Movie S1.

Spontaneous calcium transients in SL cells (the imaged field is 200 µm deep from the cortical surface) while the mouse breathes clean air, measured using Cal-520 with excitation at 810 nm and approximately 20 mW power after the objective (Nikon 16×/0.8 NA). The movie was acquired at 30 frames/s; then frames were averaged together in consecutive groups of 12, giving 400 ms between frames. The frames then were Kalman-filtered in ImageJ with the following parameters: acquisition noise variance estimate = 0.05; bias on prediction = 0.80. The final 33 frames show an average of all frames (green) merged with a static image of SR 101 labeling of astrocytes (magenta) in the same field.