Sweep-VEP measurement of contrast sensitivity in patients with either retinitis pigmentosa or macular disease

Purpose. In order to validate a model of retinal contrast sensitivity (CS) that based its predictions on the relative contributions of central and peripheral visual field, we determined with a sweep-VEP technique the CS in normal subjects, as well as in patients with retinitis pigmentosa (RP) or a diagnosis of macular disease (MD). Methods. Eleven normals, eight patients with RP, and six patients with MD were studied. VEPs were elicited by sine wave luminance gratings presented at 0.5, 1, 3, 8, 12, or 16 cpd, and counterphased in a square wave temporal profile at 7 Hz. Stimulus field sizes were 3, 6, or 12° in diameter, as well as an annulus extending from 6 to 12°. For each stimulus condition, the contrast of the gratings was swept from 0 to 50% in a 12-sec epoch, and the CS was scored as the lowest contrast in the epoch which evoked a response detectable with a lock-in amplifier referenced to the stimulus counterphase frequency. CS was also determined psychophysically in the normals. Results. In normals, the CS measured by sweep-VEP was about 0.5-0.75 log units lower than their psychophysical CS. Patients with MD had significantly (p<.05) lower CS (by sweep-VEP) in all conditions than did the normals or RP patients. A CS loss was noted in RP patients at high spatial frequencies in the 12° diameter field, and both RP and MD patients had worse CS compared to normals with the annular stimulus field. Conclusions. The sweep-VEP technique can be used to measure CS differences between normals and patients with disorders affecting function in central or peripheral retinal regions.