Spatial Integration of Objectively Equiluminous Chromatic Gratings

Abstract

Contrast sensitivity functions for achromatic and chromatic gratings tend to be band-pass and low-pass in shape, respectively. Our aim was to test whether spatial integration contributes to the shape difference found at low spatial frequencies. We measured binocular chromatic contrast sensitivity as a function of grating area for objectively equiluminous red - green and blue - yellow chromatic gratings. Chromatic contrast refers to the Michelson contrast of either of the two chromatic component gratings presented in counterphase against the combined background. Grating area (A) varied from 1 to 256 square cycles (Af²) at spatial frequencies (f) of 0.125 – 4.0 cycles deg⁻¹. We used only horizontal gratings at low and medium spatial frequencies to minimise the transverse and longitudinal chromatic aberrations due to ocular optics. At all spatial frequencies studied, chromatic contrast sensitivity increased with grating area. A_c was found to be constant at low spatial frequencies (0.125 – 0.5 cycles deg⁻¹) but decreased in inverse proportion to increasing spatial frequency at 1 – 4 cycles deg⁻¹. Thus, spatial integration depends similarly on spatial frequency for achromatic (Luntinen et al, 1995 Vision Research 35 2339 – 2346) and chromatic gratings, and differences in spatial integration do not contribute to the shape difference of the respective contrast sensitivity functions.