Prefrontal projections to the mediodorsal nucleus of the thalamus in the rhesus monkey

Bilateral thalamic infarcts: Percheron territory

Blood supply to the human thalami is complex and multiple variants exist. The artery of Percheron is one of those variants and is characterized by a solitary arterial trunk that branches from one of the proximal segments of either posterior cerebral artery and sup- plies blood to the paramedian thalami. Its occlusion results in bilateral paramedian thalamic infarction sometimes extending to the midbrain. We report a case of bithalamic infarction secondary to occlusion of the artery of Percheron. We will illustrate the complex clinical symptomatology and underscore the role of imaging, especially MRI, for diagnosis.

Diaschisis in the human brain reveals specificity of cerebrocerebellar connections

Anatomical studies in animals and imaging studies in humans show that cerebral sensorimotor areas map onto corresponding cerebellar sensorimotor areas and that cerebral association areas map onto cerebellar posterior lobe regions designated as the representation of the association (cognitive and limbic) cerebellum. We report a patient with unilateral left hemispheric status epilepticus, whose brain MRI revealed diffuse unihemispheric cerebral cortical FLAIR and diffusion signal hyperintensity but spared primary motor, somatosensory, visual, and to lesser extent auditory cerebral cortices. Crossed cerebellar diaschisis (dysfunction at a site remote from, but connected to, the location of the primary lesion) showed signal hyperintensity in the right cerebellar posterior lobe and lobule IX, with sparing of the anterior lobe, and lobule VIII. This unique topographic pattern of involvement and sparing of cerebral and cerebellar cortical areas matches the anatomical and functional connectivity specialization in the cerebrocerebellar circuit. This first demonstration of within-hemispheric specificity in the areas affected and spared by cerebrocerebellar diaschisis provides further confirmation in the human brain for topographic organization of connections between the cerebral hemispheres and the cerebellum.

High-Resolution Tractography Protocol to Investigate the Pathways between Human Mediodorsal Thalamic Nucleus and Prefrontal Cortex

Animal studies have established that the mediodorsal nucleus (MD) of the thalamus is heavily and reciprocally connected with all areas of the prefrontal cortex (PFC). In humans, however, these connections are difficult to investigate. High-resolution imaging protocols capable of reliably tracing the axonal tracts linking the human MD with each of the PFC areas may thus be key to advance our understanding of the variation, development, and plastic changes of these important circuits, in health and disease. Here, we tested in adult female and male humans the reliability of a new reconstruction protocol based on in vivo diffusion MRI to trace, measure, and characterize the fiber tracts interconnecting the MD with 39 human PFC areas per hemisphere. Our protocol comprised the following three components: (1) defining regions of interest; (2) preprocessing diffusion data; and, (3) modeling white matter tracts and tractometry. This analysis revealed largely separate PFC territories of reciprocal MD-PFC tracts bearing striking resemblance with the topographic layout observed in macaque connection-tracing studies. We then examined whether our protocol could reliably reconstruct each of these MD-PFC tracts and their profiles across test and retest sessions. Results revealed that this protocol was able to trace and measure, in both left and right hemispheres, the trajectories of these 39 area-specific axon bundles with good-to-excellent test-retest reproducibility. This protocol, which has been made publicly available, may be relevant for cognitive neuroscience and clinical studies of normal and abnormal PFC function, development, and plasticity.SIGNIFICANCE STATEMENT Reciprocal MD-PFC interactions are critical for complex human cognition and learning. Reliably tracing, measuring and characterizing MD-PFC white matter tracts using high-resolution noninvasive methods is key to assess individual variation of these systems in humans. Here, we propose a high-resolution tractography protocol that reliably reconstructs 39 area-specific MD-PFC white matter tracts per hemisphere and quantifies structural information from diffusion MRI data. This protocol revealed a detailed mapping of thalamocortical and corticothalamic MD-PFC tracts in four different PFC territories (dorsal, medial, orbital/frontal pole, inferior frontal) showing structural connections resembling those observed in tracing studies with macaques. Furthermore, our automated protocol revealed high test-retest reproducibility and is made publicly available, constituting a step forward in mapping human MD-PFC circuits in clinical and academic research.

Gradients of thalamic connectivity in the macaque lateral prefrontal cortex

In the primate brain, the lateral prefrontal cortex (LPF) is a large, heterogeneous region critically involved in the cognitive control of behavior, consisting of several connectionally and functionally distinct areas. Studies in macaques provided evidence for distinctive patterns of cortical connectivity between architectonic areas located at different dorsoventral levels and for rostrocaudal gradients of parietal and frontal connections in the three main architectonic LPF areas: 46d, 46v, and 12r. In the present study, based on tracer injections placed at different dorsoventral and rostrocaudal cortical levels, we have examined the thalamic projections to the LPF to examine to what extent fine-grained connectional gradients of cortical connectivity are reflected in the topography of thalamo-LPF projections. The results showed mapping onto the nucleus medialis dorsalis (MD), by far the major source of thalamic input to the LPF, of rostral-to-caudal LPF zones, in which MD zones projecting to more caudal LPF sectors are located more rostral than those projecting to intermediate LPF sectors. Furthermore, the MD zones projecting to the rostral LPF sectors tended to be much more extensive in the rostrocaudal direction. One rostrolateral MD sector appeared to be a common source of projections to caudal prefrontal areas involved in the oculomotor frontal domain, a more caudal and ventral MD sector to a large extent of the ventral LPF, and middle and dorsal MD sectors to most of the dorsal LPF. Additional topographically organized projections to LPF areas originated from the nucleus pulvinaris medialis and projections from the nucleus anterior medialis selectively targeted more rostral sectors of LPF. Thus, the present data suggest that the topography of the MD-LPF projections does not adhere to simple topological rules, but is mainly organized according to functional criteria.

The spatial extent of anatomical connections within the thalamus varies across the cortical hierarchy in humans and macaques

Each cortical area has a distinct pattern of anatomical connections within the thalamus, a central subcortical structure composed of functionally and structurally distinct nuclei. Previous studies have suggested that certain cortical areas may have more extensive anatomical connections that target multiple thalamic nuclei, which potentially allows them to modulate distributed information flow. However, there is a lack of quantitative investigations into anatomical connectivity patterns within the thalamus. Consequently, it remains unknown if cortical areas exhibit systematic differences in the extent of their anatomical connections within the thalamus. To address this knowledge gap, we used diffusion magnetic resonance imaging (dMRI) to perform brain-wide probabilistic tractography for 828 healthy adults from the Human Connectome Project. We then developed a framework to quantify the spatial extent of each cortical area's anatomical connections within the thalamus. Additionally, we leveraged resting-state functional MRI, cortical myelin, and human neural gene expression data to test if the extent of anatomical connections within the thalamus varied along the cortical hierarchy. Our results revealed two distinct corticothalamic tractography motifs: 1) a sensorimotor cortical motif characterized by focal thalamic connections targeting posterolateral thalamus, associated with fast, feed-forward information flow; and 2) an associative cortical motif characterized by diffuse thalamic connections targeting anteromedial thalamus, associated with slow, feed-back information flow. These findings were consistent across human subjects and were also observed in macaques, indicating cross-species generalizability. Overall, our study demonstrates that sensorimotor and association cortical areas exhibit differences in the spatial extent of their anatomical connections within the thalamus, which may support functionally-distinct cortico-thalamic information flow.

Anatomical organization of forebrain circuits in the primate

The primate forebrain is a complex structure. Thousands of connections have been identified between cortical areas, and between cortical and sub-cortical areas. Previous work, however, has suggested that a number of principles can be used to reduce this complexity. Here, we integrate four principles that have been put forth previously, including a nested model of neocortical connectivity, gradients of connectivity between frontal cortical areas and the striatum and thalamus, shared patterns of sub-cortical connectivity between connected posterior and frontal cortical areas, and topographic organization of cortical-striatal-pallidal-thalamocortical circuits. We integrate these principles into a single model that accounts for a substantial amount of connectivity in the forebrain. We then suggest that studies in evolution and development can account for these four principles, by assuming that the ancestral vertebrate pallium was dominated by medial, hippocampal and ventral-lateral, pyriform areas, and at most a small dorsal pallium. The small dorsal pallium expanded massively in the lineage leading to primates. During this expansion, topological, adjacency relationships were maintained between pallial and sub-pallial areas. This maintained topology led to the connectivity gradients seen between cortex, striatum, pallidum, and thalamus.

Mapping the subcortical connectome using in vivo diffusion MRI: Feasibility and reliability

Tractography combined with regions of interest (ROIs) has been used to non-invasively study the structural connectivity of the cortex as well as to assess the reliability of these connections. However, the subcortical connectome (subcortex to subcortex) has not been comprehensively examined, in part due to the difficulty of performing tractography in this complex and compact region. In this study, we performed an in vivo investigation using tractography to assess the feasibility and reliability of mapping known connections between structures of the subcortex using the test-retest dataset from the Human Connectome Project (HCP). We further validated our observations using a separate unrelated subjects dataset from the HCP. Quantitative assessment was performed by computing tract densities and spatial overlap of identified connections between subcortical ROIs. Further, known connections between structures of the basal ganglia and thalamus were identified and visually inspected, comparing tractography reconstructed trajectories with descriptions from tract-tracing studies. Our observations demonstrate both the feasibility and reliability of using a data-driven tractography-based approach to map the subcortical connectome in vivo.

Frontotemporal thalamic connectivity in schizophrenia and schizotypal personality disorder

Schizotypal personality disorder (SPD) resembles schizophrenia, but with attenuated brain abnormalities and the absence of psychosis. The thalamus is integral for processing and transmitting information across cortical regions and widely implicated in the neurobiology of schizophrenia. Comparing thalamic connectivity in SPD and schizophrenia could reveal an intermediate schizophrenia-spectrum phenotype to elucidate neurobiological risk and protective factors in psychosis. We used rsfMRI to investigate functional connectivity between the mediodorsal nucleus (MDN) and pulvinar, and their connectivity with frontal and temporal cortical regions, respectively in 43 healthy controls (HCs), and individuals in the schizophrenia-spectrum including 45 psychotropic drug-free individuals with SPD, and 20 individuals with schizophrenia-related disorders [(schizophrenia (n = 10), schizoaffective disorder (n = 8), schizophreniform disorder (n = 1) and psychosis NOS (n = 1)]. Individuals with SPD had greater functional connectivity between the MDN and pulvinar compared to individuals with schizophrenia. Thalamo-frontal (i.e., between the MDN and rostral middle frontal cortex) connectivity was comparable in SPD and HCs; in SPD greater connectivity was associated with less symptom severity. Individuals with schizophrenia had less thalamo-frontal connectivity and thalamo-temporal (i.e., pulvinar to the transverse temporal cortex) connectivity compared with HCs. Thalamo-frontal functional connectivity may be comparable in SPD and HCs, but abnormal in schizophrenia, and that this may be protective against psychosis in SPD.

The human mediodorsal thalamus: Organization, connectivity, and function

The human mediodorsal thalamic nucleus (MD) is crucial for higher cognitive functions, while the fine anatomical organization of the MD and the function of each subregion remain elusive. In this study, using high-resolution data provided by the Human Connectome Project, an anatomical connectivity-based method was adopted to unveil the topographic organization of the MD. Four fine-grained subregions were identified in each hemisphere, including the medial (MDm), central (MDc), dorsal (MDd), and lateral (MDl), which recapitulated previous cytoarchitectonic boundaries from histological studies. The subsequent connectivity analysis of the subregions also demonstrated distinct anatomical and functional connectivity patterns, especially with the prefrontal cortex. To further evaluate the function of MD subregions, partial least squares analysis was performed to examine the relationship between different prefrontal-subregion connectivity and behavioral measures in 1012 subjects. The results showed subregion-specific involvement in a range of cognitive functions. Specifically, the MDm predominantly subserved emotional-cognition domains, while the MDl was involved in multiple cognitive functions especially cognitive flexibility and inhibition. The MDc and MDd were correlated with fluid intelligence, processing speed, and emotional cognition. In conclusion, our work provides new insights into the anatomical and functional organization of the MD and highlights the various roles of the prefrontal-thalamic circuitry in human cognition.

Disentangling the influences of multiple thalamic nuclei on prefrontal cortex and cognitive control

The prefrontal cortex (PFC) has a complex relationship with the thalamus, involving many nuclei which occupy predominantly medial zones along its anterior-to-posterior extent. Thalamocortical neurons in most of these nuclei are modulated by the affective and cognitive signals which funnel through the basal ganglia. We review how PFC-connected thalamic nuclei likely contribute to all aspects of cognitive control: from the processing of information on internal states and goals, facilitating its interactions with mnemonic information and learned values of stimuli and actions, to their influence on high-level cognitive processes, attentional allocation and goal-directed behavior. This includes contributions to transformations such as rule-to-choice (parvocellular mediodorsal nucleus), value-to-choice (magnocellular mediodorsal nucleus), mnemonic-to-choice (anteromedial nucleus) and sensory-to-choice (medial pulvinar). Common mechanisms appear to be thalamic modulation of cortical gain and cortico-cortical functional connectivity. The anatomy also implies a unique role for medial PFC in modulating processing in thalamocortical circuits involving other orbital and lateral PFC regions. We further discuss how cortico-basal ganglia circuits may provide a mechanism through which PFC controls cortico-cortical functional connectivity.

Altered microglia and neurovasculature in the Alzheimer's disease cerebellum

Traditionally regarded to coordinate movement, the cerebellum also exerts non-motor functions including the regulation of cognitive and behavioral processing, suggesting a potential role in neurodegenerative conditions affecting cognition, such as Alzheimer's disease (AD). This study aims to investigate neuropathology and AD-related molecular changes within the neocerebellum using post-mortem human brain tissue microarrays (TMAs). Immunohistochemistry was conducted on neocerebellar paraffin-embedded TMAs from 24 AD and 24 matched control cases, and free-floating neocerebellar sections from 6 AD and 6 controls. Immunoreactivity was compared between control and AD groups for neuropathological hallmarks (amyloid-β, tau, ubiquitin), Purkinje cells (calbindin), microglia (IBA1, HLA-DR), astrocytes (GFAP) basement-membrane associated molecules (fibronectin, collagen IV), endothelial cells (CD31/PECAM-1) and mural cells (PDGFRβ, αSMA). Amyloid-β expression (total immunolabel intensity) and load (area of immunolabel) was increased by >4-fold within the AD cerebellum. Purkinje cell counts, ubiquitin and tau immunoreactivity were unchanged in AD. IBA1 expression and load was increased by 91% and 69%, respectively, in AD, with no change in IBA1-positive cell number. IBA1-positive cell process length and branching was reduced by 22% and 41%, respectively, in AD. HLA-DR and GFAP immunoreactivity was unchanged in AD. HLA-DR-positive cell process length and branching was reduced by 33% and 49%, respectively, in AD. Fibronectin expression was increased by 27% in AD. Collagen IV, PDGFRβ and αSMA immunoreactivity was unchanged in AD. The number of CD31-positive vessels was increased by 98% in AD, suggesting the increase in CD31 expression and load in AD is due to greater vessel number. The PDGFRβ/CD31 load ratio was reduced by 59% in AD. These findings provide evidence of molecular changes affecting microglia and the neurovasculature within the AD neocerebellum. These changes, occurring without overt neuropathology, support the hypothesis of microglial and neurovascular dysfunction as drivers of AD, which has implications on the neocerebellar contribution to AD symptomatology and pathophysiology.

Topographic organization of connections between prefrontal cortex and mediodorsal thalamus: Evidence for a general principle of indirect thalamic pathways between directly connected cortical areas

Our ability to act flexibly, according to goals and context, is known as cognitive control. Hierarchical levels of control, reflecting different levels of abstraction, are represented across prefrontal cortex (PFC). Although the mediodorsal thalamic nucleus (MD) is extensively interconnected with PFC, the role of MD in cognitive control is unclear. Tract tracer studies in macaques, involving subsets of PFC areas, have converged on coarse MD-PFC connectivity principles; but proposed finer-grained topographic schemes, which constrain interactions between MD and PFC, disagree in many respects. To investigate a unifying topographic scheme, we performed probabilistic tractography on diffusion MRI data from eight macaque monkeys, and estimated the probable paths connecting MD with each of all 19 architectonic areas of PFC. We found a connectional topography where the orderly progression from ventromedial to anterior to posterolateral PFC was represented from anteromedial to posterolateral MD. The projection zones of posterolateral PFC areas in MD showed substantial overlap, and those of ventral and anteromedial PFC areas in MD overlapped. The exception was cingulate area 24: its projection zone overlapped with projections zones of all other PFC areas. Overall, our data suggest that nearby, functionally related, directly connected PFC areas have partially overlapping projection zones in MD, consistent with a role for MD in coordinating communication across PFC. Indeed, the organizing principle for PFC projection zones in MD appears to reflect the flow of information across the hierarchical, multi-level PFC architecture. In addition, cingulate area 24 may have privileged access to influence thalamocortical interactions involving all other PFC areas.

Variability and anatomical specificity of the orbitofrontothalamic fibers of passage in the ventral capsule/ventral striatum (VC/VS): precision care for patient-specific tractography-guided targeting of deep brain stimulation (DBS) in obsessive compulsive disorder (OCD)

Deep Brain Stimulation (DBS) is a neurosurgical procedure that can reduce symptoms in medically intractable obsessive-compulsive disorder (OCD). Conceptually, DBS of the ventral capsule/ventral striatum (VC/VS) region targets reciprocal excitatory connections between the orbitofrontal cortex (OFC) and thalamus, decreasing abnormal reverberant activity within the OFC-caudate-pallidal-thalamic circuit. In this study, we investigated these connections using diffusion magnetic resonance imaging (dMRI) on human connectome datasets of twenty-nine healthy young-adult volunteers with two-tensor unscented Kalman filter based tractography. We studied the morphology of the lateral and medial orbitofrontothalamic connections and estimated their topographic variability within the VC/VS region. Our results showed that the morphology of the individual orbitofrontothalamic fibers of passage in the VC/VS region is complex and inter-individual variability in their topography is high. We applied this method to an example OCD patient case who underwent DBS surgery, formulating an initial proof of concept for a tractography-guided patient-specific approach in DBS for medically intractable OCD. This may improve on current surgical practice, which involves implanting all patients at identical stereotactic coordinates within the VC/VS region.

MAPBOT: Meta-analytic parcellation based on text, and its application to the human thalamus

Meta-analysis of neuroimaging results has proven to be a popular and valuable method to study human brain functions. A number of studies have used meta-analysis to parcellate distinct brain regions. A popular way to perform meta-analysis is typically based on the reported activation coordinates from a number of published papers. However, in addition to the coordinates associated with the different brain regions, the text itself contains considerably amount of additional information. This textual information has been largely ignored in meta-analyses where it may be useful for simultaneously parcellating brain regions and studying their characteristics. By leveraging recent advances in document clustering techniques, we introduce an approach to parcellate the brain into meaningful regions primarily based on the text features present in a document from a large number of studies. This new method is called MAPBOT (Meta-Analytic Parcellation Based On Text). Here, we first describe how the method works and then the application case of understanding the sub-divisions of the thalamus. The thalamus was chosen because of the substantial body of research that has been reported studying this functional and structural structure for both healthy and clinical populations. However, MAPBOT is a general-purpose method that is applicable to parcellating any region(s) of the brain. The present study demonstrates the powerful utility of using text information from neuroimaging studies to parcellate brain regions.

Network-selective vulnerability of the human cerebellum to Alzheimer's disease and frontotemporal dementia

SEE SCHMAHMANN DOI101093/BRAIN/AWW064 FOR A SCIENTIFIC COMMENTARY ON THIS ARTICLE: Neurodegenerative diseases are associated with distinct and distributed patterns of atrophy in the cerebral cortex. Emerging evidence suggests that these atrophy patterns resemble intrinsic connectivity networks in the healthy brain, supporting the network-based degeneration framework where neuropathology spreads across connectivity networks. An intriguing yet untested possibility is that the cerebellar circuits, which share extensive connections with the cerebral cortex, could be selectively targeted by major neurodegenerative diseases. Here we examined the structural atrophy in the cerebellum across common types of neurodegenerative diseases, and characterized the functional connectivity patterns of these cerebellar atrophy regions. Our results showed that Alzheimer's disease and frontotemporal dementia are associated with distinct and circumscribed atrophy in the cerebellum. These cerebellar atrophied regions share robust and selective intrinsic connectivity with the atrophied regions in the cerebral cortex. These findings for the first time demonstrated the selective vulnerability of the cerebellum to common neurodegenerative disease, extending the network-based degeneration framework to the cerebellum. Our work also has direct implications on the cerebellar contribution to the cognitive and affective processes that are compromised in neurodegeneration as well as the practice of using the cerebellum as reference region for ligand neuroimaging studies.

Characterization of Behaviour and Remote Degeneration Following Thalamic Stroke in the Rat

Subcortical ischemic strokes are among the leading causes of cognitive impairment. Selective atrophy of remote brain regions connected to the infarct is thought to contribute to deterioration of cognitive functions. The mechanisms underlying this secondary degenerative process are incompletely understood, but are thought to include inflammation. We induce ischemia by unilateral injection of endothelin-I into the rat dorsomedial thalamic nucleus, which has defined reciprocal connections to the frontal cortex. We use a comprehensive test battery to probe for changes in behaviour, including executive functions. After a four-week recovery period, brain sections are stained with markers for degeneration, microglia, astrocytes and myelin. Degenerative processes are localized within the stroke core and along the full thalamocortical projection, which does not translate into measurable behavioural deficits. Significant microglia recruitment, astrogliosis or myelin loss along the axonal projection or within the frontal cortex cannot be detected. These findings indicate that critical effects of stroke-induced axonal degeneration may only be measurable beyond a threshold of stroke severity and/or follow a different time course. Further investigations are needed to clarify the impact of inflammation accompanying axonal degeneration on delayed remote atrophy after stroke.

Damage to the dorsomedial thalamic nucleus, central lateral intralaminar thalamic nucleus, and midline thalamic nuclei on the right-side impair executive function and attention under conditions of high demand but not low demand

This study reports a patient, OG, with a unilateral right-sided thalamic lesion. High resolution 3T magnetic resonance imaging revealed damage to the parvicellular and magnocellular subdivisions of the dorsomedial thalamus (DMT), the central lateral intralaminar nucleus (also known as the paralamellar DMT), the paraventricular and the central medial midline thalamic nuclei. According to the neuropsychological literature, the DMT, the midline and intralaminar thalamic nuclei influence a wide array of cognitive functions by virtue of their modulatory influences on executive function and attention, and this is particularly indicated under conditions of low arousal or high cognitive demand. We explored this prediction in OG, and compared his performance on a range of low and high demand versions of tests that tapped executive function and attention to a group of 6 age- and IQ-matched controls. OG, without exception, significantly under performed on the high-demand attention and executive function tasks, but performed normally on the low-demand versions. These findings extend and refine current understanding of the effects of thalamic lesion on attention and executive function.

Cortico-amygdala-striatal circuits are organized as hierarchical subsystems through the primate amygdala

The prefrontal and insula cortex, amygdala, and striatum are key regions for emotional processing, yet the amygdala's role as an interface between the cortex and striatum is not well understood. In the nonhuman primate (Macaque fascicularis), we analyzed a collection of bidirectional tracer injections in the amygdala to understand how cortical inputs and striatal outputs are organized to form integrated cortico-amygdala-striatal circuits. Overall, diverse prefrontal and insular cortical regions projected to the basal and accessory basal nuclei of the amygdala. In turn, these amygdala regions projected to widespread striatal domains extending well beyond the classic ventral striatum. Analysis of the cases in aggregate revealed a topographic colocalization of cortical inputs and striatal outputs in the amygdala that was additionally distinguished by cortical cytoarchitecture. Specifically, the degree of cortical laminar differentiation of the cortical inputs predicted amygdalostriatal targets, and distinguished three main cortico-amygdala-striatal circuits. These three circuits were categorized as "primitive," "intermediate," and "developed," respectively, to emphasize the relative phylogenetic and ontogenetic features of the cortical inputs. Within the amygdala, these circuits appeared arranged in a pyramidal-like fashion, with the primitive circuit found in all examined subregions, and subsequent circuits hierarchically layered in discrete amygdala subregions. This arrangement suggests a stepwise integration of the functions of these circuits across amygdala subregions, providing a potential mechanism through which internal emotional states are managed with external social and sensory information toward emotionally informed complex behaviors.

Mapping changes of in vivo connectivity patterns in the human mediodorsal thalamus: correlations with higher cognitive and executive functions

The mediodorsal thalamic nucleus is recognized as an association hub mediating interconnections with mainly the prefrontal cortex. Tracer studies in primates and in vivo diffusion tensor tractography findings in both humans and monkeys confirm its role in relaying networks that connect to the dorsolateral prefrontal, orbitofrontal, frontal medial and cingulate cortex. Our study was designed to use in vivo probabilistic tractography to describe the pathways emerging from or projecting to the mediodorsal nucleus; moreover, to use such information to automatically define subdivisions based on the divergence of remote structural connections. Diffusion tensor MR imaging data of 156 subjects were utilized to perform connectivity-based segmentation of the mediodorsal nucleus by employing a k-means clustering algorithm. Two domains were revealed (medial and lateral) that are separated from each other by a sagittally oriented plane. For each subject, general assessment of cognitive performance by means of the Wechsler Abbreviated Scale of Intelligence and measures of Delis-Kaplan Executive Function System (D-KEFS) test was utilized. Inter-subject variability in terms of connectivity-based cluster sizes was discovered and the relative sizes of the lateral mediodorsal domain correlated with the individuals' performance in the D-KEFS Sorting test (r = 0.232, p = 0.004). Our results show that the connectivity-based parcellation technique applied to the mediodorsal thalamic nucleus delivers a single subject level descriptor of connectional topography; furthermore, we revealed a possible weak interaction between executive performance and the size of the thalamic area from which pathways converge to the lateral prefrontal cortex.

Contribution of the central thalamus to the generation of volitional saccades

Lesions in the motor thalamus can cause deficits in somatic movements. However, the involvement of the thalamus in the generation of eye movements has only recently been elucidated. In this article, we review recent advances into the role of the thalamus in eye movements. Anatomically, the anterior group of the intralaminar nuclei and paralaminar portion of the ventrolateral, ventroanterior and mediodorsal nuclei of the thalamus send massive projections to the frontal eye field and supplementary eye field. In addition, these parts of the thalamus, collectively known as the 'oculomotor thalamus', receive inputs from the cerebellum, the basal ganglia and virtually all stages of the saccade-generating pathways in the brainstem. In their pioneering work in the 1980s, Schlag and Schlag-Rey found a variety of eye movement-related neurons in the oculomotor thalamus, and proposed that this region might constitute a 'central controller' playing a role in monitoring eye movements and generating self-paced saccades. This hypothesis has been evaluated by recent experiments in non-human primates and by clinical observations of subjects with thalamic lesions. In addition, several recent studies have also addressed the involvement of the oculomotor thalamus in the generation of anti-saccades and the selection of targets for saccades. These studies have revealed the impact of subcortical signals on the higher-order cortical processing underlying saccades, and suggest the possibility of future studies using the oculomotor system as a model to explore the neural mechanisms of global cortico-subcortical loops and the neural basis of a local network between the thalamus and cortex.

Neuroanatomical correlates of executive dysfunction in the at-risk mental state for psychosis

Deficits in executive functioning have been described as a core feature of schizophrenia and have been linked to patterns of fronto-temporo-limbic brain alterations. To date, such structure-cognition relationships have not been explored in a clinically defined at-risk mental state (ARMS) for psychosis using whole-brain neuroimaging techniques. Therefore, we used voxel-based morphometry in 40 ARMS and 30 matched healthy control (HC) individuals to investigate whether gray and white matter volumes (1) correlated with the performance in the Trail-Making Test B (TMT-B), an established measure of executive functioning, and (2) were volumetrically linked to the ventromedial prefrontal cortex (VMPFC), found to be associated with TMT-B in the ARMS during the first analysis step. We found the ARMS subjects to be specifically impaired in their TMT-B performance versus HC. Brain-cognition associations involving the insular cortices were observed in the HC, but not in the ARMS individuals. Conversely, TMT-B correlations in the VMPFC, the cerebellum, the fronto-callosal white matter were detected in the ARMS, but not the HC group. The VMPFC was linked to the temporo-limbic cortices in HC, whereas the connectivity pattern in the ARMS involved the left temporal and dorsolateral prefrontal cortex, the cerebellum, the right SMA and extended portions of the fronto-callosal white matter. These findings suggest that executive deficits are already present in the ARMS for psychosis and may be subserved by structurally altered networks of interconnected cortical and subcortical brain regions in line with the disconnectivity hypothesis of schizophrenia.

Morphological abnormalities of the thalamus in youths with attention deficit hyperactivity disorder

OBJECTIVE

The role of the thalamus in the genesis of attention deficit hyperactivity disorder (ADHD) remains poorly understood. The authors used anatomical MRI to examine the morphology of the thalamus in youths with ADHD and healthy comparison youths.

METHOD

The authors examined 46 youths with ADHD and 59 comparison youths 8-18 years of age in a cross-sectional case-control study. Conventional volumes and measures of surface morphology of the thalamus served as the main outcome measures.

RESULTS

A mixed-effects model comparing whole thalamic volumes revealed no significant differences between groups. Maps of the thalamic surface revealed significantly smaller regional volumes bilaterally in the pulvinar in youths with ADHD relative to comparison subjects. Post hoc analyses showed that ADHD patients who received stimulants (N=31) had larger conventional thalamic volumes than untreated youths with ADHD, and maps of the thalamic surface showed enlargement over the pulvinar in those receiving stimulants. Smaller regional volumes in the right lateral and left posterior thalamic surfaces were associated with more severe hyperactivity symptoms, whereas larger regional volumes in the right medial thalamic surfaces were associated with more severe symptoms of inattention.

CONCLUSION

These findings demonstrate reduced pulvinar volumes in youths with ADHD and indicate that this same area is relatively enlarged in patients treated with stimulants compared to those untreated. Associations of hyperactivity scores with smaller regional volumes on the lateral thalamic surface and inattention scores with larger regional volumes on the medial thalamic surface suggest the differential involvement of thalamic subcircuits in the pathogenesis of differing ADHD symptoms.

Effects of normal aging on prefrontal area 46 in the rhesus monkey

This review is concerned with the effects of normal aging on the structure and function of prefrontal area 46 in the rhesus monkey (Macaca mulatta). Area 46 has complex connections with somatosensory, visual, visuomotor, motor, and limbic systems and a key role in cognition, which frequently declines with age. An important question is what alterations might account for this decline. We are nowhere near having a complete answer, but as will be shown in this review, it is now evident that there is no single underlying cause. There is no significant loss of cortical neurons and although there are a few senile plaques in rhesus monkey cortex, their frequency does not correlate with cognitive decline. However, as discussed in this review, the following do correlate with cognitive decline. Loss of white matter has been proposed to result in some disconnections between parts of the central nervous system and changes in the structure of myelin sheaths reduce conduction velocity and the timing in neuronal circuits. In addition, there are reductions in the inputs to cortical neurons, as shown by regression of dendritic trees, loss of dendritic spines and synapses, and alterations in transmitters and receptors. These factors contribute to alterations in the intrinsic and network physiological properties of cortical neurons. As more details emerge, it is to be hoped that effective interventions to retard cognitive decline can be proposed.

Substantia Nigra Output to Prefrontal Cortex Via Thalamus in Monkeys. II. Activity of Thalamic Relay Neurons in Delayed Conditional Go/No-Go Discrimination Task

The present report investigated the involvement of primate nigro-thalamo-cortical projections in discrimination of visual signals with behavioral meaning. We tested the extracellular unit activity of mediodorsal (MD) and ventral anterior (VA) thalamic neurons monosynaptically receiving inhibitory input from the substantia nigra pars reticulata (SNr) and projecting to the frontal cortex in Japanese monkeys performing a delayed conditional go/no-go discrimination task. In the task two colored stimuli (S1, S2) intervened by delay period required the monkeys lifting a lever (go) or not (no-go); the same and different colored pairs of S1 and S2 meant go and no-go signals, respectively. Prominent task-relevant responses were sustained activity with color preference to S1 during delay period and S2-related activity with different firing rates between go and no-go trials. In particular, a high proportion of such go/no-go differential S2-related activity was found in thalamic relay neurons, receiving input from the caudolateral SNr and projecting to the prefrontal area (PSv) ventral to the principal sulcus, in the rostrolateral MD. The findings suggest that the caudolateral SNr–rostrolateral MD–PSv pathways may be possible conduits of signals coding the behavioral meaning of the visual stimuli and thus may be responsible for generating similar neuronal activity in the PSv.

Bilateral thalamic stroke due to occlusion of the artery of Percheron in a patient with patent foramen ovale: a case report

Introduction

Bilateral thalamic infarcts are rare presentations of stroke. They are the result of a complex combination of risk factors and a predisposing vessel distribution. The artery of Percheron, characterized by a single arterial trunk that irrigates both paramedian thalamic regions, can be occluded as a result of embolic diseases leading to bilateral paramedian thalamic infarcts. Clinical and image findings of this uncommon form of posterior circulation infarct are presented along with their anatomic and pathophysiologic correlates.

Case presentation

A 27-year-old Mexican man with no relevant medical history was admitted to hospital after he was found deeply stuporous. On admission, an urgent neuroimaging protocol for stroke, including magnetic resonance imaging and magnetic resonance imaging angiography, was performed. The scans revealed symmetric bilateral hyperintense paramedian thalamic lesions consistent with acute ischemic events. The posterior circulation was patent including the tip of the basilar artery and both posterior cerebral arteries, making the case compatible with occlusion of the artery of Percheron. Further evaluation with an aim to define the etiology revealed a patent foramen ovale as the cause of embolism.

Conclusion

Bilateral thalamic infarcts are unusual presentations of posterior circulation stroke; once they are diagnosed by an adequate neuroimaging protocol, a further evaluation to define the cause is necessary. Cardioembolism should always be considered in relatively young patients. A complete evaluation should be conducted by an interdisciplinary team including neurologists, cardiologists and neurosurgeons.

Laminar and modular organization of prefrontal projections to multiple thalamic nuclei

The prefrontal cortex projects to many thalamic nuclei, in pathways associated with cognition, emotion, and action. We investigated how multiple projection systems to the thalamus are organized in prefrontal cortex after injection of distinct retrograde tracers in the principal mediodorsal (MD), the limbic anterior medial (AM), and the motor-related ventral anterior/ventral lateral (VA/VL) thalamic nuclei in rhesus monkeys. Neurons projecting to these nuclei were organized in interdigitated modules extending vertically within layers VI and V. Projection neurons were also organized in layers. The majority of projection neurons to MD or AM originated in layer VI ( approximately 80%), but a significant proportion ( approximately 20%) originated in layer V. In contrast, prefrontal neurons projecting to VA/VL were equally distributed in layers V and VI. Neurons directed to VA/VL occupied mostly the upper part of layer V, while neurons directed to MD or AM occupied mostly the deep part of layer V. The highest proportions of projection neurons in layer V to each nucleus were found in dorsal and medial prefrontal areas. The laminar organization of prefrontal cortico-thalamic projections differs from sensory systems, where projections originate predominantly or entirely from layer VI. Previous studies indicate that layer V cortico-thalamic neurons innervate through some large terminals thalamic neurons that project widely to superficial cortical layers. The large population of prefrontal projection neurons in layer V may drive thalamic neurons, triggering synchronization by recruiting several cortical areas through widespread thalamo-cortical projections to layer I. These pathways may underlie the synthesis of cognition, emotion and action.

Disconnection syndromes of basal ganglia, thalamus, and cerebrocerebellar systems

Disconnection syndromes were originally conceptualized as a disruption of communication between different cerebral cortical areas. Two developments mandate a re-evaluation of this notion. First, we present a synopsis of our anatomical studies in monkey elucidating principles of organization of cerebral cortex. Efferent fibers emanate from every cortical area, and are directed with topographic precision via association fibers to ipsilateral cortical areas, commissural fibers to contralateral cerebral regions, striatal fibers to basal ganglia, and projection subcortical bundles to thalamus, brainstem and/or pontocerebellar system. We note that cortical areas can be defined by their patterns of subcortical and cortical connections. Second, we consider motor, cognitive and neuropsychiatric disorders in patients with lesions restricted to basal ganglia, thalamus, or cerebellum, and recognize that these lesions mimic deficits resulting from cortical lesions, with qualitative differences between the manifestations of lesions in functionally related areas of cortical and subcortical nodes. We consider these findings on the basis of anatomical observations from tract tracing studies in monkey, viewing them as disconnection syndromes reflecting loss of the contribution of subcortical nodes to the distributed neural circuits. We introduce a new theoretical framework for the distributed neural circuits, based on general, and specific, principles of anatomical organization, and on the architecture of the nodes that comprise these systems. We propose that neural architecture determines function, i.e., each architectonically distinct cortical and subcortical area contributes a unique transform, or computation, to information processing; anatomically precise and segregated connections between nodes define behavior; and association fiber tracts that link cerebral cortical areas with each other enable the cross-modal integration required for evolved complex behaviors. This model enables the formulation and testing of future hypotheses in investigations using evolving magnetic resonance imaging techniques in humans, and in clinical studies in patients with cortical and subcortical lesions.

Adult onset leukodystrophy with neuroaxonal spheroids: clinical, neuroimaging and neuropathologic observations

Pigmented orthochromatic leukodystrophy and hereditary diffuse leukoencephalopathy with spheroids are two adult onset leukodystrophies with neuroaxonal spheroids presenting with prominent neurobehavioral, cognitive and motor symptoms. These are familial or sporadic disorders characterized by cerebral white matter degeneration including myelin and axonal loss, gliosis, macrophages and axonal spheroids. We report clinical, neuroimaging and pathological correlations of four women ages 34-50 years with adult onset leukodystrophy. Their disease course ranged from 1.5-8 years. Three patients had progressive cognitive and behavioral changes; however, one had acute onset. Neuroimaging revealed white matter abnormalities characterized by symmetric, bilateral, T2 hyperintense and T1 hypointense Magnetic Resonance Imaging signal involving frontal lobe white matter in all patients. Extensive laboratory investigations were negative apart from abnormalities in some mitochondrial enzymes and immunologic parameters. Autopsies demonstrated severe leukodystrophy with myelin and axonal loss, axonal spheroids and macrophages with early and severe frontal white matter involvement. The extent and degree of changes outside the frontal lobe appeared to correlate with disease duration. The prominent neurobehavioral deficits and frontal white matter disease provide clinical-pathologic support for association pathways linking distributed neural circuits sub-serving cognition. These observations lend further support to the notion that white matter disease alone can account for dementia.

Sustained attention and serotonin: a pharmaco-fMRI study

OBJECTIVE

Evidence suggests that stimulation of serotonergic function in healthy humans causes an impairment of sustained attention. The present study assessed the influence of increased serotonin levels on brain areas involved in sustained attention.

METHODS

Ten healthy volunteers (5 females, 5 males) received the selective serotonin reuptake inhibitor (SSRI) escitalopram (20 mg) and placebo in a balanced, double blind, two-way crossover design. Participants performed the Mackworth Clock Test to measure sustained attention during functional MRI measurements at 3 Tesla. Subjective measurements after pharmacological manipulation were conducted with the Bond and Lader Questionnaire.

RESULTS

Independent of treatment, brain areas associated with task performance on a sustained attention task were activated, including right prefrontal and parietal areas. After escitalopram administration, less activation was shown in the caudate nucleus, thalamus, and frontal areas. No effect of escitalopram was shown on behavioral data although subjective measurements showed decreased alertness after escitalopram.

CONCLUSIONS

The results of the current pharmaco-functional magnetic resonance imaging (fMRI) study give a first indication of involvement of serotonin in sustained attention through modulating activation of selective brain areas including the thalamus and caudate nucleus. Possibly, these areas are involved in a subcortical network for sustained attention, but further research is necessary.

Distinguishing specific sexual and general emotional effects in fMRI-subcortical and cortical arousal during erotic picture viewing

Sexual activity involves excitement with high arousal and pleasure as typical features of emotions. Brain activations specifically related to erotic feelings and those related to general emotional processing are therefore hard to disentangle. Using fMRI in 21 healthy subjects (11 males and 10 females), we investigated regions that show activations specifically related to the viewing of sexually intense pictures while controlling for general emotional arousal (GEA) or pleasure. Activations in the ventral striatum and hypothalamus were found to be modulated by the stimulus' specific sexual intensity (SSI) while activations in the anterior cingulate cortex were associated with an interaction between sexual intensity and emotional valence. In contrast, activation in other regions like the dorsomedial prefrontal cortex, the mediodorsal thalamus and the amygdala was associated only with a general emotional component during sexual arousal. No differences were found in these effects when comparing females and males. Our findings demonstrate for the first time neural differentiation between emotional and sexual components in the neural network underlying sexual arousal.

Role of the Lateral Prefrontal Cortex in Executive Behavioral Control

The lateral prefrontal cortex is critically involved in broad aspects of executive behavioral control. Early studies emphasized its role in the short-term retention of information retrieved from cortical association areas and in the inhibition of prepotent responses. Recent studies of subhuman primates and humans have revealed the role of this area in more general aspects of behavioral planning. Novel findings of neuronal activity have specified how neurons in this area take part in selective attention for action and in selecting an intended action. Furthermore, the involvement of the lateral prefrontal cortex in the implementation of behavioral rules and in setting multiple behavioral goals has been discovered. Recent studies have begun to reveal neuronal mechanisms for strategic behavioral planning and for the development of knowledge that enables the planning of macrostructures of event-action sequences at the conceptual level.

Efferent association pathways from the rostral prefrontal cortex in the macaque monkey

The different prefrontal cortical regions exert executive control over processing occurring in posterior cortical regions. We examined with the autoradiographic method, in the macaque monkey, the course and terminations of the efferent corticocortical connections of the rostral prefrontal region, the function of which is least understood. Three efferent streams of fibers organized into three distinct fasciculi convey rostral prefrontal influences on posterior cortical areas. These connections provide powerful insights into the cortical regions on which executive control is being exercised. The lateral stream of fibers via the extreme capsule targets the midsection of the auditory superior temporal region and the multisensory areas of the superior temporal sulcus, thus permitting control over the most integrated aspects of cognitive processing. The fibers coursing through the extreme capsule originating in areas 10 and 9 continue as part of the white matter of the superior temporal gyrus (i.e., the middle longitudinal fasciculus) to target the midportion of the superior temporal gyrus (areas TAa, TS2, and TS3) and adjacent multisensory area TPO within the upper bank of the superior temporal sulcus. Some of the fibers from areas 10 and 9 that enter the extreme capsule terminate in the ventral part of the insula. The dorsomedial limbic stream via the cingulate fasciculus targets the anterior and posterior cingulate cortex, as well as the retrosplenial cortex, allowing control over motivational and memory processes. A ventral limbic stream via the uncinate fasciculus targets the temporal proisocortex and the amygdala, indicating an additional powerful influence over the emotional motivational sphere.

Transforming bottom-up topographic representations with top-down signals in the brain

There has been considerable success in allocating function to the different parts of the brain. We also know much about brain organisation in different regions of the brain and how different brain regions connect to one another. One of the most important next steps for modern neuroscience is to work out how different areas of the brain interact with one another. In particular we need to know how sensory regions communicate with association areas and vice versa. This article explores how top-down signals originating from association areas may be used to process and transform bottom-up representations originating from sensory areas of the brain. Simple models of networks containing topographically organised ensembles of neurons used to integrate and process information are described. The different models can be used to process information in a variety of different ways that could be used as the starting point for a variety of cognitive operations, in particular the extraction of abstract information from sensory representations.

Thalamic chronotaraxis: isolated time disorientation

BACKGROUND

Acute isolated disorientation of time, chronotaraxis, is an uncommon manifestation of thalamic stroke. To our knowledge, acute thalamic chronotaraxis with MRI findings has not previously been reported.

OBJECTIVE

To describe five patients with chronotaraxis after thalamic stroke and attempt to demonstrate the correlation between lesion location and neurological findings.

PATIENTS, METHODS AND RESULTS

Isolated time disorientation and loss of time sense were found in five of 120 patients (4%) with ischaemic thalamic stroke in our centre. All patients had disorientation to actual date, inability to know the exact time of the day and under or overestimation of the time passed during examination. Patients expressed themselves as having time blindness with an inability to estimate and guess the actual time.

CONCLUSION

Acute thalamic chronotaraxis is a specific clinical picture that accurately predicts a small artery disease of the thalamus involving the mediodorsal nucleus of the thalamus. This clinical syndrome appears to have a good clinical recovery.

Cerebral white matter--historical evolution of facts and notions concerning the organization of the fiber pathways of the brain

Gross and microscopic studies by early investigators led the cerebral white matter from being regarded as an amorphous mass to an intricately organized system of fasciculi that facilitate the highest expression of cerebral activity. Here we pay homage to the anatomists whose observations resulted in the evolution of ideas about the cerebral white matter. We also draw attention to limitations of the earlier methodologies and to some of the conflicts and controversies that have characterized this field and that persist in the current literature. We conclude with brief reference to the principles of organization of the fiber pathways derived from our studies in the monkey using the autoradiographic tract tracing technique, another step in the ongoing investigations of the cerebral white matter. This historical review has contemporary relevance because the fiber pathways of the brain are crucial components of the distributed neural circuits that subserve nervous system function; the clinical manifestations of white matter damage are recognized with greater frequency and clarity; and magnetic resonance imaging tractography has made it possible to view these fiber bundles within the living human brain.

Neurobiological mechanisms for the regulation of mammalian sleep-wake behavior: reinterpretation of historical evidence and inclusion of contemporary cellular and molecular evidence

At its most basic level, the function of mammalian sleep can be described as a restorative process of the brain and body; recently, however, progressive research has revealed a host of vital functions to which sleep is essential. Although many excellent reviews on sleep behavior have been published, none have incorporated contemporary studies examining the molecular mechanisms that govern the various stages of sleep. Utilizing a holistic approach, this review is focused on the basic mechanisms involved in the transition from wakefulness, initiation of sleep and the subsequent generation of slow-wave sleep and rapid eye movement (REM) sleep. Additionally, using recent molecular studies and experimental evidence that provides a direct link to sleep as a behavior, we have developed a new model, the cellular-molecular-network model, explaining the mechanisms responsible for regulating REM sleep. By analyzing the fundamental neurobiological mechanisms responsible for the generation and maintenance of sleep-wake behavior in mammals, we intend to provide a broader understanding of our present knowledge in the field of sleep research.

Efferent association pathways originating in the caudal prefrontal cortex in the macaque monkey

The efferent association fibers from the caudal part of the prefrontal cortex to posterior cortical areas course via several pathways: the three components of the superior longitudinal fasciculus (SLF I, SLF II, and SLF III), the arcuate fasciculus (AF), the fronto-occipital fasciculus (FOF), the cingulate fasciculus (CING F), and the extreme capsule (Extm C). Fibers from area 8Av course via FOF and SLF II, merging in the white matter of the inferior parietal lobule (IPL) and terminating in the caudal intraparietal sulcus (IPS). A group of these fibers turns ventrally to terminate in the caudal superior temporal sulcus (STS). Fibers from the rostral part of area 8Ad course via FOF and SLF II to the IPS and IPL and via the AF to the caudal superior temporal gyrus and STS. Some fibers from the rostral part of area 8Ad are conveyed to the medial parieto-occipital region via FOF, to the STS via Extm C, and to the caudal cingulate gyrus via CING F. Fibers from area 8B travel via SLF I to the supplementary motor area and area 31 in the caudal dorsal cingulate region and via the CING F to cingulate areas 24 and 23 and the cingulate motor areas. Fibers from area 9/46d course via SLF I to the superior parietal lobule and medial parieto-occipital region, via SLF II to the IPL. Fibers from area 9/46v travel via SLF III to the rostral IPL and the frontoparietal opercular region and via the CING F to the cingulate gyrus.

Metabolic thalamocortical correlations during a verbal learning task and their comparison with correlations among regional volumes

Methods based on the analysis of metabolic and volumetric interregional correlations have been used in neuroimaging research, yet metabolic and volumetric interregional correlations for identical regions of interest have never been compared in the same group of subjects. Magnetic resonance and [18F]-fluorodeoxyglucose positron emission tomography brain images were acquired in 59 healthy subject. Correlation matrices for relative glucose metabolic rates during a verbal learning task and for relative gray matter volumes were compiled between the manually traced mediodorsal, centromedian, and pulvinar nuclei of the thalamus and 39 cortical Brodmann's areas. Metabolic correlations between the cortex and these thalamic nuclei followed the known patterns of anatomical connectivity in non-human primates. Intercorrelations of the mediodorsal nucleus were widespread with the prefrontal cortex (9 out of 10 Brodmann's areas in the left hemisphere) and temporal lobe (10 out of 11 Brodmann's areas in the left hemisphere) while the pulvinar correlated only with the parietal and occipital cortical areas. Different correlation patterns were observed for the regional gray matter volumes whereby only the pulvinar yielded extensive cortical intercorrelations, primarily with the occipital, parietal, anterior cingulate, and orbitofrontal areas in the right hemisphere. Metabolic thalamocortical correlations were much more extensive for the mediodorsal and centromedian nuclei whereas structural correlations were more extensive for the pulvinar. Therefore, metabolic and volumetric correlational methods are sensitive to different aspects of interregional relations in the brain and their comparison in the same group of subjects may render complementary and only partially overlapping connectivity information.

Volume and neuron number of the mediodorsal thalamic nucleus in schizophrenia: a replication study

Previous neuropathological studies on the mediodorsal thalamic nucleus (MD) in schizophrenia have yielded conflicting results. While some studies suggested that patients with schizophrenia have a pronounced reduction of the volume and neuron number in the MD, more recent data have not found anatomical alterations in this thalamic nucleus. However, most studies have considerable methodological shortcomings. In the present study, we investigated the volume, neuron density and neuron number in the left and right MD in patients with schizophrenia (n=20) and normal control subjects without neuropsychiatric disorders (n=18). Patients with schizophrenia showed no significant difference in neuron density and total neuron number in the MD. Compared with the control group, patients with schizophrenia had a smaller MD volume in both hemispheres, a difference that approached significance in the left MD (-7.3%) when the whole brain volume was included as a covariate. No significant main group effect of diagnosis was found for the right MD volume. There were no significant correlations between MD volume, neuron density, total neuron number and the duration of illness or the age of the patients. Taken together, the present results suggest that schizophrenia is associated with a moderate volume reduction in the left mediodorsal thalamic nucleus, while the neuron density and the total neuron number are unchanged.

Correlations between MRI-assessed volumes of the thalamus and cortical Brodmann's areas in schizophrenia

BACKGROUND

We compared the thalamic-cortical volumetric correlational patterns in patients with schizophrenia and normal comparison subjects, and evaluated their relations to outcome.

METHODS

High-resolution MR images were acquired in patients with schizophrenia (n=106) and normal comparison subjects (n=42). Patients were divided into good-outcome (n=52) and poor-outcome (Kraepelinian, n=54) subtypes based on their ability for self-care. Correlations between the relative gray and white matter volumes of the individual cortical Brodmann's areas and five dorsoventral levels of the thalamus were assessed.

RESULTS

Compared to normal subjects, schizophrenia patients lacked significant thalamic gray matter volume correlations with the prefrontal and medial temporal cortical regions in the right hemisphere, and with frontal, cingulate, posterior parietal and occipital regions in the left hemisphere, while normal white matter volume cortical-thalamic correlations along the cingulate gyrus and in the temporal lobe were not found in schizophrenia patients in both hemispheres. In contrast to both normal comparison subjects and good-outcome group, schizophrenia patients with poor outcomes showed significant bilateral gray matter volume correlations between the dorsal thalamus and ventral prefrontal cortex, while the group differences in the white matter volume correlations were mostly restricted to the cingulate arch.

CONCLUSIONS

Whereas patients with schizophrenia exhibit deficiencies in cortical-thalamic correlational patterns, poor outcome is associated with abnormal interregional correlations not observed in either normal subjects or patients with good outcomes. This latter finding may be explained by a core neurodevelopmental disturbance that results in aberrant cortical-thalamic connectivity in poor-outcome schizophrenia.

Morphometric characterization of synapses in the primate prefrontal cortex formed by afferents from the mediodorsal thalamic nucleus

The main thalamic afferentation of the prefrontal cortex (PFC) originates in the mediodorsal nucleus (MD). Although it is suggested that this pathway is affected in schizophrenia, there is a lack of functional and structural data regarding its synaptic organization. The scope of this study was to characterize the ultrastructural features of thalamocortical synapses formed by afferents from the MD by applying anterograde tract tracing, immunohistochemical detection of parvalbumin (PV, a probable marker of thalamocortical endings), and quantitative electron microscopic techniques to the PFC of the macaque monkey. Our findings indicate that anterogradely-labeled and PV-immunoreactive boutons exhibit similar ultrastructural properties, characterized by their larger size, higher incidence of release sites and a higher occurrence of mitochondria when compared to non-labeled, excitatory-like endings in the middle layers of the PFC. Although most of the contacts were made on spines in both cases, PV-immunopositive axon terminals apparently targeted dendritic shafts at about twice the frequency found for anterogradely-labeled afferents from the MD (20.5% and 9.5%, respectively). This result suggests diversity among thalamocortical and/or PV-immunoreactive axon terminals of the PFC. In accordance with studies in other cortical areas, our findings suggest that corollary discharge through the mediodorsal thalamocortical projection is also adapted to synaptic transmission with high efficacy and probably exhibits marked short-term temporal dynamics in the PFC.

Cortical connections of the lateral mediodorsal thalamus in cynomolgus monkeys

The prefrontal cortex has been defined as that cortical territory that has "essential or sustaining" connections with the mediodorsal (MD) nucleus of the thalamus. However, recent studies in the monkey have documented projections from MD to the more caudal, agranular regions of the frontal cortex, suggesting that the connections of MD may be characterized by a breadth of distribution and diversity of functional roles too great to be useful as a unifying and defining feature for a specific cortical territory. In this study, we placed tracer injections in the lateral divisions of MD in cynomolgus monkeys (Macaca fascicularis) to assess the relative proportions of connections devoted to diverse regions of the frontal cortex (FC). Three different patterns of label were observed in the cortex, associated with different locations within lateral MD. We have designated these as the ventrolateral MD-arcuate FC circuit, having most label in areas 8 and 6; the caudoventral MD-dorsomedial FC circuit, having most label in areas 24 and presupplementary motor area (SMA); and the anterodorsal MD-anterior FC circuit, with the most label in areas 9, 46, 12, and 10. Only two of the nine cases injected in lateral MD were predominantly connected with the anterior FC. Thus, particular locales within lateral MD are connected with multiple, functionally diverse cortical regions, including several not classically recognized as "prefrontal" areas. This divergence may distinguish MD-frontocortical and reciprocal corticothalamic pathways from the largely segregated pathways arising from the other thalamic nuclei that are interconnected with the frontal cortex, such as those from the ventrolateral nuclear group.

Subcortical afferents to the lateral mediodorsal thalamus in cynomolgus monkeys

The mediodorsal (MD) nucleus of the thalamus has long been known to provide the principal source of subcortical input to the primate prefrontal cortex, as well as to other areas of the frontal lobe that are thought to contribute to higher-order cognitive functions. In this study, we used injections of retrograde tracers in the lateral portion of the monkey MD to assess the locations of labeled cells in subcortical structures. Three main patterns were identified in the distribution of subcortical connections. We found that the claustrum, superior colliculus and ventral midbrain regions were heavily labeled in the cases with injections in caudoventral MD. In these cases, labeled cells were also found in either the periaqueductal gray or zona incerta, depending on the specific case. In one case with an injection in anterodorsal MD, labeled cells were most numerous in the structures of the ventral midbrain, especially the ventral tegmental area. Finally, the claustrum and superior colliculus contained the largest percentage of labeled subcortical cells in cases with injections in ventrolateral MD. These three patterns of subcortical label corresponded to three equally distinctive trends in the distribution of MD connections with the cortex in these same cases [J Comp Neurol 473 (2004) 107]. Very few labeled cells were found in other areas such as the amygdala, globus pallidus and deep cerebellar nuclei, suggesting that pathways leading from these structures to dorsolateral and dorsomedial frontal cortices are not likely to include the lateral divisions of MD. In concert, these findings show that particular locales within lateral MD receive distinct profiles of subcortical afferents, and project into specific neocortical domains, suggesting that these different sites within lateral MD may participate in functionally distinct circuits of information processing.

Differential thalamic connections of the posteroventral and dorsal posterior cingulate gyrus in the monkey

Previous functional studies suggest that the posterior cingulate gyrus is involved in spatial memory and its posteroventral part, in particular, is also involved in auditory memory. However, it is not clear whether the neural connections of the posteroventral part differ from those of the rest of the posterior cingulate gyrus. Here, we describe the thalamic connections of the posteroventral part of monkey area 23b (pv-area 23b), the main component of the posteroventral posterior cingulate gyrus. We compare these thalamic connections with those of the more dorsal area 23b (d-area 23b) and of adjoining retrosplenial areas 29 and 30. Thalamocortical projections to pv-area 23b originate mainly from the anterior nuclei, nucleus lateralis posterior and medial pulvinar. In contrast, projections to d-area 23b originate from the nucleus lateralis posterior, medial pulvinar, nucleus centralis latocellularis, mediodorsal nucleus and nucleus ventralis anterior and lateralis and weakly from the anterior nuclei. Projections to retrosplenial areas 29 and 30 originate from the anterior nuclei. Corticothalamic projections from pv-area 23b terminate in the anterior and laterodorsal nuclei, nucleus lateralis posterior and medial pulvinar. Projections from d-area 23b terminate in these nuclei as well as the nucleus ventralis anterior and lateralis. Projections from area 30 terminate mainly in the anterior nuclei and, to a lesser extent, in the medial pulvinar. These results show that the connections of pv-area 23b differ from those of d-area 23b or areas 29 and 30. This suggests that pv-area 23b may play distinct functional roles in memory processes, such as spatial and auditory memory.

From rule to response: neuronal processes in the premotor and prefrontal cortex

The ability to use abstract rules or principles allows behavior to generalize from specific circumstances (e.g., rules learned in a specific restaurant can subsequently be applied to any dining experience). Neurons in the prefrontal cortex (PFC) encode such rules. However, to guide behavior, rules must be linked to motor responses. We investigated the neuronal mechanisms underlying this process by recording from the PFC and the premotor cortex (PMC) of monkeys trained to use two abstract rules: "same" or "different." The monkeys had to either hold or release a lever, depending on whether two successively presented pictures were the same or different, and depending on which rule was in effect. The abstract rules were represented in both regions, although they were more prevalent and were encoded earlier and more strongly in the PMC. There was a perceptual bias in the PFC, relative to the PMC, with more PFC neurons encoding the presented pictures. In contrast, neurons encoding the behavioral response were more prevalent in the PMC, and the selectivity was stronger and appeared earlier in the PMC than in the PFC.