Chance and Motivation
...like ourselves, our reason has in it a large element of chance. -Plato, Timaeus (ca. 360 BCE)
What chance is in Nature, so will is in Man. -Francis Bacon, De Interpretatione Naturae (d. 1626)
It is impossible for a Die, with such determin’d force and direction, not to fall on such determin’d side, only I don’t know the force and direction which makes it fall on such determin’d side, and therefore I call it Chance, which is nothing but the want of art. - John Arbuthnot, intro to Huygens' Of the Laws of Chance (1692)
Chance in decision making
For perception and motor control, accuracy is key and noise is a problem. Our work on Drosophila courtship suggests that decision-making circuitries exploit the probabilistic nature of neuronal communication to make behaviors flexible and scalable.
A coin-flip model for behavioral selection
Every time the male taps a female with his pheromone-receptor-bearing foreleg, there is a fixed probability of initiating courtship immediately afterward. This probability is a product of the quality of the target (likely to be receptive?) and the motivational state of the male (reflecting his reproductive potency). This provides a simple model for cue-triggered binary decision making.
Excitatory and inhibitory inputs to a decision center
Work from Vanessa Ruta's and Kristin Scott's labs showed that excitation from a tap is split into excitatory and inhibitory channels that are delivered to P1 courtship command neurons. We previously showed that a dopaminergic signal conveys the male's motivational state to P1.
Motivation weights the coin
In a highly motivated male, the dopamine tone received by P1 is elevated. This desensitizes P1 to the inhibitory input recruited by a tap, and increases the fraction of taps that, by chance, can stimulate P1 beyond threshold and throw the male into the courtship state. Artificial stimulation of P1 further increases the odds.
Motivation, perception, and chance converge to make and sustain a binary decision
After courtship initiation, dopamine maintains P1 activity by promoting recurrent activity. Depending on the level of the motivating dopamine tone, the male may persist until the goal is achieved, or eventually abandon the attempt.
What does it mean?
When we conjure up a mental image of something, are we performing mental taps that send excitatory and inhibitory inputs to a decision making center for that behavior? Does our motivation for that behavior desensitize the decision making center to the inhibition? Are people with manic, addictive or compulsive disorders stuck on the right tail of the probability distribution? Are depressed people stuck on the left tail? Is this, when stripped down to its fundamental basis, what motivation is?