Mental Chronometry encoding comparison decision making response selection

Transcription

1 Part 2: Methods of Cognitive Neuroscience Psy393: Cognitive Neuroscience Prof. Anderson Department of Psychology Week 3 Cognitive Psychology Lesion method: Cognitive Neuropsychology Brain recording Single cell, Intracranial recording, Scalp recording (EEG, ERP) Metabolic imaging (PET,fMRI) Cognitive Psychology Information processing depends on internal mental representations Say goodbye to behaviorism! Mental representations undergo systematic transformations Flowchart models of operations Characterizing mental operations: Mental chronometry Same or different category? Consonant/vowel 5 different conditions: - physical identity: A A - phonetic identity: A a - same category - both vowels: A U - both consonants: S C - different category: A S Mental Chronometry Mental chronometry Accuracy Response time Sternberg memory scanning mental operations encoding - visually process letter comparison - match to template decision making - make category decision response selection - execute action Derive multiple representations from same stimulus (physical, phonetic identity, conceptual category) Each require finite amount of time (serial stages) Donder s method: Functional independence Additive factors logic Red line high luminance Blue line low luminance Does not interact with memory set size 1

2 Does functional decomposition map onto structure? Flow chart of transformations relate to neuroanatomy? Functional independence Additive factors Concurrent task Supported by different brain regions? Brain lesion method IF: Function X is disrupted by lesion to brain region Y THEN: Brain region Y supports function X Shared Independent Human & nonhuman lesion Human Neuropsychology Not under control of experimenter Acquired brain damage Naturally occurring neurological condition or surgical treatment of condition Single-case or group studies Nonhuman animals Under control of experimenter Lesioning of selected brain structures Surgical or neurotoxic procedures Much more precise Immunotoxic lesions studies Single dissociation Single & Double dissociations Patient with damage to area X is impaired in function A but not function B Double dissociation Patient with damage to area Y is impaired in function B but not cognition A lesion to Broca s area (X) impairs speech production (A) but not comprehension (B) lesion of Wernicke s area (Y) impairs comprehension (B) but not production (A) X Y Wernicke s area Why are double dissociations so important? Why are double dissociations so important? May not reflect distinct functions supported by brain regions Differences in task difficulty, required attention, etc. E.g., Prosopagnosia Faces have less distinctive features, more difficult classification than objects 2

3 Lesion method: Limitations Issues Variability in patients E.g. Due to IQ differences Variability in lesion Due to adjacent cortex Achromatopsia/Prosopagnosia Quasi-Correlational in humans Possible solutions group studies can control for age, IQ, etc. Lesion overlap across patients Lesion Venn diagram More limitations: Disconnection Syndromes Is a brain region critical for a specific function? Lesion may disconnect two brain regions that are critical for function A Split-brain patients Severing the corpus callosum leads to certain cognitive impairments But it s not the corpus callosum that carries out these functions Lesion method: Sitting on a 2 legged stool Function not of area X but of brain without area X E.g., Ascribe function to missing leg: hold up stool on own? All legs participate Falling is a result of system level dysfunction X Brain measurement: Intra/extracellular recording Rather than disrupt function Measure neural correlates during normal operation Electrode inserted into brain near neuron or inside of neuron (intracellular) Records voltage changes pooled over just a few neurons (or a single neuron) Record # of action potentials Logic More AP, more participate in function Receptive fields The area of space in which a neuron can be influenced (maximally) Visual, auditory, somatosensory Cellular recording: Limitations Done in nonhuman animals Generalize to humans? How do workings of a few neurons relate to macroscopic/population level Multi-cellular recording 100+ neurons simultaneously Still correlational How relate to observed behavior Correlated but not causally related Solution? Record/stimulate: e.g., Motion perception 3

4 Intracranial recording: Humans Epilepsy patients Cortical mapping for cortical resection: Stim & Record exposed cortex of epilepsy patient Cons: Neurologically dysfunctional brain grid work of electrodes laid over the surface for stimulation and recording Functional imaging Brain recording in neurologically intact brains Not static: Anatomical/structural imaging CT, MRI Dynamic: Physiological imaging How vary over time (function) Electrical Scalp EEG, ERP Metabolic PET, fmri Electroencephalography (EEG) 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 voice Cheering crowd EEG signal: Dipoles Excitatory inputs (EPSPs) Relative depolarization of dendrites relative to cell body Creates voltage difference dipole EEG signal: Brainwaves Important for studying sleep, diagnosing epilepsy and brain damage Signature rhythms relate to state of arousal Beta: alert, low amplitude, high frequency (13-30 Hz) Alpha: resting with eyes closed, high amplitude (8-12 Hz) Theta: deeply relaxed (4-8 Hz) Evoked Response Potentials: Evoked brainwaves EEG records global brain activity over long time period Represents neural rhythms Not relative to a stimulus ERPs are a special case of EEG Align signal to onset of a stimulus or response Event-Related Potential (ERP) Average EEG trace from a large number of trials Noise cancels out 4

5 Its all in the timing: Endogenous & Exogenous components ERP: The good and the bad Downward waves: positive (P) Upward waves: negative (N) Each wave produced by a different generator Serial order Exogenous components I V: brainstem generators Detect infant deafness Endogenous components N1, P2, N2 Cognitive Endogenous Exogenous Within the first few milliseconds Pros Really good temporal resolution Specific physiological markers (components) e.g., N1, P3 etc., can be linked to known cognitive processes Cons Poor spatial resolution Largely cortical Difficult to get at some brain regions e.g., medial temporal lobes, subcortical structures MRI: Magnetic Resonance Imaging Quest for better resolution, brain coverage Requires very, very strong magnet Source: 1 Tesla (T) = 10,000 Gauss Earth s magnetic field = 0.5 Gauss 4 Tesla = 4 x 10, = 80,000 X Earth s magnetic field x 80,000 = 4T Structural MRI Many organic elements are magnetic Hydrogen most abundant human body 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 Can stimulaye/excite different slices Localization: Resonance freq depends on strength of magnetic field Magnetic gradient Signal: Loss of RF energy ( Relaxation) Reminder: MRI vs. fmri MRI vs. fmri MRI studies brain anatomy. Functional MRI (fmri) studies brain function. MRI fmri High resolution (1 mm) One image Low resolution (~ 3 mm) Many images (e.g. every 2 s for 5 minutes) 5

6 Where does the signal come from? The first brain imaging exp Origin of fmri signal: BOLD Blood Oxygenation Level Dependent signal (BOLD) Angelo Mosso Italian physiologist ( ) E = mc 2??? [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) neural activity blood flow/ O 2 fmri signal Why? Deoxy hemoglobin has increased magnetic properties (paramagnetic) Created magnetic distortions in image Ratio of oxygenated blood (arteries) to deoxy (veins) increases with neural activity Due to increased blood flow, but same O 2 extraction Results in decreased magnetic susceptibility Increased fmri signal Hemodynamic Response fmri: Subtractive logic Hemodynamic response (HR): Blood flow change Neural response: milliseconds HR: peak 5-10 s Anatomical image Functional images Block designs Examine extended HR across same trial type Event-related designs (ER) HR for individual trials Slow vs Rapid ER ER allows examination of trial specific HR E.g., Can examine what brain response predicts later memory Statistical map of difference Condition 1 Condition 2 - Contrast cond1 and cond2 - Functional images are subtracted from one another. - Superimposed on anatomical image. Group activation vs ROIs Brains are different in size, shape, etc. Can warp into common brain space See what is consistent across people Regions of interest (ROI) Predefine anatomical regions Examine signal No warping Pros Non-invasive, no radiation Multiple sessions with same subject High spatial resolution Good temporal resolution Cons Expensive Correlational MRI: Pros and cons 6

7 Positron Emission Tomography Measures local changes in cerebral blood flow (rcbf) Measures rcbf over a few minute period Much worse temporal resolution than fmri (PET) Radioactive isotope tracers carried in blood Positron Emission Tomography (PET) Isotopes rapidly decay (~2 min half life) Emit positrons Positrons collide with electrons 2 photons (or gamma rays) are emitted Photons travel in opposite directions Allows location of collision to be determined More neural activity, more blood flow, more positron emission PET: Pros and cons Pros Track multiple metabolic processes Not just blood flow labeling of various substances imaging of some neurotransmitters Raclopride competes with DA receptors Cons Invasive radioactive isotopes can only be administered limited number of times Limited spatial resolution Highly limited temporal resolution Limited by the half life of the isotope used Break Perception and Encoding Eye to brain: Evidence for parallel processing Visual maps: Multiple neural representations of reality Brain to mind: How does neural organization relate to human perception? Review: Is vision analytic or synthetic? 7

8 Vision as analytic vs. synthetic Analytic/constructivist Construct perception through assembly of its parts Feature extraction > Object perception Synthetic/gestalt Whole more than sum of parts Object perception > feature extraction Neural divergence Neural convergence Overview of visual neural pathways Parallel processing I: Two main receptor types Two types of vision Cones: High acuity, lower sensitivity Rods: Low acuity, higher sensitivity Different topography Origin of M & P Cones: Parvo Rods: Magno Fovea Ganglion cells Middle layer Receptor cells Eye to CNS: Parallel processing II Two pathways Retino-geniculate-striate pathway Retino-collicular-pulvinar pathway Rods Cones Rods Rods Retino-geniculate-striate path Vision for perception: What systems Conscious vision Cortical blindness: Hemianopia Blindsight Weiskrantz Nonconscious sight May be due to spared Cortex Spared retino-collicular Path Eyes can locate unseen lights 8

9 Retino-collicular-pulvinar path Vision for action: Where systems Evidence for action vs. perception Stimulus present in intact and blind fields slowed during eye movement, not detection (button press) Retino-geniculate pathway Organization of LGN: Laminar structure Retinal origin: Segregation of ocular inputs Temporal/Nasal adjacent (Same VF) Retino-geniculate pathway: Parallel processing III Organization of LGN: 2. Retinotopy Retino-geniculate pathway Organization of LGN: 3. Morphology Not all retinal maps the same Parvocellular (P) Small cells Top 4 layers Magnocellular (M) Large cells Bottom 2 layers 6 representations of retina in register Organization of visual cortex: Divide & Conquer! Bifurcations and more bifurcations LGN > V1 2 divisions M & P V1 > extrastriate Even greater divergence Maintain M & P origin Differ in features (Parallel) & complexity (Hierarchical) Increase in RF size Magno Parvo Primary visual cortex: Striate cortex/v1/area 17 First cortical synapse in vision: Calcarine sulcus 9

10 Striate cortex (V1): Retinotopy 6 LGN maps > 1 striate map Striate cortex (V1): M & P segregation Distinct laminar projections As well 35+ maps in surrounding cortex Striate cortex (V1): Eye, orientation selectivity Ocular dominance columns (Retained from LGN) Diff from LGN Orientation selectivity Increase in complexity to LGN (center-surround) Higher order visual cortex: Extrastriate cortex Cytoarchitecture Cellular correlates More complex features E.g., Motion, MT/V5 Direction and speed selective Polar plot Responds to left downward motion Speed tuning What about humans? Human visual cortex: Striate (V1) Retinotopy (traveling wave method) Eccentricity Foveal distortion Polar angle Defines distinct areas Human visual cortex: Ocular dominance columns High resolution fmri distinguishes ODC 10

11 Human visual cortex: Extrastriate cortex Human V4: Isoluminant color Lingual, fusiform gyrus Human MT (V5): Motion Middle temporal gyrus V1 and MT activation QuickTime and a YUV420 codec decompressor are needed to see this picture. 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 Conceptual motion: MT What about no percept of motion? Moving vs. static rings Static images Implied motion vs. no motion Musical epilepsy? (Sacks book) Neuropsychological evidence: Retinotopy Visual field deficits: Scotomas Cortical blindness Neuropsychological evidence: Color (V4) and Motion (MT) Fractionation of perception Achromatopsia Hemiachromatopsia Akinetopsia Distinguish between peripheral (retinal) and central (cortical) blindness? 11

12 Parallel processing, feature maps, and perceptual experience Evidence 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 orientation Look for green T Parallel ( Pop out ) E.g., Color, orientation Response time Conjunction Feature Set size W Perceptual primitives: What makes a feature? Perceptual primitives Building blocks of perception Relation to cortical feature Maps? Luminance, orientation, color, Motion, depth Higher-order objects Synthesis of primitives Objects defined by conjunctions of primitives Unique primitives Share 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 luminance Not color (isoluminant) Conclusion: Vision as a synthetic process How does it all come together? Independence (analytic) and convergence (synthetic) Depth Motion Synthesis/Convergence Visual perception of form Multiple representations Luminance, color, motion, depth come together to produce form How come together: 1) Neural convergence, 2) Temporal Synchrony 12

13 End of lecture 3 13