Research Methods in Cognitive Neuroscience Flashcards
definitions
- Cognitive psychology: Understanding human cognition through observation of behaviour during performance on cognitive tasks.
- Cognitive neuroscience: Using behaviour and the brain to understand human cognition
- Cognitive neuropsychology: Studying brain-damaged patients to understand human cognition in general.
- Computational cognitive science: Using computational models (e.g. algorithms) to understand cognition.
Brain Anatomy
- Functional properties of different lobes of the brain- learn through research
- Cortex of the brain is not smooth- characterized by sulci and gyri
- Gyri- ridge like elevations which are surrounding by sulci
- Gyri are made up of grey matter
- Each lobe of the brain processes different information
- Cortex made of white matter- bundles of nerves and fibres which keep together the two hemispheres
Brain anatomy and functions
- Frontal Lobe > Executive functions; Planning; Problem solving;
- Temporal Lobe > Auditory functions; Language; Memory; Emotions;
- Occipital lobe > Visual perception; Object representation;
- Parietal Lobe > Attention; Sensation; Body position;
- Basal ganglia > Movement coordination;
- Brain stem > Physiological functions (e.g., digestion, breathing;
Cerebellum > Balance; Coordination; Voluntary movements;
functional areas in the brain
- Korbinian Brodmann
- 52 Brodmann areas based on their cytoarchitectonic (histological) characteristics
- Divided the brain into areas based on functions
- Can also split and organise the brain according to structure/characteristics of cells
- Brodmann- histological characteristic of brain regions in the cortex to subdivide the brain into different regions
Research later found that it also has some correspondence with function
Modern methods to studying the brain
- The electrophysiological brain (EEG, MEG, iEEG and ECoG)
- Study of electrical activity from neurons in different ways
- The imaged brain
- Structural neuroimaging (CT, MRI)
- Functional neuroimaging (fMRI, PET)
- The ‘lesioned’ brain
- Virtual lesions (TMS)- used in cognitive neuroscience to stimulate patients brains to stimulate a performance in a Cognitive task
Spatio-temporal trade off
- Spatial and temporal resolution of techniques
- (PS Churchland and TJ Sejnowski, 1988, American Association of the Advancement of Science)
- No best technique to study the brain
- Evaluation on invasiveness, spatial resolution (how specific it can be localised) and temporal resolution (how often we can record a signal)
Graph shows all of this evaluation for each technique
The electrophysiological brain
- Non-Invasive
- Electroencephalography (EEG)- captures electrical activity from postsynaptic potentials recorded from electrodes on the scalp
- Magnetoencephalography (MEG)- captures magnetic fields from electrical currents
- Somatic nervous system activity (muscles tension, eye movements)
- Autonomic nervous system activity (skin conductance, cardiovascular activity)
- Invasive
Invasive methods on humans (e.g., intracranial EEG)
Neuron structure and functioning
- Neurons communicate between each other via electro-chemical signals
- Neurons are brain cells- have a body (soma), nucleus, dendrites
- This is grey matter
- Also have another structure called an axon- long projections that originate from the soma that transmit the electrical signals
- Neural activity being measured depends on the effectiveness of communication between the different neurons
- Stable electrochemical gradient at rest (no communication or stimulus).
- Difference of positive (Na+) and negative (K-) charges between inside and outside environments
- Membrane potential: Vm = Vin – Vout = -70 µV
- Neuron B is in a stable environment at rest- characterised by a specific difference in concentration of ions between the intra and extra cellular space
- When the neuron receives a strong signal, the flow of Na+ increases into the neuron, and the one of K- outside the neuron.
- It modifies the general gradient, which affects membrane permeability and enables its depolarisation (i.e., becomes more positive).
- If reaches a threshold (-55mV), it causes a rapid upward spike in voltage.
- The positive spike propagates along the axon and is called the “Action potential”.
- Action potential is generated at the level of the soma and propagates along the axon, increasing in speed due to the myelin sheath
- Reaches the axon terminal and then is released
- Way in which neurons communicate with each other
- Neurons behave like electric dipoles
- This creates an electric field along the dipole, which allows conducting the current.
This in turn creates a magnetic field around the dipole.
Electroencephalography (EEG)
- Modern systems using soft caps
EEG measures the summation of electr(ochem)ical activity on the scalp over time by means of recording-electrodes attached to the surface of the scalp.
EEG waves
- EEG provides a useful overview of the electrical activity of the brain.
- ## EEG is often used in the clinical diagnosis of brain damage and neurological disorders, e.g. epilepsy
How EEG works
- EEG measures the post-synaptic activity (postsynaptic potentials, PSPS) around the dendrites of pyramidal neurons in the cerebral cortex
- An EEG electrode sums the electrical potentials occurring from many thousands of brain cells
Neuron structure and functioning- EEG
- Pre Synaptic Potential (PESP):
- Action potential
- Excitatory only
- Fast
- Post Synaptic Potential (POSP):
- Excitatory (Depolarising) or Inhibitory (Hyperpolarising), depending on the neurotransmitter received from Neuron A
- Slow and long lasting
- Enable if sufficient inputs towards Neuron B.
- EEG capture POSP, not PESP
- Axons from neighbouring neurons synapse with the pyramidal neurons, triggering a local depolarisation: POSTSYNAPTIC POTENTIALS (PSPs)
Pyramidal cells in the human brain cortex
- Pyramidal cells are distributed and spatially aligned in the most superficial layers of the human brain cortex.
- Pyramidal cells in layers 3, 4, 5 and 6 are the generators of EEG signals
- Layers 1, 2 contain the dendrites of the pyramid cells -
- the EEG sources are the slow post-synaptic signals generated in these layers
- Their activity is synchronous; this produces a large signals that can be detected from the scalp
EEG positioning
- The internationally standardized 10-20 system
- Electrodes on the EEG cap are positioned in very specific locations.
- Reference electrode- located around the brain which do not record activity but still record noise and other sorts of electrical input from the environment
- Sort of a baseline for analysis
Tip of the nose, behind the ears
EEG- differential amplifier
- EEG recordings are translated into line tracings – i.e. brain waves
EEG signal is always a RELATIVE measure (i.e. a difference in potential between one electrode and another electrode used as reference)
EEG and Event related potentials (ERPs)
- Response measured in relation to a specific event (e.g. sensory cognitive or motor stimulus/ task).
- Fundamental element for a clean and reliable ERP response is averaging signals from many trials and many participants.
- Lots of trials conducted
Trade off- lots of trials might induce boredom and reduce attention in ppts - ERPs provide excellent temporal resolution.
- Great tool to study fast cortical processes (e.g. vision, attention)
- ERP waveforms are characterised by several components
- Critical elements of ERP components:
- Amplitude (and difference amplitude between conditions)
- Latency (and difference latency between conditions)
Scalp distribution
Other psychophysiological techniques
- Electromyography (EMG)
- Detects the electrical potential of the muscle cell (membrane potential is ~-90mV)
- Electrooculography (EOG)
- Electrodes are placed near the eyes and record the membrane potential of the retina
- Skin Conductance Response (SCR)
- Change in the electrical properties of the skin, associated mainly with sweat gland activity.
- Elicited by stimuli that cause (emotional) arousal.
- Electrocardiography (ECG)
- Measure of heartbeat.
Average heart rate of healthy adult is 70 beats per minute.
- Measure of heartbeat.
Magnetoencephalography (MEG)
- Detects the electromagnetic field generated by the neurons’ electrical activity
- Superconducting quantum interference device(SQUID sensors) allow recording of the small neuromagnetic signals generated in the brain
What can we see with MEG? Almost all of the cortex, fissural activity emphasized
differences between EEG and MEG
- EEG:
- Signal affected by skull, meninges…
- Detects both tangential dipoles (at sulci) and radial dipoles (at gyri)
- High temporal resolution (ms)
- Poor spatial resolution
- “Cheap” and widely available
- MEG:
- Signal unaffected by skull, meninges…
- Detects only tangential dipoles
- High temporal resolution (ms)
- Good spatial resolution (combined with MRI)
Very expensive and limited availability
structural imaging vs functional imaging
- Structural imaging: To explore brain structure and changes in it (e.g. Contrast X-ray Computed Tomography, Magnetic Resonance Imaging)
- Different types of tissues (skull, gray matter, white matter, cerebrospinal fluid) have different physical properties, which are exploited to create static maps of the physical structure of the brain
- Functional imaging: To explore brain cognitive (dis-)functioning (e.g. functional Magnetic Resonance Imaging, Positron Emission Tomography
Neural activity produces physiological changes on site, which are used to create dynamic maps of the moment-to-moment activity of the brain.
Magnetic Resonance Imaging (MRI)
- What tissues’ property is exploited?
- The amount of water (H2O <-> H+ OH-) in each tissue
- How does it work?
- Different tissues contain different amount of water. The single protons (H+) in the water have magnetic fields, which are randomly oriented.
- When a strong magnetic field is applied from the scan (constantly), the magnetic fields of protons align with it.
- At this point a brief radio-wave pulse is applied, and the orientation of the protons is knocked by 90 degrees.
- As the protons spin (precess) in the new state, they produce the detectable signal.
- The protons are eventually pulled back to the original state of alignment with the magnetic field (relaxation)
Evaluation of MRI
- NOT Invasive: no emission of ionizing radiation. Completely safe!
- Expensive.
- Variable scanning time, ranging from ~15 minutes up to 2 hours. High sensitivity to movements. Very high risk of claustrophobic symptoms (anxiety).
- High spatial resolution. It distinguishes between gray and white matter and discerns between the folds of individual gyri.
- Highly detailed and can detect very small alterations in soft tissue.
Best for detecting brain tumours, causes of dementia or neurological diseases.
Functional imaging reflects energy metabolism
- Neural Activity requires a lot of energy, which needs to be metabolised.
- Energy metabolism: Rate at which neurons produce and consume ATP (Adenosine triphosphate)
ATP production in neurons requires GLUCOSE and OXYGEN uptake from the blood.
Positron Emission tomography (PET)
- A carrier molecule (e.g. deoxyglucose) is combined with a radioisotope (e.g. 18-Fluoro[F]), which is an unstable radionuclide
- This tracer is injected in the blood stream and can be absorbed by tissues (≠ molecules: ≠ tissues).
- Unstable radionuclide?
- It is a radioactive atom, also called RADIO- ISOTOPE.
- Unstable ratio neutron/proton in the nucleus (too many neutrons or too many protons).
- RADIOACTIVE DECAY: spontaneously going back to stable state by throwing particles in the space = releases positrons\
- Once the tracer is inside the cell, it undergoes radioactive decays (releases positrons).
- Each positron interacts with an electron in the surrounding cell milieu. This interaction (= collision) causes annihilation of both particles, releasing two photons that speed off in opposite directions
Produces gamma rays detectable by the PET machine.
Functional MRI
- Oxygen consumption during high metabolic activity.
- When neurons are very active, there is an increase in oxygenated blood supply.
- The active areas take up more oxygenated hemoglobin than they need for their energy requirements > blood rush.
- There is then greater proportion of oxygenated hemoglobin to deoxygenated hemoglobin in active areas.
- fMRI detects differences in magnetic properties between oxygenated and deoxygenated blood, which alters the relaxation time of H+ protons.
- Deoxyhemoglobin normally creates magnetic inhomogeneity (this alters the relaxation time of nearby H+ protons).
In active areas there is more oxygenated hemoglobin which restore a more homogenous magnetic field. This results in a longer T2 relaxation time and brighter signal in active areas.
PET vs fMRI
- PET
- Invasive: involves radioactive tracers (although radiation passes quickly out of the body)
- Expensive
- Very low temporal resolution = 30 sec.
- Good spatial resolution = 10mm
- Only block design (cognitive subtraction: experimental vs baseline conditions, subtraction of activated areas)
- Sensitive to whole brain
- Not affected by movements (good for producing over speech)
- fMRI
- NOT Invasive: Completely safe!
- Expensive
- Better temporal resolution = 1-2 sec (still very low compared to EEG/EMG)
- Better spatial resolution = 1mm
- Both block design and event-related design
- Some brain regions are hard to image due to different magnetic properties (e.g. near sinuses, oral cavity)
Very sensitive to movements
EEG vs MRI
- EEG
- Not invasive: Completely safe!
- “Cheap” – widely used
- High temporal resolution (ms)
- Low Spatial Resolution (only surface of brain)
- Signal affected by skull, meninges…
- event-related design, continuous recording, frequency measure,
- Very sensitive to movements
- fMRI
- NOT Invasive: Completely safe!
- Expensive
- Better temporal resolution = 1-2 sec (still very low compared to EEG/EMG)
- Better spatial resolution = 1mm
- Both block design and event-related design
- Some brain regions are hard to image due to different magnetic properties (e.g. near sinuses, oral cavity)
Very sensitive to movements
The issue of causality
- Neuroimaging methods tell us what areas are involved in a process = correlation; NOT what areas are necessary for that process = causation.
- > Correlation is NOT Causation
- Lesion studies or Transcranial Magnetic Resonance (TMS) can be more appropriate to test for causation
Transcranial magnetic stimulation (TMS)
- TMS disrupts activity in a brain area by creating a magnetic field under a coil, placed over the targeted area.
- A large electrical field is passed through the coil, and generates magnetic pulse that passes through the skull.
- Magnetic fields induce electrical activity in the target area, and affect its normal function.
- > TMS simulates a short-lasting “virtual lesion” in a target region to investigate its role in a particular mechanism.
TMS evaluation
- Advantages over Lesions:
- In real damaged brain, reorganization and compensatory strategies may have developed.
- No problem of too difficult tasks for patients
- It recruits general population (often the number of patients with a similar lesion is low)
- Advantages over Neuroimaging:
- Does not investigate mere correlations, but causal relations.
- Allows investigation on timing of cognitive functions,
- Allows investigation on the necessity of a brain area
Allows investigation on dynamic connectivity between areas