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Parkinson's patients can run from fire
Running from fire
The motivation signal suddenly ramps up high enough to engage the action system. That gives a different framing for the slowing of movement in Parkinson's disease, called bradykinesia: it may reflect weak motivation to act, along with the loss of motor control.
The basal ganglia
The disease damages the basal ganglia. They matter for action, and they also carry the motivation signals that link goals, like getting out of a burning house, to the outcome.
Eye movements and reward
A controlled version of the idea used eye movements, which are pre-programmed and ballistic, each with a particular velocity. When the reward for a rapid eye movement went up, the velocity increased in patients, just as in healthy people. Adding dopamine for a patient raised it further. Incentives drive action, and dopamine is part of how they do it.
What is paradoxical kinesia in Parkinson's disease?
It is the temporary ability of a patient who is normally slow or frozen to move quickly and well under strong external stimulation. French neurologist Andre Souques described it in 1921, and it has been seen in emergencies such as fires. Patients have also been reported to catch a ball or climb stairs on cue when they could barely walk across a room.
How much dopamine is lost before symptoms appear?
Oleh Hornykiewicz found in the 1960s that the striatum of Parkinson's patients was severely depleted of dopamine. Later studies suggested that symptoms usually appear after most of the dopamine in the striatum has gone. The brain compensates for a long time as the neurons die, so the disease can progress for years before the first visible sign.
Why use eye movements to study this?
Saccades, the quick jumps the eyes make between points, follow a stereotyped relation between their size and their peak speed. That makes them easy to measure precisely and hard to alter by deliberate effort. In monkeys, Masaki Takikawa and colleagues showed in 2002 that expected reward speeds up saccades, with neurons in the basal ganglia carrying the reward signal.
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