Summary of points for WS04 – from Weiss & Jeannerod (1998) Reaching & Grasping Summary of points for WS04 – from Weiss & Jeannerod (1998)

Nature of the problem How do we control multiple DoF (degrees of freedom) within this motor task that requires a reach and grasp... involves torso, shoulder, upper arm, lower arm, hand, fingers all coordinated together to stretch out to a target, close around it and lift it Could be a problem of getting to the end position (system seeks control of end posture) Could be a problem of the way we move to the end position (system seeks control of trajectory)

Nature of the problem Research looks at time Limb segments organized in terms of timing of muscle contractions Research looks at synergies Muscle contractions are repeatedly activated together within practiced movements, thus forming “joint systems” that come together at the level of the movement (coordinative structures) Thus movements might be planned at the level of these synergies (Bernstein, 1967) This is probably reflected by the formation of neural networks accompanying the specific joints in the movement

Nature of the problem Viability of these solutions is examined by creating models based on these assumptions and seeing whether they match up to people’s movement behaviors Several such models have been tested...

Some controversy Model 1: Model 2: Movement planned via movement of hand through space (Hogan & Flash, 1987) – predicts flat trajectory Model 2: Movement planned as series of joint configurations – predicts curved trajectories Overall, seems to depend on the task – we use both kinds of trajectories.

Simplifying the problem Freezing degrees of freedom When the target position varies, only some parts of the movement are changed. Others remain the same – implying that we tend to only change aspects of a movement when we have to...simplifying the coordination issue.

Underlying representations One view suggests we memorize stored final postures, in terms of stiffness of muscles required for particular positions. Movements then consists of simply “gliding” to that final stiffness of the muscles, and hence the final posture.

Underlying representations Another view points out that the nature of movement is not consistent with this, and instead suggests that the memory includes information about future task intention think of how you grasp a door handle, and how the grasp changes depending on whether you want to twist the handle clockwise or anti-clockwise

More on representations Representation = feedforward control Something that can start a movement and anticipate the feedback that it will generate (based on past experience), using this to drive further movement. Seems to be localized in the cerebellum Seems to rely on updating through proprioception in order to continually control multi-joint movements (with the implied awkward set of torques involved)