Biological Psychology - The Control of Action

The Control of Action: Part 1

• Introduction

  • The lecture discusses the biological psychology of motor control, focusing on the sensorimotor system.

• Learning Objectives

  • Explain the hierarchical organization of the sensorimotor system (LO 8.1).

  • Describe the general model of sensorimotor function (LO 8.4).

  • Explain the role of the posterior parietal cortex in sensorimotor function and the effects of damage or stimulation (LO 8.5).

  • Describe the functions of the primary motor cortex (LO 8.9, 8.10).

  • Describe the structure and connectivity of the cerebellum and its function (LO 8.11).

  • Describe the anatomy of the basal ganglia and its function (LO 8.12).

  • Explain the hierarchy of central sensorimotor programs and its importance (LO 8.21).

  • Note the importance of reading the textbook section on reflexes (LO 8.14, 8.15, 8.16, 8.17, 8.18).

• Lecture Overview

  1. Hierarchical organization of the sensorimotor system.

  2. Secondary and primary motor areas, and the cerebellum.

  3. Primary motor cortex (A/Prof James Coxon).

  4. Basal ganglia.

  5. Higher-order motor functions.

The Complexity of the Motor System

• Humans possess wide range of motor skills, from everyday actions to specialized athletic feats.

• The motor system is a complex system including:

  • ~ 206 bones

  • ~ 650 skeletal muscles

  • ~ 86,000,000,000 neurons, each with up to 10,000 synapses

• Achieving a motor goal requires coordinating these elements.

• The system must control multiple joints, muscles, and motor units precisely.

Hierarchical and Parallel Organization

• Multiple Levels of CNS: Cerebrum & cerebellum, brainstem, spinal cord.

• Hierarchical control: Higher-order areas (e.g., cortex) send commands to lower levels (e.g., spinal cord). Lower levels can generate movement patterns (e.g., walking).

• Parallel control: Signals flow between levels over multiple paths, enabling higher-order areas to exert control in multiple ways.

Sensorimotor System Organization

• The sensorimotor system is organized hierarchically, similar to a company structure.

• The president (association cortex) gives direction to lower levels.

• Lower levels (motor neurons and muscles) take care of details.

• This organization allows higher levels to focus on complex functions and decision-making.

Key Motor Cortical Areas Involved

• Dorsolateral prefrontal association cortex

• Posterior parietal association cortex

• Areas of secondary motor cortex

• Frontal eye field

• Primary motor cortex

General Model of Sensorimotor System

• Association Cortex: involved in high-level planning and decision-making.

• Secondary Motor Cortex: involved in programming sequences of movements.

• Primary Motor Cortex: involved in executing movements.

• Brain Stem Motor Nuclei: relay motor commands to the spinal cord.

• Basal Ganglia and Cerebellum: involved in modulating and coordinating movements.

• Spinal Motor Circuits: execute motor commands and generate reflexes.

• Descending Motor Circuits: carry motor commands from the brain to the spinal cord.

• Feedback circuits: provide sensory information about movement to higher-level motor areas.

Control of Action: Part 2 - Secondary and Primary Motor Areas, and the Cerebellum

• This section details the roles of secondary and primary motor areas, as well as the cerebellum, in motor control.

Descending and Feedback Circuits

• Descending Motor Circuits: These circuits transmit signals from the brain to the spinal cord, initiating and controlling voluntary movements.

• Feedback Circuits: Essential for refining movements, these circuits relay sensory information from muscles back to the brain, allowing for adjustments and corrections.

Secondary Motor Areas

• There are at least eight areas of secondary motor cortex.

• Two areas of premotor cortex

• Three supplemental motor areas

• Three cingulate motor areas

• These areas project to the primary motor cortex, each other, the basal ganglia, and the brainstem.

• They produce complex movements before and during voluntary movements.

• Premotor areas encode spatial relations and program movements.

Supplementary Motor Complex

• Supplementary Motor Area (SMA), pre-SMA, supplementary eye field (SEF):

  • Role in planning, preparing, and initiating movement.

  • Monkey SMA & SEF neurons active before movement.

  • Movement sequencing:

    • Monkey SMA & pre-SMA neurons fire before specific sequence.

    • Human fMRI – SMC active during tasks requiring complex motor sequencing.

  • SMC project to ipsilateral & contralateral motor cortex, & to the contralateral SMC.

  • Bimanual coordination.

Primary Motor Cortex & Motor Homunculus

• The primary motor cortex is located on the precentral gyrus of the frontal lobe, anterior to the central fissure.

• It is somatotopically organized (Penfield).

• The body is diffusely represented on the motor homunculus.

• There is not a 1:1 relationship between a location on the body and its representation; regions can overlap.

• This organization enables brain-computer interfaces.

Cerebellum

• The cerebellum is a subcortical sensorimotor structure.

• It constitutes only 10% of the brain's mass but contains over half of its neurons.

• It is organized systematically into lobes.

• The cerebellum does not transmit signals directly to the spinal cord.

• It integrates and coordinates activity within the sensorimotor system.

• It receives inputs from the primary and secondary motor cortex, brainstem motor nuclei, and somatosensory and vestibular systems.

• The cerebellum corrects deviations from intended movements and is involved in motor learning.

• It also influences diverse sensory, cognitive, and emotional responses.

Effects of Cerebellar Damage

• Diffuse damage to the cerebellum results in:

  • Loss of the ability to precisely control movement.

  • Inability to adjust motor output to changing conditions.

  • Inability to maintain steady posture.

  • Inability to exhibit coordinated locomotion.

  • Inability to maintain balance.

  • Impaired speech clarity.

  • Inability to control eye movements.

Control of Action: Part 3 - The Primary Motor Cortex

• The primary motor cortex (M1) is crucial for movement control.

• Neurons in M1 code movement direction.

Primary Motor Cortex (M1) Functions

• Provides the command to drive motoneurons to make muscles move.

• Different subregions control specific body parts.

• Direction is a function of summed activity (vector) across the population of neurons.

• There is a debate over what M1 neurons code for:

  • Trajectory & distance to target?

  • Sensory-motor integration?

• Motor cortex also organised for ethologically relevant behaviour

Ethologically Relevant Behaviors Encoded in M1

• Reach to grasp

• Defense

• Climbing/leaping

• Hand in lower space

• Hand to mouth

• Manipulate in central space

• Chewing/licking

What Aspects of Movement Does the Brain (M1) Encode?

• Trajectories were examined in experiments by Georgopoulos, using animals trained to make pointing movements.

Cell Firing in Primary Motor Cortex (M1)

• Raster Plots of ONE M1 neuron are shown from intracellular recordings (shoulder region).

• 5 movements x 8 directions = 40 trials.

• Note- neurons are almost always firing, but the firing rate increases or decreases depending on the direction of movement.

Neuronal Preferred Direction

• Neurons have a 'Preferred Direction' for maximum firing.

• Fitted with a cosine curve: firing\ rate = k \cos \theta

Population Vector

• Each neuron is represented by a vector.

• The vector sum of all cells = Population Vector.

• The Population Vector specifies movement direction/goal!

Brain-Computer Interface Research

• Research at UPMC Rehabilitation Institute and the University of Pittsburgh School of Medicine focuses on brain-computer interfaces.

• Study participant Jan Scheuermann was able to feed herself using a brain-computer interface.

Summary of M1 Contributions

• Movement direction and trajectories are encoded by the integration of large numbers of M1 neurons.

• Redundancy exists in the system.

• Bernstein stated that there can be no unambiguous relationship between movements and the neural signals that give rise to them!

Control of Action: Part 4 - The Basal Ganglia

• This section discusses the role of the basal ganglia in motor control and the effects of its dysfunction.

Basal Ganglia Dysfunction

• Dysfunction can result in:

  • Parkinson’s disease

  • Huntington’s Disease

  • Tourette Syndrome

  • Hemiballismus

  • Multiple Systems Atrophy

  • Progressive Supranuclear Palsy

  • Dystonia

  • Drug overdose

  • Head injury

  • Infection

  • Liver disease

  • Metabolic problems

  • Side effects of certain medications (e.g., haloperidol and risperidone)

  • Stroke

  • Tumors

  • Environmental toxins

  • Neuropsychiatric disorders

Parkinson’s Disease

• Parkinson’s disease is characterized by specific motor symptoms.

• Deep brain stimulation can be used as a treatment.

Clinical Symptoms of Parkinson’s Disease

• Main clinical symptoms:

  • Bradykinesia: slowness of movement

  • Rest tremor: 4-6 Hz, present at extremities (e.g., hand)

  • Rigidity: ‘stiffness’ or increased resistance to movement

• Diagnosis is made by clinical assessment (e.g., by neurologist).

• First-line treatment is dopamine replacement medications (levodopa aka L-dopa), but over time this becomes less effective.
  * “OFF” phenomenon = dopamine medications wear off over time
  * Secondary side-effects (e.g., dyskinesia)

Pathology of Parkinson's Disease

• Second most common neurodegenerative disorder (behind Alzheimer’s disease).

• Age of onset varies, normally around 65 years.

• Major pathologic feature – profound loss of pigmented dopamine neurons, mainly in the substantia nigra (SN).

• Symptoms appear when dopamine neuronal death reaches a critical threshold:

  • 70-80% striatal nerve terminals

  • 50-60% SNc

Basal Ganglia Overview

• Basal = base; ganglia = group of nerve cell bodies

• Subcortical nuclei

• Interact closely with cerebral cortex, thalamus, & brainstem to guide behavior

• Multiple, parallel, largely segregated, cortico-cortical re- entrant pathways

• Dysfunction associated with movement disorders & neuropsychiatric disorders

Basal Ganglia Neuroanatomy

• Includes:

  • Caudate nucleus

  • Putamen

  • Striatum

  • Globus pallidus

  • Subthalamic nucleus

  • Substantia nigra

Basal Ganglia Organization

• Input:

  • From cortex (excluding primary auditory & visual cortex) to striatum (putamen, caudate, nucleus accumbens – ventral striatum).

  • From motor cortex to subthalamic nucleus (STN).

• Output:

  • From Globus pallidus internal (GPi) & substantia nigra (SNr).

  • To thalamic nuclei which project to frontal cortex, pedunculopontine nucleus (PPN), and superior colliculus (SC).

Internal Connections

• Internal connections of the basal ganglia (BG) contain both direct and indirect pathways from the striatum to BG output nuclei.

• The indirect pathway goes via the Globus pallidus external (Gpe) & subthalamic nucleus (STN).

Direct and Indirect Pathways

• Direct pathway:

  • Activation of the direct pathway results in increased facilitation of the cortex.

• Indirect pathway:

  • Activation of the indirect pathway results in reduced facilitation of the cortex.

BG Dysfunction in Parkinson’s Disease

• Parkinson’s disease involves specific disruptions in the basal ganglia circuitry, affecting motor control.

Dopamine Medications for Parkinson's

• Medications used to treat Parkinson's disease:

  • Levodopa: replaces dopamine

  • Dopamine agonists: mimic dopamine

  • MAO-B inhibitors: preserve existing dopamine

  • COMT inhibitors: preserve levodopa

Deep Brain Stimulation

• Deep brain stimulation (DBS) leads are typically implanted in the subthalamic nucleus.

Freezing of Gait in Parkinson’s Disease

• Basal ganglia connections with:

  • Supplementary motor cortex = internally guided movement

  • Lateral premotor cortex = externally guided movement

Control of Action: Part 5 - Higher-Order Motor Functions

• This section focuses on the higher-order cognitive processes involved in motor control.

Pathways of Posterior Parietal Association Cortex

• Posterior parietal association cortex interacts with:

  • Dorsolateral prefrontal association cortex

  • Frontal eye field

  • Areas of secondary motor cortex

  • Somatosensory cortex

  • Auditory cortex

  • Visual cortex

Function of Posterior Parietal Association Cortex

• Provides information on where body parts are in relation to the external world.

• Receives input from visual, auditory, and somatosensory systems.

• Output goes to the secondary motor cortex.

• Stimulation of this area makes subjects feel they are performing an action.

Apraxia

• Apraxia is the inability or difficulty performing movements on command, despite intact primary motor processes.

• Occurs when the posterior parietal association cortex is lesioned.

• Associated with left hemisphere damage.

• Symptoms are often bilateral, indicating deficits in higher-order motor control.

Selection of Movement

• Much of the brain is important for motor control.

• All areas promote movement, but relative contribution varies as a function of task demands.

• Concurrently representing many possible actions may offer a speed advantage by allowing the brain to begin preparing an action before the arrival of full information.