Thalamus

The thalamus, the yellow section on the drawing above, is an important brain nucleus and is considered one of the intermediate brain, together with the pituitary gland and the hypothalamus. It is the most 'wired' portion of the brain.

The thalamus consists of six functional separate core groups where information from other parts of the nervous system is transferred to specific areas of the cerebral cortex.

The thalamo cortical connections are bidirectional; they lead from the thalamus to the cortex and from the cortex back to the thalamus. This is called Thalamo-cortico-thalame circuit. The result of this is that small thalamus injury may "mimic" cortex syndromes. In some syndromes the specialization of the brain hemisphere can be recognized.

Gateway to the Cerebral Cortex

The connections from the thalamus to the cerebral cortex return from the cortex to the thalamus.
This is called the thalamo-cortico-thalamic circuit. As a result, minor thalamic lesions can mimic cerebral cortex syndromes, and in some syndromes, the specialization of the cerebral hemisphere can be recognized.

As a gateway to the cerebral cortex, the thalamus must transmit many sensory stimuli (except smell), but it must also filter stimuli, allowing healthy individuals to "shut off" certain stimuli. It is therefore one of the so-called "filters" (stimulus filters).

The thalamus is also associated with wakefulness, sleep, consciousness, vigilance (alertness), motor skills, emotions related to an event, and akinetic mutism. This is a syndrome, resulting from brain injury, in which the person no longer speaks or speaks less (mutism) and has little or no movement (akinesia). The person can follow you with their eyes but otherwise appears apathetic.

Six core groups

The thalamus has six distinct core groups that produce specific symptoms when injured. These groups transmit information from other parts of the nervous system to specific areas in the cerebral cortex.

The core groups of the thalamus:

Location of the lesion in the thalamus and associated syndrome:
1. Anterior thalamic nucleus: Inability to memorize information (anterograde amnesia)
2. Anterior medial nucleus: Filling a gap in memory with fabrications (anterograde amnesia). Forgetting sequences (sequence amnesia). Executive function disorders (executive function disorders).
Executive functions include: goal-oriented and flexible planning, monitoring and adjusting actions, logic, strategy, and problem solving. 3. Intralaminar nuclei: Apathy and attention deficit disorder
4. Left pulvinar: word-finding disorders
5. Right pulvinar: Left-sided spatial neglect/failure to notice (neglect)
6. Large bilateral medial thalamic lesions including the
intralaminar nuclei: Coma and somnolence

7. Contralateral Motor Nuclei: Hemilateral motor neglect, temporary lack of movement without loss of strength
8. Visual Somatosensory and Nociceptive Nuclei (Contralateral):

• Visual field loss (sector anopia) on the opposite side of the body
  
• Reduced body sensation (hemihypesthesia)
  
• Reduced pain sensitivity on one side of the body (hemihypalgesia)
  
• Reduced sense of taste
  
• Thalamic pain on the opposite side of the body
  
• Pain on one side of the body
  
• Pain may be felt in the legs, arms, and face. Pain, burning, stabbing, and pricking sensations are present
  Changes in the weather or certain movements or activities may further aggravate the condition.
  The pain caused by a stroke in the thalamus is called "Déjerine-Roussy syndrome."

Possible consequences of damage to the thalamus

Problems with wakefulness, alertness, and consciousness

The thalamo-cortico-thalamic circuit is associated with awakening, vigilance and consciousness. Damage to a portion of the thalamus is associated with risk of coma.

Sensory disturbances

Damage to a part of the thalamus can lead to  sensory changes and pain on the opposite side of the body.

Damage here can also cause movement disorders (motor disorders) such as bradykinesia.

Two  nuclei: the pulvinar at the back of the thalamus and a nucleus in the middle at the bottom, the nucleus reticularis thalami, have partially overlapping functions:

The pulvinar, a nucleus at the back of the thalamus

• Sensory processing. The pulvinar helps combine information from different senses, such as sight, hearing, and touch. This ensures that the brain gets a clear picture of the environment.

   

• Visual processing. The pulvinar works in conjunction with the visual parts of the brain. It receives information from the superior colliculi (a part of the midbrain) and helps process visual reflexes. It helps focus attention on important visual details such as color, shape, or location. It helps filter out less important information. In doing so, it collaborates with other brain regions, such as the parietal lobe. For example, when  you focus on the color red or something to the right in your field of vision, the pulvinar enhances these features in visual areas.

Damage to the pulvinar may  disrupt visual reflexes, which can lead to problems focusing attention on important visual details.

Another part of the thalamus, the nucleus reticularis thalami, likely also helps the brain focus on the most important details of what you see.

• Attention and focus. As we wrote under 'processing of visual stimuli', the pulvinar helps us maintain our attention. It ensures that we can concentrate on what is important and become less distracted by other stimuli around us, such as visual stimuli.

• Spatial awareness. This part of the brain helps us understand where things are located and how we move through a space. This is important for planning and navigating.

• Memory and learning. Research shows that the pulvinar also plays a role in learning and remembering, especially when it comes to things we see and visual memories.

• Dealing with emotions. The pulvinar helps us process emotional stimuli. This plays a role in how we deal with emotions and how we connect with others.
  

Note! In case the pulvinar is damaged, it affects one and likely several of these functions!

Other reported problems following injury to the thalamus include:

• visual field loss

• reduced sense of taste
  
• reduced sensation of pain
  
• reduced sensation on one side of the body
  
• poverty of movement
  
• word-finding difficulties
  

Symptoms depend on the location of the injury in the thalamus.

Behavioral changes

Depending on the location of the thalamic lesion, symptoms may consist of:

• apathy
  
• perseveration: inability to stop a thought or action: 'inability to detach from what occupies the mind and continuation of an action' 
  
• personality changes may occur
  
• overstimulation is one of the most frequently reported complaints
  
• emphasizing unrelated information
  
• memory loss

Déjerine-Roussy syndrome / Thalamic pain syndrome

Thalamic pain occurs in the thalamus following a stroke (CVA).

The pain is felt on the opposite side of the body from where the stroke occurred, i.e., in one half of the body.

The pain may be localized or felt over a larger area in the legs, arms, or face.

The pain involves burning, stinging, pricking, and tingling sensations. Changes in the weather or certain movements or activities may aggravate it.

More information on this can be found here. 

Here you can download a document by Emmanuel Carrera M.D. and Julien Bogousslavsky, M.D., containing more information about thalamus injury and behavior.

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