Metastability and Eye Movements: A Dynamical‑Systems Interpretation

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  1. Why metastability is a natural language for gaze behavior The idea that perception, attention, and neural activity evolve through transiently stable states has deep roots in cognitive science and neuroscience. Concepts such as attractors , basins , noise‑induced transitions , and escape times appear in work on perceptual switching, decision‑making, and neural population dynamics (Kelso, 1995; Rabinovich et al., 2008; Deco & Jirsa, 2012). Eye movements, especially the alternation between fixations and saccades ,   fit remarkably well into this metastable picture. The Freidlin–Wentzell theory of rare events (Freidlin & Wentzell, 2012) provides a rigorous mathematical language for these intuitions. 2. Fixations as metastable states  Consider the gaze position X t as the state of a stochastic dynamical system. During a fixation the  gaze remains confined to a small region, microsaccades and noise generate small fluctuations, and the gaze tends to return...
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INVOLVEMENT OF THE INFERIOR FRONTAL CORTEX IN THE LEVODOPA-INDUCED DYSKINESIAS

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Parkinson’s disease (PD) is one of the most prevailing neurodegenerative disorders affecting populations all over the world.  Almost from 4.1 to 4.6 million patients suffering from PD are counted across the  fifteen most crowded countries where  live the two thirds of the world population. The average age of the PD onset is 55. The incidence grows along with the age: from 20/100000 cases in the overall population to 120/100000 in the elder population above 70 years old.  

This number is expected to reach 8.7 / 9.3 million in 2030. China is expected to increase from two million to five million by 2030. (E. R. Dorsey, et al.; Neurology 2007)

Gender difference  in the rate of incidence of PD was reported in many epidemiological studies:  from 1.5 to 2 times higher in men than in women. In Japan, however, there is a higher incidence in women.

Environmental factors are critical in determining the incidence rate of the disease. Some studies report that, among the cases in which Parkinson's disease is attributable to factors of environmental exposure, 10% is associated with the use of herbicides in the professional field. US epidemiological studies show a significant association between mortality rates for parkinsonian years 1986-1988 and the presence of the chemical in the territory. Only in 5% of cases ensures inheritance of the disease.

It is a disease that involves functions such as control of movements and balance. But what exactly is happening in the brain? The substantia nigra is a structure of Basal Ganglia [see here the basal ganglia circuit] that is rich of dopaminergic neurons.  In Parkinson these neurons get sick and die, and when the cell loss reachs 80% the symptoms of the disease appear: tremor, slowness of movement and rigidity.

The main pharmacological therapy involves the administration of a drug, levodopa, which is converted to dopamine in the brain. By increasing the concentration of this substance symptoms diminish, slow movements and tremors disappear.

For the first five to six years the patient returns to a normal condition and a good quality of life. Then, however, begin new problems. The major complications related to the assumption of levodopa or dopamine agonists include dyskinesias such as fluctuations of the movement, involuntary movements very fast like tics affecting the face, mouth, tongue, upper limbs and sometimes even lower. These are side effects of the drug that interfere so heavy with normal daily activities.

An italian research team has investigated what happens in the brain in patients suffering from severe dyskinesia. The studies began a few years ago and were conducted over hundreds of patients.

The main findings showed that to be involved is the right inferior frontal gyrus, which modulates voluntary movements. In case of prolonged intake of levodopa anatomic and functional alterations were observed and this would cause the onset of motor disorders. 
The researchers applied repetitive transcranic magnetic stimulation (R-TMS) to PD patients. The stimulation of the inferior frontal gyrus yielded promising: the brain area in question  returned to work and involuntary movements disappeared.

See more on this research here

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