Closer look at the thalamus

Like an archaeologist noticing low hummocks on the ground, he can theorize but will not get very far in understanding until he digs below the surface. So it is with the brain. The activity on the surface of the cortex is unlikely to be understood until the activity below it is studied too. It is now possible to look more closely to the thalamus with higher powered fMRI and this development is examined by C. Metzger and others. (citation below)



While a continuing debate on segregated networks generally focuses on cortical regions, the interaction of subcortical structures - foremost thalamus and basal ganglia - with these cortical networks, their influence and control has hardly been investigated and therefore remains poorly understood. … For fMRI, insufficient spatial resolution in most studies limited the interpretation of thalamic activation, while continuous innovation in high resolution fMRI (hr-fMRI) now enables the functional investigation of small, anatomically well-described subcortical structures including the thalamus - also in humans.”



The thalamus is difficult to study. It is small and has different functional areas packed closely together. It is vital and cannot be interfered with in any major way. Until recently fMRI could not measure activity in small enough regions to ‘see’ the functional structure of the thalamus. It is connected to almost every other part of the brain (cortico-striatal-thalamo-cortical loops, sensory input, reticular formation and other tracks) and involved in a great many brain functions (consciousness, memory, cognition, perception, motor control, emotion at the very least) so it is important not to treat it as one, ‘the thalamus’, but as its individual functional and anatomical parts.



An example of recent research is the separation of the parts of the thalamus connected to the task oriented network and the default mode network. These networks are far-flung on the cortex but connected to two separate but adjacent parts of the thalamus. The area of the thalamus called the mediodorsal nucleus (MD) appears to be connected to cortical areas of the default network. The area called the centromedian/parafasicular complex (CM) appears to be connected to the task attention network. These thalamus areas may, in fact, be orchestrating the two networks. The illustration from the paper shows the two networks. (Click on image to enlarge)

FIGURE 2 | (adapted from Eckert et al., 2011): Network segregation based on relative fiber counts. (A) Sagittal plane; (B) cornal plane; (C) transversal plane (D) color bars: indicating the level of T-values for each region shown in (A–C). Regions with preferential connectivity to the MD are shown in blue and those connecting stronger to the CM/Pf complex are shown in red, the strength of the connectivity are visualized in the brightness of the blue and red colors. The PCC and the nucleus accumbens do not show significant preferences and appear in green. Abbreviation MD, mediodorsal thalamic nucleus; CM, centromedian/parafasicualar complex of the thalamus; amy, left amygdala; hipp, left hippocampus; PCC, posterior cingulate cortex; put, right putamen; pall, right pallidum; NAcc, right nucleus accumbens; caudate, right caudate nucleus; dlPFC, right dorsolateral prefrontal cortex; dACC, dorsal anterior cingulate cortex; pgACC, pregenual anterior cingulate cortex; aI/fo, left anterior insula-frontal operculum.



The authors also have examples of clearer identification of areas involved in memory, emotion and motor control. I look forward to more high resolution studies.



Here is the abstract:


The thalamus, a crucial node in the well-described cortico-striatal-thalamo-cortical circuits, has been the focus of functional and structural imaging studies investigating human emotion, cognition and memory. Invasive work in animals and post-mortem investigations have revealed the rich cytoarchitectonics and functional specificity of the thalamus. Given current restrictions in the spatial resolution of non-invasive imaging modalities, there is, however, a translational gap between functional and structural information on these circuits in humans and animals as well as between histological and cellular evidence and their relationship to psychological functioning. With the advance of higher field strengths for MR approaches, better spatial resolution is now available promising to overcome this conceptual problem. We here review these two levels, which exist for both neuroscientific and clinical investigations, and then focus on current attempts to overcome conceptual boundaries of these observations with the help of ultra-high resolution imaging.

C.D. Metzger, Y.D. van der Werf, M. Walter (2013). Functional mapping of thalamic nuclei and their integration into cortico-striatal-thalamo-cortical loops via unltra-high resolution imaging - from animal anatomy to in vivo imaging in humans Frontiers of Neuroscience, 7

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