[Effect of an early visual encoding defect on attentional function: a study of daltonism using the Stroop test]

The Stroop color-word test is often used to assess attentional function. In this study we investigated whether dyschromatopsia affects the results of this test, and in that case which are the attentional consequences of this color-vision trouble. Event-related potentials were recorded on 19 dyschromatopsic subjects when submitted to a video-displayed Stroop test, and compared to those of 19 control subjects. Results showed that reaction times of dyschromatopsic subjects, although generally longer than those of controls, had normal interference and facilitation effects. Potentials evoked by neutral stimulations were delayed for P2 and P3 waves, and those evoked by word-containing stimulations showed delays for N2b (N320) and P3 waves. The premotor potential appeared modified in dyschromatopsic patients, whereas error negativities, or correct response negativities were identical to those of control subjects. We conclude that dyschromatopsia affects negatively the performance in the Stroop test, at sensory levels which are reflected by evoked potentials, but without important consequences on attentional function. In particular, dyschromatopsia did not alter significantly the attentional focusing on the chromatic aspects of the stimulus.