Macular pigment: a test of the acuity hypothesis

Schültze, in 1866, originally proposed that macular pigment (MP) could improve acuity by reducing the deleterious effects associated with the aberration of short-wave (SW) light. Although proposed well more than a century ago, the hypothesis has never been empirically tested. The authors chose to begin evaluating the acuity hypothesis by measuring MP levels, gap, and hyperacuity in the same observers.

Eighty healthy young subjects were assessed. Forty subjects were assigned to the gap acuity experiment and 40 to the hyperacuity experiment. Peak MP optical density (MPOD) was measured using heterochromatic flicker photometry (HFP). Resolution and hyperacuity were measured as the minimum perceivable gap between two solid black lines (1'' width) vertically separated and as a vernier offset, respectively. These targets were presented on a 0.5 degrees circular diffusing background that appeared either white (17 cd/m2) or yellow (16 cd/m2). The yellow background was produced by using light-emitting diodes (LEDs) with a peak lambda = 570 nm. The white background was produced by combining the yellow with a blue LED (peak lambda = 460 nm). The subject's head (5.33 m from the stimulus) was stabilized with a head-rest assembly, and the adaptive state was controlled with the use of a constant white surround (11 cd/m2). Thresholds were determined based on probit analysis of psychometric functions generated using a two-alternative forced-choice procedure.

MPOD ranged from 0.14 to 1.00 measured at 30' eccentricity. Gap and hyperacuity measures each varied by a factor of approximately 5 to 6. Average gap acuity (N = 38) for the white condition (filtered by MP) was 31.2'' (SD = 9.4) and did not differ from the average (N = 38) for the yellow condition (not filtered by MP), which was 32.1'' (SD = 10.9). Similarly, average hyperacuity for the white condition (7.0''; SD = 2.9) did not differ from that of the yellow condition (6.8''; SD = 3.5).

MPOD did not correlate significantly with gap or hyperacuity measured in the yellow or white conditions. These data, therefore, do not support the predictions of the acuity hypothesis.