Before recording the Ca2+ signals, spikes, and LFPs, mice were required to have recovered and be in good condition for 10 days after the surgery. Two horizontal bars (fixed to the head plate by 2 screws) were used to head-fix awake mice, which were able to maneuver on an air-supported free-floating Styrofoam ball (Thinkerbiotech). After the mice had adapted for a period of time, Ca2+ signals, spikes, and LFPs were recorded simultaneously.

A fiber photometry system (ThinkerTech) was used to record fluorescence emissions. A laser beam from a laser tube (488 nm; OBIS 488LS; Coherent) was reflected by a dichroic mirror, focused through a 10× objective lens (NA = 0.3; Olympus) and then coupled to an optical commutator (Doric Lenses). An optical fiber (200 mm O.D., NA: 0.37, 1.5 m long) guided the light between the commutator and the implanted optical fiber. Laser power was modulated to 40–60 μW at the tip of the optical fiber. GCaMP6s fluorescence emission was band-pass filtered (MF525-39, Thorlabs) and detected by a photomultiplier tube (R3896, Hamamatsu). An amplifier (C7319, Hamamatsu) was used to convert the photomultiplier tube current output to voltage, which was further filtered through a low-pass filter (35 Hz cutoff; Brownlee, 440). The analog voltage signals were digitized at 500 Hz and recorded by fiber photometry software.

In vivo electrophysiological data from the tetrodes were sent to the headstage and amplified by a 16-channel amplifier (Plexon DigiAmp; bandpass filtered at 0.1–5000 Hz; 2000× gain) and sampled at 40 kHz by a Plexon OmniPlex recording system. The procedure for spike recordings was similar to that for recordings made during the tetrode implantation described above. The LFP signals were amplified (2000× gain; Plexon DigiAmp), filtered at 0.1–300 Hz, and sampled at 1 kHz. Odor stimulation event markers were recorded alongside the spike/LFP data via the Plexon OmniPlex recording system.

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