Week 8 - basal ganglia

• Basal ganglia: old system, predates the neo cortex - has important connections to brainstem. Exerts influence on motor and prefrontal areas via thalamus. Exerts influence on limbic circuits.
• striatum - "striped" where the cortex projects to and where info enters the basal ganglia.

  globus pallidus - pale globe. Has an internal and external part.

  Subthalamic nucleus (STN). Sits just below the thalamus.

  Substantia nigra - black substance. Sub division of pars compacta.

  
• basal ganglia consists of a direct pathway and indirect pathway. Cerebral cortex and motor cortex has projections to the spinal cord.
• Basal ganglia loop begins w the cortex projecting to the striatum (excitatory). Then goes on to the GPi (global pallidus) and thalamus however these connections are inhibitory. Indirect pathway via the striatum to GPe and STN then linking back to GPi.

  
• direct pathway is net excitatory and its connection back to the cortex excites the motor cortex helping to select the motor plan for execution.
• Indirect pathway is net inhibitory, its connections back to the cortex will suppress a given motor plan and through a balance of excitation and inhibition one motor plan will be selected and executed
• striatum inhibits the GPi which itself inhibits the thalamus. This is double inhibition causing the thalamus to fire back to the cortex
• the result of an increase in activity due to double inhibition is known as disinhibition as it isnt the same as excitation
• D1 receptors excited by dopamine and innervate the direct pathway. D2 receptors are inhibited by dopamine and they directly innervate the indirect pathway.
• a burst of dopamine coming from the substantia nigra can be thought of as facilitating movement as it will activate the direct pathway and inhibit the indirect
• distinctive black band in the basal ganglia corresponds to the substantia nigra. If this dies the black staining vanishes and can cause cell loss, dopamine to the striatum and ultimately parkinsons
• substantia nigra is key to the circuit of the excitatory and inhibitory mechanisms working for the basal ganglia
• parkinsons caused by degeneration of dopamine neurons in substantia nigra
• neurological disease caused by alteration in a single neurochemical transmitter - dopamine
• disordered signals sent to SMA: motor disorder. Also influences limbic system: motivation and emotional disturbance
• key symptoms of parkinsons: slowness of movement, stiffness and tremors
• treatments for parkinsons - levodopa (LDOPA) increases dopamine. Transplants - bone marrow derived stem cells. Neuro-surgery to rebalance connections between striatum and SMA.
• strong direct pathway with too much dopamine is hyperkinesia and weak direct is akinesia
• basal ganglia acts as a filter to add contextual info onto reflex control of eye movements - generalised inhibition of the sup colliculus except for target of interest
• basal ganglia acts as a filter with broad cortical input and motor output. Receives diverse information and its outputs shape the actions we perform.
• cerebellum - learning through error corrections
• basal ganglia - reinforcement learning
• reinforcement signals are given to the basal ganglia which then allows you to choose the strategy to maximise future reward
• rewards include natural reinforcers or stimulated rewards but all release dopamine. Substances of abuse will increase DA release in nucleus accumbens
• animal presses lever for intra-cortical electric stimulation (ICS) of dopamine neurons. Most reliable ICS sites include DA fibres.
• ICS causes dopamine release in NAcc / striatum / cortex. Potentiates the glutamate transmission in cortex leading to strengthening of cortical inputs to striatum
• LTP causes a strengthening of synapses and reinforcement of behaviour over time leading to habits forming and actions performed even without the reward (addiction)
• those with parkinsons on LDOPA seem to be more sensitive to learning through punishment then reward. Levels of dopamine directly impact our ability to learn and the style through which we learn.
• dorsalateral is at the top side part of your frontal cortex. Undergoes prolonged maturation relatively late in adolescence. Has dense connections with basal ganglia
• patients with dlpfc lesions persist with unsuccessful strategy at the Wisconsin card sort test.
• central executive traditional view believes we respond to our environment by first perceiving the world around us then making an executive decision then implementing that decision through the motor system
• view 1: central exec. Strictly serial one after the other

  
• affordance competition believes we have parallel specification and selection systems. Potential actions compete against each other within sensorimotor map, influenced by a variety of biased factors (reward/dopamine). Decision is then made through a distributed consensus - motor control is decision making.
• View 2: affordance model

  
• when an action is performed this gives rise to prediction of the outcome of the action which is then supported by sensory input, tactile or visual input then the process begins again
• affordance model implies that decisions result from the consensus of wide and distributed set of different regions all contributing different computations. Basal ganglia is at the heart of the process providing behavioural biasing.
• affordance model provides a clear model with testable hypothesis. Perception cognition and action should be simultaneous and not serial.
• first two targets appear, then one then the circle of green where you will pick top right due to the same place as original red. Cells in the PMC responsible for red and blue direction become active long before movement takes place. Increase their firing rate in anticipation. This supports affordance and goes against CE as there is activity before the action has been selected showing this parallel weighing up that occurs.

  
• animal performs simple task receiving 2 types of cues. Either to go or not to go. The timings of divergence between the two cues show that there is parallel not serial processing supporting affordance
• cognition leaking into the motor system has ramifications for free will. Means that perhaps our conscious self does not select behaviour but instead alerted to a given behaviour the body has already planned.