Waking state, as indexed by pupil size, varies over a wide range. A, Basic experimental setup for the head-fixed mouse preparation. An LCD monitor is mounted to the mouse's left, presenting visual stimuli to the left eye, and a USB camera is mounted to the mouse's right, capturing video of the right eye, which is illuminated by 2 IR LEDs. A photodiode mounted to the upper-left corner of the LCD monitor gives precise timestamps of stimulus onsets. An optical mouse mounted to the side of the wheel acquires wheel speed. B, Representative traces of normalized pupil diameter and locomotion speed for an entire example recording session (∼90 min). The horizontal dashed line plotted with wheel speed indicates the 5 cm/s threshold for qualifying as a locomotion bout (see Materials and Methods, “State-dependent electrophysiological metrics”). Insets magnify the numbered regions and show regions of interest (ROIs) around the eye for selected times. C, Normalized pupil diameter distributions, sorted according to whether mice were still or walking, for all task-engaged (N = 20) or passively behaving (N = 25) animals in the dataset. Vertical dotted lines indicate the peaks of the two distributions.