Cognitive Neuroscience Talks

On-line and off-line visuomotor behaviour in hemispatial neglect

Left hemispatial neglect manifests itself in a rightward bias in perceptual tasks (e.g., line bisection), yet the presence of this neglect-specific bias in visuomotor control remains a matter of debate. In a series of experiments we compared patients with and without neglect after right-hemisphere stroke when performing on-line and off-line reaching tasks (e.g., delayed or anti-pointing). We found that despite their perceptual impairments, patients with left neglect were not specifically impaired when performing on-line reaches, even towards targets located on the left side of space and even when visual feedback of the hand were removed. Further voxel-based lesion analysis confirmed that motor impairments in neglect patients are lesion-specific rather than neglect-specific. However, when neglect patients were asked to perform off-line actions they were specifically disrupted in the end-point of their movements, especially towards the left side of space. Importantly in this case, lesion-symptom analysis revealed that neglect-associated lesions (around the temporo-parietal junction) are critical nodes for the ‘perceptual control’ of action involved in off-line reaches. These observations agree with the controversial suggestion that hemispatial neglect might reflect a breakdown in a system for scene representation that puts together visual information received via the ventral visual stream, but not the dorsal (Milner & Goodale, 2006).