W9: Cerebellum and Motor Learning

Learning Objectives

• anatomy of cerebellum + divisions

• the principal input and output pathways

• the organisation of the cerebellar cortex

• the parallel-fibre / Purkinje model of learning

• roles of the cerebellum in motor control

• the ability of the cerebellum to predict

hierarchical sensory-motor organisation

Cerebellum - little brain

• massive cortical area = much simpler than cerebral cortex (only 1 output layer)

• heavily folded

• exerts influence on movement via influence on motor + pre-motor cortex

• also connects with brainstem + spinal cord

• adds:

  • coordination

  • fine control

  • skill

  • ^ to basic movement patterns

• a powerful learning machine

Flattening the cortical sheet

• length of cerebellar cortex reflects body mass

• width may reflect cognitive properties

• e.g. cow is larger so cerebellum is longer, but it is narrower bc humans perform more cognitive functions than cows

Human Cerebellum: cell numbers

• total = 102 billion

  • granule cells

  • purkinje cells

  • golgi, basket, stellate

  • nuclear cells

• purkinje cell inputs:

  • from parallel fibres

  • from climbing fibres

• cerebellum is smaller than cerebral cortex = but has more cells

layers in cerebellum pic

cerebellar cortical connections

• purkinje cells = place where motor learning takes place

• shows that sensory info is coming from different sensory parts of cortex

• arriving through mossy fibres

• cerebellum can predict using sensory info

  • e.g. you need to catch a ball - cerebellum uses sensory info (visual) to know where you need to be in space (from climbing fibre)

cerebellum circuit

this circuitry allows the cerebellum to:

• refine movement

• detect errors

• facilitate motor learning

through precise modulation of output signals

Deep Cerebellar Nuclei

• output from the cerebellum to the rest of the brain = through deep cerebellar nuclei

• they send outputs to the motor structures of the cerebellar cortex

Inputs: circuit pic

• info about the motor action you want to perform would arrive at the purkinje cells

• output from purkinje to nuclei = inhibitory = reduces activity of cerebellar nuclei

Cortex-Cerebellar Loops

• MRI studies

• can see what parts of the cerebellum are active when receiving info from different areas (e.g. visual cortex, auditory, somatosensory)

Cerebellar Damage

• Hypermetria (overshoot) = finger-to-nose

• Intention tremor = esp during action

• Ataxia = loss of coordination + skill

• Nystagmus, balance, gait, speech

• Cerebellar affective disorder = executive, emotional, personality (children are affected this way)

Cerebellum + Motor Learning: Marr-Albus model of learning

• The synapse between granule cells (parallel fibres) + Purkinje cells = is plastic = can undergo Long Term Depression (LTD)

• The trigger for LTD is simultaneous activity of parallel fibres

  • climbing fibres (associative learning) via mossy fibres

  • error signal via climbing fibres

• LTD reduces P-cell inhibition of cerebellar nuclei + dis-inhibits the direct pathway

  • = cerebellar nuclei more active

• LTD = connection gets weaker over time w certain activity

4 examples of cerebellar learning

1. Vestibular Ocular Reflex (VOR)

2. Eye Blink conditions

3. Skill Learning

4. Visuo-motor recalibrating

Vestibular Ocular Reflex (VOR)

• VOR is a mechanism that helps stabilise vision during head movement

• does this by producing eye movements that counteract head motion

• makes sure image on retina is stationary

VOR gain

Input: vestibular system signal of head motion

Output: modulation of direct path to ocular motor neurons

Consider: VOR gain too weak

• retinal slip drives LTD

• reduced excitation of P-cell

• dis-inhibition of vestibular nucleus = stronger drive = higher gain

Training signal: retinal slip signal on climbing fibres

Eye Blink conditioning

Classical conditioning: lesion of cerebellum = failure to learn

Unconditioned stimulus: puff of air into eye (also activates climbing fibres)

Unconditioned response: eyeblink

Conditioned stimulus: a tone/light (activates parallel fibres)

Conditioned response: learned eyeblink to avoid air puff

explained:

• the cerebellum helps link the CS (tone) with US (air puff) using LTD at parallel fibre - Purkinje cell synapse

• this weakens Purkinje cell inhibition = cerebellar nuclei generates a times blink in response to tone

• if the cerebellum is lesion = this learning doesnt happen

Eye Blink CC

Skill Learning

• monkey in a lab grasping device that animal could twist

• animals task was to stabilise the weights

• recoding directly from Purkinje cells in cerebellum - animal cant feel bc brain doesn’t have any pain receptors

• first time being able to see learning take place in real time - activity of Purkinje cells

• first bump = action, underneath = action potentials

Visuo-motor recalibration

• prism glasses distort visual inputs - need to adjust movement output to recalibrate

  • imagine you reach out for object below

  • with prism glasses you see your hand moving upwards

  • if you reach out to 12:00 and hit 4:00, you know you need to reach 8:00 to get to 12:00 = recalibrating the motor system

• short term learning of visuo-motor relationship

• learning blocked by cerebellar lesions

Summary

• Cerebellar cortex = big

• important for all skilful movement

• huge numbers of parallel fibres bring very diverse sensory-motor input to Purkinje cells

• climbing fibres induce parallel fibre = P-cell LTD

• adjusts VOR gain - support classical conditioning of eye-blink

• predictive control

Normal throwing

• need to throw a ball at target

• a signal activates muscles = move limbs to throw

• a copy of the motor command that goes to the body also goes to the cerebellum (so it knows what we about to do) = efference copy

• when the ball hits target - sensory input goes to cerebellum = it can compare predicted vs actual movements

• if there’s a match = no learning needs to take place

• if you miss = mismatch = error signal which drives cerebellum to update motor command = changes next movements = compensates so next time get closer to target

The role of the cerebellum in prediction - study

• if you have damage to cerebellum = you shouldn’t be able to see mismatch + learn from mistakes

• patients performed visual motor rotation - e.g. controlling cursor and place at specific point on screen

• researchers slightly changed it so forward = moves slightly to the side etc

• patients struggle to adapt their movements to hit the target

• also tried where they move their hand slightly so they have to use more force = but still couldn’t move hand with more force in opposite direction

The role of the cerebellum in prediction

• cerebellar patients show impairments in both force-field + visuomotor adaptation

• impairments are related to specific degradation of cerebellar areas

• force-field = anterior lobe of cerebellum

• visuomotor = posterior lobe of cerebellum

• SO - the cerebellum appears to perform the same function (prediction) across tasks with each being related to specific regions

Study: motor prediction

• ppts had to move hand to a target (sweeping motion with hand)

• they created a virtual lesion by exciting brain with a magnetic pulse = temporarily disrupted the function of the cerebellum

• showed that the cerebellum is also involved in a process called ‘state estimation’ = estimating where the limb is throughout the entire range of movement

another study:

• had to reach target + some got a sensation in limb as reward

• some did without

• found - cerebellum still predicts movement irrespective of any sensory input

The importance of prediction: Ronald vs Ronaldo

• the ability to predict the sensory outcomes of your motor commands

1. fast responses as you do not need to ‘wait’ for slow sensory feedback to update your behaviour (basc the ability to correct errors fast)

2. performing actions when sensory feedback is not available / poor quality (e.g. in darkness)

3. ability to predict upcoming actions based on the outcomes of the current one (increase planning horizon)

• the ability is associated with highly skilled behaviour - athletes show greater cerebellar grey matter volume

Study: cerebellum disrupting language prediction

• TMS study - disrupted functioning of cerebellum during language task

• = impaired ability for students

• impaired ability to generate an appropriate verb in response to a noun = was a delay in generating the verb

• e.g. banana (choose from eat, car, drive etc)

Summary

• the cerebellum is essential for prediction within the motor system - enables the performance of fast + accurate movements

• this ability is key for skilled performance (athletes + musicians) - appears to develop across childhood

• this ability is severely impaired in patients with cerebellar disease (apraxia)

• the cerebellum has reciprocal connections to all areas of the cortex - it is hypothesised that the cerebellum plays a similar role in prediction across domains

  • language, working memory, arithmetic

• non-invasive brain stimulation (tdcs) of the cerebellum can enhance our ability to predict but results are difficult to replicate