Non-Invasive Techniques

Electroencephalography (EEG):

  • electrodes placed at scalp to record signals
  • reflection of sum of postsynaptic potentials
  • cannot record signals from tangential to scalp/deep inside the brain
  • poor spatial resolution (cm), great temporal resolution (ms)

    Conditions

  • weak signals, need amplification/processing and avoid movements
  • cap with recording electrode along with position references: mastoids   (behind ear), nasion (on top of nose), inion (base of skull) -- to measure skull circumference​

The 10-20 System

A standard method used to identify locations of the various lobes of the brain with respect to each other

https://en.wikipedia.org/wiki/10-20_system_(EEG)#/media/File:21_electrodes_of_International_10-20_system_for_EEG.svg

EEG Methodology

  • Bipolar (measure potential difference of electrodes) or unipolar (compare charge to neutral or average)
  • one plugs in to differential amplifier and the other to reference electrode, amplifying by 1000-100000 times
  • anti-aliasing filter then A/D (analog to digital), then another filter (1-50 Hz)
  • can capture "brain waves" at various frequencies
  • neurons have characteristic frequency ranges
      i.e. alpha waves - 8-13 Hz in occipital region of awake person when relaxed
             beta waves, delta waves, gamma waves

Magnetoencephalography (MEG)

  • measures magnetic field of brain via superconducting quantum interfacing devices (SQUID)
  • only sensitive to the neurons tangential to scalp due to Maxwell's equations
  • also high temporal resolution
  • offers better spatial resolution than EEG
  • independence from head geometry
  • more expensive, non-portable

Maxwell's Equations

The neural stimuli in specific directions as a result of an applied magnetic field

https://upload.wikimedia.org/wikipedia/commons/0/0e/Magnetoencephalography.png

Functional Magnetic Resonance Imaging (fMRI)

  • detect change in blood flow from activation of neurons during specific tasks
  • deoxygenated hemoglobin is more magnetic
  • blood oxygenation level dependent (BOLD) response.
  • present stimuli to subjects
  • high spatial resolution, low temporal resolution

Functional Near Infrared (fNIR) Imaging

  • detect near-infrared light absorbance of hemoglobin in blood (comparing oxygenated or deoxygenated)
  • can penetrate skull and reflect back to detectors
  • subjects aren't as strict with movements because of optical measurements rather than electrical

Positron Emission Tomography (PET)

  • measures emission of radioactively labeled chemical injected into blood stream transporting to brain (radiotracer)
  • great spatial resolution, low temporal resolution
  • drawback requires injection of radioactive chemicals and also have to account for rapid decay of radioactivity

Stimulating the Brain

Invasive Techniques

Microelectrodes

  • glass microelectrodes can inject current into cell to excite or inhibit action potentials
  • platinum-iridium microelectrodes can also be used
  • can be used for DBS and can relieve tremors and gait problems

Direct Cortical Electrical Simulation (DCES)

  • electrodes used on the surface of the cortex much like ECoG
  • electric current (<15 mA) delivered across bipolar electrodes
  • can generate movements or cause particular sensations, or do the exact opposite.
  • helps locate where each brain function is located

 

Optical Simulation

  • two photon laser illumination- illuminate the tangent at the membrane of the cell and creates excitation
  • optogenetic stimulation - use genetic manipulation to make only certain neurons responsive to illumination
  • two photons can selectively excite single neurons while optogenetic can selectively excite a class of neurons

Transcranial Magnetic Stimulation (TMS)

  • uses electromagnetic induction and generates a magnetic field to create a current in the brain and excite the neurons
  • penetrates 3-5 cm into the brain- only for superficial layers
  • advantage is non-invasive

Transcranial Ultrasound

  • low-intensity pulsed ultrasound to influence neural activation without thermal effects or tissue damage
  • worked up to 92% of mice tested

Simultaneous Recording and Stimulation

Multielectrode Array

  • Good for both recording spike activity and delivering currents to excite or inhibit neurons
  • Some can be used specifically for recording, others for stimulation

Neurochip

  • integrated chip to serve multiple purposes
  • records from neurons, performs isgnal processing and delivers appropriate stimulation to other neurons
  • preprocessor to do spike sorting
  • instruct circuit to deliver electrical pulses
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