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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