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Apathy shows up at small rewards
Weighing every option
Motivation works like a running calculation. On a beautiful day there are twenty things a person could do and only one can win, so each gets weighed: how rewarding will it be, and how effortful, whether the effort is physical, cognitive, social, or emotional? The option with the best trade becomes the behavior.
Turning down low rewards
People with apathy from many different neurological disorders share one signature when asked whether a task is worth a given reward for a given amount of physical effort. Everyone agrees to the high rewards. Apathetic people turn down the low ones. In the most extreme case, a man with strokes in the brain's motivation circuit could almost never be moved to act unless someone prompted him.
The activation hill
Much of the weighing happens below awareness, and past experience tilts it. Being bad at learning Spanish makes learning French look like a mountain before the thought of starting has even formed. Every behavior has an activation hill to cross first, and the same hill is a mountain for one person and a mound for another.
How do researchers measure effort against reward?
Effort-based decision tasks often ask people to squeeze a handgrip, with a stronger squeeze earning a bigger reward. Participants choose between an easy option with a small payoff and a harder one with a larger payoff. By varying both, researchers find the point where a person is equally willing to accept or refuse, which gives a number for how much effort a given reward is worth to them.
Why does past failure make new tasks look harder?
The brain keeps running estimates of how well an action will turn out and updates them by comparing expectation with outcome. Dopamine neurons in monkeys were found to signal the gap between expected and received reward, in recordings by Wolfram Schultz and colleagues in the 1990s. A history of poor outcomes in one area drags down the expected value of related actions. Those lowered expectations then enter the weighing before a person is aware of having decided anything.
What happens to effort choices when dopamine is lowered?
John Salamone's experiments on rats showed that depleting dopamine in the nucleus accumbens changes what animals will work for. Rats offered a choice between a preferred food behind a barrier and a plainer food freely available switched to the plain food. The rats still ate and still preferred the better food when it was easy to reach. Only the willingness to pay the effort cost dropped.
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