Automatic online control of motor adjustments

-P NANDHA KUMAR

Overview

There exists an automatic online process for reaching and grasping

Understanding a simple event such as pointing at a moving object requires a thorough understanding of various control systems in the human body

Movement execution in humans can be studied through various processes

Movement Execution

Key contribution by Marc Jeannerod

Estimation of hand position relies mostly on visual rather than proprioceptive feedback

Movement executionPre-planned phase

Final Corrective phase

Saccadic Suppression

The natural sequence of pointing at an object is an eye saccade with average latency of 200ms followed 50-100ms later by limb movement

During orientation the nervous system partly inhibits further transient retinal signals

It was observed that when the target was slightly displaced at the onset of orienting saccade, the subject was unable to report the displacement but were still able to point at it correctly

Motor Oriented Visual stream

Jeannerod identified two different visual streams, a ventral stream for perception and a dorsal stream for action

This idea is empirically supported

Modules for online correction and reaching

The modules in the brain (regions) for online control and for reaching have been identified individually.

However the relation between these two is still a matter of debate

Unaware double-step pointing paradigm

Method to investigate natural control of planned movement after initiation

i. Amplitude double step

- A real time psychophysical problem is given

- The target jumps were randomly missed with the no jumps and hand vision was suppressed at onset of hand movement.

-The experimental setup is explained in the following slide

Setup for the experiments

Results of the experiment

I. No subject was able to report a target jump

II. Saccadic gain remained unchanged

III. Hand endpoint shifted in direction of target displacement

Directional double step

The previous experiment is now performed with visual feedback of the hand.

The results concur with the previous one. Subjects never detected any target movement

Reach-to-grasp

Various experiments and clinical studies indicate a critical role of the PPC (Posterior Parietal Cortex) in production of fast, automatic adjustments of on-going limb responses and in reach to grasp movements

The experimental findings are as follows

Rehabilitation Perspective

Mostly useful for patients with Hemiparesis (weakness of the entire left or right side of the body)

The stimulation of the automatic visuo-motor pathway can preserve muscular activity in such patients

A less effortful way for recovery of hand movements following frontal lobe damage can be found.

Further possible research

The direct electrophysiological counterparts of the discussed processes have to be recorded using real-time EEG or MEG methods or cell recording in primates.

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