When we typically think of how decision-making works in the brain, we think of new input coming in, perhaps through the eyes or ears, being processed in the relevant sensory areas, and then sent to the ‘decision-making’ areas (the basal ganglia, prefrontal cortex, or anterior cingulate cortex) where this information is used to make a decision. Although useful and intuitive, this modular view ends up giving short shrift to some areas that do heavy lifting.
Sensory areas are not actually the ruthless calculating machines that we tend to think of, but are in fact quite plastic. This ability of sensory cortex to modify its own responses allows it to participate in certain decisions: for instance, it can learn how long to wait in order to get a reward. If a rat receives two visual cues that predict how long it will have to wait in order to receive a reward – either a short time or a long time – neurons in the initial part of visual cortex, V1, will maintain a heightened firing rate to match that duration.
This is accomplished through something like reinforcement learning. When learning whether a visual cue is giving an animal information about how long it will have to wait for a reward, acetylcholine acts as a ‘reinforcement signal’. The effect is to change encoding of the reward by modifying the strength of the synapses in the network.
Although we tend to think of certain ‘decision-making’ areas of the brain, in reality all of the brain is participating in every decision at some level or another. In certain cases – perhaps when speed is of the essence or maybe when you want other areas of the brain to be involved in the computations and processing of that decision – even sensory portions of the brain are learning how to make decisions. It is not always dopamine, the ‘rewarding’ or ‘motivational’ chemical in the brain that supports this decision-making: other neuromodulators like acetylcholine often play the very same role.
Chubykin, A., Roach, E., Bear, M., & Shuler, M. (2013). A Cholinergic Mechanism for Reward Timing within Primary Visual Cortex Neuron, 77 (4), 723-735 DOI: 10.1016/j.neuron.2012.12.039