A virtual reality system was designed using an air-supported spherical treadmill for head-restrained mice19 (link) in combination with a projection-based visual display system20 (link), in which a toroidal screen presented an image from a projector via an angular amplification mirror21 (link). Custom software to control the virtual reality system was developed based on the Quake2 game engine. Rotations of the spherical treadmill, measured by an optical computer mouse, were used to update the visual display. Water-scheduled C57BL/6J mice (8–12 weeks old) were trained using operant conditioning to run from end-to-end of a virtual linear track (180 cm long) to obtain water rewards. For electrophysiology measurements, a small craniotomy (~0.5 mm diameter) was made centered over dorsal hippocampus (2.2 mm caudal, 1.7 lateral to bregma). The craniotomy was sealed with silicone grease and then covered with silicone elastomer to allow recordings across multiple days. Extracellular recordings were made using a glass electrode (filled with 0.5 M NaCl, ~2.5 MΩ pipette resistance) mounted on a micromanipulator positioned behind the mouse. Whole cell recordings were obtained using standard blind patch methods. Patch pipettes were pulled with a long taper (~100 μm diameter at 1 mm from the tip), to minimize damage to the overlying cortical tissue, and were mounted on a micromanipulator positioned outside the field of view. Firing rate maps were calculated for 80 spatial bins along the virtual track as the number of spikes in a bin divided by the time spent in that bin. Changes in baseline membrane potential in the place field were measured from membrane potential traces excluding spikes. Theta oscillations were analyzed following band-pass filtering (6–10 Hz) of the membrane potential recording using a linear phase finite impulse response filter.