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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
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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 Earth’s magnetic field
Source: www.spacedaily.com
1 Tesla (T) = 10,000 Gauss
Earth’s 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)
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E = mc2
???
Where does the signal come from?The first brain imaging exp
“[In Mosso’s 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
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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
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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
“Blindsight”Weiskrantz
Nonconscious sightMay be due to spared
CortexSpared retino-collicular
path
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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
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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
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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
QuickTime™ and a YUV420 codec decompressor are needed to see this picture.
V1 and MT activation Levels of cortical processing:Early vs. late
Does motion perception depend on V1 (early processing)?Motion illusions: No retinal motion
Doesn’t activate V1 activate MT
Musical epilepsy? (Sacks book)
Conceptual motion: MT
What about no percept of motion?Moving vs. static ringsStatic images
Implied motion vs. no motion
Neuropsychological evidence:
RetinotopyVisual field deficits: Scotomas
Cortical blindness
Distinguish between peripheral (retinal) and central (cortical) blindness?
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Neuropsychological evidence:
Color (V4) and Motion (MT)Fractionation of perceptionAchromatopsia
HemiachromatopsiaAkinetopsia
Parallel processing, feature maps, and perceptual
experienceEvidence from human performance (Cognitive psychology)
How does the existence of multiple parallel cortical feature maps relate to human perception?
Thought experiment: If brain organized differently, what would perception be like?
Convergence between perceptual and neural evidence ?
Feature maps: Evidence from visual search
Feature vs. conjunction search (Treisman)Serial
Color and orientationParallel (“Pop out”)
E.g., Color, orientation
WSet size
Res
pons
e tim
e
Conjunction
Feature
Look for green T
Perceptual primitives:What makes a “feature”?
Perceptual primitivesBuilding blocks of perceptionRelation to cortical feature
Maps?Luminance, orientation, color,
Motion, depth
Higher-order objectsSynthesis of primitivesObjects defined by
conjunctions of primitives Share primitives
Unique primitives
Integration (binding) across feature maps
Synthesis requires attention—allows coherence across feature maps: Objects
• W/out attention Illusory conjunctions
Human perception: M & P pathways
Do M & P pathways represent different modes of perception?
Isoluminance studies Depth/Motion when defined
by luminanceNot color (isoluminant)
Depth Motion
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Conclusion: Vision as a synthetic process
How does it all come together? Independence (analytic) and convergence (synthetic)
Synthesis/ConvergenceVisual perception of form
Multiple representationsLuminance, color, motion, depth come together to produce “form”How come together: 1) Neural convergence, 2) Temporal Synchrony
End of lecture 3