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									    JHU BME 580.422 Biological Systems II


        Adaptation of Visuomotor Maps
          Disorders of parietal cortex


               Reza Shadmehr




1
    Schematic of the computations involved
                  in reaching




                      hand location in       Displacement vector
                      fixation coordinates   in fixation
                                             coordinates

                                                                                Force (motor output)

                  q             xh
                                                  q                q
                                             xa           Dq                f
                                     xt
    Proprioceptive
    state of the body:
    arm, head, and                                    Displacement vector
    eye positions          Target location in         in proprioceptive
                           fixation coordinates       coordinates



2
        p x  l1 cos(q1 )  l2 cos(q1  q2 ) 
    p                                        
        p y   l1 sin( q1 )  l2 sin( q1  q2 ) 
        
    xh  s  p                                                              l2
                                                                       p
    x a  xt  x h                                                 s
        q                                                                l1
    q   1
         q2 
                 dp x dp x 
                               
        dp  dq1 dq2 
    J       
        dq  dp y dp y 
                               
                 dq1 dq2 
                               
         l1 sin( q1 )  l2 sin( q1  q2 ) l2 sin( q1  q2 ) 
    J                                                        
         l1 cos(q1 )  l2 cos(q1  q2 ) l2 cos(q1  q2 ) 
           dp
    Dp        Dq
           dq
          dp
    xa       Dq
          dq
    Dq  J 1x a
3
         PPC neurons encode target of intended movement during
                           the delay period
                     PPC cell
                                                  Somatosensory cortex cell
    A   Control Task
                                          D   Control Task




                                0.5 sec                            0.5 sec

    B   Delay Task
                                          E   Delay Task




    C   Delay Task
                                          F   Delay Task




4
                                                             Crammond and Kalaska 1989
    PPC neurons encode target of intended movement even
                    after it disappears




5
                                         Kalaska JF Can J Physiol Pharmacol 1996
    Human PPC neurons code for target location in fixation
                  centered coordinates
                            Delayed pointing with
        Delayed pointing    intervening saccade




                                                    Left cortex   Right cortex




                                      Activation area when          …left of fixation
                                      the remembered target
                                      is to the right of
6                                     fixation                          Medendorp et al.
                                                                        2003
                                     PPC neurons encode target location and not the forces
                                               necessary to reach that target


                                                                    Opposing load   No load   Assisting load



                                                 Deltoid muscle
Kalaska et al. J Neurosci 1989




                                                                                                      500 ms

                                                        Primary
                                                          motor




                                                                                                           100 imp/s
                                                      cortex cell



                                                       Posterior
                                                  parietal cortex
                                                    cell (area 5)




                                 7
                                                                                                Kalaska JF (1988)
    Newts show an inability to adapt to radical changes in the
                        optics of vision


    Experiment: rotate each eye by 180o. After 4-5 months, no
    sign of adaptation.
    “When the piece of meat was moved back and forth in the
    water several centimeters above and a little to one side of
    the animals, they tilted their heads downward on that side
    and began to move toward the bottom of the aquarium.
    Even though the newts happened to be resting on the
    bottom when the lure was thus waved above them, they
    cocked their heads down under them and began pushing
    about among the pebbles of the bottom with the nose and
    forefeet. If the lure was placed below the animals, the head
    and forebody were tilted upward and the newts started
    toward the surface.”


8
        Primates adapt to radical changes in optics of vision

    A




                                Latency (s)
                                              Pre-   Days of reversing Post-

    B




    C



9
                        Day 3                             Day 34
                        camera




          target
                         xdv
             xt


                        xee       q2

     c2                          q1
                   c1




10
        Wedge prisms: rapid adaptation and equally rapid de-
     adaptation indicates short-term changes in existing networks



                A                         B




11
     With extensive
training, throwing with
   wedge prisms can
 become a skill. This
indicates formation of
   a new map for the
 prism and the ability
 to switch on context.




 12
     Patients with lesion in the right hemisphere may exhibit
                  neglect of the left visual space



     Model       Copy




13
                                          Marshall JC & Halligan PW (1995) Nature 373:521
               Neglect of the extra-personal and personal space

 Test for neglect of extra-personal space: Line bisection test, figure copying,
 reading a sentence.
 Test for neglect of personal space: use a comb, use a razor to shave the face,
 use a lipstick.


 Neglect of extra-personal
 space: lesion of the right
 frontal lobe, ventral
 premotor cortex
 Neglect of personal
 space: lesion of the right
 inferior parietal cortex


 STG: superior temporal gyrus
 MFG: medial frontal gyrus
 WM: white mater
 SMG: supramarginal gyrus
14
                                                 Committeri, G. et al. Brain 2007 130:431-441
     Prism glasses produce after affects that improves the sense
               of “straight ahead” in neglect patients




15
                                                 Rossetti Y. et al. (1998) Nature 395:166
       Prism glasses produce after affects that reduce the neglect
                       observed in PPC lesions



                      Prisms                        Controls


     original

     pre-

     post-


     late




16
                                                   Rossetti Y. et al. (1998) Nature 395:166
                                       Apraxia

     Apraxia is an inability to perform skilled movements, particularly tool
     use, in the absence of elementary motor deficits (weakness, normal
     posture or tone).
     It is most commonly associated with damage to the parietal cortical
     areas of the left hemisphere.
     When the patient is asked to demonstrate use of a screwdriver, the
     patient may position his hand as if holding a pen. When given a
     partially driven nail into a piece of wood, and a collection of tools, they
     may select a scissor to drive the nail rather than a hammer.
     In performing a task that requires a sequence of actions, these
     patients may have difficulty in putting the acts in the proper order. For
     example, demonstrate how to prepare a letter and envelope for
     mailing.



17
         Understanding actions of others: the mirror neuron system
     Social skills and theory of mind: the awareness that other people have beliefs
     and desires as we do, but different from our own, and that these beliefs and
     desires guides their actions. By observing their actions, we can guess their
     goals and intentions.




18
                                                  D




19   Fogassi et al. (2005) Science 308:662-667.
20   Fogassi et al. (2005) Science 308:662-667.
                    Summary of the posterior parietal cortex

 Actions are planned in fixation centered coordinates:
          Position of the hand and the target are represented in terms of their
          location with respect to the fixation.
 Neurons combine proprioceptive information with visual information using a gain
 field.
 Movements are planned in terms of goals, not in terms of detailed forces.
 Lesion of the right parietal cortex can result in neglect. Lesion of the left parietal
 cortex can result in apraxia.
 Mirror neurons might provide a mechanism through which we understand the
 intention of others.




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