Visual acuity threshold was measured daily with the optokinetic tracking (OKT) response using Optometry software and apparatus (Cerebral Mechanics Inc., Alberta, Canada) as previously described [7 (link),14 (link)]. Briefly, mice were surrounded by a virtual cylinder consisting of vertical lines rotating at varying frequencies, and tracking behavior was assessed by an investigator blinded to genotype. Visual acuity is represented as the highest spatial frequency at which mice track the rotating cylinder. Because tracking is a temporal to nasal-specific reflex, counter-clockwise and clockwise rotations exclusively test the right eye and the left eye, respectively. Occasionally, the mice were too unsteady due to motor deficits to adequately perform OKT analysis, in which case a measurement was not recorded for that mouse on that day.