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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

M-ganglion cells

P-ganglion cells

MagnoLGN

ParvoLGN

V1

V1

V2

V2

V3

V4

MTV5

ITcortex

ParietalDorsal system is fast, but color blind.Helps with motor control (Where/How).

Ventral system is slow, but detailed.Helps with object identification (What).

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Where versus What

Dorsal damage: action– Akinetopsia: Motion Blind– Spatial perception problems– Problems with reaching, eye movements

Ventral damage: recognition– Achromatopsia: Colorblind– Agnosia: object identification

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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 ControlsPosting task

Perceptual matching

Posting

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Ventral vs Dorsal damage (Goodale et al. [1994] Curr Biol. 4:604-610)

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

DF RV

chance

100%

0%

DFControl RVF

req

uen

cy

25%

0%

Distance from centre (mm)0 15 0 15 0 15 30

When asked to grasp an object, DF grasped near the centre (like healthy people), RV was poor at this task.

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Ataxia

Ataxia (from Greek ataxiā, 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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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.

Unlike DF, AT’s 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

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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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 online trajectory of their hand movement towards moving target 300 ms before they are visually aware of target’s change of location.

Visual awareness of object subsequent to, and not necessary for, accurate visual control of action on object (cf. DF).

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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.