Figure 1. Visual performance declines with age.

Spatial frequency threshold (SFT) and contrast sensitivity (CS) measurements were performed on an optokinetic tracking (OKT) device built in-house. Left: An oblique view of the OKT device, with the mirrored ceiling removed to show the typical position of a compliant rat: standing on an elevated perch, with its head roughly centered in the device. Movement was unrestricted during testing, and rats were gently placed back on the perch if needed. Tracking was recorded only if the rat remained on the perch and the stimulated eye (left eye if the virtual barrel was moving clockwise; right if counter-clockwise) was inside a small central portion of the device (see Fig. S1 in File S1). Additional device details – including illustrations, dimensions, reproducibility of measurements, and comparisons to a commercially-available system [47] – are provided as Supplemental Figure S1 (in File S1). Center and Right: Scatter plots showing longitudinal changes in SFT and CS. Spatial frequency of the stimulus is reported by “cycles/barrel degree” (c/bd) such that a (40/360° = ) 0.111 c/bd setting means that the three screens, together, generated a 360° virtual barrel with 40 dark-light-dark cycles of the sine-wave grating. Mean±SEM are indicated by the horizontal lines overlaid on each scatter plot. Longitudinal measures from the same subject are connected by gray lines, while a white mark within a point denotes the lack of follow-up data (e.g., due to animal death). Note that values from rats lost to follow-up are evenly distributed among those from animals retained for longitudinal comparisons. § indicates a significant (q<0.05; paired two-tailed tests) effect of age. Comparisons of ∼7 to ∼19 mo data are not labeled since patterns of significance (SFT: P = 0.57, CS: P = 2.1e-7) are well-depicted by the ∼7 vs. ∼11.5 mo comparisons. SFT tends to stabilize after the initial decline from young to mid-adulthood. CS progressively declines with age.