Research methods, plasticity & development, intro to vision

• Development of brain

  • much of it in utero
  • neurons must:

  1. proliferate (multiply)
  2. migrate, to where they will remain

  • glia often guides migration

  1. differentiate, into specific types of neuron
  2. myelinate (axon is covered in myelin)
  3. form synapses

  • ongoing process throughout life
  • many synapses are formed and then lost
  • dendritic spines are added/lost
• Brain plasticity

  • Part of the brain respond to experience
  • congenitally blind ppl: 'visual' areas of the brain respond to auditory stimuli
• Neural Darwinism

  • initially brain forms lots of random connections
  • connections that are useful are retained
  • connection that are not, die off
  • in the blind, the visual cortex is 'starved' for connections
• Plasticity in the synapse

  • Synapses that are more active are strengthened
  • by neurotrophins (BDNF)
  • Synapses that are not strengthened, die
  • cell may also die off (apoptosis)
• Nativism vs. Empiricism (nature/nurture)

  • can plasticity processes determine the entire wiring diagram of the brain?
  • empiricism; yes
  • the brain begins as a blank slate (random connections)
  • experience shapes the connections that survive
  • nativism; no
  • some things are hardwired from birth
  • these basic things are not plastic
  • these things take place in brain modules
  • some evidence for both
  • Visual read in blind patients rewire for sound
  • areas involved in localizing visual stimuli activate to localize sounds
  • areas involved in identifying objects activate pitch
• Neurogenesis (creation of neurons)

  • plasticity decreases with age
  • new neurons are formed in some areas, even in adults
  • adult neurogenesis
  • new neurons must proliferate, migrate, incorporate into functional circuits
• aging

  • neurons and synapses are lost with age
  • some brain regions begin to shrink
  • plasticity within neurons decreases
  • less flexibility in modulating synaptic strength
  • less reliable transmission of signals
  • size and density of dendritic spines decreases
  • synthesis of neurotransmitters declines or becomes more erratic
  • dopamine, serotonin, glutamate all decrease
  • remaining synapse become more sensitive (denervation supersensitivity)
  • huge inter-individual variable
• Brain damage

  • Brain is sensitive
  • can be damaged by drugs, diseases, cell death, physical trauma
• Plasticity after damage

  • patient can learn to perform similar functions in diff ways (work around problem)
  • some parts of the brain can rewire to take over lost functions
• Rewiring

  • areas with no incoming connections can elicit new synapses (with BDNF)
  • nearby axons grow branches to connect
  • collateral sprouting
• wiring errors

  • Re-growing axons may form synapses with the wrong neurons
  • info is addressed incorrectly
  • neurons can the reassign themselves to new functions
  • can cause phantom limb effects
• Phantom limbs

  • After amputation:
  • axons of damaged neurons die off
  • axons from nearby regions attach to vacant synaptic sites
  • usually painful
  • new messages are in the wong 'syntax' for the old system
• Modularity

  • Are all areas of the brain equally plastic?
  • brain has 2 main challenges:

  1. doing things very fast (escaping triggers)
  2. doing flexible things (learning new responses)

  • some tasks have to be repeated frequently, and quickly
  • these tasks can be automated
  • performed by dedicated modules in the brain
• The nature of modules

  • according to Fodor (1983), modules are:
  • domain specific- only process one type of sensory info
  • encapsulated
  • not accessible to other systems
  • get input, give output. the process cannot be affected by other areas of the brain
  • obligatory- always active
  • fast- performing their processing very fast
  • shallow- give simple outputs
  • localized- on one place in the brain
  • example of module; muller-lyer illusion
• Caveats

  • complex cognitive tasks are usually not module based
  • modules do no necessarily have to be fodorian
  • Other ppl have definitions
• Perception and awareness

  • perception depends on:
  • environmental stimulation (lights, odors)
  • processing by the brain
  • diff stages of processing are responsible for diff part of perception
  • only some of the stages are accessible to consciousness (later stages)
  • damage to one are can disrupt only part of the process (blindsight)
• Blindsight

  • patients with damage to primary visual cortex (V1) are blind (cortical blindness)
  • if asked to guess where or what an object is they guess much better than chance
  • some unconscious visual processing is still possible without V1
• Seeing is believing

  • visions created in the brain, not eye
  • some stimuli trick the brain; optical illusions
  • cannot be consciously overcome