Unilateral posterior parietal lobe lesions disrupt kinaesthetic representation of forearm orientation.

OBJECTIVE To apply the lesion method to assess neuroanatomical substrates for judgments of forearm orientation from proprioceptive cues in humans. METHODS Participants were 15 subjects with chronic unilateral brain lesions and stable behavioural deficits, and 14 neurologically normal controls. Subjects aligned the forearm to earth fixed vertical and trunk fixed anterior-posterior (A-P) axes ("straight ahead"), with the head aligned to the trunk and with head and shoulder orientations varied on each trial. RESULTS Most subjects with posterior parietal lobe lesions made larger variable errors than controls in aligning the forearm to the earth fixed vertical axis and the trunk A-P axes, whether the head was held upright or oriented in different positions. Lesion subjects and controls made similar constant errors for aligning the forearm to gravitational vertical. Variable error magnitude correlated positively with greater lesion volume of right and left superior parietal lobules (SPL), but not with lesions in other brain areas. Larger variable errors for aligning the forearm to the trunk fixed A-P axis were also correlated with the volume of SPL lesions, but constant error magnitude correlated with larger volume lesions in premotor areas, inferior parietal lobules, and posterior regions of the superior temporal gyri, but not with SPL lesion volume. CONCLUSIONS The findings suggest that the right and left superior and inferior parietal lobules, posterior superior temporal gyri, and premotor areas play a role in defining higher level coordinate systems for specifying orientation of the right and left forearm.