H4 the Imaged brain

• Structural imaging:

  Measures of the spatial configuration of different types of tissue in the brain (principally CT and MRI). CT related to X-ray absorption
• Functional imaging: 


  Measures temporary changes in brain physiology associated with cognitive processing; the most common method is fMRI and is based on a hemodynamic measure.
• Hemodynamic refers to changes in blood flow, blood volume, and oxygenation. 


  Hemodynamic methods do not measure the neural activity as in EEG and MEG, but rather measure downstream gevolgen of neural activity
• Magnetic resonance imaging (MRI)

  • Does not use ionizing radiation (so is non-invasive)
  • Better spatial resolution (e.g. folds of individual gyri can be onderscheiden)
  • Better discrimination between white matter and gray matter
  • Adapted for use in fMRI
• Functional Imaging


  • Measures neural activity and generating electrical signals, which consumes oxygen
  • In order to compensate for increased oxygen consumption, more blood is pumped into the active region
• PET


  Measures the blood flow in a region
• fMRI

  Measures the blood oxygenation (zuurstofvoorziening)
• Functional imaging

  • Has a poor temporal resolution, but a good spatial resolution
  • Complementary to ERP
• Positron Emission Tomography (PET)


  • Measures local blood flow (rCBF)
  • Radioactive tracer injected into blood stream – regarded as invasive
  • Tracer takes up to 30 seconds to peak
  • Spatial resolution around 10mm
  • Different radioactive tracers makes PET more versatile for exploring a variety of physiological and pathological conditions
• Voxel-based Morphometry (VBM)

  • A technique for segregating and measuring differences in white matter and gray matter concentration
  • VBM divides the brain into tens of thousands of small regions, several cubic millimeters in size (called voxels), and the concentration of white/gray matter in each voxel is estimated (geschat)
• Voxel


  A volume-based unit (cf. pixels, which are 2D); in imaging research the brain is divided into many thousands of these
• Diffusion Tensor Imaging (DTI)

  • Uses MRI to measure white matter connectivity between brain regions
  • Water molecules trapped in axons only diffuse within the axon. When many such axons are arranged together it is possible to quantify this effect with MRI (using a measure called fractional anisotropy)
• Fractional anisotropy (FA)


  A measure of the extent to which diffusion takes place in some directions more than others
• The brain has a constant supply of blood and oxygen; if it didn't, it would die
• This means we cannot literally stick someone in a scanner and read their thoughts (because the whole brain would look active)
• To infer functional specialization


  One needs to compare RELATIVE differences in brain activity between two or more conditions
• 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 isn't foolproof
• Functional Magnetic Resonance Imaging (fMRI)

  • Does not use radioactivity, but directly measures the concentration of deoxyhemoglobin in the blood
  • This is called the BOLD response (Blood Oxygen Level Dependent contrast)
  • The change in BOLD response over time is called the Hemodynamic Response Function and it has a number of distinct phases (not to be confused with the ERP waveform, which is completely unrelated)
  • The Hemodynamic Response Function peaks in 6–8 seconds. This limits the temporal resolution of fMRI
• The Hemodynamic response function (HRF)

  1. Initial dip

  2. Overcompensation

  3. Undershoot
• Over the last ten years fMRI has overtaken PET scans in functional imaging experiments