Control over conflict during motor preparation: role of the posterior parietal cortex Elizabeth Coulthard, Parashkev Nachev and Masud Husain Institute of Cognitive Neuroscience and Institute of Neurology, University College London and Imperial College London 1. Background 4. Directional hypokinesia in the instructed condition is 7. Parietal damage causes paradoxical speeding of After right hemisphere stroke, neglect patients respond preferentially to objects associated with parietal damage rightward movements in the incongruent condition on their right. Competitive bias toward rightward motor plans, as well as toward "frontal" neglect rightward sensory stimuli, could contribute to this. group n=8 normal Lesion overlay of 10 neglect 0.20 Previous studies have been inconsistent, but there is a suggestion that such elderly "parietal" non-neglect patients slower to move left controls neglect right patients demonstrate directional hypokinesia - slow initiation of leftward than right with the other 6 RT costs in 0.15 n=11 group hemisphere n=8 movements using the intact right arm (eg. Mattingley, Husain et al 1998). A neglect patients subtracted. incongruent 0.10 stroke n=8 possible reason for such a deficit is failure to resolve competition between condition/neutral reaction time conflicting rightward and leftward motor plans. 0.05 0.00 Here we investigate whether conflicting motor programmes slow instructed In instructed trials, parietal patients perhaps encounter greatest conflict from alternative -0.05 leftward movements and internally generated leftward movements. directional motor plans plans when moving leftward. To assess this further we used a conflict task. rightward movements 5. Conflict task Lesion overlay of the 8 neglect patients with 2. Free choice vs instructed movements Imperative cue Congruent flankers a benefit of incongruence signalling Instructed trial move left for right movements after Time subtraction of 8 neglect Subjects responded using a centrally positioned joystick to stimuli presented in the vertical Time patients with a cost of midline. This removed the possible confound incongruence bilaterally of lateralised perceptual biases. Subjects moved as fast as possible left or right according to the direction Choice trial In the instructed condition, they responded in the direction of the arrows, right or left, as of a central arrow. In the incongruent condition, the central arrow was flanked vertically by arrows Incongruent flankers 8. Conflict from motor plans causes directional differences fast as possible. In the choice condition subjects were instructed to choose left or pointing in the opposite direction. In the congruent condition, the central arrow was flanked by arrows in movement initiation in parietal patients whereas visual right movements as quickly possible. They were told not to make a plan in advance but pointing in the same direction. In the neutral condition, the central arrow was flanked by squares comprising distraction slows leftward movement in frontal patients to move with their first instinct. Stimuli were components of the arrow stimuli rotated to remove any presented for 200ms and the intertrial directional associations. Incongruent These different patterns of performance interval was at least 2000ms. Stimuli were presented for 200ms and the intertrial interval Neutral * suggest susceptibility to different types was at least 2000ms. Neutral of conflict. Competition from conflicting flankers 60 * motor plans is maximal in the incongruent 3. Patients with left neglect were disproportionately slowed Left - Right RTs (ms) condition. The patients with parietal 40 Congruent lesions show the greatest difference between when they chose the direction of movement. across different leftward and rightward movement initation The neglect patients were slow to intitiate 6. Two abnormal patterns for RIGHTWARD movements in the conditions 20 times in the incongruent condition. Neglect patients n=16 freely chosen movements whether they neglect group 0 If susceptibility to visual distraction is were rightward or leftward. RT (ms) responsible for leftward movement slowing, RT (ms) • One group were slower to make Incongruent one would expect the biggest difference in ** Ten of the sixteen neglect patients both right and left movements Incongruent -20 "frontal neglect" n=8 movement initation times in the neutral Non-neglect were slower to move left than right 800 Age- stroke in the presence of incongruent "parietal" neglect n=8 condition (most visually distracting) and matched controls in the instructed condition i.e. leftward flankers; similar to normal age matched controls n=11 progressively less in the incongruent and 700 Neutral non-neglect stroke n=8 normal n=8 directional hypokinesia controls, but with disproportionate congruent conditions. This pattern is seen controls median n=11 Neglect cost in the right incongruent reaction Neutral Neglect patients with R * = signficantly slower leftward than rightward movements in neglect patients with more frontal lesions. 600 slower to condition compared with neutral. move times incongruence cost (n=8) left (n=10) Neglect Congruent not slower • The second group paradoxically 600 Congruent 9. Conclusions 400 700 * to move left (n=6) were faster to move rightward Neglect patients with no R when the flankers were incongruence cost (n=8) 0 Neglect patients have directionally specific deficits in motor control. The data presented here incongruent than when they RT show there are two types of impairment depending upon the nature of the conflict; lesions 600 were neutral or congruent. 500 Non-neglect stroke controls (ms) in different brain areas are associated with these behavioural patterns. During motor left instructed They showed a normal cost for (n=8) preparation, parietal regions play a role in control over conflict between action plans. right instructed leftward movements. left choice 500 Age matched controls right choice 0 (n=11) • These two groups were associated 400 References with frontal and parietal damage leftward movements rightward movements respectively. Leftward Rightward Mattingley, J. B., M. Husain, et al. (1998). "Motor role of human inferior parietal lobe revealed in unilateral neglect patients." Nature 392: 179-182.
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