A comparison of psycholphysical and VECP increment threshold functions of a rod monochromat.

Increment thresholds of a rod monochromat were obtained over a wide range of background luminances. Within each experimental session, visual-evoked cortical potentials (VECP's) were computed at several luminance levels above the increment threshold for each background level. The results indicate that as the background level approached 1,000 scotopic trolands (td.), the VECP began to decrease in amplitude and it was unrecordable above 2,000 scotopic td. The VECP increment threshold function was determined with latency used as the criterion response. Good agreement was found between the psychophysical and VECP functions, although the VECP function did show a tendency to underestimate the threshold at the two dimmest background levels. It is concluded that the dramatic drop in the VECP amplitude at high background luminances is a reflection of the saturation of the rod system and that the VECP appears to be a sensitive correlate of the psychophysical data.