Normative data for onset VEPs to red-green and blue-yellow chromatic contrast

Objective: To better characterize the properties of chromatic VEPs to onset-offset of red-green and blue-yellow equiluminant patterns, and establish normative values for a set of stimuli able to elicit robust and reliable responses, suitable for the clinical application. Methods: Chromatic VEPs have been recorded (Oz lead) from 28 normal subjects (age range 20-53 years) in response to monocular presentation of both red-green and blue- yellow equiluminant sinusoidal gratings. Stimuli were generated by a Cambridge VSG/2 card and displayed on a Barco CCID monitor (14 x 14 deg field size). Spatial frequency, chromaticity, contrast and onset-offset duration were varied. Results: For both red-green and blue-yellow equiluminant stimuli, robust responses have been obtained with gratings of 2 c/deg, presented in onset (300 ms) offset (700 ms) mode, at contrasts ranging from 90 to 6%. In all observers, the VEP waveform consisted mainly of a negative wave at stimulus onset, with a latency rapidly increasing with decreasing contrast. For both red-green and blue-yellow stimuli, the VEP contrast threshold coincided with the psychophysical threshold. Conclusions: The results complement previous studies aimed at characterizing the properties of chromatic VEPs. In addition, normative data are provided for a set of stimulus characteristics suitable for the clinical routine.