Cognitive Neuroscience Toolbox PDF

The Cognitive Neuroscience Toolbox – How we measure Brain Function Lesion studies, Brain stimulation Single neuron recording (animals) EEG: Electroencephalography – ERPs: Event-Related Potentials MRI: Magnetic Resonance Imaging – fMRI: Fun...

The Cognitive Neuroscience Toolbox – How we measure Brain Function Lesion studies, Brain stimulation Single neuron recording (animals) EEG: Electroencephalography – ERPs: Event-Related Potentials MRI: Magnetic Resonance Imaging – fMRI: Functional Magnetic Resonance Imaging  Neuropsychology – Brain Lesions Explains normal brain function by examining what changes when part of the brain is damaged Stroke or brain injury in humans Induced lesions in animals (electrical/chemical) Assumption: Whatever changes in behaviour/cognition must rely on that part of the brain that is damaged  Single Neuron Recording Place a thin electrode into an animal’s brain (rat, cat, monkey) record action potentials “firing” from a single neuron Measure what that neuron encodes or detects – What causes it to fire? – What stimulus / action / thought (?) does it represent?  Visual Cortex: Single neuron recording Hubel and Wiesel, 1959 First recordings from visual cortex neurons in cats Nobel Prize in 1981 https://www.youtube.com/watch?v=8VdFf3egwfg  Neural encoding of actions in motor cortex Decoding movement plans/intentions from neural activity https://www.youtube.com/watch?v=Y6fug4pzU4Q Velliste et al. Nature 2008  Single Neuron Recording Best Localisation and Timing of brain function – Directly measuring action potentials from individual neurons Problems – Highly invasive electrodes directly into brain – Animals only (although some studies now measure single neuron firing in humans during brain surgery)  EEG – Electroencephalography Summed activity from action potentials of neurons in the cortex cause electrical activity change on the scalp (skin of the head) Measure voltage changes from electrodes placed on the scalp “Brainwaves”  EEG – Electroencephalography Waveforms vary with brain states: Brain activity in EEG shows constant - Sleep and Alertness oscillations (waves) Frequencies of oscillations change with alertness and sleep 2 Hz) a (8-1 Clinical Uses: Alph Detecting stages of sleep Monitoring for Epileptic seizures Epileptic seizure 3 Hz) (1- Delta ERPs – Event-Related Potentials Brain activity related to a specific event or stimulus Average together >100 trials of EEG in response to the stimulus Peaks represents different stages of processing of the stimulus Example: Auditory Event-Related Potentials Activity over time in different parts of the brain auditory pathway for processing sounds Clinical Use: Detecting deafness in babies  Face Processing – N170 ERPs can show precise time of information processing in the brain 100ms Viewing any stimuli peak of brain activity 100ms after seeing visual stimulus Early brain processing of general visual features (brightness, colours, edges) 170ms Viewing faces peak of brain activity 170ms Activity for faces after seeing face (170ms) located brain processing for face- over lateral recognition in visual cortex occipital regions  ERPs – Event-Related Potentials ERPs can show precise time of information processing in the brain Direct measure of electrical activity (neuron firing) in the brain Problems Difficult to accurately localise activity to specific brain areas Poor spatial resolution Measures electrical potential conducted across the scalp Hard to determine exactly where in the brain this activity comes from  Functional Brain Imaging Change in blood flow associated with neural activity PET: Positron Emission Tomography – 1980 to late 90’s – Uses radioactive substances injected into bloodstream – Used now to map neurotransmitters or receptors (radioactively labelled “tracers”) in the brain fMRI: Functional Magnetic Resonance Imaging – 1992 to current – Measures change in blood oxygen level  MRI versus fMRI Functional MRI (fMRI) MRI studies brain anatomy. studies brain function. Brain activation in frontal lobes  fMRI – Change in blood oxygen Functional MRI detects change in Blood Oxygen Level (BOLD signal) Active neurons use Oxygen Oxygen is carried in blood – delivered to active neurons Increased Brain Activity Change in Blood Oxygen Level = Change in Brain Activity Increased Blood Flow (more oxygen delivered) Increased fMRI signal  First fMRI ever (1992) Participants viewed checkerboard – Off for 60s – Flashing for 60s Visual Cortex – Off for 60s – Flashing for 60s Increased BOLD signal (é blood flow = é brain activity) in visual cortex when flashing Source: Kwong et al., 1992  fMRI – Functional Magnetic Resonance Imaging fMRI measures changes in blood oxygen level that accompany changes in brain activity Good localisation of brain activity Problems Indirect measure of brain activity BOLD signal from change in blood oxygen level Not precise timing of neural activity Cunnington & Windischberger, 7 Tesla MRI Very expensive !!  Brain measurement is NOT mind-reading Cannot infer what people are thinking or doing or feeling based on measurement of their brain activity !!! Reverse Inference = bad! Back to the Future, 1985 Good Experimental Design: Manipulate one factor – independent variable (eg. task or behaviour) and Measure the effect on the dependent variable (eg. brain activity). Cannot do reverse: Cannot look at brain activity (dependent variable) to determine what the independent variable was (i.e. task people were doing)  “Brain scans revealed that when men are shown pictures of scantily clad women, the region of the brain associated with tool use lights up. And in a ‘shocking’ finding, Fiske noted, some of the men studied showed no activity in the part of the brain that usually responds when a person ponders another's intentions. This means that these men see women ‘as sexually inviting, but they are not thinking about their minds,’ Fiske said.” “Tool area” in brain active when men look at women in bikinis Take home message: Does that mean men are thinking Brain scans are not mind-reading!! about tools? Or are there alternative Can’t tell what someone is thinking explanations? or feeling based on their brain scan BUT … The Future: MRI reading the contents of consciousness MRI “Brain Decoding” methods: Attempting to use MRI to reconstruct what people are seeing based on activity in their visual cortex Reconstructing images during dreaming? Excellent article on latest research at: http://www.nature.com/news/brain-decoding-reading-minds-1.13989  Jennifer Aniston Grandma How are these things related?  Brain Plasticity – How the brain changes with learning – Definition: The capability of the brain to alter its functional organisation as a result of experience. Neurogenesis and Synaptogenesis – Generation of new neurons and synapses (connections) Integration of signals – whole brain Neuron receives many, many inputs – has only one output – What combination of inputs will cause this neuron to “fire” and pass on it’s signal? Brain is enormous “integrator” of information – adapts with learning (billions of neurons with millions of billions of connections) When sum of all inputs is high enough, triggers output (cell “fires”) Integration of information in the brain Imagine this neuron represents memory of your grandmother When this neuron “fires” you consciously recall your grandmother What information does this neuron need to receive to “fire” and give conscious recall of your grandmother?  Grandmother cells “Grandmother Cells” – all theoretical – Neurons could “represent” (encode or “fire” to) a specific concept, such as your grandmother (Jerry Lettvin, 1969) – Billions of neurons can encode billions of concepts – Memory may be represented by groups of neurons each encoding specific concepts or objects “Jennifer Aniston cells” found in Hippocampus – Recording from neurons in hippocampus in epilepsy patients – Fire specifically to pictures of Jennifer Aniston Quiroga et al, 2005 Nature Halle Berry cell  It’s all about connections Spreading Activation Model - Theory – Neurons represent a specific concept (eg. Grandmother cells) – Share connections with neurons that represent related concepts – Eg. Fire-engine èRed, Truck, Fire, Siren – Activation (firing) of one neuron leads to spreading activation to related or connected neurons (concepts) Learning and Memory – Making and strengthening connections between neurons that represent associated concepts Grandma Jennifer Aniston  Ramón y Cajal Studied growth of neurons and axons during brain development Neurons do not regenerate “In the adult centers the nerve paths are something fixed, ended and immutable. Everything must die, nothing may be regenerated. It is for the science of the future to change, if possible, this harsh decree.” - Cajal was mostly right, but not entirely … Ramón y Cajal, 1852-1934 Nobel Prize 1906 Neurogenesis – growing new brain cells Neurons never regenerate or repair: damaged brain areas never “re-grow” BUT New neurons constantly “born” throughout life from Neural stem cells Only two areas in adult brain: – Hippocampus (learning and memory) – Subventricular zone for olfactory bulb Synapses – Forming and Strengthening connections Synaptogenesis – Definition: Generation of new synapses: brain connections New synapses are constantly formed and strengthened with experience and learning  Donald Hebb – Neuroscience of learning Pet rats are smarter than laboratory rats 1940’s Synaptogenesis – Environmental Enrichment “Enriched” conditions lead to growth of dendrites and more extensive synaptic connections  Learning and Memory – Strengthening Synapse Connections Long-Term Potentiation (LTP) – Change in the structure of synapses to give stronger signal from pre-synaptic to post-synaptic neuron – Many mechanisms Eg. More post-synaptic receptors – focus of cellular/molecular neuroscience research on memory and learning Graded Potentials Excitatory and Inhibitory inputs (via dendrites) sum together – Change membrane potential at axon hillock – Graded Potentials Graded Potential at axon hillock depends on strength of synapse connection (on dendrite) – Strong stimulus causes large change in membrane potential – Weak stimulus causes small change Hebb’s Law – “Hebbian Learning” “Neurons that fire together wire together” “When an axon of cell A is near enough to excite cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased.” Donald Hebb, 1949. “Hebbian Learning” Repeated firing of pre-synaptic and post-synaptic neuron “firing together” strengthens synaptic connection Brain “learns” associations through repeated pairings Strengthens connections between paired stimuli or events Basis of Neuroplasticity Learning: Brain pathways (connections) that are used often are strengthened  “Homunculus” Sensory Motor Primary Sensory cortex and Primary Motor cortex Size of area on cortex determines sensitivity or fine motor control Neuroplasticity: Brain re-organisation with experience Studied sensory cortex in musicians who play stringed instruments Measured activity for index and little finger sensation for left and right hand String players had larger area on primary sensory cortex for left hand fingers than non string players Sensory cortex finger areas expand with use and experience Elbert et al., 1995, Science  Neuroplasticity: Brain re-organisation after damage Lesioned motor cortex in monkeys and used electrical stimulation to map hand area With no rehabilitation: (no movement) motor cortex area for hand got smaller: maladaptive plasticity With rehabilitation: (movement training) motor cortex area for hand expanded and movement improved. After damage, motor cortex can re-organise with use to Nudo et al., 1996, Science recover function. Neuroplasticity: Brain re-organisation in blind people Studied brain activity in blind people while reading Braille Brain activity in visual cortex while reading Braille Visual cortex changed only in blind participants. Sighted participants did not show visual cortex activity when reading Braille Brain areas lacking their normal input can take new Sadato et al., 1996, Nature functions with use.