VEP latency and RT as power functions of luminance in the peripheral visual field

Behavioral correlates of P1 latency of visual evoked potentials (VEPs) were investigated using visual reaction time (RT) as a measure at the fovea, 20 degrees and 40 degrees nasal retinal eccentricities along the horizontal meridian of the right eye. Three luminances of the target in steps of 1 log units were used: 26, 260 and 2600 cd/m2. As luminance increased VEP latency and RT decreased. Further, as eccentricity increased VEP latency and RT increased. The highest product-moment coefficient of correlation (r = 0.998) was found between RT and P1 latency in the fovea, whereas the coefficient of correlation was found to be slightly small in the peripheral loci. The data (both VEPs and RTs) were fitted by the power function of the form: T = kLB + T0 where T, k, L T0 and B indicate VEP latency (or RT, scaling constant, luminance, asymptotic latency and exponent of the power function, respectively. This shows how the VEP latency and the RT can be related to luminance with exponent B. It was found that psychophysical power law governs VEP latency as well as RT with an exponent of about -0.32. It was suggested that both RT AND P1 latencies of VEP originate in the similar nonlinear visual system.