Special issue: Original articleDisconnection syndromes of basal ganglia, thalamus, and cerebrocerebellar systems
Section snippets
Principles of organization of fiber pathways in the cerebral cortex
Anatomical investigations of the non-human primate brain reveal that there is a consistent pattern of white matter fiber tracts that emanate from every region of the cerebral cortex (Schmahmann and Pandya, 2006). Neurons within any cortical area give rise to five distinct categories of efferent fibers. These are (1) association fibers, (2) striatal fibers, and a confluence of fibers (the “cord”) that carries (3) commissural fibers, and subcortical (projection) fibers to (4) thalamus, and (5)
Clinical features
Subcortical dementia as a clinical entity was first recognized in progressive supranuclear palsy and Huntington's disease (Albert et al., 1974), characterized by slowness of mental processing, forgetfulness, apathy, and depression. This notion was later expanded when it became apparent that focal subcortical lesions play a role in arousal, attention, mood, motivation, language, memory, abstraction, and visuospatial skills (Cummings and Benson, 1984). Patients with Parkinson's disease experience
Clinical features
In the first detailed account of the behavioral consequences of thalamic hemorrhage, Fisher (1959) described neglect (“modified anosognosia and hemiasomatognosia”), global “dysphasia”, confusion and visual hallucinations. Accounts followed of thalamic dementia from prion diseases (Martin, 1997), and behavioral changes in patients with thalamic tumors (Ziegler et al., 1977, Nass et al., 2000), but these lesions are seldom confined to thalamus. There are four thalamic vascular syndromes that
Reticular thalamic nucleus
This nuclear shell surrounds thalamus and conveys afferents from cerebral cortex. It contributes to synchrony and rhythms of thalamic neuronal activity, and is relevant in the pathophysiology of epilepsy (Huguenard and Prince, 1997), and the neural substrates of consciousness (Llinas and Ribary, 2001).
Intralaminar thalamic nuclei
The paracentral (Pcn), central lateral (CL), centromedian (CM), parafascicular (Pf) and midline nuclei such as paraventricular, rhomboid and reunions play a role in autonomic drive. They receive afferents from brainstem, spinal cord, and cerebellum, and have reciprocal connections with cerebral hemispheres (Brodal, 1981, Jones, 1985). The CM/Pf nuclei are also linked with the basal ganglia in tightly connected functional circuits. A sensorimotor circuit links putamen with CM through the
Limbic thalamic nuclei
The functions of the anterior nuclear group – ventral, medial, and dorsal (AV, AM and AD nuclei), and the lateral dorsal (LD) nucleus, reflect their reciprocal anatomical connections with limbic structures in the cingulate gyrus, hippocampus, parahippocampal formation, entorhinal cortex, retrosplenial cortex, orbitofrontal and medial prefrontal cortices, and with subcortical structures including the mamillary bodies and amygdala (Yakovlev et al., 1960, Locke et al., 1961, Yakovlev and Locke,
Specific sensory thalamic nuclei
The specific sensory nuclei include the medial geniculate nucleus (MGN), lateral geniculate nucleus (LGN), and ventroposterior nuclei (lateral, medial, and inferior – VPL, VPM VPI).
Medial geniculate nucleus connections with primary and association auditory cortices infer a role in higher level auditory processing, as well as in elementary audition (Mesulam and Pandya, 1973, Pandya et al., 1994, Hackett et al., 1998).
The lateral geniculate projects to primary and secondary visual cortices (
Effector thalamic nuclei
Motor nuclei include the ventral anterior (VA), ventromedial (VM), and ventral lateral (VL) nuclei. Subregions within VA receive afferents from the internal globus pallidus (Ilinsky and Kultas-Ilinsky, 1987), are linked with premotor cortices (Jones, 1997), and may be responsible for dystonia in rostral thalamic lesions. Neurons in VA receiving afferents from the substantia nigra pars reticulata (Jones, 1985, Francois et al., 2002) are linked with premotor, supplementary motor (Schell and
Associative thalamic nuclei
The lateral posterior, medial dorsal, and pulvinar nuclei are interconnected with cerebral association areas, and have no peripheral afferents or links with primary sensorimotor cortices.
The lateral posterior (LP) nucleus is reciprocally linked with the posterior parietal (Weber and Yin, 1984, Yeterian and Pandya, 1985, Schmahmann and Pandya, 1990), medial and dorsolateral extrastriate (Yeterian and Pandya, 1997), and posterior cingulate and medial parahippocampal cortices (Yeterian and Pandya,
Thalamic connectional topography
Sensorimotor, effector, limbic, and associative regions of cerebral cortex are therefore linked with distinctly different sets of thalamic nuclei. Thalamic projections to the posterior parietal lobe exemplify this concept (Schmahmann and Pandya, 1990). Connections become progressively elaborated as one moves from rostral to caudal within both the superior and the inferior parietal lobules. Rostral areas concerned with intramodality somatosensory processing are related to modality-specific
Clinical features
The cerebellum is subcortical only in the sense that it is distinct from cerebral cortex. The traditional view that cerebellar function is confined to the coordination of voluntary motor activity has evolved in recent years (Schmahmann, 1997). Evidence from patients has made it plain that cerebellar pathology is related to intellectual and emotional deficits in addition to motor incoordination. The wider role of the cerebellum in nervous system function has far-reaching implications for
Discussion
This overview of the neurology and neuropsychiatry of basal ganglia, thalamus and cerebellum makes it clear that “higher cortical functions” are not exclusively the domain of the cerebral cortex. The principle of organization of cerebral cortex, apparent from tract tracing studies, is that each cortical area has association, striatal, and projection fibers linking it in a precise manner to topographically arranged sectors within other cortical regions and subcortical areas. How does the
Conclusion
We have reviewed anatomical, clinical, and imaging data implicating the thalamus, basal ganglia and cerebellum in cognition and emotion, in addition to their roles in motor control. We consider these clinical phenomena as disconnection syndromes, extending the earlier notions of Wernicke and Geschwind to the subcortical nodes. We introduce a new theoretical framework in which to consider the distributed neural systems, based on general, and specific, principles of anatomical organization, and
Acknowledgements
Supported in part by RO1 MH067980, and the Birmingham Foundation. The assistance of Jason MacMore BA is greatly appreciated.
List of abbreviations+
- AD
- anterior dorsal thalamic nucleus
- AM
- anteromedial thalamic nucleus
- AS
- arcuate sulcus
- Assn
- association fibers
- AV
- anteroventral thalamic nucleus
- CC
- corpus callosum
- CCAS
- cerebellar cognitive affective syndrome
- Cd
- caudate nucleus
- CeM
- central medial thalamic nucleus
- CL
- central lateral thalamic nucleus
- Cl
- claustrum
- Cld
- central lateral dorsal thalamic nucleus
- CM
- centromedian thalamic nucleus
- Comm
- commissural fibers
- CS
- central sulcus
- Csl
- centralis superior lateralis thalamic nucleus
- DM
- medial dorsal thalamic nucleus
- DSI
- diffusion
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