Thalamus
The thalamus is a cluster of nuclei that contain dense populations of neurons
and dense interconnections with every other part of the brain. There are two
thalami, lying deep inside each cerebral hemisphere. I compare the Thalamus to a
multi-channel mixer using templates to regulate complex interactions of all
brain systems. The templates are based on innate patterns and are modified by
learning. Mease wrote:" A major synaptic input to the thalamus originates from neurons in
cortical layer 6 (L6); during sensory processing... L6 input to the thalamus can
shape both the overall gain and the temporal dynamics of sensory responses that
reach the cortex."
Consciousness is a monitor image that samples brain activities. In my
discussion of consciousness, I suggested that these monitor images and
sensations depend on the recursive interaction of the thalamus and cortex. Other
modulating information is fed into the mix from the smaller nuclei surrounding
the thalamus contributing feelings, mood and other information. Because the
thalamus is so complexly and recursively connected with all other parts of the
brain, focusing on the thalamus to explain executive functions will be
misleading to some extent. The interaction of frontal lobe and thalamic circuits
is essential, for example, to anticipatory planning - one of the more recent attributes of cognition. The intellectual challenge is to become
comfortable with complex, self-regulating, recursive systems that do not require
an overseer.
Halassa and Kastner wrote:” “We highlight recent studies across rodents and
primates showing how thalamus contributes to attentional control. In addition to
high-fidelity information relay to or between cortical regions, thalamic
circuits shift and sustain functional interactions within and across cortical
areas. This thalamic process enables rapid coordination of spatially segregated
cortical computations, thereby constructing task-relevant functional networks.
Because such function may be critical for cognitive flexibility, clarifying its
mechanisms will likely expand our basic understanding of cognitive control and
its perturbation in disease.” (Michael M. Halassa & Sabine Kastner. Thalamic
functions in distributed cognitive control. Nature Neuroscience 20, 1669–1679
(2017)
The “executive system” is a popular construct, a hypothetical entity that
integrates different cognitive processes. The identification of real functional
components remains difficult. We would like to explain a variety of manifest
properties of the brain such as alerting, orienting mechanisms, goal directed
behaviors, strategizing, decision making, planning, sequencing, and social
interactions. Patients with frontal lobe damage, for example, display variations
of the “dysexecutive syndrome.” As with all broad, metaphorical categories the
term, executive function, may obstruct, rather than enhance understand of how different brain modules interact
to perform different tasks. All attempts to localize executive function are
misleading since brains are inherently recursive, self-regulating, integration
systems.
In their review of 10 years of studying the connections of thalamic nuclei in
rats, Van der Werf et al stated that the thalamic midline and intralaminar
nuclei form awareness, arousing system, involved in cognitive, sensory and motor
functions. They proposed that the thalamic nuclei as a whole play a role in
awareness. They studied the connections between frontal lobes and the thalamus
in rats and suggested that the medial prefrontal cortex is involved in
high-order cognitive processes such as decision-making, goal directed behavior,
and working memory. They showed the infralimbic, prelimbic, anterior cingulate
cortices connect to midline/medial structures of the thalamus; the medial
agranular cortex connects to the intralaminar nuclei, the ventromedial, and
ventrolateral nuclei of thalamus; all four divisions of the medial prefrontal
cortex project densely to the nucleus reuniens of the thalamus.
The nucleus reuniens is the major source of thalamic input to the
hippocampal formation, transferring information from the medial prefrontal
cortex to the hippocampus. Their evidence suggests that thalamic nuclei act as
mixers and executive controllers of diverse subcortical and cortical structures
in the brain. Although thalamocortical synapses are less dense than
corticocortical synapses, their effect on cortical activity is important.
Van der Werf et al stated that the thalamic midline and intralaminar nuclei
form an awareness, arousing system, involved in cognitive, sensory and motor
functions. They proposed that the thalamic nuclei as a whole play a role in
awareness, with each of the groups subserving a role in different aspects of
awareness:
(1) a dorsal group is involved in viscero-limbic functions
(2) a lateral group is involved in cognitive functions
(3) a ventral group is involved with multimodal sensory processing
(4) a posterior group is involved in limbic functions.
The Pulvinar is about 30% of the thalamic mass and has 4 subsections
The pulvinar’s main role is creating spatial visual attention. Gatta et al
reviewed pulvinar functions: “There are at least two aspects in which the
pulvinar seems to be instrumental for selective visual processes. The first
aspect concerns pulvinar connectivity pattern. The pulvinar is connected with
brain regions known to be playing a role in attentional mechanisms, such as area
V4, the superior colliculus (SC), and the inferior parietal cortex (IP).
Additionally, the pulvinar is richly interconnected with multiple cortical
areas. This enables the pulvinar to serve as a hub for brain communication,
potentially gating the flow of information across different regions. The second
aspect concerns neuronal circuits intrinsic to the pulvinar. We claim these
circuits are subserving three basic steps regarding the allocation of spatial
attention: disengaging from the current focus of attention, moving it to a new
target, and engaging it at a new position.”
(Gattaa, R et al. The Role of the Pulvinar in Spatial
Visual Attention. Advances in anatomy, embryology, and cell biology. 2018, DOI:
10.1007/978-3-319-70046-5_12, PMID: 29116453)
