Thalamus

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MRI cross-section, with thalamus marked

The thalamus [1] is a midline symmetrical structure in the brains of vertebrates. It is between the cerebral cortex and midbrain.

It relays sensory and motor signals to the cerebral cortex,[2][3] and regulates consciousness, sleep, and alertness.

The thalamus sits above the hypothalamus, and below the cerebral cortex. It is a collection of nuclei with various functions. It acts as a relay station, gathering sense information of all kinds (except olfactory) and passes it on to the cerebral cortex.

There are action systems for several types of behaviour, including eating, drinking, defecation, and copulation.[4] These behaviours satisfy short-term needs, and are called 'consummatory' behaviours.

Main functions[change | change source]

The thalamus has many functions. First of all, it acts as a relay station, or hub. It relays information between subcortical areas and the cerebral cortex.[5] In particular, every sensory system except smelling) has a thalamic nucleus that receives sensory signals and sends them to the related areas in the cortex. For sight, for example, inputs from the retina are sent to the thalamus, which in turn sends them to the visual cortex in the occipital lobe.

The thalamus is believed to process sensory information as well as relay it—each of the primary sensory relay areas gets strong feedback from the cerebral cortex.[6]

The thalamus also plays an important role in regulating states of sleep and wakefulness.[7] Thalamic nuclei have strong connections with the cerebral cortex. These circuits are believed to be involved with consciousness.[8] The thalamus plays a major role in regulating arousal, the level of awareness, and activity. Damage to the thalamus can lead to permanent coma.[9]

The thalamus has a role in the basal ganglia system but this is poorly understood. The thalamus has been thought of as a "relay" that just forwards signals to the cerebral cortex. But research suggests that thalamus is more selective.[10] Many different functions are linked to regions of the thalamus. This is the case for most sensory systems (except the olfactory system), such as the auditory, somatic sensory system, visceral, eating and visual systems. There specific lesions cause specific sensory deficits. A big role of the thalamus is to support the motor and language systems. Much of the circuitry for these systems is shared with the thalamus. The thalamus is functionally connected to the hippocampus.[11] This is part of the hippocampal system which is crucial for human episodic event memory.[12][13][14]

The information for motor control is a network involving the thalamus as a subcortical motor center.[15] In the brains of primates, the thalamus provides the specific channels from the basal ganglia and cerebellum to the cortical motor areas.[16][17][18] In an investigation of the eye movement motor response in three monkeys, the thalamic regions were found to cause antisaccade eye-movement. That is the ability to inhibit the reflexive jerking movement of the eyes in the direction of a presented stimulus.[19][20] They still look in the direction of the stimulus, but do so in a more controlled manner.

Recent research suggests that the mediodorsal (MD) thalamus may "amplify the connectivity (signaling strength) of just the circuits in the cortex needed for the current context. This helps the flexibility (of the mammalian brain) to make complex decisions by wiring the many associations on which decisions depend into weakly connected cortical circuits".[21] Researchers founds that "enhancing MD activity magnified the ability of mice to "think".[21] This lowered by more than 25 percent their error rate in deciding which conflicting sensory stimuli to follow to find the reward".[22]

In short, the thalamus helps to make mammals more effective at making decisions and living in their natural environment.

The half-second delay[change | change source]

The thalamus is a key areas which is involved in what is known as the "half-second delay". This is a perceptual illusion which has only recently been discovered.[23][24] The discovery is as follows: we perceive events in the world as instantaneous. They happen when we see (hear, sense) them happening. But the processing of visual signals at least is what the thalamus does. Experientially, we see events happening (so we think) without any delay. But in reality it takes half a second for the brain to organise the data which comes in from, let us say, vision. But we do not sense that half-second delay at all. Our perception of events is that we see them as they happen.[25]

All this applies to consciousness and conscious behaviour. Automatic responses to sudden pain, bare foot on a pin for example, happen instantaneously, and are consciously perceived later.

References[change | change source]

  1. from Greek θάλαμος = inner chamber) Douglas Harper - index & University of Washington Faculty Web Server & Search engine search page + Perseus Project tufts.edu Retrieved 2012-02-09
  2. Sherman, S. (2006). "Thalamus". Scholarpedia. 1 (9): 1583. doi:10.4249/scholarpedia.1583.
  3. S. M. Sherman & Ray Guillery -ISBN 0-12-305460-5Elsevier B.V [Retrieved 2012-02-10]
  4. Jones E.G. 1985. The thalamus. Plenum Press. ISBN 9780306418563 [1]
  5. Gazzaniga; Ivry; Mangun, Michael, S.; Richard B.; George R. (2014). Cognitive Neuroscience - The Biology of The Mind. New York: W.W. Norton. pp. 45. ISBN 978-0-393-91348-4.CS1 maint: multiple names: authors list (link)
  6. "The thalamus, middleman of the brain, becomes a sensory conductor". The University of Chicago Medicine. Retrieved 10 Sep 2020.
  7. Steriade, Mircea; Llinás, Rodolfo R. (1988). "The functional states of the thalamus and the associated neuronal interplay". Physiological Reviews. 68 (3): 649–742. doi:10.1152/physrev.1988.68.3.649. PMID 2839857.
  8. Coma and disorders of consciousness ISBN 978-1-447-12439-9 p. 143
  9. The Neurology of Consciousness: cognitive neuroscience and neuropathology ISBN 978-0-123-74168-4 p. 10
  10. Leonard, Abigail W. (August 17, 2006). "Your Brain Boots Up Like a Computer". LiveScience.
  11. Stein, Thor; Moritz, Chad; Quigley, Michelle; Cordes, Dietmar; Haughton, Victor; Meyerand, Elizabeth (2000). "Functional connectivity in the thalamus and hippocampus studied with functional MR imaging". American Journal of Neuroradiology. 21 (8): 1397–401. PMID 11003270.
  12. Aggleton, John P.; Brown, Malcolm W. (1999). "Episodic memory, amnesia, and the hippocampal–anterior thalamic axis" (PDF). Behavioral and Brain Sciences. 22 (3): 425–44, discussion 444–89. doi:10.1017/S0140525X99002034. PMID 11301518.
  13. Aggleton, John P.; O'Mara, Shane M.; Vann, Seralynne D.; Wright, Nick F.; Tsanov, Marian; Erichsen, Jonathan T. (2010). "Hippocampal-anterior thalamic pathways for memory: Uncovering a network of direct and indirect actions". European Journal of Neuroscience. 31 (12): 2292–307. doi:10.1111/j.1460-9568.2010.07251.x. PMC 2936113. PMID 20550571.
  14. Burgess, Neil; Maguire, Eleanor A; O'Keefe, John (2002). "The human hippocampus and spatial and episodic memory". Neuron. 35 (4): 625–41. doi:10.1016/S0896-6273(02)00830-9. PMID 12194864. S2CID 11989085.
  15. Evarts, E.V; Thach, W T (1969). "Motor mechanisms of the CNS: cerebrocerebellar interrelations". Annual Review of Physiology. 31: 451–98. doi:10.1146/annurev.ph.31.030169.002315. PMID 4885774.
  16. Orioli, PJ; Strick, PL (1989). "Cerebellar connections with the motor cortex and the arcuate premotor area: An analysis employing retrograde transneuronal transport of WGA-HRP". The Journal of Comparative Neurology. 288 (4): 612–26. doi:10.1002/cne.902880408. PMID 2478593. S2CID 27155579.
  17. Asanuma C, Thach WT, Jones EG (May 1983). "Cytoarchitectonic delineation of the ventral lateral thalamic region in the monkey". Brain Research. 286 (3): 219–35. doi:10.1016/0165-0173(83)90014-0. PMID 6850357. S2CID 25013002.
  18. Kurata, K (2005). "Activity properties and location of neurons in the motor thalamus that project to the cortical motor areas in monkeys". Journal of Neurophysiology. 94 (1): 550–66. doi:10.1152/jn.01034.2004. PMID 15703228.
  19. "Archived copy". Archived from the original on 2017-09-16. Retrieved 2014-01-21.CS1 maint: archived copy as title (link)
  20. Kunimatsu, J; Tanaka, M (2010). "Roles of the primate motor thalamus in the generation of antisaccades" (PDF). Journal of Neuroscience. 30 (14): 5108–17. doi:10.1523/JNEUROSCI.0406-10.2010. PMC 6632795. PMID 20371831.
  21. 21.0 21.1 "New role discovered for brain region". Neuroscience News. 2017-05-03. Retrieved 2017-12-03.
  22. Schmitt, L. Ian; Wimmer, Ralf D.; Nakajima, Miho; Happ, Michael; Mofakham, Sima; Halassa, Michael M. (11 May 2017). "Thalamic amplification of cortical connectivity sustains attentional control". Nature. 545 (7653): 219–223. Bibcode:2017Natur.545..219S. doi:10.1038/nature22073. ISSN 1476-4687. PMC 5570520. PMID 28467827.
  23. References can be got from Norretranders T. 1999. The user illusion: cutting consciousness down to size. Penguin, London, chapter 8.
  24. Libet, Benjamin 1965. Cortical activation in conscious and unconscious experience. Perspectives in Biology and Medicine. 9, 77–86.
  25. Libet B; Pearl D.K; Morledge D; Gleason D.A; Morledge Y & Barbaro N.M. 1991. Control of the transition from sensory detection to sensory awareness in Man by the duration of a thalamic stimulus. Brain 14, 1731–57.