Vector analysis of visual evoked potentials elicited by pattern reversal and photic stimuli

Objective: The 2D VEPs to pattern reversal (PR) and LED goggle were studied in order to obtain a stable parameter for the functional assessing of posterior visual pathways regardless of the stimulus type used. Design and methods: Apex c latency, bc segment amplitude (V), and bc vector orientation angle (θ) in voltage space were computed from VEPs recorded in 50 normal human beings and two patients with left posterior brain lesions, in an orthogonal Fpz–Oz and T3–T4 montage and displayed as a two channel Lissajous’ trajectory. The effects of stimulus type and stimulated eye were analyzed in the normal group by a two-way ANOVA. Results: The stimulated eye had no effect on any parameter. Apex c latency was slightly longer, and V was greater and more variable in the responses to goggle stimuli, but there was no significant difference in θ, oriented to mid-occipital scalp with very low variability for both stimulus types. The patients showed significant deviations of θ towards the affected hemisphere. Conclusions: The bc vector orientation (θ) is a stable parameter for the evaluation of the posterior visual pathways using both pattern reversal and LED stimuli, specially the latter, useful in unconscious or uncooperative patients.