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Memory and Cognition PSY 324 Topic 2: Cognition and the Brain Dr. Ellen Campana Arizona State University A Brain Gray & White Matter Solid tissue Made up of neurons Golgi showed by staining slices with dye Neurons Similarities with other cells of the body Have a nucleus containing DNA Surrounded by a cell membrane Contain mitochondria and other organelles Do basic cell stuff (protein synthesis, energy production) Unique characteristics Do not reproduce Structure, function, chemicals (details to come) Structure of a Neuron Structure of a Neuron Varieties of neurons Function = transmit information to other cells Sensory / Afferent neurons: info TOWARD CNS Motor / Efferent neurons: info AWAY from CNS Interneurons: info to other neurons in the CNS Info Transmission: Simple Story Neurons are transducers – convert environmental energy to electrical energy (starting with receptors) Energy is propagated from the dendrites into the cell body. Energy is propagated to the end of the axon. When it goes above a threshold, it triggers the release of neurotransmitters into the synapse The neurotransmitters in the synapse trigger the same process (or a different one) in the next cell Synapses Info Transmission: Deeper Story Background concepts from physics Matter composed of molecules (always moving) Molecules can have +/- charge Like charges repel, opposite charges attract + - + - Info Transmission: Deeper Story Each neuron has a resting potential, the voltage difference across the cell membrane, caused by the chemicals inside/outside the cell when the cell is not firing + + + + + + + + - + + + + + + + + Info Transmission: Deeper Story Axon lined with ion channels (sodium channels, potassium channels) that open and close during an action potential to propagate the signal Depolarization phase: Sodium (Na+) Channels Repolarization phase: Potassium (K+) Channels Depolarization Phase Depolarization + Repolarization Info Transmission: Deeper Story Action potential moves down the axon, as gates open and close in sequence Synapses Action Potential reaches the end of the axon, triggering release of neurotransmitters •Excitatory neurotransmitters increase firing rate in next neuron •Inhibitory neurotransmitters decrease firing rate in next neuron •NOTE: Other neurotransmitters do other things (less well understood, less relevant to cognition, especially to models we will talk about) Method: Single-Cell Recording It is possible to record activity of a single cell Tiny wires (called microelectrodes) stuck into axon, attached to oscilloscope for data display Time is a factor De/Repolarization cycle = 1/1000 S or 1 ms Activities of cognition take at least 100ms – at that resolution action potentials show up as spikes Often most useful to talk about firing rate Method: Single-Cell Recording Pictures of “spikes” – http://viperlib.york.ac.uk/ , keyword single cell recording Video clip from Hubel & Weisel (they got the 1981 Nobel Prize in physiology and medicine for this work) http://viperlib.york.ac.uk/ , keyword single-cell recording History of Single-Cell Recording Participants in experiments: 1880s – People injured by accident with exposed brains, also patients with epilepsy 1950s – Fully anesthetized animals (cats, squirrels, monkeys, apes) 1980s – Awake, active monkeys and apes Invasive, destructive procedure Data: there is a cell in the [animal] that increases firing under [conditions] Value of Single-Cell Recording By itself, the data doesn’t tell us much Can find cells that do almost anything The value of single-cell recording for understanding human cognition depends on: Functional organization of the brain Consistencies of organization within species Meaningful mapping from animal models to organization of human brain Fortunately, much evidence that these exist Clarification from last time Question came up about diffusion and connection to neuron behavior Clarification from last time Background concepts from physics Matter composed of molecules (always moving) Molecules can have +/- charge Like charges repel, opposite charges attract + - + - Clarification from last time Background concepts from physics Matter composed of molecules (always moving) Molecules can have +/- charge Like charges repel, opposite charges attract Cell membrane maintains an imbalance At rest, negative inside and positive outside (pumps maintain) Ion channels open / close quickly Particles rush in / out (like a hole in a boat) Sodium and Potassium are +, but other chemicals create the negativity inside the axon at rest Focus on Sodium and Potassium because of the gates Clarification from last time + - + + - + + + + + - - - - + + + + + + + + Axon Cell membrane (imbalance) Diffusion and charge drive the process – cells rush in/out There are mechanical pumps but they just restore resting potential at the end Clarification from last time Things to note All action potentials are the same size, in terms of voltage (all-or-nothing principle) Most useful to think of them as on / off, or to think about firing rates (spikes per second) These are from a specific study. Neuron A responds when the stimulus is ON. Neuron B responds when the stimulus is OFF. Neuron C responds to changes in the stimulus. Why study neurons? Everything we see, hear, do, smell, remember, taste, touch, pay attention to, and think about is represented physiologically by neurons firing All sensations, perceptions and thoughts are neural activation All of our actions arise from neural signals Study of cognition is about both physiological and functional models Increasingly uses brain imaging and neuroscience methods (later today) Brain Organization Hierarchical Structure Smallest unit: Neuron Neurons form Circuits (many levels) Convergence, Inhibition, Excitation Related circuits contribute to localized function Brain areas for different functions Hemispheric specialization Neurons as part of circuits Neural processing occurs when neurons synapse together to form a neural circuit Convergence Interaction of excitation and inhibition Neurons as part of circuits Neural processing occurs when neurons synapse together to form a neural circuit Convergence Interaction of excitation and inhibition Hubel & Weisel Single-cell recording of feature detectors Simple neurons (from the video last time) Complex neurons Orientation (thickness, location of line) Orientation, direction of motion End-stopped Length, direction of motion Feature Detectors Lines (shapes and orientations) Directed Motion Complex Stimuli Geometrical figures Common objects in the environment (houses, manmade objects, birds) Faces Depend on selectivity – neurons firing at some times and not at others Neural Codes in Daily Life Consider the case of recognizing the face of a specific person – how could that happen? Hypothesis 1: specificity tuning – a particular neuron could selectively fire when you see that person Specificity Coding Difficulties with Specificity Coding Hypothesis Related idea: Grandmother cell (coined by Lettvin) Too many different faces, concepts, etc. to have a neuron for each one Depends on experience – would have to learn each face (because neurons don’t reproduce) Neurons selective for faces are active for many different faces Cell responds to image of a grandmother, general concept of grandmothers, your own grandmother Some evidence that these might exist in Hippocampus – associated with memory storage, not vision For recognition (and many other types of cognition), specificity coding is not enough Neural Codes in Daily Life Hypothesis 2: Distibuted Coding – code for a specific face is distributed across a set of neurons Distributed Coding Advantages Efficient -- firing of fewer neurons can represent many more different stimuli Similar items can have similar neural codes Helps with learning Graceful degradation -- if one or two neurons do not fire, it is still possible to recognize a face Reconciling types of coding Evidence for Specificity Coding Feature detectors Concept cells in hippocampus (memory area) Argument for Distributed Coding in recognition Clear theoretical advantages in recognition Will see a lot of evidence later Both are happening in the brain – in different areas at the same time (parallel processing) Pattern across (+ interaction btwn) areas=cognition The Whole Brain Localization of function - Different parts of the brain serve different functions Many, many ways to divide the brain Like an onion, many layers Like a fractal, the closer you look the more complex it seems Descriptions may seem contradictory and/or overlapping because of this Cerebral Cortex Most important for Cognition Cerebral Cortex Temporal Lobe Language Memory Hearing Perceiving forms Occipital Lobe Visual information (early processing) – feature detectors Cerebral Cortex Parietal Lobe Touch Vision Attention Frontal Lobe Proportionately larger in humans than in other species Language Thought Memory Motor functioning Subcortical Structures Subcortical Structures Hippocampus Amygdala Forming memories Emotions, emotional memories Thalamus Processing sensory information (vision, hearing, touch) Hemispheres Brain separated into sides (hemispheres) Corpus Collosum connects them Structurally and functionally very similar Lateralization – specific functions occurring in one hemisphere or the other Note: Sperry studied “split-brain patients”, who had had their corpus collosum severed as a treatment for epilepsy. He shared Hubel & Weisel’s Nobel Prize for this work. Lateralization Vision of left part of the world lateralized to the right side (opposite also true) Motor Control of left side of body lateralized to the right side (opposite also true) Touch on left side of body lateralized to the right side (opposite also true) Lateralization Are there “right-brained” and “left-brained” people? Analytical/Logical processing (syntax of language) usually on the left side (not always) Analogy and Broad Thinking usually on the right side (not always) Everyone has (and uses) both Patients who have had a hemispherectomy Other side usually takes over missing functionality Coglab “Brain Assymetry” Localized Function Parietal Lobe Sensory Homunculus (near the front, somatosensory cortex) Motor Homunculus (near the back, motor cortex) These guys aren’t actually IN your brain, they’re representations of how much cortex area is devoted to different body parts Sensory Homunculus Each side of the brain has a copy, which processes touch from the other side Localized Function Parietal Lobe Sensory Homunculus (near the front, somatosensory cortex) Motor Homunculus (near the back, motor cortex) Temporal Lobe Wernicke’s Area – metaphor, meaning in language Broca’s Area – logical structure of language Fusiform Face Area (FFA) – specialized for faces (or is it things we’re experts at recognizing???) Researching Localized Function Neuropsychology – comparing patients with localized brain damage Single dissociation– single patient has some things impaired, other things not impaired Single Dissociation: Phineas Gage Construction accident – 1848 Harlow (doctor) wrote a lot about his condition Gage lived, could talk, act, and do all “normal” activities, but suffered impairment of emotional, social, and personal traits Evidence for some separation of language and social traits, etc. Researching Localized Function Neuropsychology – comparing patients with localized brain damage Single dissociation – single patient has some things impaired, other things not impaired Alice: Short-Term Memory OK, Long-Term Memory impaired (like in Memento) Double dissociation -- two (or more) patients show opposite single impairments Bert: Long-Term Memory OK, Short-Term Memory impaired Double Dissociation Alice (temporal lobe damage) Short-term memory Long-term memory OK Impaired Bert (frontal lobe Impaired damage OK Double Dissociation Naming Living Things Naming Nonliving Things Group 1 OK (damage to area 1) Impaired Group 2 Impaired (damage to area 2) OK What can we conclude? Double dissociation Two functions involve different mechanisms Two functions involve different brain areas Mechanisms are independent Single dissociation Two functions involve different mechanisms Two functions involve different brain areas Mechanisms may not be independent Limitations of Neuropsychology At least for human processing, brain damage comes about from natural means (accident, etc.) Members of groups rarely have exactly the same damage (location or extent) No record of processing or brain organization before the damage Difficult to assess all possible types of functional impairment Damage may cause reorganization (plasticity) Imaging Methods EEG- Electrodes on outside of head continuously measure electrical activity PET- Radioactive dye injected, accumulates in different regions over time and can be read by a scanner. Essentially measures metabolism of neurons fMRI- Brief magnetic pulses used to give a snapshot of ratio of oxygenated to deoxygenated blood (metabolism) TMS- New measure. Magnetic field can disable specific portions of the brain for a short time, simulating damage. Temporal resolution: Detail with respect to time Spatial Resolution: Detail with respect to physiology Image from an fMRI scan Image from a PET scan Imaging Methods EEG Spatial Resolution Temporal Resolution Poor Good PET Excellent Poor fMRI Good Good TMS Good Good Subtraction Technique Used for fMRI studies Method similar to Donder’s study Compared two situations that included different cognitive processes Data = blood glucose level Relative measure Visual Stimulus (light flashing) Visual Stimulus (light flashing) Perception of the light Perception of the light Response DECISION Response Subtraction Technique Activation in control condition is subtracted from experimental condition to get activity due to stimulation in the experimental condition Effects of experience Experience-dependant plasticity Developmental environment can affect neuron specialization Kittens raised in environment with only vertical lines had more of their brain devoted to recognizing vertical lines in adulthood (and none devoted to horizontal) Learning happens through changes in connections and relationships between neurons, even in adulthood Greebles study (back to the FFA) Greebles Recall discussion of localization of function Fusiform Face Area (FFA) was an area in the Temporal lobe devoted to recognizing faces… or was it things we’re experts at recognizing??? Kanwisher has demonstrated, using fMRI, that the area does selectively respond to faces Gauthier and colleagues showed fMRI evidence for experience-based plasticity in this area (Greebles study) Greebles Study Step 1: measure brain activity in FFA when viewing Greebles Step 2: train people to recognize individual Greebles and families of Greebles Step 3: measure brain activity in FFA when viewing Greebles Analysis: compare activity in FFA before and after training Greebles Study Conclusions Plasticity of FFA FFA selects for things we’re experts about Faces are things we’re experts about