Cone adaptation in age-related maculopathy

Cone adaptation was measured in patients with age-related maculopathy (ARM) and age-matched controls. A red stimulus was used to selectively bias responses toward cone function and adaptation was measured 5, 10, 20, and 40 degrees eccentric to fixation. The ARM patients' thresholds were elevated by 1 log unit at 5 and 10 degrees, with the threshold difference reducing to 0.5 log units at 40 degrees. Curves of the form log (threshold luminance) = ae-bt + c (where t is time and a, b, and c are constants) were fitted to each adaptation curve; a procedure involving differentiation of the above equation was used to estimate the constant parameters. Time constants of cone recovery (1/b) were derived for each fitted curve. No significant difference was found between the time constants of the two test groups. Our data suggest that cone adaptation is significantly affected in patients with ARM, with raised cone thresholds occurring for areas well into the peripheral retina. We suggest that the effect is mediated by a reduction in the number of functional cones, which elevates the final cone thresholds without altering the adaptation time constant. Our findings imply that age-related "maculopathy" may not be confined to the macular area, and that early signs of the degeneration may be present in the peripheral retina even when central acuity is normal.