Brain potential correlates of supraliminal contrast functions and defocus

In six subjects with normal corrected vision, perceived suprathreshold contrast of a sine‐wave grating pattern was assessed by an intermodal matching technique in combination with the method of ratio estimation. Vertical gratings were produced on an oscilloscope screen at spatial frequencies of 1.0, 2.0, 3.5, 5.0, 9.0 and 15.0 cycles per degree (c/deg) and at contrast levels between 0.03 and 0.55. Steady‐state visual evoked potentials (VEP) were recorded from the occiput to phase‐reversing gratings of the same spatial frequency and contrast range. A linear relation was found in logarithmic coordinates between perceived and physical contrast and between VEP amplitude and physical contrast. The slopes varied with spatial frequency and were steeper at low and high spatial frequencies than in the intermediate range for both subjective contrast and VEP.

In a second group of six subjects, the effect of retinal image defocus (myopia) on the psychophysical power functions was determined by placing spherical plus lenses in front of the eyes. A marked change in slope was obtained with blurred stimuli, and this effect was more pronounced with an increase in spatial frequency. Power transformations can thus be induced by neural and optical factors. Growth rate of supraliminal response functions seems to be inversely related to detection threshold of sensory systems.