Action selection & the motor cortex

Distinguish Sulci & Gyri

***Sulci and gyri***

* Gyri are the folds or bumps in the brain
* Sulci are the indentations or grooves
* Folding of the cerebral cortex creates gyri and sulci which separate brain regions and increase the brain's surface area and cognitive ability

Describe neural measures: experiments on the brain

* Investigating brain-behaviour relationship requires
* Measuring neural activity correlated with behaviour (correlational)-
* does pattern of neural activity reappear each time a behaviour occurs OR
* Interfering with neural activity & assessing consequences (causal)
* Correlational and inference methods have different degrees of spatial and temporal resolution
* Use aggregate information of many adjacent neurons, pool information

Technique focus: Microstimulation

* Electrical neuronal stimulation: activation of single cell or cell population with small currents through microelectrodes placed in brain area (mimicking cell activity)
* Interference method (constructive interference rather than destructive- creates meaningful patterns artificially)
* Conducted at the level of neurons, with higher spatial resolution by passing a current through an electrode
* Stimulation can be achieved at the single-neurone level, by using small electrode (microstimulation)
* Location of stimulation can be marked by metal deposit, achieved by high-frequency current, which makes a small lesion
* One of the interference approaches with the highest temporal and spatial resolution
* Infer causal involvement of cell (population) in behaviour with high temporal resolution
* Determine functional connectivity between cell (populations)
* Limitation: stimulation can be too artificial

Describe Penfield & Rasmussen, 1952

* Conscious stimulation of brain
* No pain nerves in the brain- no pain
* Stimulation of different brain area has effects on different muscle groups
* Size of brain area dedicated to body part proportional to dexterity and technicality, not size
* Specialized brain region concerned with control of movement
* somatopic mapping of the body
* Different neurons in different parts of M1 are carrying out similar jobs but in relation to different body parts.

Describe the anatomy of the primary motor cortex: Betz Cells

M1 is able to exert its influence over movements because of its anatomical connexions; it projects via Betz cells (pyramidal cells) in layer V and then via the corticospinal tract.  

Betz cells:

* Large neurons
* May support very large axons
* Project all the way down spinal cord to control motor neurons (descending tract)
* Contralateral/ decussation

Describe the anatomy of the primary motor cortex: descending pathways for movement control

* Lateral group
* Corticospinal tract
* independent limb control
* decussates to travel through contralateral spinal cord
* synapse in lateral part of ventral horn on interneurons and alpha motor neurons
* important for distal limb control
* Corticobulbar tract
* face movements via the medulla or bulb and the cranial nerves (speech, swallowing)
* Rubrospinal tract
* via red nucleus, muscle movement of trunk/ limbs
* Ventromedial group
* Vestibulospinal tract
* trunk, proximal muscles and upper leg posture
* Tectospinal tract
* coordinating head and neck movement
* Ventral corticospinal tract
* non-decussating part of corticospinal tract mainly concerned with trunk

Describe the theory that the motor cortex encodes limb position

* Motor cortex may determine an equilibrium point for a joint (or set of joints)
* Forces produced by antagonistic muscle pairs leads to rotational forces (torques) at a joint that oppose and balance one another
* This can be achieved if a given pattern of motor cortex activity leads to a given activation level for each muscle
* Joint will return to this position after a perturbation because of muscle's viscoelastic springlike properties producing a restoring force

Motor cortex – coding postures or movement directions?

Describe Polit & Bizzi, 1978

* Behavioural evidence for postural control

* Does brain tell arm to assume a particular position, or specify and distance and direction to move?
* Deafferented monkeys can still place their hand at the position of an extinguished light in the dark
* Deafferented monkeys can still place their hands at position of extinguished light after application of an opposing torque


* When an assisting torque is applied the hand initially moves towards target, then back (as the torque is removed), then forwards again suggesting that equilibrium point control proceeds by shifting from commands specifying one equilibrium point to another
* If opposing torque prevents movement (but not visible or felt), hand springs into correct location due to muscle activity when opposed
* If assisting torque provided, arm forced closer to target (overshot), then force removed, arm springs back to midpoint
* Motor cortex may encode a specific posture: controls equilibrium point between agonist and antagonist muscles

Describe Graziano et al., 2002- Stimulation evidence for postural control

* Short train of stimulation to M1
* Caused monkey’s arm to move to a particular location in space regardless of starting position
* M1 may also specify position of mouth as well as of limbs.
* Similar effect for mouth movement

Summary: does the motor cortex encode position?

* Motor cortex may encode motor primitives
* A small number of equilibrium points are genetically encoded
* When two networks are activated simultaneously, then sum to an equilibrium point between the points produced independently by each network
* But, controversial:
* Repetitive stimulation may lead to misleading results
* Experiments use 500ms stimulation trains that can lead to propagation of activity changes beyond the stimulated area – Strick (2000)

Technique focus: Single/ multi-unit activity

* Single-unit activity (SUA): records extracellular currents associated with action potentials of individual neurons.
* It is also possible to record from multiple units, either combining or individuating neuronal activity (multi-unit)
* Microelectrodes are used to record the action potentials of single units (neurons) from outside the cell (extracellular) near the tip of a microelectrode. 


* Field potentials (FP): records extracellular currents associated with synaptic potentials (mainly) averaged over neuronal population.

Uses:

* measure correlation of activity in cells (populations) with specific behaviours / experimental parameters
* measure population dynamics (eg synchronisation)
* Limitation: spatial resolution can be “too good”

Describe the coding of movement direction:

Georgopoulos, 1984

* Eight direction centre-out task
* Neurons in M1 have a broad tuning for direction
* Different neurons are tuned to different directions
* Each neuron has a preferred direction
* The neuron’s activity during a particular movement can be represented by a vector where the vector’s direction corresponds to the neuron’s preferred direction and the vector’s magnitude corresponds to the degree of activity in the neuron during the movement
* Population vector is sum of individual vectors and corresponds to the actual direction of movement on a trial by trial basis
* Neuronal activity when monkey moving joystick measured
* M1 and area 5 parietal neurons encode the position of the hand WRT the shoulder
* Neurons have preferred direction of movement, so show tuning curves
* Whole population of neurons' activities encodes the precise direction of the reaching  movement (population coding)

Describe Kakei and Strick (1999, 2001)

* argued that there might be a confound between the arm’s posture and the arm’s direction in many reaching tasks
* controlled for this by training monkeys to reach in 8 different directions with three different arm postures
* the same direction is associated with different joint postures and different muscle movements by the task
* PMv neurons encode extrinsic direction, M1 neurons are more likely to have activity that is modulated by posture

Describe Brain machine interface (BMI)

Velliste et al (2008)

* Signals recorded from macaque motor cortex used to control a multi-joint prosthetic arm and gripper that could interact with the environment and pick up food items

Velliste et al (2013)

* similar system used to control prosthetic arm in quadriplegic human patient

Describe the premotor cortex and action selection

Premotor neurons may also encode how neurons represent  information related to selecting an action not just executing it.

(Mirror neurons, Selecting actions to perform together in a sequence)

* Premotor cortex may encode information for action selection as well as action execution.
* PMv contains mirror neurons that are active both when an action is made and when it is observed
* Mirror neuronsdi Pelligrino et al, Exp Brain, Res, 1992, Gallese et al, Brain, 1996, Umilta, et al, Neuron, 2002
* Pre-supplementary motor area (pre-SMA) neurons encode sequences of actions. 
* A neuron may only be active when a particular movement is made when it is part of a certain sequence.