Figure 1.

Free choice task. Variable- and fixed-reward conditions were randomly assigned for each trial in a 70% / 30% ratio, respectively. In both conditions, each trial was initiated when a rat poked its nose into the center hole (C), after it had to keep poking for 1600–2600 ms until a “go” tone sounded (Hold). During the fixed-reward condition only, a short light stimulus was presented during the center-hole poking to inform rats that the trial was a fixed-reward-condition trial. After the presentation of “go” tone (Go), rats had to choose either a left (L) or a right (R) hole, and a reward of a food pellet was dispensed stochastically (D) (Choice). In the variable-reward condition, reward probabilities were selected randomly (90–50%, 50–10%, 10–50%, and 50–90% for left-right) and the reward setting changed based on the choice performance of rat. In the fixed-reward condition, the reward probability was constant with either 90–50% or 50–90% for all the sessions, and the reward setting was pre-determined for each rat.