Movement Flashcards
What is a forward model?
Takes into consideration current state to predict how a set of motor commands will influence the state of the animal or environment
What is a inverse model?
Outputs necessary motor commands to achieve or maintain a desired goal
What are the problems associated with controlling the movement of a limb?
Main: how does a motor command turn into a movement?
- Under-determined joint configuration.
- Unknown muscle tensions.
- Unknown dynamics – how to get to desired end configuration.
Multiple joints that con move in different way just knowing the vector doesn’t tell you the set of commands – it depends what the muscle tension is doing – muscle tension is different if you’re carrying something – and also, we have unknown dynamics you can learn different changes – you can relatively quickly update the rule of how your hand moves in space
Solution: in the spine?
How did Georgopolis demonstrate that population vectors are present in the brain?
Monkey reaching task with sensor on upper reaching limb + record from M1 - neuronal populations respond depending on reaching direction
Firing rate of neurons in a particular population increase when reach is being initiated in a specific direction
Georgopolis 1986
What does assessing population vector activation allow?
weighting direction can be encoded by which population is being activated thus allowing a decoding of direction
What experiment demonstrated that population vectors could be used to decode direction?
Monkey trained to reach at 90 degree to direction of LED. In this experiment, a light would come on and rather than reaching towards the light, the monkey had to reach 90 degrees to the light
Recording from M1 in 100 ms intervals -
decode what reaching directions you’ve got what you see for preferred reaching direction. The preferred reaching direction points towards the light – activation of population vectors slowly rotates to direction where monkey has to reach
Georgopolis 1989
What are convergent force fields?
• Provide motor primitives that span workspace of the limb.
force is larger the further you get from equilibrium position and second that the equilibrium position tend to be in different places
Why are convergent force fields useful?
- Can be combined linearly to produce CFFs with intermediate equilibrium positions.
- i.e. could be used to control movements to arbitrary locations simply by varying the relative activations of a small number CFF generators in the spine.
What is a CFF generator in the spine?
Point in the spine where stimulation brings about movement through production of CFF with different equilibrium position
How are CFF’s useful for generating movement trajectories?
If you stimulate a different position – if you let the limb go it will be moving to different places – you can add these convergent force field linearly. CFFs combine linearly to produce another CFF with an intermediate equilibrium position.
You can add these force field and that gives you a point where the equilibrium is – go through the procedure where you generate the convergent force
CFF’s act as basic building blocks and adding them together will leads to building sequences which spinal network will convert to a set of movement trajectories.
What predicts actual trajectory?
Movement of equilibrium position predicts actual trajectory
What is the equilibrium point control hypothesis?
- CNS learns muscle tensions associated with equilibrium positions.
- Trajectory defined by specifying equilibrium points along the desired trajectory. Less relevant to primate motor control? – Primates have more descending connections directly onto spinal motor neurons.
How can we build a BMI?
Electrodes in M1 to record from a subset of neurons feeds into a data acquisition box which uses ANN and linear fit to real time predictions via a server to a client which determines the position, velocity and force of a robotic arm gripper -> if we take the scenario where monkey is used to moving a dot on a screen with a joystick - monkey will see where the dot is moving which provides visual feedback -> back to M1
What network outputs CFF activity?
At the core of the spinal motor system are sets of local interneurons which assemble themselves into ordered networks capable of controlling the activity and output of spinal motor neurons. These networks are usually referred to as central pattern generators (CPGs).
Who first proposed CPG’s didn’t require peripheral input?
In the early 20th century, T.G. Brown proposed that the rhythms of CPG’s were generated centrally, i.e. by neural networks that do not require input from the periphery (e.g. sensory neurons) for generating cyclic behavior.
