16: Skin, Touch, and Movement #

• Somatosensory receptors occur in the skin
  • Dorsal-root ganglion (DRG) is a somatosensory neuron that carries ’touch’ information
• Somatosensory cortex is where information is processed
  • Located in postcentral gyrus
  • Somatosensory body map pairs region in cortex and location on body
    • Discovered by Wilder Penfield
    • Size in cortex isn’t 1:1 with size (index finger has an extra-large area)
      • “Distortion” reflects sensory sensitivity
      • E.x. difficult to sense two points of contact (on back); Two-point discrimination test
    • Finger amputation in monkey increased cortical response for adjacent fingers (somatosensory map reorganization)
      • Phantom limb’s may occur when one loose’s their arm and ‘feels’ said arm when receives information from cortical-adjacent areas
  • Primary somatosensory cortex (S1) lesions lead to loss of sensation in specific region
  • Posterior somatosensory cortex (S2, S3, etc) lesions lead to neglect syndrome: one is aware, but unable to focus on a body region (ex. one doesn’t dress their left-side of their body)
• Primary motor cortex (M1)
  • Located anterior to the postcentral gyrus
  • Contains a motor-body map
    • Contralateral; left-right, right-left
  • Supplementary motor / premotor areas: Involves planning of movement; lesions cause apraxias, impairment in organizing movements (to solve problems)
  • Mirror neurons activate during movement and during observation of movement!
    • Vast interconnectivity between posterior sensory areas and prefrontal motor areas
• Cerebellum
  • Involved in timing and coordination of movement
  • More nerve cells here than the rest of the brain
• Frontal-parietal lesion
  • Causes paralysis of right half of body
  • Lead to loss of sensation, ‘weirdness’ on right side
  • If on the right side, they become denial!
    • Anosognosia, loss of knowledge of one’s disease
    • Exhibits a kind of exaggeration of psychological defenses
    • Hypothesis: Left hemisphere controls defense mechanisms and is kept in check by the right hemisphere

17: Imaging the Brain #

Static/structural brain imaging: #

Popularized by Andreas Vesalius (1514-1564) in De Humani Corporis Fabrica (1543)

• X-ray imaging
  • Discovered by William Rontgen in 1895
  • First Nobel Prize in Physics (1901)
  • Xrays are higher energy than UV and natural light, passing through skin/tissue more-so than bone
  • Lesions show as change in contrast
• Computed axial tomography (CAT, CT)
  • Began in 1970s
  • 16A
• Magnetic resonance imaging (MRI)
  • Looks similar to CAT scan
  • Operates by Nuclear Magnetic Resonance (NMR) to measure/alter the nuclear spin of atoms in molecules
  • Magnetic field strength:
    • Tesla = 10,000 Gauss
    • Earth’s magnetic field = 0.5 gauss
  • No known toxic effects on the body

Dynamic/functional brain imaging #

• Electroencephalography (EEG)
  • Developed in 1920s by Hans Berger
  • Give’s you a ‘summed’ signal of the brain
    • Seizure causes erratic behavior
  • Wilder Penfield, epileptogenic tissue
• Electrocorticography (ECoG)
  • Temporary implanted electrodes on brain itself
  • Allows higher frequency range + higher resolution
• Magnetoencephalography (MEG)
  • Magnetic fields are far weaker, so a stronger detector is needed
  • Very expensive and can only operate in special, shielded room
• fMRI
  • Hemoglobin: Protein in our red blood; has spin-flip energy depending on oxygen contents
  • BOLD signal: Blood Oxygen Level Dependent Signal
    • Represents changes in (de)oxygenated hemoglobin from blood flow and cell metabolism
    • Correlated with neural activity
• Positron emission tomography (PET)
  • Radioactive isotope injected into blood; Florine-18 (HL: 2hr), Oxygen-15 (HL: 2min), Carbon-11 (HL: 20min)
    • These unstable elements emit positrons (anti-electron): the antimatter particle corresponding to the electron
    • Matter-antimatter collision results in annihilation: complete conversion of mass to energy
    • 2 gamma-ray photons released when positron-electron collision occur
  • Enables 3d reconstruction of where most energy is occurring (glucose intake, directly)
  • Only difference/change in brain activity is shown
    • “dark energy” of the brain is the abnormally large background noise
• Ernest Lawrence invented, at Berkeley, the Cyclotron in the 1930s
  • Accelerates atoms to very high speeds
  • Won Nobel Prize in 1939 in Physics
  • Transuranium elements 104-118 found by Russians
    • Live for a fraction of a second
  • Able to create unstable elements, required in PET scans

18: Connectivity, Language, and Meaning #

• Aphasia:
  • Broca’s;
    • Left frontal lobe
    • Production aphasia for spoken and written language
    • Mouth/tongue etc are all uneffected
  • Wernicke’s:
    • Comprehension aphasia for spoken and written language
      • Prevent analysis at a higher level
    • Part of auditory cortex

• Cortical layers and connectivity
• Hemispheric asymmetry
• Wada test:
  • Bartbiturate sedative-hypnotic injected into right/left hemisphere
    • via right/left catheter threaded from femoral artery in thigh
  • Asked to count after injection
    • Right causes slight pause when it hits
    • Left causes speaking to stop when it hits
• Language lateralization and handedness
  • Right handers: LH 97%; RH 3%
  • Non-right handers: LH 70%; RH 15%; Both 15%
• Linguistic mirror neurons
  • Sound activate the auditory cortex, A1
  • Sounds that are clearly language activate A1, Wernicke’s
  • Language with meaning activates A1, Wernicke’s, Broca’s
  • Language occurs all across the brain
    • Origin of intentionality isn’t real clear
    • Prefrontal motor areas generate movement