NEUR 2.3 - Measuring Brain Function

• Summary of single-neuron recording

  
  

  We know where we place the electrode so we know its exact location

  + directly measuring the action potentials from individual neurons = most accurate for localisation and timing of brain activity 

  1. implant thin electrode into animal brain
  2. record action potentials firing from a single neuron
  3. Measure what neuron detects / encodes
• Single neuron recording Example - Neuron detecting in Visual cortex (cats)

  Result: Whole perception is built up by thousands of neurons responding or tuned to different kind of characteristics of aspects of vision 
• Single neuron recording Example - Neural encoding of actions in Motor cortex (monkeys)

  
  

  Result: Reading out from firing of the motor cortex on homunculus to try to decode plans, intentions for movements --> feed into computer that can then control a robot arm

  
  

  
  

  Future: Develop neuro prosthetic arms, that patients can control directly from their brain activity

  
  

  • Controlling robot arm through the firing pattern in its primary cortex
  • (great example of neuroplasticity) 
• EEG

  Detect and measure (on the surface of the head), activity of neurons in the brain that generates small electrical currents
• EEG measurement

  1. Electrode sensors fitted in elastic cap on participant's head (64 EEG sensors)

  2. Connect with skin to detect small electrical currents

  3. Conductive electrolyte gel used for each electrode sensor
• Brain activity

  • Changes over time in tasks associated with our perception, cognition, decision-making, and planning actions
• Brain waves

  Rhythmic oscillations of brain activity representing the fluctuating activity of the brain going on over time
• Uses of EEG

  • Assessing level of sleep, alertness, arousal
• EEG Clinical Use - Detecting stages of sleep/disturbance

  Alpha (8-12 Hz)

  • relaxed and sleepy (particularly with eyes closed)
  • suppressed when person's eyes are open and alert, engaged in highly demanding cognitive tasks

  
  

  Delta (1-3 Hz)

  • indication of really deep sleep
• EEG Clinical Use - Monitoring epileptic seizures

  • signs of abnormal spiking electrical activity
• ERPs

  Derived from EEG (cut out segments of data (epochs), but they represent the brain activity related to a specific event or stimulus
• Measuring ERPs

  1. Present particular stimulus

  2. Take short windows or epochs of the EEG activity following stimulus

  3. Average together many trials in response to that stimulus
• ERPs

  • Represent different stages of information processing in the brain as neurons fire their signals passing from 1 level to another in the brain
  • Show earliest stages of sensory perception - later and higher-order stages of cognitive and decision-making processes
• EEP/ERPs Disadvantages

  1. Difficult to localise activity to specific brain areas

  • Poor spatial resolution
  • Broad: measures electrical potential conducted across scalp
  • Hard to determine exactly where in brain
• ERP Clinical Use - Detecting deafness in babies

  
  

  Auditory ERPs: activity over time in different parts of the brain auditory pathway for processing sounds

  • Detect at which level there is deficit in baby's auditory system
• ERPs - Face processing - N170

  Well known peak and component of information processing is for faces

   

  • Part of the visual system: area quite specialised for ability to process + recognise people's identity from their faces and face processing
  • Peak at 100ms after seeing visual stimulus (general features: brightness, colours, edges)
  • Peak at 170ms after seeing face = brain processing for face-recognition in visual cortex
• fMRI

  PURPOSE: measure brain activity (changes in the blood oxygen level that happens as brain activity increases), to investigate brain function associated with task

   

  • Neurons in brain activity increase in activity = firing more rapidly = consume more energy
  • Energy in the brain is from glucose and oxygen (carried in the blood) - areas of the brain with increased activity = receive an increased supply of oxygenated blood
• fMRI Disadvantages

   

  1. Relatively indirect (measuring brain activity, not electrical activity of neurons or action potentials directly at all
  2. Bit slow and delayed relative to electrical activity of the neurons themselves (measuring blood oxygen and its changes that happen as a result of brain activity)

   

  = not precise timing of neural activity

   

  1. Very expensive technique
• fMRI Example - checkerboard

  edges between light and dark = very effectively activates visual cortex
• fMRI experiment

  Can show brain activity levels during a problem-solving task
• What an fMRI experiment can determine

  • Statistical map of what brain areas are involved in the task
  • Cannot determine neurotransmitter systems and aspects of cognition involved
• fMRI technique

  Technique: measuring techniques in blood oxygen level that accompany changes in brain activity. Really excellent for our ability to localise where the brain activity changes with a particular task.
• Dependent variable

  Brain activity
• Independent variable (IV)

  Task/behaviour we get people to do
• Good experimental design

  1. Manipulate 1 factor - independent variable

  2. Measure effect on dependent variable
• Cannot infer what people are thinking/doing/feeling based on measurement of brain activity