The Acting Brain

The acting brain.

Dr Progress Njomboro

                To move things is all mankind do, and for this task the sole executant is a muscle, whether it be whispering a syllable or felling a tree

                                                                                                Charles Sherrington.

introduction

Why do animals have a brain?

Thinking is expressed through action.

Processes that generate and control action also generate and control thought.

How does our brain transform our intentions, desires, decisions, and plans into actions?

How do we learn a new motor skill?

What's happening in our brain while we are learning the new motor skill?

Why are people’s motor skills different?

Why does practice makes perfect

Movement and action

Movement

A physical act, which can occur in the absence of cognition  e.g., reflexes

Action

A cognitive act

The embedding of movement within an environmental reality

By early adulthood, most actions are effortless – until injury or disease strikes parts of the motor system.

Reflex arc

The case of Ian Waterman (see video)

Lost all somatosensory and proprioceptive input

Initially unable to make any coordinated movement

After years of practice he trained himself to make movement under visual guidance

Requires total concentration to move and even maintain posture

Collapses to ground when lights unexpectedly go out

Components

The motor system mediates all behavior by acting on the environmental representations from our sensory system

Three components of the motor system make this possible

Planning

Execution

Feedback

Different regions of the brain are involved in these 3 tasks.

Action propagation

Action propagation

The action brain

The action brain

Primary motor cortex

Frontal eye fields

Premotor cortex

Lateral

Medial (SMA)

Prefrontal

The Action Brain

Primary motor cortex

Executes all voluntary movements

Somatotopically organized

L = R

R = L

Damage to one side produces Hemiplegia

The Action Brain

The Action Brain

Lateral vs. medial premotor cortex

Prefrontal cortex

Planning and higher control of action

Mediates selection and goal maintenance of action

Applying attention to action

i.e., in novel or complex situations

Prefrontal vs. Premotor contributions

Premotor

Prepare action to internally and externally triggered events

Using activity in the primary motor cortex to guide a prosthetic limb

PLANNING ACTIONS: The SAS Model

The SAS Model:
How does it work?

Schemas

Well-learned/automatic/habitual actions

Scripts

A collection of schemas that go together

The SAS model contd…

Contention scheduling

Controls schema selection

Selects limited number of compatible schemas

2 problems

1.) Not all tasks are routine

2.) Even if were routine, organizing different schemas together is not routine

The supervisory activating/ attentional system

An attentionally limited controller

Organizes, co-ordinates, monitors schemas

Deficits from disordered SAS functioning

Frontal apraxia/ action disorganisation syndrome

Failure to maintain routine activities, e.g., making a cup of tea

Damage to scripts; maintenance of scripts; or some combination

Perseveration

No deactivation of activated schemas; or

No change in goal once accomplished

Utilization behaviour

Activation of schemas by environmental cues without SAS regulation

Summary – the action brain

Primary motor cortex – execution of voluntary movements

Premotor cortex – preparation of actions

Lateral premotor: external cues

Medial premotor (SMA): internal cues

Prefrontal cortex: action/goal selection

damage results in dysexecutive behavior.

Ownership & awareness of action

How much of our behavior is actually ‘willed?’

Homunculus problem

There is a little “I” in your head that decides on actions

Libet et al. (1983)

Readiness potential of action neurons fires before reports of intent to act

Haggard & Eimer (1999)

Awareness of action relates to performing a specific action

To do or not to do: ‘Checking mechanisms’ and whether to make an action or not

Prediction of actions

Forward models

A representation (efference copy) of the action is used to predict the consequences of the action

Alien hand contd…

Hand produces unintentional movements

Damage to

SMA

Corpus callosum

Eg., Left SMA damage leave the right hand under the control of the intact left lateral premotor regions (which control more object-based action than the SMA.

Action comprehension and imitation

Mimicry

Simple, rote copying of the actual movement itself

Shallow level of processing

Imitation

Reproducing actions based on goals

Deeper level of processing

Infants use goals too

The mirror neuron system

Neurons that respond during the performance observation of an action

disregard the distinction between self other.

tuned to specific goal directed actions

Important for

Understanding actions of others

Assist imitation

PARIETOFRONTAL CIRCUITS FOR SENSORY-MOTOR TRANSFORMATION

Parietal lobes

Two routes of visual processing

Ventral (“what”) – occipital to temporal

Medial structures – non-living objects

Lateral structures – living objects

Dorsal (“”where/how”) – occipital to parietal

Damage

to ventral stream

Can’t tell what you see

E.g., Patient DF Could not tell/report orientation of a slot

to dorsal stream

Cannot reach accurately towards objects

Optic ataxia

Double-dissociation between ventral and dorsal damage

Double dissociations evident in normal people

Evidenced by visual illusions, e.g., Titchener circles

NEURAL MECHANISMS OF SENSORY-MOTOR TRANSFORMATION

Coding specific types of actions

an action vocabulary (grasping, tearing, holding)

Area F5 in primates

Coding action-relevant properties of objects

Coding sensory info across modalities

Deficit of sensory-motor transformation: The Phantom Limb

Amputees experience that their amputated limb is still there

It was previously thought this was the result of irritated nerve endings from the amputation

Ramachandran’s theory

Phantom limb sensations due to cross wiring in somatosensory cortex

Eg., Brain areas near to hand (arm & face) take over (remap) cortical region that no longer has input

Phantom limb pain

Using objects and tools

A 3D description of the object is matched to a store of ‘typical’ Visuospatial object representations (in infero-temporal cortex).

And representations of factual info in semantic memory

And object-based actions in the (left) inferior parietal lobe.

Eg., activated during pantomiming

Why left hemisphere??

Deficits of tool use: Ideomotor apraxia

Patients cannot adequately produce actions on verbal command

e.b., wave goodbye, brush your teeth; comb your hair

Often leave out the object and use hand (e.g., for combing hair) or finger (e.g., for brushing teeth)

**Can the patient copy meaningless action though?

Objects for action: Affordances (Gibson, 1979)

What is the influence of semantic knowledge?

Object use is sometimes spared in spite of damage to its semantic reps.

What are affordances?

Evidence from semantic dementia

Patients have lost all or some of semantic knowledge about objects, but are still able to use them correctly

Summary

How can we apply what we have learnt

ROLE OF SUBCORTICAL STRUCTURES FOR PREPARATION AND EXECUTION OF ACTIONS

Cerebellar loop

Involved in the co-ordination of movements

Basal ganglia loop

Initiation and execution of internally generated movements and linking one action to the next

Hypokinetic disorders of the basal ganglia – Parkinson’s

Loss of dopaminergic brain cells in the pathway linking basal ganglia and substantia nigra

Increased output of the indirect pathway (brakes) and decreased output on direct pathway (accelerator)

Result in poverty of self-initiated movement and slowness of movement

Hypokinetic disorders of the basal ganglia: Huntington’s disease

Excessive movements, dance-like flailing limbs (chorea), and contorted postures

Mechanism

Reduction in ‘brakes’ – but normal ‘acceleration’ results in promotion of movement

Hyperkinetic disorders – Tourette’s syndrome

Excessive and repetitive actions

Simple eg., jerks and tics

Complex eg., OCD

Coprolalia - verbal obscenities, barking also common

Echolalia – repeating after someone’s words

Palilalia – repeating own words

Failure to inhibit initiation of action

summary