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Elizabeth Coulthard_ Parashkev Nachev and Masud Husain

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					          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.