action selection

  • basal ganglia in the oldest structure in the CNS
    • it is a deep part of the basal ganglia

challenges

  • limbs and digits need to move in complex ways
    • need to face gravity at every point in time as terrestrial beings
  • facial expressions need to be handles
    • eye movement have to be carefully controlled
  • humans have a large behavioral repertoire
    • and a complex social structure
  • the ultimate selector of action is the basal ganglia
    • it navigates the complex hierarchy of options available

basal ganglia anatomy

  • the outer rind of the telencephalon is the cerebral cortex
  • the inner brain telencephalon has
    • striatum
    • pallidum
  • the core structures of basal ganglia are
    • telencephalon
    • fore brain

pathways

  • two keys parts of the basal ganglia are striatum and pallidum
    • the pallidum is always active, even at rest
      • they inhibit the entire range of movement at rest
      • they contain GABA and are therefore inhibitory
  • when a signal from the cerebral cortex reaches striatum,
    • it inhibits the inhibition by pallidum as needed per the excitatory signal
    • this double inhibition opens up the pathway to allow a movement through he basal ganglia
    • this is the direct pathway through the basal ganglia
  • once a particular pathway is opened
    • a corresponding movement is preferred
    • and it is harder to change to a different movement
    • this is the neuro-biological contribution to behavioral inertia
  • behavioral inertia is the condition where
    • the organism continues doing what it was dong before
  • activation of the direct pathway results in a focused action being initiated and performed,
    • often for an extended period of time,
    • such as reading a book for the afternoon

  • recovering from a stumble involves reflexes and the cerebellum to modulate gait

  • the competing movements could to be suppressed
    • indirect pathways produce additional inhibitions on movements that are not on the direct pathways
      • making them even more unlikely to happen
    • this helps in fine tuning the actual movement chosen
  • parkinson’s is a poverty of movement
    • where indirect pathway inhibition works
    • but direct pathway dis-inhibition doesn’t
    • most common basal ganglia disorder known
  • the reverse, where the indirect pathways are blocked
    • lead to extra competing movements in addition to the direct pathway preferred movement
    • movements are not fine tuned
    • hemibulismus caused by a stroke
  • practising makes motor mulitasking easy
    • basal ganglia inhibits motor multitasking by default

basal ganglia function

  • basal ganglia is a learning machine
    • it tries to learn the outcome of the effect of it’s action towards a stimuli
  • if it’s good it’s done again and the preferred pathway strengthens
    • else inhibit pathway strengthens
    • this is operational learning
    • dopamine neuro-transmitter is responsible for enhancing positive outcome pathways and inhibiting negative outcome pathways
  • dopamine creates craving
    • craving is allegedly a more evolutionarily advantageous state compares to nirvana
    • food was scarce evolutionarily, so craving for food makes people eat
    • similarly for water, drinking water was evolutionarily more advantageous and dopamine enforced this behavior

chunking

  • neurotransmitter dopamine is used to put together a series of actions
    • lots of actions are grouped together into a chunk
  • chunking is an important function of the basal ganglia
    • dopamine helps chunk actions together
  • example: the way one moves their helps recognize faces
    • moving ones eyes has a lot of separate components
    • which have been chunked into scanning an individual’s face
    • driving a car for a long time - experience at working a job
      • everything is one big chunk

  • mental effort is reduced due to chunking

  • disadvantage to chunking
    • once started, it has to go through he whole thing
      • it is outcome independent
    • when started learning it, it was outcome dependent
    • these chunks are habits
      • habits don’t depend on outcome
      • habits are fast, but are inflexible
      • doing things deliberately is slow, but flexible
    • refer to learning how to learn course material for more one habits, zombies and chunk formation
  • while harnessing the power of chunking can be especially useful for learning new things,
    • because of how inflexible it is, it is hard to correct it
    • drug abuse victims suffer from inflexibility in changing habits
      • and self defeating behaviors are reinforced by dopamine

dopamine

  • substantia nigra cells produce dopamine

  • dopamine contains a pigment called neuro-melanin
    • that pigment makes dopamine black
  • if more than 90% of these substantia nigra cells die,
    • a person starts to show the signs of parkinson
    • even if 80% of them are dead, they are not going to be symptomatic
  • dopamine does two main things for movements
    • it provides a motor oil function for motor functions
      • it happens all the time
      • if animals dont make dopamine, they dont feed and they die within a few days
    • facilitates the direct pathway in the basal ganglia
      • dopamine helps disinhibit a particular motion pathway in pallidum
  • amphetamines increase dopamine
    • and they make the consumers move more
  • so, if 90% of the dopamine making cells die, (parkinson)
    • there is no motor oil
    • and no direct pathways are inhibited
    • this causes poverty of movement
      • and even if triggered is extremely slow
    • movements that are chunked into habit have to be done in component movements
    • basic movements can be frozen due to lack of dopamine
      • multitasking in can be disabled
      • mental effort has be applied to each component action
  • even if the motor hierarchy works fine, an action cant be selected

non-motor function

  • basal ganglia is like a selector chip
    • it considers many options and selects one
  • it gets sensations from outer rind ~ the cortex
    • and communicates back in thoughts
  • helps makes choice between perceptions of the same scenario or sensory snapshot

cerebellum and basal ganglia

  • they are two great loops of the brain
    • there is a ton more information that comes in that goes out into both at any point in time
    • 40 inputs : 1 output
  • the cerebellum plays a larger role than basal ganglia for coordinated movements, such as
    • reaching out to press a doorbell
    • one walks on a boat on rocky seas
    • coordinated eye movement such as smooth pursuit when you track an animal running
  • basal ganglia for chunking and multitasking