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    Psy393: Cognitive Neuroscience

    Prof. AndersonDepartment of Psychology

    Week 3

    Functional imagingBrain recording in neurologically intact brains

    Not anatomical/structural imaging: Static CT, MRI

    Physiological/functional imaging: Dynamic2 classes

    ElectricalEEG, ERP

    MetabolicfMRI, PET

    Large populations of synchronous neural firing

    Produce electrical potentials

    Skull and scalp passively conduct signals that can be amplified and measured

    Stadium/microphone analogy

    Single voiceCheering crowd

    Electroencephalography (EEG) EEG signal: Dipoles

    Excitatory inputs (EPSPs)Relative depolarization of dendrites relative to cell bodyCreates voltage

    differencedipole

    Important for studying sleep, diagnosing epilepsy and brain damage

    Signature rhythms relate to state of arousal

    Beta: alert, low amplitude, high frequencyAlpha: resting with eyes closed, high amplitudeTheta: deeply relaxed

    EEG signal: Brainwaves

    EEG records global brain activity over long time periodRepresents neural rhythmsNot relative to a stimulus

    ERPs are a special case of EEGAlign signal to onset of a stimulus or response

    Event-Related Potential (ERP)

    Average EEG trace from a large number of trials

    Noise cancels out

    Evoked Response Potentials: Evoked brainwaves

  • 2

    Downward waves:positive (P)Upward waves:negative (N)Each wave produced by a different generator

    Serial orderExogenous components

    I V: brainstem generatorsDetect infant deafness

    Endogenous componentsN1, P2, N2Cognitive

    Its all in the timing: Endogenous & Exogenous components

    Within the first few milliseconds

    Exogenous

    Endogenous ProsReally good temporal resolutionSpecific physiological markers (components)

    e.g., N1, P3 etc., can be linked to known cognitive processes

    ConsPoor spatial resolutionLargely cortical

    Difficult to get at some brain regionse.g., medial temporal lobes, subcortical structures

    ERP: The good and the bad

    MRI: Magnetic Resonance Imaging

    Quest for better resolution, brain coverageRequires very, very strong magnet

    x 80,000 =

    4 Tesla = 4 x 10,000 0.5 = 80,000 X Earths magnetic field

    Source: www.spacedaily.com

    1 Tesla (T) = 10,000 Gauss

    Earths magnetic field = 0.5 Gauss

    Protons spin around a given axis (random axis): Precession

    When placed in a magnetic field the protons become aligned in parallel

    Resonance: A Radio Frequency (RF) pulse is used in MRI to push protons out of alignment with the magnetic field

    Imagine tuning fork

    Localization: Resonance freq depends on strength of magnetic fieldSignal: Loss of RF energy (Relaxation)

    Many organic elements are magneticHydrogen most abundant human body

    Structural MRI

    MRI studies brain anatomy. Functional MRI (fMRI) studies brain function.

    Reminder: MRI vs. fMRI MRI vs. fMRIMRI fMRI

    High resolution (1 mm)

    One image

    Low resolution (~ 3 mm)

    Many images (e.g. every 2 s for 5 minutes)

  • 3

    E = mc2???

    Where does the signal come from?The first brain imaging exp

    [In Mossos experiments] the subject to be observed lay on a delicately balanced table which could tip downward either at the head or at the foot if the weight of either end were increased. The moment emotional or intellectual activity began in the subject, down went the balance at the head-end, in consequence of the redistribution of blood in his system.

    -- William James, Principles of Psychology (1890)

    Angelo MossoItalian physiologist

    (1846-1910)

    Origin of fMRI signal: BOLDBlood Oxygenation Level Dependent signal (BOLD)

    Why? Deoxy hemoglobin has increased magnetic properties (paramagnetic)Ratio of oxygenated blood (arteries) to deoxy (veins) increases with neural activity

    Do to increased blood flow, but same O2 extraction Results in decreased magnetic susceptibility

    Increased fMRI signal

    neural activity blood flow/ O2 fMRI signal

    Hemodynamic ResponseHemodynamic response (HR): Blood flow changeNeural response: millisecondsHR: peak 5-10 s

    Block designsExamine extended HR

    across same trial typeEvent-related designs (ER)

    HR for individual trialsSlow vs Rapid ER ER allows examination of trial specific HR

    E.g., Can examine what brain response predicts later memory

    - Contrast cond1 and cond2- Functional images are

    subtracted from one another.- Superimposed on anatomical

    image.

    Anatomical image Functional images

    Condition 1 Condition 2

    Statistical map of difference

    fMRI: Subtractive logic

    Group activation vs ROIs

    Brains are different in size, shape, etc.Can warp into common brain spaceSee what is consistent across people

    Regions of interest (ROI)Predefine anatomical regionsExamine signalNo warping

    ProsNon-invasive, no radiationMultiple sessions with same subjectHigh spatial resolutionGood temporal resolution

    ConsExpensiveCorrelational

    MRI: Pros and cons

  • 4

    Measures local changes in cerebral blood flow (rCBF)

    Measures rCBF over a few minute period

    Positron Emission Tomography (PET)

    Radioactive isotopes tracers Isotopes rapidly decay

    (~2 min half life)Emit positronsPositrons collide with electrons

    2 photons (or gamma rays) are emitted

    Photons travel in opposite directions Allows location of collision to be determined

    Positron Emission Tomography (PET)

    ProsTrack multiple metabolic processes

    labeling of various substances imaging of some neurotransmitters

    ConsInvasive

    radioactive isotopes can only be administered limited number of times

    Limited spatial resolutionHighly limited temporal resolution

    Limited by the half life of the isotope used

    PET: Pros and cons End of Part 1

    Perception and Encoding

    Eye to brain: Evidence for parallel processing

    Brain to mind: How does neural organization relate to human perception?

    Review: Is vision analytic or synthetic?

    Visual maps: Multiple neural representations of reality

  • 5

    Vision as analytic vs. synthetic

    Analytic/constructivistConstruct perception through assembly of its partsFeature extraction > Object perception

    Synthetic/gestaltWhole more than sum of partsObject perception > feature extraction

    Neural divergence

    Neural convergence

    This week

    Overview of visual neural pathwaysParallel processing I: Two main receptor types

    Two types of visionCones: High acuity, lower sensitivityRods: Low acuity, higher sensitivity

    Different topographyOrigin of M & P

    Cones: ParvoRods: Magno

    Other receptor types as well:Retina-SCN: Regulation of circadian rhythms

    Ganglion cells

    Middle layer

    Receptor cells

    Fovea

    Cones

    Rods

    RodsRods

    Eye to CNS: Parallel processing II

    Two pathwaysRetino-geniculate-striate pathway Retino-collicular-pulvinar pathway

    Retino-geniculate-striate pathVision for perception: What systems

    Conscious visionCortical blindness: Hemianopia

    BlindsightWeiskrantz

    Nonconscious sightMay be due to spared

    CortexSpared retino-collicular

    path

  • 6

    Retino-collicular-pulvinar pathVision for action: Where systems

    Evidence for action vs. perceptionStimulus present in intact and blind fieldsslowed during eye movement, not detection

    Retina>Suprachiasmatic nucleus

    Other forms of nonconscious visionNon-rod, non-cone, melatonin based photoreceptors

    Regulation of circadian behaviorMutant mice lacking rods and cones demonstrate phase shifting to lightSupported by connection between retina and SCNConclude: Many types of vision

    Retino-geniculate pathwayOrganization of LGN: Laminar structure

    Retinal originTemporal/Nasal adjacent (Same VF)

    Retino-geniculate pathway: Parallel processing III

    Organization of LGN: 2. Retinotopy

    6 representations of retina in register

    Retino-geniculate pathway

    Organization of LGN: 3. MorphologyNot all retinal maps the same

    Parvocellular (P)Small cellsTop 4 layers

    Magnocellular (M)Large cellsBottom 2 layers

    Organization of visual cortex: Divide & Conquer!

    Bifurcations and more bifurcations

    LGN > V12 divisions

    M & P

    V1 > extrastriate Even greater divergenceMaintain M & P origin

    Differ in features (Parallel)& complexity (Hierarchical)

    Increase in RF size

    Parvo

    Magno

  • 7

    Primary visual cortex: Striate cortex/V1/Area 17

    First cortical synapse in vision: Calcarine sulcus

    Striate cortex (V1): Retinotopy

    6 LGN maps>1 striate map

    Striate cortex (V1): M & P segregation

    Distinct laminar projections

    Striate cortex (V1):Eye, orientation selectivity

    Ocular dominance columns (Retained from LGN)Diff from LGN:Orientation selectivity

    Increase in complexityto LGN (center-surround)

    Higher order visual cortex:Extrastriate cortex

    CytoarchitectureCellular correlates

    More complex featuresE.g., Motion, MT/V5Direction and speed

    selective

    What about humans?Human visual cortex: Striate (V1)

    Retinotopy (traveling wave method)Eccentricity

    Foveal distortionPolar angleDefines distinct areas

  • 8

    Human visual cortex: Ocular dominance columnsHigh resolution fMRI distinguishes ODC

    Human visual cortex:Extrastriate cortex

    Human V4: Isoluminant colorLingual, fusiform gyrus

    Human MT (V5): MotionMiddle temporal gyrus

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