Headstage

Extracellular recordings were made acutely using 64-channel silicon probes (ASSY-77 H2, Cambridge Neurotech). The 64-channel voltage signals were amplified, filtered and digitized (16 bit) on a headstage (Intan Technologies), recorded on a 512-channel Intan RHD2000 recording controller (sampled at 20 kHz), and stored for offline analysis. At 12–24 h before recording, a small (1.5-mm diameter) craniotomy was made unilaterally over ALM centred on the fiducial. The probes were targeted stereotaxically to ALM, lowered to a depth of 1,000–1,100 μm. Recording depth from the pial surface was inferred from micromanipulator readings. To minimize brain movement, 1.8% low-melt agarose (A9793–50G, Sigma Aldrich) in 1× phosphate-buffered saline (Corning) was pipetted in the craniotomy following probe insertion. Three to seven recordings were made from each craniotomy. After each recording session, the craniotomy was filled with silicone gel (Kwik-Cast, World Precision Instruments). Carprofen (0.05 mg kg⁻¹) was given daily to reduce inflammation.

Subjects were habituated to two neutral (no rewarding or aversive stimuli presented) recording chambers prior to electrophysiological recordings. The chambers were contextually unique but sized similarly (~sixty cm long and forty-five cm long, the walls were forty-five cm tall). The affixed recording array was attached to a headstage (Intan Technologies, Los Angeles, CA) and digitized electrophysiological signals were sent through tether cables into an RHD 2000 USB interface board (Intan Technologies). Local field potentials (LFPs) were sampled at thirty kHz and bandpass filtered between 1–6000 Hz with the Open Ephys GUI (V0.5.0, Cambridge, MA). Behavior was tracked using Bonsai (Lopes et al., 2015 (link)). Running speed was calculated distance over time (ms). Running speed between groups was examined using a one-way ANOVA and second by second behavioral states were grouped using unsupervised learning (kmeans clustering; Python V3.7, Scikit Learn) to identify still or slow, walking, or running states.