Abstract: In an apparatus for assessing visually-evoked response, wherein stimulus means are provided to present a variable visual pattern to an observer and analyzing means are provided to analyze scalp potentials of the observer responsive to said pattern; the improvement wherein the stimulus means comprise means for temporal modulation of the pattern at a determined rate and simultaneously sweeping another visual parameter, in a determined period, in a range which crosses an observer's visual threshold for said other parameter, said analyzing means comprising means for synchronously demodulating said scalp potentials employing said determined rate of the temporal modulation as a reference, at a plurality of different phases with respect to said reference, for producing a display of the scalp potentials demodulated with different phase reference.