Electrophysiology and Functional Brain Imaging

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Lecture 2

• Electrophysiology and Functional Brain Imaging

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Brain Reading?

• To what extent can we tell what someone is
  thinking by monitoring momentary changes in their
  brain?
• What do techniques such as fMRI actually measure?
• What are the limitations and usefulness of the
  various methods?

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The Active Brain

• Cognitive activity is associated with increased
  activity of neurons
• Neurons performing similar functions tend to
  cluster together (functional specialization)
• Neural activity generates electrical signals
  measured by electrophysiological techniques
• Neural activity leads to oxygen consumption and
  this leads to localized changes in blood flow (a
  haemodynamic response) measured by functional
  imaging (PET, fMRI)

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Revision of Neural Electrical Activity

• Axons propagate action potentials (sudden
  depolarization of membrane)
• The electrical input from lots of different
  neurons is summed together. If it exceeds a
  threshold then the receiving neuron will also
  generate an action potential.

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Two Main Electrophysiological Techniques

• (1) Single-cell recordings
• Electrode/s placed in or near a neuron (invasive)
• Measure number of action potentials per second
• (2) Event-related potentials (ERP)
• Electrode/s placed on the skull
• Measures summed electrical potentials from
  millions of neurons (sensitive to dendritic
  currents)

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Single-Cell Recordings

• Enable researchers to understand how individual
  neurons code information
• How specific is the response of neurons? For
  example, could one find a neuron that responded
  to just one face (a "grandmother cell")?
• Neurons may respond to faces more than objects,
  and to some faces more than others, but they
  typically respond to several faces in a graded
  fashion (sparse coding)

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Single-Cell Recordings
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Single-Cell Recordings

• Neurons may respond to conceptual properties of a
  stimulus too
• For example, this neuron responds to an averted
  gaze even though the physical pose is very
  different

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Event-Related Potentials (ERPs)

• Based on EEG (electroencephalography) recordings
• EEG signal is averaged over many events (to
  reduce effects of random neural firing) and
  synchronized to some aspect of the event (e.g.
  onset of stimulus, pressing a button)
• Electrodes record a series of positive and
  negative peaks
• Timing and amplitude of the peaks is related to
  different aspects of the stimulus and task (e.g.
  consider face recognition)

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Event-Related Potentials (ERPs)
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Advantages and Disadvantages of ERP

• ERP signal is directly related to neural activity
  and this electrical activity is conducted
  instantaneously to the scalp
• Therefore, ERP has an excellent temporal
  resolution
• The ERP signal is derived from different sources
  in the brain and it is not possible to infer
  exactly where these sources are from the scalp
• Therefore, ERP has a poor spatial resolution

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Functional Imaging

• Neural activity consumes oxygen as well as
  generating electrical signals
• In order to compensate for increased oxygen
  consumption, more blood is pumped into the active
  region
• PET measures the blood flow in a region, whereas
  fMRI measures the blood oxygenation
• The time taken for this response is slow (several
  seconds) and so functional imaging has a poor
  temporal resolution, but a good spatial
  resolution
• This is the complementary profile to ERPs

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Positron Emission Tomography (PET)

• Measures local blood flow (rCBF)
• Radioactive tracer injected into blood stream
• Tracer takes up to 30 seconds to peak

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Functional Magnetic Resonance Imaging (fMRI)

• Does not use radioactivity, but directly measures
  the concentration of deoxyhaemoglobin in the
  blood
• This is called the BOLD response (Blood Oxygen
  Level Dependent contrast)
• The change in BOLD response over time is called
  the haemodynamic response function and it has a
  number of distinct phases (not to be confused
  with the ERP waveform, which is completely
  unrelated)
• The Haemodynamic Response Function peaks in 68
  seconds and so this is the temporal resolution of
  fMRI

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Functional Magnetic Resonance Imaging (fMRI)

• Haemodynamic response function (change in BOLD
  signal over time)

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What does it mean to say a brain region is
"active"?

• The brain has a constant supply of blood and
  oxygen if it didnt, it would die
• This means we cannot literally stick someone in a
  scanner and read their thoughts (because the
  whole brain would look active)
• In order to infer functional specialization, one
  needs to compare RELATIVE differences in brain
  activity between two or more conditions
• This involves selecting a baseline or comparison
  condition
• A region is "active" if it shows a greater
  response in one condition relative to another
• If the experimenter chooses inappropriate
  conditions the regions of activity will be
  meaningless (junk in, junk out) functional
  imaging isnt foolproof

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Peterson et al. (1988) PET Study
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Peterson et al. (1988) PET Study
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Lie Detection
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• Traditional polygraph measures bodily response
  (e.g. sweating, heart rate), but what if someone
  doesnt feel guilty?
• Brain is the organ that creates the lie
• Anterior cingulate cortex active when asked to
  generate false answers to questions relative to
  truthful ones (e.g. where was your last
  vacation?)
• This region believed to be involved in monitoring
  conflicts between responses
• BUT, not necessarily active if lie is memorized
  in advance

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Is Brain Reading Possible?

• Most experiments manipulate cognitive processing
  and measure changes in brain response
• Can the reverse be done, i.e. can the brain
  response be used to infer what people are
  thinking?
• Brain response can be used to predict which
  object is seen or imagined from a limited set
  (e.g. face v. house v. cats v. shoes) with 90
  accuracy
• However, can only be done if experimenter
  pre-tests the participants on these items (to
  know where to look in that particular person)

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Limits on Brain Reading

• Each brain is subtly different anatomically
• Always need a baseline for comparison cant
  infer from brain activity online
• May be able to measure general contents of
  thought (e.g. a face) rather than specific
  contents (e.g. Tony Blairs face)
• Not easy to distinguish between conscious and
  unconscious cognition just from looking at the
  brain