News Section | Neurobiology
Ventrolateral nucleus area of brain linked to sensory processing
Sept 24 : The ventrolateral nucleus (VL) of the thalamus in the brain, which is connected to the cerebellum and motor cortex and is thought to be involved in motor function, also plays a role in sensory processing, according to a new study.
The study to be published in Annals of Neurology, the official journal of the American Neurological Association, also suggests that damage to the VL thalamic brain leads to functional and neural changes.
Examination of the human VL have been restricted to date due to a lack of tools available to investigate its function, and also because lesions that occur in this region tend to be larger and affect more than one nucleus of the thalamus.
Thalamic nuclei are dense clumps of nerve cells found where the fibres from sensory systems terminate in the thalamus.
Led by Tony Ro of Rice University in Houston, Texas, the researchers conducted a series of behavioural and neuroimaging studies on a patient who had suffered a stroke affecting only the right VL, a rare occurrence.
The patient reported changes to her sensory abilities, such as bumping into the left sides of doorways or veering right when driving as a result of decreased sensations on the affected (left) side, but was otherwise normal.
She was tested using visual and tactile stimuli, and also underwent diffusion tensor imaging (DTI) scans, an imaging method that uses a magnetic resonance imaging (MRI) scanner to visualize the neuron fibres and connectivity in the brain. Experiments were conducted one, three and six years following the patient’s stroke.
In the years following her stroke, the patient experienced a dramatic change in her sensory perception. When she would hear certain sounds, she felt tingling and other sensations in the left side of her body, especially her left arm.
The results suggested that the VL lesion resulted in a significant amount of functional and neural reorganization that influenced sensory perception.
For a year and a half after her stroke, the patient demonstrated anti-extinction, in which she detected sensations significantly more on the impacted side when she was also stimulated on the unaffected side.
She also developed tactile sensations induced by sound, (sound-touch synesthesia), which may have been due to even further weakened pathways between the thalamus and cortex, a finding that was also suggested by the DTI results. The synesthesia persisted, and was still present six years following her stroke.
“Regardless of the exact neural mechanisms, this phenomenon of brain-damage induced feelings of sound suggests that other forms of synesthesia, in which reportedly neurologically normal individuals feel, taste or see something qualitatively different than the actual sensory input, may be due to cross-wiring in the brain, especially subcortically,” the researchers said.
The results support previous studies that suggested that acquired forms of synesthesia might appear after months or years following brain damage.
The researchers observed that in addition to demonstrating a previously unknown role for the VL in sensory processing, the results suggested that connections between the thalamus and other brain areas might be important for the ability of the nervous system to change in terms of sensory processing following brain damage.
“Our results suggest that local disruption of the thalamus causes large-scaled changes in remotely connected regions of the brain, perhaps including excitatory connections between auditory and somatosensory cortex leading to the patient’s synesthesia,” researchers stated.
They further said that synesthesia might also have been caused by altered connections within the thalamus that help the processing of sensory information from the body to the brain when normal processing is impaired.
The researchers are now planning to use other behavioural and neuroimaging methodas to shed light on the consequences of VL thalamic brain damage. (ANI)
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