7: Motor Control

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7 Motor Control

• Chris Rorden
• Ataxia
• Apraxia
• Motor Neurons
• Coordination and Timing

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• M1 Primary Motor Strip
• M1 sends outputs to muscles
• Motor strip most posterior portion of frontal
  cortex
• Topic of future lecture
• Damage to M1 leads to paresis (weakness, partial
  loss of movement, impaired movement) or plegia
  (complete paralysis). For example, after stroke
  many patients have hemiparesis (poor use of
  contralesional limbs).

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Cortical visual processing
Dorsal system is fast, but color blind. Helps
with motor control (Where/How).
Parietal
MT V5
Magno LGN
V1
V2
V3
M-ganglion cells
Parvo LGN
P-ganglion cells
V1
V2
IT cortex
V4
Ventral system is slow, but detailed. Helps with
object identification (What).
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Where versus What

• Dorsal damage action
• Akinetopsia
• Spatial perception problems
• Problems with reaching, eye movements
• Ventral damage recognition
• Achromatopsia
• Agnosia object identification

5
Visual Form Agnosia

• DF has ventral damage
• Profound agnosia can not even tell orientation
  of object
• Motor control accurate motor system functions
  accurately.

Patient DF
Controls
Posting task
Perceptual matching
Posting
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Ventral vs Dorsal damage (Goodale et al. 1994
Curr Biol. 4604-610)

• When shown two shapes (left), DF was poor at
  saying if the shapes were same or different, RV
  was good at this task.

100
chance
0
DF
RV
When asked to grasp an object, DF grasped near
the centre (like healthy people), RV was poor at
this task.
DF
RV
Control
25
Frequency
0
0 15
0 15
0 15 30
Distance from centre (mm)
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Ataxia

• Ataxia (from Greek ataxia, meaning failure to put
  in order) is unsteady and clumsy motion of the
  limbs, with poorly coordinated movements.
• Optic ataxia results from a lesion in the
  superior parietal lobe, which can be bilateral
  it causes an impairment in the automatic
  visuo-motor transformation (either or both in
  reaching and/or grasping visual targets) in
  actions with the hand contralateral to the lesion
  site

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• In spite of visuomotor impairment, optic ataxic
  patient AT can recognize shape and size of visual
  objects (Jeannerod, Decety, Michel, 1994).
• AT cannot reach and/or grasp object with
  precision grip, but she can estimate size of
  object by scaling the distance between thumb and
  index.
• Contrary to DF, ATs performance in task of
  pointing (index) is improved if delayed by 5
  seconds between occurrence of stimulus and
  launching of action.

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Perception and action dissociate

• In spite of visuomotor impairment, optic ataxic
  patient AT can recognize shape and size of visual
  objects (Jeannerod, Decety, Michel, 1994).
• AT cannot reach and/or grasp object with
  precision grip, but she can estimate size of
  object by scaling the distance between thumb and
  index.
• Contrary to DF, ATs performance in task of
  pointing (index) is improved if delayed by 5
  seconds between occurrence of stimulus and
  launching of action.

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• Maximum grip aperture seen at 60 of movement to
  target.
• After lesion of superior parietal lobe, optic
  ataxic patients are impaired (bilaterally) in (B)
  grasping with precision grip and in (C) reaching

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• Apraxia (Greek without action or working) loss
  of the ability to execute or carry out learned
  purposeful movements, despite having the desire
  and the physical ability to perform the
  movements.
• Apraxic patients are unable to use and recognize
  tools, to pantomime the use of imaginary tools
  and to understand others pantomimed actions
  using imaginary tools but their visuo-motor
  transformation is intact.

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• Most argue apraxia results from a lesion in the
  left inferior parietal lobe
• A careful study by Goldenberg shows pantomime
  specific to left insula.

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Mirror Neurons

• Rizzolatti observation and action use same
  neurons.

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Perception versus Action

• Haffenden, Schiff Goodale (2001)
• Titchener illusion is perceptually powerful
• However, grasping shows reduced bias.

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Perception versus action

• Bridgeman (1975) et Goodale et al. (1986) find
  that healthy subjects can point accurately index
  finger towards target even though they are not
  visually aware of target motion because it
  coincided with saccadic eye movement.
• Castiello et al. (1991) find that healthy
  subjects can correct on line trajectory of their
  hand movement towards moving target 300 ms before
  they are visually aware of targets change of
  location.
• Visual awareness of object subsequent to, and not
  necessary for, accurate visual control of action
  on object (cf. DF).

16
Coordination and Timing

• Movement coordination requires timing.
• In particular, bimanual movements.
• Amazingly, split brain patients show precisely
  timed movements when moving both hands.
• Many believe cerebellum plays key role in
  sequencing and timing.