled cells were also found in cingulate motor and sensory areas on the medial wall of the hemisphere. Previous studies revealed a similar pattern of projections to area 1 in Old World macaque monkeys, suggesting a pattern of cortical inputs to area 1 that is common across anthropoid primates.

Somatosensory maps

Somatosensory areas containing topographic maps of the body surface are a major feature of parietal cortex. In primates, parietal cortex contains four somatosensory areas, each with its own map, with the primary cutaneous map in area 3b. Rodents have at least three parietal somatosensory areas. Maps are not isomorphic to the body surface, but magnify behaviorally important skin regions, which include the hands and face in primates, and the whiskers in rodents. Within each map, intracortical circuits process tactile information, mediate spatial integration, and support active sensation. Maps may also contain fine-scale representations of touch submodalities, or direction of tactile motion. Functional representations are more overlapping than suggested by textbook depictions of map topography. The whisker map in rodent somatosensory cortex is a canonic system for studying cortical microcircuits, sensory coding, and map plasticity. Somatosensory maps are plastic throughout life in response to altered use or injury. This chapter reviews basic principles and recent findings in primate, human, and rodent somatosensory maps.

Pain syndromes and the parietal lobe

Pain was considered to be integrated subcortically during most of the 20^th century, and it was not until 1956 that focal injury to the parietal opercular-insular cortex was shown to produce selective loss of pain senses. The parietal operculum and adjacent posterior insula are the main recipients of spinothalamic afferents in primates. The innermost operculum appears functionally associated with the posterior insula and can be segregated histologically, somatotopically and neurochemically from the more lateral S2 areas. The Posterior Insula and Medial Operculum (PIMO) encompass functional networks essential to initiate cortical nociceptive processing. Destruction of this region selectively abates pain sensations; direct stimulation generates acute pain, and epileptic foci trigger painful seizures. Lesions of the PIMO have also high potential to develop central pain with dissociated loss of pain and temperature. The PIMO region behaves as a somatosensory area on its own, which handles phylogenetically old somesthetic capabilities based on thinly myelinated or unmyelinated inputs. It integrates spinothalamic-driven information - not only nociceptive but also innocuous heat and cold, crude touch, itch, and possibly viscero-somatic interoception. Conversely, proprioception, graphesthesia or stereognosis are not processed in this area but in S1 cortices. Given its anatomo-functional properties, thalamic connections, and tight relations with limbic and multisensory cortices, the region comprising the inner parietal operculum and posterior insula appears to contain a third somatosensory cortex contributing to the spinothalamic attributes of the final perceptual experience.

ALE meta-analysis reveals dissociable networks for affective and discriminative aspects of touch

Emotionally-laden tactile stimulation-such as a caress on the skin or the feel of velvet-may represent a functionally distinct domain of touch, underpinned by specific cortical pathways. In order to determine whether, and to what extent, cortical functional neuroanatomy supports a distinction between affective and discriminative touch, an activation likelihood estimate (ALE) meta-analysis was performed. This meta-analysis statistically mapped reported functional magnetic resonance imaging (fMRI) activations from 17 published affective touch studies in which tactile stimulation was associated with positive subjective evaluation (n = 291, 34 experimental contrasts). A separate ALE meta-analysis mapped regions most likely to be activated by tactile stimulation during detection and discrimination tasks (n = 1,075, 91 experimental contrasts). These meta-analyses revealed dissociable regions for affective and discriminative touch, with posterior insula (PI) more likely to be activated for affective touch, and primary somatosensory cortices (SI) more likely to be activated for discriminative touch. Secondary somatosensory cortex had a high likelihood of engagement by both affective and discriminative touch. Further, meta-analytic connectivity (MCAM) analyses investigated network-level co-activation likelihoods independent of task or stimulus, across a range of domains and paradigms. Affective-related PI and discriminative-related SI regions co-activated with different networks, implicated in dissociable functions, but sharing somatosensory co-activations. Taken together, these meta-analytic findings suggest that affective and discriminative touch are dissociable both on the regional and network levels. However, their degree of shared activation likelihood in somatosensory cortices indicates that this dissociation reflects functional biases within tactile processing networks, rather than functionally and anatomically distinct pathways.

Intracortical connections are altered after long-standing deprivation of dorsal column inputs in the hand region of area 3b in squirrel monkeys

A complete unilateral lesion of the dorsal column somatosensory pathway in the upper cervical spinal cord deactivates neurons in the hand region in contralateral somatosensory cortex (areas 3b and 1). Over weeks to months of recovery, parts of the hand region become reactivated by touch on the hand or face. To determine whether changes in cortical connections potentially contribute to this reactivation, we injected tracers into electrophysiologically identified locations in cortex of area 3b representing the reactivated hand and normally activated face in adult squirrel monkeys. Our results indicated that even when only partially reactivated, most of the expected connections of area 3b remained intact. These intact connections include the majority of intrinsic connections within area 3b; feedback connections from area 1, secondary somatosensory cortex (S2), parietal ventral area (PV), and other cortical areas; and thalamic inputs from the ventroposterior lateral nucleus (VPL). In addition, tracer injections in the reactivated hand region of area 3b labeled more neurons in the face and shoulder regions of area 3b than in normal monkeys, and injections in the face region of area 3b labeled more neurons in the hand region. Unexpectedly, the intrinsic connections within area 3b hand cortex were more widespread after incomplete dorsal column lesions (DCLs) than after a complete DCL. Although these additional connections were limited, these changes in connections may contribute to the reactivation process after injuries. J. Comp. Neurol. 524:1494-1526, 2016. © 2015 Wiley Periodicals, Inc.

Online repetitive transcranial magnetic stimulation (TMS) to the parietal operculum disrupts haptic memory for grasping

The parietal operculum (OP) contains haptic memory on the geometry of objects that is readily transferrable to the motor cortex but a causal role of OP in memory-guided grasping is only speculative. We explored this issue by using online high-frequency repetitive transcranial magnetic stimulation (rTMS). The experimental task was performed by blindfolded participants acting on objects of variable size. Trials consisted in three phases: haptic exploration of an object, delay, and reach-grasp movement onto the explored object. Motor performance was evaluated by the kinematics of finger aperture. Online rTMS was applied to the left OP region separately in each of the three phases of the task. The results showed that rTMS altered grip aperture only when applied in the delay phase to the OP. In a second experiment a haptic discriminative (match-to-sample) task was carried out on objects similar to those used in the first experiment. Online rTMS was applied to the left OP. No psychophysical effects were induced by rTMS on the detection of explicit haptic object size. We conclude that neural activity in the OP region is necessary for proficient memory-guided haptic grasping. The function of OP seems to be critical while maintaining the haptic memory trace and less so while encoding it or retrieving it.

Anatomic organization of the auditory cortex

The auditory cortex is a network of areas in the part of the brain that receives inputs from the subcortical auditory pathways in the brainstem and thalamus. Through an elaborate network of intrinsic and extrinsic connections, the auditory cortex is thought to bring about the conscious perception of sound and provide a basis for the comprehension and production of meaningful utterances. In this chapter, the organization of auditory cortex is described with an emphasis on its anatomic features and the flow of information within the network. These features are then used to introduce key neurophysiologic concepts that are being intensively studied in humans and animal models. The discussion is presented in the context of our working model of the primate auditory cortex and extensions to humans. The material is presented in the context of six underlying principles, which reflect distinct, but related, aspects of anatomic and physiologic organization: (1) the division of auditory cortex into regions; (2) the subdivision of regions into areas; (3) tonotopic organization of areas; (4) thalamocortical connections; (5) serial and parallel organization of connections; and (6) topographic relationships between auditory and auditory-related areas. Although the functional roles of the various components of this network remain poorly defined, a more complete understanding is emerging from ongoing studies that link auditory behavior to its anatomic and physiologic substrates.

Patterns of cortical input to the primary motor area in the marmoset monkey

In primates the primary motor cortex (M1) forms a topographic map of the body, whereby neurons in the medial part of this area control movements involving trunk and hindlimb muscles, those in the intermediate part control movements involving forelimb muscles, and those in the lateral part control movements of facial and other head muscles. This topography is accompanied by changes in cytoarchitectural characteristics, raising the question of whether the anatomical connections also vary between different parts of M1. To address this issue, we compared the patterns of cortical afferents revealed by retrograde tracer injections in different locations within M1 of marmoset monkeys. We found that the entire extent of this area is unified by projections from the dorsocaudal and medial subdivisions of premotor cortex (areas 6DC and 6M), from somatosensory areas 3a, 3b, 1/2, and S2, and from posterior parietal area PE. While cingulate areas projected to all subdivisions, they preferentially targeted the medial part of M1. Conversely, the ventral premotor areas were preferentially connected with the lateral part of M1. Smaller but consistent inputs originated in frontal area 6DR, ventral posterior parietal cortex, the retroinsular cortex, and area TPt. Connections with intraparietal, prefrontal, and temporal areas were very sparse, and variable. Our results demonstrate that M1 is unified by a consistent pattern of major connections, but also shows regional variations in terms of minor inputs. These differences likely reflect requirements for control of voluntary movement involving different body parts.

Corticocortical projections to representations of the teeth, tongue, and face in somatosensory area 3b of macaques

We placed injections of anatomical tracers into representations of the tongue, teeth, and face in the primary somatosensory cortex (area 3b) of macaque monkeys. Our injections revealed strong projections to representations of the tongue and teeth from other parts of the oral cavity responsive region in 3b. The 3b face also provided input to the representations of the intraoral structures. The primary representation of the face showed a pattern of intrinsic connections similar to that of the mouth. The area 3b hand representation provided little to no input to either the mouth or the face representations. The mouth and face representations of area 3b received projections from the presumptive oral cavity and face regions of other somatosensory areas in the anterior parietal cortex and the lateral sulcus, including areas 3a, 1, 2, the second somatosensory area (S2), the parietal ventral area (PV), and cortex that may include the parietal rostral (PR) and ventral somatosensory (VS) areas. Additional inputs came from primary motor (M1) and ventral premotor (PMv) areas. This areal pattern of projections is similar to the well-studied pattern revealed by tracer injections in regions of 3b representing the hand. The tongue representation appeared to be unique in area 3b in that it also received inputs from areas in the anterior upper bank of the lateral sulcus and anterior insula that may include the primary gustatory area (area G) and other cortical taste-processing areas, as well as a region of lateral prefrontal cortex (LPFC) lining the principal sulcus.

Patterns of afferent input to the caudal and rostral areas of the dorsal premotor cortex (6DC and 6DR) in the marmoset monkey

Corticocortical projections to the caudal and rostral areas of dorsal premotor cortex (6DC and 6DR, also known as F2 and F7) were studied in the marmoset monkey. Both areas received their main thalamic inputs from the ventral anterior and ventral lateral complexes, and received dense projections from the medial premotor cortex. However, there were marked differences in their connections with other cortical areas. While 6DR received consistent inputs from prefrontal cortex, area 6DC received few such connections. Conversely, 6DC, but not 6DR, received major projections from the primary motor and somatosensory areas. Projections from the anterior cingulate cortex preferentially targeted 6DC, while the posterior cingulate and adjacent medial wall areas preferentially targeted 6DR. Projections from the medial parietal area PE to 6DC were particularly dense, while intraparietal areas (especially the putative homolog of LIP) were more strongly labeled after 6DR injections. Finally, 6DC and 6DR were distinct in terms of inputs from the ventral parietal cortex: projections to 6DR originated preferentially from caudal areas (PG and OPt), while 6DC received input primarily from rostral areas (PF and PFG). Differences in connections suggest that area 6DR includes rostral and caudal subdivisions, with the former also involved in oculomotor control. These results suggest that area 6DC is more directly involved in the preparation and execution of motor acts, while area 6DR integrates sensory and internally driven inputs for the planning of goal-directed actions. They also provide strong evidence of a homologous organization of the dorsal premotor cortex in New and Old World monkeys.

Cortical connections to single digit representations in area 3b of somatosensory cortex in squirrel monkeys and prosimian galagos

The ventral posterior nucleus of thalamus sends highly segregated inputs into each digit representation in area 3b of primary somatosensory cortex. However, the spatial organization of the connections that link digit representations of areas 3b with other somatosensory areas is less understood. Here we examined the cortical inputs to individual digit representations of area 3b in four squirrel monkeys and one prosimian galago. Retrograde tracers were injected into neurophysiologically defined representations of individual digits of area 3b. Cortical tissues were cut parallel to the surface in some cases and showed that feedback projections to individual digits overlapped extensively in the hand representations of areas 3b, 1, and parietal ventral (PV) and second somatosensory (S2) areas. Other regions with overlapping populations of labeled cells included area 3a and primary motor cortex (M1). The results were confirmed in other cases in which the cortical tissues were cut in the coronal plane. The same cases also showed that cells were primarily labeled in the infragranular and supragranular layers. Thus, feedback projections to individual digit representations in area 3b mainly originate from multiple digits and other portions of hand representations of areas 3b, 1, PV, and S2. This organization is in stark contrast to the segregated thalamocortical inputs, which originate in single digit representations and terminate in the matching digit representation in the cortex. The organization of feedback connections could provide a substrate for the integration of information across the representations of adjacent digits in area 3b.

Imaging the where and when of tic generation and resting state networks in adult Tourette patients

Introduction: Tourette syndrome (TS) is a neuropsychiatric disorder with the core phenomenon of tics, whose origin and temporal pattern are unclear. We investigated the When and Where of tic generation and resting state networks (RSNs) via functional magnetic resonance imaging (fMRI).

Methods: Tic-related activity and the underlying RSNs in adult TS were studied within one fMRI session. Participants were instructed to lie in the scanner and to let tics occur freely. Tic onset times, as determined by video-observance were used as regressors and added to preceding time-bins of 1 s duration each to detect prior activation. RSN were identified by independent component analysis (ICA) and correlated to disease severity by the means of dual regression.

Results: Two seconds before a tic, the supplementary motor area (SMA), ventral primary motor cortex, primary sensorimotor cortex and parietal operculum exhibited activation; 1 s before a tic, the anterior cingulate, putamen, insula, amygdala, cerebellum and the extrastriatal-visual cortex exhibited activation; with tic-onset, the thalamus, central operculum, primary motor and somatosensory cortices exhibited activation. Analysis of resting state data resulted in 21 components including the so-called default-mode network. Network strength in those regions in SMA of two premotor ICA maps that were also active prior to tic occurrence, correlated significantly with disease severity according to the Yale Global Tic Severity Scale (YGTTS) scores.

Discussion: We demonstrate that the temporal pattern of tic generation follows the cortico-striato-thalamo-cortical circuit, and that cortical structures precede subcortical activation. The analysis of spontaneous fluctuations highlights the role of cortical premotor structures. Our study corroborates the notion of TS as a network disorder in which abnormal RSN activity might contribute to the generation of tics in SMA.

The neural correlates of risk propensity in males and females using resting-state fMRI

Men are more risk prone than women, but the underlying basis remains unclear. To investigate this question, we developed a trait-like measure of risk propensity which we correlated with resting-state functional connectivity to identify sex differences. Specifically, we used short- and long-range functional connectivity densities to identify associated brain regions and examined their functional connectivities in resting-state functional magnetic resonance imaging (fMRI) data collected from a large sample of healthy young volunteers. We found that men had a higher level of general risk propensity (GRP) than women. At the neural level, although they shared a common neural correlate of GRP in a network centered at the right inferior frontal gyrus, men and women differed in a network centered at the right secondary somatosensory cortex, which included the bilateral dorsal anterior/middle insular cortices and the dorsal anterior cingulate cortex. In addition, men and women differed in a local network centered at the left inferior orbitofrontal cortex. Most of the regions identified by this resting-state fMRI study have been previously implicated in risk processing when people make risky decisions. This study provides a new perspective on the brain-behavioral relationships in risky decision making and contributes to our understanding of sex differences in risk propensity.

Topographically organized projection to posterior insular cortex from the posterior portion of the ventral medial nucleus in the long-tailed macaque monkey

Prior anterograde tracing work identified somatotopically organized lamina I trigemino- and spinothalamic terminations in a cytoarchitectonically distinct portion of posterolateral thalamus of the macaque monkey, named the posterior part of the ventral medial nucleus (VMpo; Craig [2004] J. Comp. Neurol. 477:119-148). Microelectrode recordings from clusters of selectively thermoreceptive or nociceptive neurons were used to guide precise microinjections of various tracers in VMpo. A prior report (Craig and Zhang [2006] J. Comp. Neurol. 499:953-964) described retrograde tracing results, which confirmed the selective lamina I input to VMpo and the anteroposterior (head to foot) topography. The present report describes the results of microinjections of anterograde tracers placed at different levels in VMpo, based on the anteroposterior topographic organization of selectively nociceptive units and clusters over nearly the entire extent of VMpo. Each injection produced dense, patchy terminal labeling in a single coherent field within a distinct granular cortical area centered in the fundus of the superior limiting sulcus. The terminations were distributed with a consistent anteroposterior topography over the posterior half of the superior limiting sulcus. These observations demonstrate a specific VMpo projection area in dorsal posterior insular cortex that provides the basis for a somatotopic representation of selectively nociceptive lamina I spinothalamic activity. These results also identify the VMpo terminal area as the posterior half of interoceptive cortex; the anterior half receives input from the vagal-responsive and gustatory neurons in the basal part of the ventral medial nucleus.

Impairment and recovery of hand use after unilateral section of the dorsal columns of the spinal cord in squirrel monkeys

Damage to the ascending forelimb afferents in the dorsal columns (DCs) of the cervical spinal cord in monkeys impairs forelimb use, particularly hand dexterity. Although considerable recovery has been reported, interpretation of the results is complicated by the reproducibility of the lesion and behavioral assessment. Here, we examined the effects of a unilateral DC lesion at the C4-C6 spinal cord level in four adult squirrel monkeys. Behavioral performance was assessed on a reach-to-grasp task over 5-13 weeks after lesion. Retrograde tracers were injected into the skin of the fingertips to determine the distribution of axon terminals in the cuneate nucleus and estimate the effectiveness of lesion at the conclusion of each case. The size and level of DC lesion was reflected in the proportion of spared afferents, which ranged from 1 to 25% across monkeys. The experiments produced two major findings. First, the extent of deafferentation in the DC is directly related to the degree of reaching and grasping impairments, and to the reactivation profile and somatotopic reorganization in contralateral primary somatosensory cortex. Second, considerable behavioral recovery and cortical reorganization occurred even in the monkey with only 1% of axons spared in the DC. Our findings suggest that cutaneous inputs from the hand and forelimb are critical to the integrity of functions such as reaching and grasping. In addition, axon branches from peripheral afferents that terminate on neurons in the dorsal horn of the spinal cord are likely central to the functional recovery.

fMRI in the awake marmoset: somatosensory-evoked responses, functional connectivity, and comparison with propofol anesthesia

Functional neuroimaging in animal models is essential for understanding the principles of neurovascular coupling and the physiological basis of fMRI signals that are widely used to study sensory and cognitive processing in the human brain. While hemodynamic responses to sensory stimuli have been characterized in humans, animal studies are able to combine very high resolution imaging with invasive measurements and pharmacological manipulation. To date, most high-resolution studies of neurovascular coupling in small animals have been carried out in anesthetized rodents. Here we report fMRI experiments in conscious, awake common marmosets (Callithrix jacchus), and compare responses to animals anesthetized with propofol. In conscious marmosets, robust BOLD fMRI responses to somatosensory stimulation of the forearm were found in contralateral and ipsilateral regions of the thalamus, primary (SI) and secondary (SII) somatosensory cortex, and the caudate nucleus. These responses were markedly stronger than those in anesthetized marmosets and showed a monotonic increase in the amplitude of the BOLD response with stimulus frequency. On the other hand, anesthesia significantly attenuated responses in thalamus, SI and SII, and abolished responses in caudate and ipsilateral SI. Moreover, anesthesia influenced several other aspects of the fMRI responses, including the shape of the hemodynamic response function and the interareal (SI-SII) spontaneous functional connectivity. Together, these findings demonstrate the value of the conscious, awake marmoset model for studying physiological responses in the somatosensory pathway, in the absence of anesthesia, so that the data can be compared most directly to fMRI in conscious humans.

Cortical networks for ethologically relevant behaviors in primates

Parietal-frontal networks in primate brains are central to mediating actions. Physiological and anatomical investigations have shown that the parietal-frontal network is consistently organized across several branches of primate evolution that include prosimian galagos, New World owl and squirrel monkeys, and Old World macaque monkeys. Electrical stimulation with 0.5-sec trains of pulses delivered via microelectrodes evoked ethologically relevant actions from both posterior parietal cortex (PPC) and frontal motor cortex (FMC). Reaching, grasping, defensive, and other complex movement patterns were evoked from domains that had a characteristic organization in both FMC and PPC. Although a PPC domain (e.g. reaching) may be connected with other PPC domains (e.g. grasping and defensive), its connections with FMC are preferential for a matching domain (reaching). Similarly, electrical stimulation of a PPC domain and concurrent optical imaging of FMC, showed activation patterns consistent with the preferential connectivity of PPC and FMC domains. The evidence for similar arrangements of interconnected functional domains in PPC and FMC of members of three major branches of the primate radiation suggests that the parietal-frontal networks emerged early in the evolution of primates. The small size of PPC in the close relatives of primates including lagomorphs, rodents, and tree shrews, suggests a limited involvement of PPC in motor behavior before archaic primates emerged. However, functional domains may have evolved in motor cortex before the emergence of archaic primates.

Within-limb somatotopic organization in human SI and parietal operculum for the leg: an fMRI study

Somatotopic organizations in human somatosensory cortex (SI and SII) for scattered portions of the leg have not been systematically observed with functional magnetic resonance imaging (fMRI). In this research we compared functional representations in the contralateral SI and bilateral parietal operculum (that contained subregions OP1, 3-4 of SII and OP2) of four acupoints in right leg in proximal-distal and medial-lateral arrangement. The results were: (1) somatotopy of SI demonstrated a lateral-to-medial and inferior-to-superior pattern when acupoints were shifting from proximal to distal or from medial to lateral; (2) the contralateral OP1 also showed a clear somatotopic organization for the four separated leg portions, and the ipsilateral OP1 showed a similar pattern to the contralateral OP1, thus arrangements of responses in the two areas were mirror-symmetric against y-axis; (3) the contralateral OP2 showed a somatotopic organization when acupoints shifting from proximal to distal, while the contralateral OP3 presented a trend of somatotopy opposite to that of the contralateral OP1. These results first show definite within-leg somatotopy of human SI for scattered leg portions in medial-lateral arrangement using fMRI. Within-limb somatotopic organization of OP1 for leg portions arranging from proximal to distal as well as from medial to lateral, and somatotopy of OP2 for leg portions arranging from proximal to distal, are also shown for the first time. Our results also reinforce the proposal of a somatotopic map existing in human OP3, and indicating a fourth somatotopic map in OP2 in human parietal operculum, which suggests that OP 2 is not just a vestibular area. In addition, separable activations in somatosensory cortex induced by adjacent acupoints should play a fundamental role in acupoint-specific effects in the brain.

The insula of Reil revisited: multiarchitectonic organization in macaque monkeys

The insula of Reil represents a large cortical territory buried in the depth of the lateral sulcus and subdivided into 3 major cytoarchitectonic domains: agranular, dysgranular, and granular. The present study aimed at reinvestigating the architectonic organization of the monkey's insula using multiple immunohistochemical stainings (parvalbumin, PV; nonphosphorylated neurofilament protein, with SMI-32; acetylcholinesterase, AChE) in addition to Nissl and myelin. According to changes in density and laminar distributions of the neurochemical markers, several zones were defined and related to 8 cytoarchitectonic subdivisions (Ia1-Ia2/Id1-Id3/Ig1-Ig2/G). Comparison of the different patterns of staining on unfolded maps of the insula revealed: 1) parallel ventral to dorsal gradients of increasing myelin, PV- and AChE-containing fibers in middle layers, and of SMI-32 pyramidal neurons in supragranular layers, with merging of dorsal and ventral high-density bands in posterior insula, 2) definition of an insula "proper" restricted to two-thirds of the "morphological" insula (as bounded by the limiting sulcus) and characterized most notably by lower PV, and 3) the insula proper is bordered along its dorsal, posterodorsal, and posteroventral margin by a strip of cortex extending beyond the limits of the morphological insula and continuous architectonically with frontoparietal and temporal opercular areas related to gustatory, somatosensory, and auditory modalities.

The organization and evolution of dorsal stream multisensory motor pathways in primates

In Prosimian primates, New World monkeys, and Old World monkeys microstimulation with half second trains of electrical pulses identifies separate zones in posterior parietal cortex (PPC) where reaching, defensive, grasping, and other complex movements can be evoked. Each functional zone receives a different pattern of visual and somatosensory inputs, and projects preferentially to functionally matched parts of motor and premotor cortex. As PPC is a relatively small portion of cortex in most mammals, including the close relatives of primates, we suggest that a larger, more significant PPC emerged with the first primates as a region where several ethologically relevant behaviors could be initiated by sensory and intrinsic signals, and mediated via connections with premotor and motor cortex. While several classes of PPC modules appear to be retained by all primates, elaboration and differentiation of these modules likely occurred in some primates, especially humans.

Cell-poor septa separate representations of digits in the ventroposterior nucleus of the thalamus in monkeys and prosimian galagos

The architectonic features of the ventroposterior nucleus (VP) were visualized in coronal brain sections from two macaque monkeys, two owl monkeys, two squirrel monkeys, and three galagos that were processed for cytochrome oxidase, Nissl bodies, or the vesicular glutamate transporter 2 (vGluT2). The traditional ventroposterior medial (VPM) and ventroposterior lateral (VPL) subnuclei were easily identified, as well as the forelimb and hindlimb compartments of VPL, as they were separated by poorly staining, cell-poor septa. Septa also separated other cell groups within VPM and VPL, specifically in the medial compartment of VPL representing the hand (hand VPL). In one squirrel monkey and one galago we demonstrated that these five groups of cells represent digits 1-5 in a mediolateral sequence by injecting tracers into the cortical representation of single digits, defined by microelectrode recordings, and relating concentrations of labeled neurons to specific cell groups in hand VPL. The results establish the existence of septa that isolate the representation of the five digits in VPL of primates and demonstrate that the isolated cell groups represent digits 1-5 in a mediolateral sequence. The present results show that the septa are especially prominent in brain sections processed for vGluT2, which is expressed in the synaptic terminals of excitatory neurons in most nuclei of the brainstem and thalamus. As vGluT2 is expressed in the synaptic terminations from dorsal columns and trigeminal brainstem nuclei, the effectiveness of vGluT2 preparations in revealing septa in VP likely reflects a lack of synapses using glutamate in the septa.

Architectonic subdivisions of neocortex in the Galago (Otolemur garnetti)

In the present study, galago brains were sectioned in the coronal, sagittal, or horizontal planes, and sections were processed with several different histochemical and immunohistochemical procedures to reveal the architectonic characteristics of the various cortical areas. The histochemical methods used included the traditional Nissl, cytochrome oxidase, and myelin stains, as well as a zinc stain, which reveals free ionic zinc in the axon terminals of neurons. Immunohistochemical methods include parvalbumin (PV) and calbindin (CB), both calcium-binding proteins, and the vesicle glutamate transporter 2 (VGluT2). These different procedures revealed similar boundaries between areas, which suggests that functionally relevant borders were being detected. These results allowed a more precise demarcation of previously identified areas. As thalamocortical terminations lack free ionic zinc, primary cortical areas were most clearly revealed by the zinc stain, because of the poor zinc staining of layer 4. Area 17 was especially prominent, as the broad layer 4 was nearly free of zinc stain. However, this feature was less pronounced in the primary auditory and somatosensory cortex. As VGluT2 is expressed in thalamocortical terminations, layer 4 of primary sensory areas was darkly stained for VGluT2. Primary motor cortex had reduced VGluT2 staining, and increased zinc-enriched terminations in the poorly developed granular layer 4 compared to the adjacent primary somatosensory area. The middle temporal visual (MT) showed increased PV and VGluT2 staining compared to the surrounding cortical areas. The resulting architectonic maps of cortical areas in galagos can usefully guide future studies of cortical organizations and functions.

Modular processing in the hand representation of primate primary somatosensory cortex coexists with widespread activation

Neurons in the hand representation of primary somatosensory cortex (area 3b) are known to have discretely localized receptive fields; and these neurons form modules that can be visualized histologically as distinct digit and palm representations. Despite these indicators of the importance of local processing in area 3b, widespread interactions between stimuli presented to locations across the hand have been reported. We investigated the relationship of neuron firing rate with distance from the site of maximum activation in cortex by recording from a 100-electrode array with electrodes spaced 400 μm apart, implanted into the area 3b hand representation in anesthetized owl monkeys. For each stimulated location on the hand, the electrode site where neurons had the highest peak firing rate was defined as the peak activation site. The lesser firing rates of neurons at all other electrode sites in the grid were compared with the firing rates of neurons at the peak activation site. On average, peak firing rates of neurons decreased rapidly with distance away from the peak activation site. The effect of distance on the variance of firing rates was highly significant (P < 0.0001). However, individual neurons retained high firing rates for distances over 3 mm. The clear decline in firing rate with distance from the most activated location indicates that local processing is emphasized in area 3b, while the distance of neurons with reduced but maintained firing rates ≤3-4 mm from the site of best activation demonstrated widespread activation in primary somatosensory cortex.

Anatomical and functional connectivity of cytoarchitectonic areas within the human parietal operculum

In monkeys, the somatosensory cortex on the parietal operculum can be differentiated into several distinct cortical fields. Potential human homologues for these areas have already been defined by cytoarchitectonic mapping and functional imaging experiments. Differences between the two most widely studied areas [operculum parietale (OP) 1 and OP 4] within this region particularly pertain to their connection with either the perceptive parietal network or the frontal motor areas. In the present study, we investigated differences in anatomical connection patterns probed by probabilistic tractography on diffusion tensor imaging data. Functional connectivity was then mapped by coordinate-based meta-analysis of imaging studies. Comparison between these two aspects of connectivity showed a good congruency and hence converging evidence for an involvement of these areas in matching brain networks. There were, however, also several instances in which anatomical and functional connectivity diverged, underlining the independence of these measures and the need for multimodal characterization of brain connectivity. The connectivity analyses performed showed that the two largest areas within the human parietal operculum region display considerable differences in their connectivity to frontoparietal brain regions. In particular, relative to OP 1, area OP 4 is more closely integrated with areas responsible for basic sensorimotor processing and action control, while OP 1 is more closely connected to the parietal networks for higher order somatosensory processing. These results are largely congruent with data on nonhuman primates. Differences between anatomical and functional connectivity as well as between species, however, highlight the need for an integrative view on connectivity, including comparison and cross-validation of results from different approaches.

Response properties of neurons in primary somatosensory cortex of owl monkeys reflect widespread spatiotemporal integration

Receptive fields of neurons in somatosensory area 3b of monkeys are typically described as restricted to part of a single digit or palm pad. However, such neurons are likely involved in integrating stimulus information from across the hand. To evaluate this possibility, we recorded from area 3b neurons in anesthetized owl monkeys with 100-electrode arrays, stimulating two hand locations with electromechanical probes simultaneously or asynchronously. Response magnitudes and latencies of single- and multiunits varied with stimulus conditions, and multiunit responses were similar to single-unit responses. The mean peak firing rate for single neurons stimulated within the preferred location was estimated to be ∼26 spike/s. Simultaneous stimulation with a second probe outside the preferred location slightly decreased peak firing rates to ∼22 spike/s. When the nonpreferred stimulus preceded the preferred stimulus by 10-500 ms, peak firing rates were suppressed with greatest suppression when the nonpreferred stimulus preceded by 30 ms (∼7 spike/s). The mean latency for single neurons stimulated within the preferred location was ∼23 ms, and latency was little affected by simultaneous paired stimulation. However, when the nonpreferred stimulus preceded the preferred stimulus by 10 ms, latencies shortened to ∼16 ms. Response suppression occurred even when stimuli were separated by long distances (nonadjacent digits) or long times (500 ms onset asynchrony). Facilitation, though rare, occurred most often when the stimulus onsets were within 0-30 ms of each other. These findings quantify spatiotemporal interactions and support the hypothesis that area 3b is involved in widespread stimulus integration.

Functional connectivity between the thalamus and visual cortex under eyes closed and eyes open conditions: a resting-state fMRI study

The thalamus and visual cortex are two key components associated with the alpha power of electroencephalography. However, their functional relationship remains to be elucidated. Here, we employ resting-state functional MRI to investigate the temporal correlations of spontaneous fluctuations between the thalamus [the whole thalamus and its three largest nuclei (bilateral mediodorsal, ventrolateral and pulvinar nuclei)] and visual cortex under both eyes open and eyes closed conditions. The whole thalamus show negative correlations with the visual cortex and positive correlations with its contralateral counterpart in eyes closed condition, but which are significantly decreased in eyes open condition, consistent with previous findings of electroencephalography desynchronization during eyes open resting state. Furthermore, we find that bilateral thalamic mediodorsal nuclei and bilateral ventrolateral nuclei have remarkably similar connectivity maps, and resemble to those of the whole thalamus, suggesting their crucial contributions to the thalamus-visual correlations. The bilateral pulvinar nuclei are found to show distinct functional connectivity patterns, compatible with previous findings of the asymmetry of anatomical and functional organization in the nuclei. Our data provides evidence for the associations of intrinsic spontaneous neuronal activity between the thalamus and visual cortex under different resting conditions, which might have implications on the understanding of the generation and modulation of the alpha rhythm.

A combined histological and MRI brain atlas of the common marmoset monkey, Callithrix jacchus

The common marmoset, Callithrix jacchus, is of growing importance for research in neuroscience and related fields. In the present work, we describe a combined histological and magnetic resonance imaging (MRI) atlas constructed from the brains of two adult female marmosets. Histological sections were processed from Nissl staining and digitized to produce an atlas in a large format that facilitates visualization of structures with significant detail. Naming of identifiable brain structures was performed utilizing current terminology. The histological sections and a simplified schematic atlas are available online at http://udn.nichd.nih.gov/brainatlas_home.html.

Multisensory connections of monkey auditory cerebral cortex

Functional studies have demonstrated multisensory responses in auditory cortex, even in the primary and early auditory association areas. The features of somatosensory and visual responses in auditory cortex suggest that they are involved in multiple processes including spatial, temporal and object-related perception. Tract tracing studies in monkeys have demonstrated several potential sources of somatosensory and visual inputs to auditory cortex. These include potential somatosensory inputs from the retroinsular (RI) and granular insula (Ig) cortical areas, and from the thalamic posterior (PO) nucleus. Potential sources of visual responses include peripheral field representations of areas V2 and prostriata, as well as the superior temporal polysensory area (STP) in the superior temporal sulcus, and the magnocellular medial geniculate thalamic nucleus (MGm). Besides these sources, there are several other thalamic, limbic and cortical association structures that have multisensory responses and may contribute cross-modal inputs to auditory cortex. These connections demonstrated by tract tracing provide a list of potential inputs, but in most cases their significance has not been confirmed by functional experiments. It is possible that the somatosensory and visual modulation of auditory cortex are each mediated by multiple extrinsic sources.

Thalamocortical connections of parietal somatosensory cortical fields in macaque monkeys are highly divergent and convergent

We examined the organization and cortical projections of the somatosensory thalamus using multiunit microelectrode recording techniques in anesthetized monkeys combined with neuroanatomical tracings techniques and architectonic analysis. Different portions of the hand representation in area 3b were injected with different anatomical tracers in the same animal, or matched body part representations in parietal areas 3a, 3b, 1, 2, and areas 2 and 5 were injected with different anatomical tracers in the same animal to directly compare their thalamocortical connections. We found that the somatosensory thalamus is composed of several representations of cutaneous and deep receptors of the contralateral body. These nuclei include the ventral posterior nucleus, the ventral posterior superior nucleus, the ventral posterior inferior nucleus, and the ventral lateral nucleus. Each nucleus projects to several different cortical fields, and each cortical field receives projections from multiple thalamic nuclei. In contrast to other sensory systems, each of these somatosensory cortical fields is uniquely innervated by multiple thalamic nuclei. These data indicate that multiple inputs are processed simultaneously within and across several, "hierarchically connected" cortical fields.

Multiple parietal operculum subdivisions in humans: tactile activation maps

We focused the present analysis on blood-oxygen-level-dependent responses evoked in four architectonic subdivisions of human posterior parietal operculum (PO) during two groups of tasks involving either vibrotactile stimulation or rubbing different surfaces against the right index finger pad. Activity localized in previously defined parietal opercular subdivisions, OP 1-4, was co-registered to a standard cortical surface-based atlas. Four vibrotactile stimulation tasks involved attention to the parameters of paired vibrations: (1) detect rare target trials when vibration frequencies matched; (2) select the presentation order of the vibration with a higher frequency or (3) longer duration; and (4) divide attention between frequency and duration before selecting stimulus order. Surface stimulation tasks involved various discriminations of different surfaces: (1) smooth surfaces required no discrimination; (2) paired horizontal gratings required determination of the direction of roughness change; (3) paired shapes entailed identifying matched and unmatched shapes; (4) raised letters involved letter recognition. The results showed activity in multiple somatosensory subdivisions bilaterally in human PO that are plausibly homologues of somatosensory areas previously described in animals. All tasks activated OP 1, but in vibrotactile tasks foci were more restricted compared to moving surface tasks. Greater spatial extents of activity especially in OP 1 and 4 when surfaces rubbed the finger pad did not support previously reported somatotopy of the second finger representation in "S2". The varied activity distributions across OP subdivisions may reflect low-level perceptual and/or cognitive processing differences between tasks.

Sources of somatosensory input to the caudal belt areas of auditory cortex

The auditory cortex of nonhuman primates is comprised of a constellation of at least twelve interconnected areas distributed across three major regions on the superior temporal gyrus: core, belt, and parabelt. Individual areas are distinguished on the basis of unique profiles comprising architectonic features, thalamic and cortical connections, and neuron response properties. Recent demonstrations of convergent auditory-somatosensory interactions in the caudomedial (CM) and caudolateral (CL) belt areas prompted us to pursue anatomical studies to identify the source(s) of somatic input to auditory cortex. Corticocortical and thalamocortical connections were revealed by injecting neuroanatomical tracers into CM, CL, and adjoining fields of marmoset (Callithrix jacchus jacchus) and macaque (Macaca mulatta) monkeys. In addition to auditory cortex, the cortical connections of CM and CL included somatosensory (retroinsular, Ri; granular insula, Ig) and multisensory areas (temporal parietal occipital, temporal parietal temporal). Thalamic inputs included the medial geniculate complex and several multisensory nuclei (suprageniculate, posterior, limitans, medial pulvinar), but not the ventroposterior complex. Injections of the core (A1, R) and rostromedial areas of auditory cortex revealed sparse multisensory connections. The results suggest that areas Ri and Ig are the principle sources of somatosensory input to the caudal belt, while multisensory regions of cortex and thalamus may also contribute. The present data add to growing evidence of multisensory convergence in cortical areas previously considered to be 'unimodal', and also indicate that auditory cortical areas differ in this respect.