Some of the following methods are similar to those previously published (43). Before behavioral training, mice were housed under stable conditions with food and water ad libitum. After the start of behavioral training, water supply was restricted. Mice could drink water only during and immediately after training. Care was taken to keep mice body weight above 80% of normal level.

In a DPA task, a sample olfactory stimulus (S1 or S2; S1: butyl formate, S2: 1-pentanol) was presented at the start of a trial, followed by a delay period (5 s) and then a testing stimulus (T1 or T2; T1: ethyl acetate, T2: methyl butenol). Odor delivery duration was set to 1 s, which was sufficient for rodents to perceive olfactory cues. Mice were trained to lick in the response window. The response window (0.5 s in duration) was started 1 s after the offset of the second odor delivery. Licking events detected in the response window in paired trials (S1-T1 or S2-T2) were regarded as Hit and will trigger instantaneous water delivery (50 ms in duration). False choice was defined as detection of licking events in the response window in unpaired (DPA) trials, and mice were not punished in “false alarm” trials. Mice were neither punished nor rewarded for “miss” (no-lick in a paired trial) nor “correct rejection” (no-lick in unpaired trial) trials. Licking events were detected by infrared beam breakers. Odor and water delivery, laser illumination, and licking events were recorded by computers through serial ports. In each day, mice were required to perform 100 trials for the DPA task in optogenetic and electrophysiological experiments. After the training sessions ended each day, mice were supplied with free water until satiety.

Before the start of training, mice were water restricted for 2 days. The behavioral training process included habituation, shaping, and task-learning phases. In the habituation phase, mice were head fixed in behavioral setups and trained to lick water from a water tube, encouraged with automatically delivered water through rodent lavage needles. Typically, mice could learn to lick for 1 to 2 min continuously in 1 to 2 days. The shaping phase was then started, in which only paired trials were applied and water was provided in all trials each day. In the beginning of the shaping phase, water was delivered manually through syringes to encourage mice to lick in the response window for only 10 trials. For the rest trials, mice were trained automatically. The shaping phase typically lasted for 2 to 3 days. The task-learning phase was then started from the next day, which was defined as day 1 in the behavioral analysis reported in all figures. All kinds of trials were applied pseudorandomly, i.e., two paired and two unpaired trials of balanced odor pairs were presented randomly in every four consecutive trials in the DPA task. No human intervention was applied in the task-learning phase to minimize any potential human bias in the behavioral results.

The performance correct rate (referred to as “performance” in labels of Fig. 2B) of each batch of 20 trials was defined byPerformance (correct rate)=Number of hit trials+Number of correct rejection trialsTotal number of trials

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