Chromostereopsis - Methods of Testing

Methods of Testing

Many different methods of testing have been employed to view the effects of chromostereopsis on depth perception in humans. Technological progress has allowed for accurate, efficient, and more conclusive testing, in relation to the past, where individuals would merely observe the occurrence.

In one method, twenty-five control subjects were tested using color-based depth effects through the use of five different colored pairs of squares. The different colors were blue, red, green, cyan and yellow. Subjects were placed in a dark room and the colored square stimuli were presented for 400 milliseconds each, and during this time the subjects were asked to attend to either the right or left square (evenly counterbalanced across subjects). Using a joystick, the subject indicated whether the square was behind, in front of, or in the same plane as its pair. According to the theory, the longer the wavelength of the color, the closer it should be perceived by the observer for positive chromostereopsis. Having a longer wavelength than the other colors, red should appear closest. To enhance this effect, subjects put on blazed-grating ChromaDepthTM glasses, which contain a prism structure to refract the light to an angle of approximately 1° and were tested again.

The use of electrodes to test brain activity is another, relatively new way to test for chromostereopsis. This form of testing utilizes EEG recordings of visual-evoked potentials through the use of electrodes. In one experiment, subjects were shown different stimuli in regard to color-contrast and were asked questions about its depth, as before. The electrodes attached to the subjects subsequently collected data while the experiment occurred.

Another more routinely used technique tests the subject's extent of chromatic aberration. In one such experiment, slits placed before the subject’s eyes measured the chromatic dispersion of the eyes as a function of the separation of the slits. Prisms in front of the eyes determined the separation of the visual and null axes. The product of these separate measurements predicted the apparent depth expected with full-pupil stereoscopy. Agreement was good with expected results, supplying additional evidence that chromostereopsis depends on chromatic dispersion.

Other experimental techniques can be used to test for reverse chromostereopsis, an occurrence seen by a minority of the population. The direction of chromostereopsis can be reversed by moving both artificial pupils in a nasal direction or temporal direction with respect to the centers of the natural pupils. Moving the artificial pupils nasally induces blue-in-front-of-red stereopsis and moving them temporally has the opposite effect. This is due to the fact that moving the pupil changes the position of the optic axis, but not the visual axis, thus changing the sign of transverse chromatic aberration. Therefore, changes in the magnitude and sign of transverse chromatic aberration brought about by changing the lateral distance between small artificial pupils are accompanied by equivalent changes in chromostereopsis