ABERRATION-FREE MEASUREMENTS OF THE VISIBILITY OF ISOLUMINANT GRATINGS

We developed a new apparatus and psychophysical technique to extend is oluminant contrast-sensitivity measurements to high spatial frequencie s. The apparatus consists of two identical laser interferometers that are designed to produce phase-locked two-color interference fringes on the retina without the influence of diffraction and most aberrations in the eye. However, even with interferometry, transverse chromatic ab erration of the eye can produce a wavelength-dependent phase shift in the interference fringes, which can be exaggerated by head movements. To reduce the effect of head movements, isoluminant red and green inte rference fringes of equal spatial frequency and orientation were drift ed slowly in opposite directions to guarantee a purely isochromatic (i n phase) and a purely isoluminant (out of phase) stimulus during each cycle of stimulus presentation. With this technique we found that obse rvers could resolve red and green stripes at spatial frequencies highe r than 26 cycles per degree (c/deg) (20-27 c/deg), substantially highe r than has previously been reported. This places a lower bound on the sampling density of neurons that mediate color vision. At all spatial frequencies, even those above the isoluminant resolution limit, a rela tive phase of the red and the green components could be found that obl iterated the appearance of luminance modulation at the fringe frequenc y. Above the resolution limit, red-green-isoluminant interference frin ges are seen as spatial noise, which may be chromatic aliasing caused by spatial sampling at some stage in the chromatic pathway.