1 Motor Control Chris Rorden Ataxia Apraxia Motor Neurons Coordination and Timing

2 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).

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

4 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

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 DFControls Posting task Perceptual matching Posting

6 Ventral vs Dorsal damage (Goodale et al. [1994] Curr Biol. 4: ) When shown two shapes (left), DF was poor at saying if the shapes were same or different, RV was good at this task. DFRV chance 100% 0% DF Control RV Frequency 25% 0% Distance from centre (mm) When asked to grasp an object, DF grasped near the centre (like healthy people), RV was poor at this task.

7 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;

8 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.

9 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

10 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.

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

12 Perception versus Action Haffenden, Schiff& Goodale (2001) Titchener illusion is perceptually powerful However, grasping shows reduced bias.

13 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).

14 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.