Dissecting the component deficits of perceptual imbalance in visual neglect: Evidence from horizontal–vertical length comparisons

IntroductionSigns of left unilateral neglect often occur after damage to the right hemisphere and entail a left–right imbalance in stimulus processing. Typically, neglect patients deviate rightward when bisecting lines. An underestimation of the left portion of the line and/or a right overestimation could explain this effect.MethodsHere we dissected their respective contribution by asking participants to compare a vertical segment to a horizontal segment, either on the left or on the right. We also tested whether neglect patients exhibited the symmetry law, whereby normal participants underestimate symmetrically bisected lines as compared to asymmetrically bisected lines.ResultsControls and patients underestimated symmetric figures. Depending on the degree of left–right horizontal competition, neglect patients underestimated left-sided stimuli or overestimated right-sided stimuli.ConclusionsWe concluded that two independent deficits contribute to neglect signs: a deficit in attentional orienting to the left, which can be worsened by left hemianopia, perhaps depending on impaired functioning of right-hemisphere attentional networks, and a tendency for attention to be captured by right-sided stimuli, possibly resulting from the activity of an isolated left hemisphere. Finally, the symmetry law was preserved in neglect patients, and thus appears to be driven by pre-attentive mechanisms.