While conflict between instructions and distractors makes normally reactions slower, right inferior parietal lobule (IPL) damage associated with left spatial neglect leads, in a visuomotor task, to the paradoxical facilitation of rightwards movements in the presence of conflicting leftward response plans (Coulthard et al., 2008). We investigated the effects of the right IPL lesion on the interference between vision and audition in a spatial context.

Behavioral data and Event-Related Potentials (ERPs) of 12 healthy adults and 12 patients with a focal right parietal post-stroke lesion, matched in age and educational level, were recorded during an audio-visual task. A pure tone through a stereo system and a white circle on a screen were presented simultaneously on the same side (left or right) in congruent trials and on opposite sides in incongruent trials. Participants had to respond to the auditory or visual target's side according to the instruction, by pressing a response-pad button with their right hand. Whereas all the patients had a left spatial neglect at the acute phase of stroke, they had recovered a normal performance on “paper-and-pencil” tests when included in our study.

Paradoxically, for right visual target with left auditory distractor, patients had a smaller reaction time “cost” than the control group. For healthy controls, two frontocentral ERPs negative components, N2 and N270, were elicited in all conflict conditions compared to congruent conditions. Whereas the amplitude of N2 and N270 was not different between the two groups for left targets, these components were not generated by the presentation of a right visual or auditory target with a left distractor in patients.

N2 and N270 might be elicited by the detection of two incongruent response plans by the prefrontal cortex. Their suppression for left targets in patients supports the hypothesis that the right IPL might be a crucial site for automatic activation of competing motor plans generated by cross-modal conflicting stimuli. In patients, the behavioral facilitation for right visual targets only may be due to additional effect of visual dominance over audition in spatial tasks.