Effect of chromatic mechanisms on the detection of mesopic incremental targets at different eccentricities

Spectral sensitivity functions for the threshold detection of mesopic incremental targets were compared for different target eccentricities (10, 20, and 30 degrees) and for different mesopic backgrounds (0.1, 0.5 and 1.0 cd m-2). Relative responsivities of achromatic mechanisms (L + M and rods) and chromatic mechanisms (S and |L-M|) were estimated for each eccentricity and background. Chromatic mechanisms contribute significantly to detection but their effect is lower at 30 degrees. A new contrast metric (C(CHC2)) is introduced to account for the selective adaptation of the photoreceptors and the effects of the chromatic mechanisms i.e. broadening of the range of spectral sensitivity with multiple local maxima and yellow sub-additivity of detection performance. The C(CHC2) metric is compared with the achromatic contrast metric of the MOVE model (C(MOVE)). For the same target, C(CHC2) generally predicts a higher visibility level than C(MOVE). However, in accordance with visual observations, for grey or yellowish incremental targets appearing at the eccentricities of 20 and 30 degrees, the visibility predicted by C(CHC2) is less than the visibility predicted by C(MOVE).