Positional acuity with chromatic stimuli.

Theoretical reasons are presented for expecting a high precision spatial acuity task (vernier acuity) to be more difficult with an equiluminous stimulus than with stimuli containing luminance cues. This prediction was verified in Experiment 1. In a second experiment, it was shown that the result of the first experiment could not be explained by reduced detectability of the equiluminous target bar. We explain these results by the conflicting demands of chromatic and spatial differencing within a single mechanism, and propose that this also explains the similarities between long and medium wavelength cones in their spectral sensitivities.