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Biopsych - Measuring Brain Activity (Y1) - Coggle Diagram
Biopsych - Measuring Brain Activity (Y1)
fMRI - Neuroimaging; Functional Magnetic Resonance Imaging)
These modes have revolutionised our ability to investigate brain anatomy and function - before the advent of neuroimaging information about localisation of function in the human brain have come from people with trauma or disease caused lesions
MRI is broadly applied to map the structure of the human brain - non-invasive and versatile
Group size - typically around 20-30 people
Neurosynth website to help locate brain areas based on meta analysis
Structural MRI - non-invasive technique of examining anatomy and pathology of the brain, and relies on different tissue types having different physical properties providing signals that can be translated into detailed maps of the brain
In an MR image, different contrasts are achieved using different pulse sequences and manipulating imaging parameters e.g. repetition time, TR; echo time, TE; pulse angle
MRI creates images of soft tissue in contrast to images provided by X-rays
Most human tissues have a large amount of water and they use these high quantities of hydrogen atoms in water to produce images of the human body
MRI is based on the magnetic excitation of a tissue and the recording of returning electromagnetic signals
How MRI scans are acquired -
Strong magnetic field applied across the part of the body being scanned, done by sliding the participant inside the MRI scanner that contains a strong magnetic field (measured in Tesla) of around 1.5, 3 or 7T (Earth's field is 0.0001T)
Protons of water molecules (hydrogen nuclei) have weak magnetic fields that react with the magnetic field of the MRI machine, and when placed inside the MRI magnet a portion of those fields align with the strong magnetic field
A short radio frequency pulse that acts as a small magnetic field is then applied that shifts the orientation of the aligned protons which causes the protons to spin or process in this new state, creating a change in the magnetic field which forms the basis of the MRI signal
When the radio frequency pulse is turned off the protons go back to their original position (they relax) and as they return to their original positions they emit energy picked up by the coils to produce an image
The scanner repeats this process of applying radio frequency pulses by sending radio waves to excite different parts of the brain - this can be a fast process, acquiring the whole brain in seconds (this is the repetition time)
Different image types are created by varying the time it takes protons to return to their original state (T1 relaxation time) and these are used to distinguish between different tissue types
Two types of image contrast -
T1 weighted image - grey matter is grey, white matter is white and CSF is black; quantify amounts of grey and white matter in the brain (cortical volume and thickness)
T2 weighted - grey is white, white is dark and CSF is white - lesion detection
Functional imaging - this has the purpose of measuring brain activity:
Blood flow and metabolism are tightly linked to neuronal activity in fMRI - as the brain is always physiologically active, as neurons would die if starved of oxygen for more than a few minutes, so if a brain function is too be mapped or a function studied, it is not enough to just place someone inside the scanner and observe which regions are receiving blood and oxygen as all neurons require this all the time
As a result, when functional imaging researchers refer to region a region being active, which means that the physiological response in one task is greater relative to some other condition - functional imaging studies must include a baseline condition to show elevated activity in the brain area to identify if it is involved in the task
Allowed us to learn how each brain area functions, including centres such as hearing and decision making
When a brain area becomes more active during a task, we know that the task at hand involves the brain area e.g. motor cortex in the left hemisphere when right hand is moved
Oxygen detection in an fMRI - when neurons become more active oxygen levels increase; the increased presence of oxyhaemoglobin as a result indicates a more active brain area and neurons
-> When neuronal activity increases, there is more oxygen delivery through increasing blood flow to that region in a process of denominated haemodynamic response - oxygen is transported by haemoglobin and when neurons consume oxygen and oxyhaemoglobin is transformed to deoxyhaemoglobin
Oxyhaemglobin has diamagnetic properties and so it causes little distortion to the magnetic field (protons spin slowly) whereas deoxyhaemglobin is paramagnetic causing magnetic field distortions as protons spin faster - these properties are used to provide an indication of oxygen consumption and therefore neural activity - this technique is known as BOLD (blood oxygen level dependent contrast) when the field becomes distorted more / quicker, neural activity has increased
BOLD signal
The way the BOLD signal evolves over time in response to an increase in neural activity which is called the haemodynamic response function (HRF) - In response to a stimuli, the BOLD signal increases as there is increased cerebral blood flow and cerebral blood volume, leading to an overcompensation of the amount of oxygen being extracted by neurons
HRF has three phases -
initial dip as neurons consume oxygen and there is a small rise in the amount of deoxyhaemglobin resulting in a reduction of the BOLD signal
Positive BOLD response / overcompensation - in response to increased consumption of oxygen there is increased cerebral blood flow and blood volume, with the increase of blood flow overcompensating for the amount of oxygen being consumed resulting in an increased BOLD signal; normally component that is measured
Undershoot - occurs after the end of the stimulus and might be explained by an accumulation of deoxyhaemglobin in the venous system while cerebral blood flow and oxxygenation extraction rate have already returned to baseline
After the presentation of a stimulus, the BOLD signal increase in activity and then plateau's about 5-8 seconds after, and will continue if neural activity is still present
Once neural activity stops, signal returns ot baseline 8-11 seconds afer
Uses radio frequency pulses to change the spin of the atom's proton / nuclei to create an image when they all line up in the magnetic field of the MRI scanner
Water detection - hydrogen 'spin' is redirected and realigned with the magnet of the machine, and the radio waves transmitted can be measured as a signal of the brain area being more active
It repeatedly measures how long the magnetic field remains synchronised and the longer is remains synced for shows increased activity
An MRI signal creates an image when all hydrogen atoms return to initial position, releasing energy
Considered an indirect measure of brain activity as it measures changes to blood oxygen and blood flow that follow neural activity
EVALUATION
Temporal resolution is in order of seconds, due to the nature of the haemodynamic response - this can be considered slow in comparison to cognitive processing speed
Spatial resolution - good, order of milimeters
fMRI does not measure the activity of neurons directly but rather measures a downstream consequence of neural activity (i.e. changes in blood flow / oxygen to meet metabolic needs) therefore making this an indirect measure of neural activity, in contrast to EEG that measures the electric fields generated by the activity of neurons
Limitations of single group studies - things that we cannot do in a scanner; information cannot be easily probed by fMRI; factors such as cost limit resresearchers
Experimental designs in fMRI:
To investigate brain activity during visual, motor or cognitive functioning tasks a paradigm is developed - a strategy to present the stimuli to subjects during an experiment, and these are usually divided into two major categories
Block design - during event onset, several stimuli of the same condition are consecutively presented and different conditions usually alternate in time, so relatively large signal changes are measured
A series of trials in one condition is presented during a discrete period of time, and the signal acquired during one block condition is then compared to other blocks of different task conditions
Three different blocks of stimulus presented
Event related design - interleaved short duration stimuli are used in the event onset; given the delayed nature of the BOLD signal, the data produced by different stimuli overlaps and thus extraction of the signal caused by each one of them becomes more difficult
individual trial events are measured, rather than collecting a block of measurement
(ABC ABC rather than AAA BBB)
Block designs are useful for high signal-to-noise information being obtained in neural processes that last a relatively long time
The advantage of event related designs is that they enable a much wider range of experimental designs and are more closely related to the typical design structure of most cognitive psychology experiments
Magnetic Resonance Spectroscopy: makes it possible to directly measure the concentration of specific molecules in localised brain regions - this technique is widely applied in clinical research, especially as a biomarker of altered brain metabolism e.g. consequence of brain tumours
More recently, its applications to neuroscience gained a new breath through measuring neurotransmitters such as GABA and glutamate
What do fMRI scans look like?
This is the result of a block design fMRI experiment showing the two different types of visual stimuli to participants
Top image - amplitude of the fMRI response; each coloured segment corresponds to one condition (rest is a baseline), white curves show BOLD responses for a selected group of voxels (volume pixels, smallest box shape) that show more activation of the magnocellular (green segments) than the parvocellular (red segments)
Middle image - specific brain slice with superimposed coloured blobs - activation map; created through statistical analysis, in this case a t-test between the two visual conditions (red and green)
Voxels responding more to the parvocellular than the magnocellular condition are coloured green to blue
Voxels responding more to the magnocellular condition are coloured red to yellow
A region of 10 voxels was selected from the activation map (white cross indicates the centre of the region) and the averaged signal time course plot corresponding to the selected region is showed in the bottom part
The horizontal axis represents time (in scans) and the vertical axis represents BOLD percentage signal change in relation to the baseline (rest condition) with green depicting the time course of the BOLD response for the magnocellular condition and red for the parvocellular condition
Group studies: detect regions that show significant increases in BOLD signal in response to a given task
Low sample sizes (20)
More liable to false positive (FP) and false negatives (FN)
There is a danger of interpreting results narrowly in the context of a limited set of studies (reverse inference)
The vmPFC is activated by processes related to self-reflection, but observing vmPFC activity does not necessarily imply that self reflection has occurred - to make claims about psychological process, one would have to know that only self-related tasks activate vmPFC activity (specificity); knowing this requires assessing the consistency of activation in all types of non self-related tasks
Difficult to do in one study
Replications of studies are rare
Meta-analysis - statistically combines and analyses quantitative studies
Large sample sizes (100-1000) - generalisability
Increases the reliability of findings and power of the statistical analysis
Provides comparisons across diverse tasks
Provides a basis for identifying consistency (but minor changes in experimental conditions may results in significant differences in brain activation)
Two general approaches to neuroimaging meta-analyses:
coordinate based - use the x,y,z coordinates of each peak location reported in the respective publication
Image based - use the full statistical images of the original studies
-> more powerful (account for intra study variance and random inter study variation)
EEG / ERPs
EEGs:
Pioneered by Hans Berger, who first recorded the electrical waves of the brain in 1920s
It is a non-invasive technique and finds its physiological basis in the electrical activity of neurons
How it works:
Neurotransmitters release from synapses and cause ion channels to open, resulting in an EPSP or an IPSP and this causes a depolarization of the postsynaptic neuronal membrane (or hyperpolarisation) generating a dipole with the action potential
Dipole - separation of positive and negative charges
A dipole itself corresponding to a single neuron is not detected by EEGs but when thousands of neurons with similar orientation have similar synaptic inputs, dipoles sum together and create voltage signals in the scalp which generates an EEG signal
EEG signals therefore reflect the voltages generated mainly by excitatory postsynaptic potentials from apical dendrites of pyramidal cells
Scalp voltages are measured with electrode caps
ISSUES - the given distribution of scalp voltages can be a result of a number of unique dipole arrangements and often two opposite orientations in two dipoles result in a cancelling out of the charge, a phenomenon made common because of folds in the cortex, making drawing conclusions about neuroanatomical regions difficult
ISSUE - two dimensional projection of three dimensional reality, meaning it is theoretically impossible to determine the location of the EEG generator based on scalp-recorded EEG generator information alone - inverse problem
EEG signals are recorded using electrodes placed on scalp with a cap and a conductive gel to improve signal; the voltage at each electrode is compared to a reference electrode and amplified to increase the signal to noise ratio, where it is sent to a computer, filtered and amplified
An EEG measure requires a comparison of voltages between two or more sites as it measures differences in potential - the reference site is typically a region that is likely not influenced by the investigated variable, commonly the mastoid bone behind the ears or the tip of the nose (nasal reference)
Electrodes are arranged in the cap in specific locations and often described with reference to the 10-20 international system
Labelled by location e.g. F=Frontal, C=Central, P=Parietal, T=Temporal and O=Occipital and by hemisphere (odd numbers for left and even for right)
EEG rhythms:
In an EEG recording we observe many waveforms that reflect cortical electrical activity - typically, it shows graph voltages on the vertical domain and time on the horizontal, providing a near real time display of ongoing cerebral activity - signal intensity for an EEG signal is typically small in the order of microvolts (mV)
Described by amplitude (intensity or size of the signal) and frequency (speed of the waves)
Often used as a diagnostic tool for sleep disorders and epilepsy
Rhythms that have certain frequencies associated with behaviour:
-> Delta - 3 Hz or below and is the dominant rhythm in infants up until the age of 1 and in stages 3 and 4 of sleep
-> Theta - 4-8 Hz - prominent during exploration, spatial navigation and is enhanced during working memory tasks
-> Alpha - 8-14 Hz - posterior regions of the head, and was the first rhythm to be identified and has a higher amplitude when the eyes are closed or during relaxation, and lower amplitude when the eyes are open
-> Beta - 15-30 Hz - electrophysiological studies of humans and monkeys associate this with preparation and inhibitory control of the motor system
-> Gamma - higher than 30 Hz - oscillations are a local phenomenon within each cortical area
Basic components needed: Brain signal, electrodes, amplifier and AC/DC converter
You measure voltage - amount of potential energy between neurons and thus the electrical activity of the brain
Also measure the linear sum of multiple brain oscillators (multiple neurons) - orchestra with one microphone
Superposition problem - voltage at an electrode is a weighted sym of all components actuvea the time
Discrete points in a time series is measured - not a continuous line, how close or far away the points are depends on frequency sampling
ERPs: same equipment as EEGs -
Small voltages generated in brain structures in response to specific events or stimuli, and are EEG changes that are time locked to sensory, motor and cognitive events
ERPs are generated by presenting stimuli of interest many times and averaging the response
This increase the signal-to-noise ratio because activity that is not related to the processing of the stimuli is averaged out
A series of particular responses to a stimulus can indicate the time course of various neural processes invoked in order to process the stimulus, understand it and decide an appropriate reaction
Researchers can compare the brain's responses to various types of stimuli or its activities as we perform certain tasks and draw conclusions about the different brain processes involved in each situation
Represented graphically by plotting time (miliseconds) on the x axis and electrode potential on the Y (mV) - graph consists of a series of positive and negative peaks, and the procedure is done for each electrode (or a selection of electrodes from the EEG cap) and each has a slightly different profile known as ERP components:
Defined by - polarity (pos or neg), latency (time or interval between presentation of stimulus and response), scalp distribution (ERPs presented below originate from different regions, denoted by the ovals that represent the scalp (nose forwards, lateral ears) and sensitivity to task manipulators
Early waves, or components peaking roughly in the first 100 milliseconds after stimulus are termed sensory or exogenous as they depend on the physical parameters of the stimulus
In contrast, ERPs generated in later parts reflect the manner in which the subject evaluates the stimulus and are known as cognitive or endogenous as they examine information processing (waveforms described according to latency and amplitude)
E.G. N100 would be a negative potential measured in the fronto-central region of the scalp approx 100 ms - associated with the unpredictability of a stimulus and this potential is weaker when stimuli are repetitive, and stronger when random
Evaluation -
Advantages;
EEGs and ERPs have high temporal resolution, which allow it to measure the precise moment at which electrical activity occurs in the brain
Non invasive technique and is a direct measure of neural activity (unlike the BOLD signal of an fMRI)
Equipment relatively inexpensive when compared to other techniques
Disadvantages;
Poor spatial resolution - signal received at the scalp is the sum of the electric field produced by a large neuron population; the spatial resolution of a single electrode is in the order of one centimeter of the cortex, containing hundreds of thousands of neurons, and so the EEG signal is not useful at pinpointing the exact source of an activity (although ERPs can better indicate specific phenomena)
tES and TMS
Transcranial Magnetic Stimulation (TMS): non-invasive brain stimulation technique:
Allow investigation of the role of certain brain structures or neuronal activity patterns for a given cognitive or motor function
The biggest advantage of these methods is that they offer unique, non-invasive methods for investigation causal brain-behaviour relationships
TMS offers direct and non invasive interaction with cortical processing - passes a brief and strong current through a stimulation coil, which induces a perpendicular magnetic field which penetrates the scalp without attenuation
It is based on Faraday's electromagnetic conduction principle
A change in the electric current in a wire, the stimulating coil, generates a magnetic field which can then induce a secondary electric current in the neurons below the stimulation site causing action potentials
The key features of this technique is that the TMS machine delivers a large current in a short period of time, and therefore can be used to induce a transient perturbation of activity in a relatively restricted area of the brain
Behavioural response elicited by a single TMS pulse depends on the exact cortical area stimulated
E.g. a twitch in the subject's muscles can be induced by a pulse in the primary motor cortex
Precisely localised pulses can lead to the movement of a singular finger, or one to the primary visual cortex can induce a sensation of seeing light, even when eyes are closed, an experience called phosphene
It is essentially a magnetic pulse that temporarily interrupts normal functioning to produce a measurable change in behaviour so we can observe the area that a function is localised too
Visual lesions or interference:
when applied at the right time during a task or sensory processing, TMS can transiently and reversibly disrupt behaviour - this can be thought of as introducing noise into or interfering with the neural processes
TMS of the human occipital cortex suppresses visual perception but only when applied 80-100 ms after presentation of the visual stimulus
Because the effect of a single pulse seems short lived, TMS can yield unique insights into the time that a brain area is essential for behaviour, or its causal chronometry
By introducing a pulse during a participant speaking which disrupts their language center, we can understand how to treat stroke victims by targeting processes that need to be interfered with to aid recovery (still able to sing due to it being on other side of the brain)
https://youtu.be/KEyYSPcdWUs
TMS has a number of advantages over studying humans with lesions caused by trauma or by disease - real brain damage may result in the reorganisation of the cognitive system, whereas the effects of TMS are brief and reversible
TMS can also be used to determine the timing of cognition - applying TMS pulses at different time intervals
Inducing plasticity like changes - TMS protocols that include multiple pulses are known as repetitive TMS - consist of structred patterns characterised by a number of pulses, the frequency in which they are given and the intensity of the stimulus
Some protocols of repetitive TMS can elicit residual effects that persist for some minutes, and these pulses that produce long lasting changes tend to emulate, in the stimulation region at least, patterns that trigger synaptic plasticity in the hippocampus
Suggests that repetitive TMS harnesses the neural processes responsible for triggering changes among synaptic connections in cortical networks
This can be very useful in therapeutic interventions, such as stroke or depression
Evaluation -
Disadvantages:
spatial resolution is poor (approx 1 cm) and cannot reach deep structures e.g. hippocampus and amygdala - effects of stimulation are limited to superficial cortical regions
It is important to remember that TMS does not record, it stimulates and outcome measures are obtained through combining it with other approaches, or by recording motor responses or accuracy in particular tasks
Advantages -
TMS has high temporal resolution but with limitation in repetitive application
TMS is less expensive than fMRI and is highly suited to answering causal questions about when cognitive processes work at a certain location
Transcranial Electrical Stimulation (tES)
This is a neuromodulatory technique in which low voltage constant or alternating currents are applied to the human brain via scalp electrodes - the basic idea of tES is that the application of weak currents can interact with neural processing, modify plasticity and entrain brain networks, and this can then modify behaviour
In contrast to TMS, tES does not cause action potentials, instead it is thought to cause changes in membrane potential, making neurons more likely to fire, meaning tES requires ongoing brain activity to cause effects
There are different modalities for tES
Transcranial direct current stimulation (tDCS) which uses a constant current (anode and catho - anode boosts, cathode inhibits - 2.0 mA)
Transcranial alternating current stimulation (tACS) which uses an alternating current
Some controversy around the effects of tES and its applications in cognition - we are unsure what it does on a physiological level, and the introduction of current changes neuron membranes, allowing them to fire more easily, and with enough stimulation and repeated exposure neurons will fire more
Unsure of long term effects - most studies are pilot studies, meta analyses and the methods are not consistent enough to compare; there is also some poor study designs
However, in effective study designs, tDCS might be an effective treatment for mood disorders, psychosis and dementia or help people recover from strokes
2016 metanalysis of 17 studies - stroke patients who did tDCS treatments rgained more motor function than patients in control groups who did fake treatments
A meta analysis of data from 6 randomised control trials with 289 patients showed that tDCS relieved symptoms of depression, even in people who felt no benefit from medication
Over time, this could help damaged or underperforming brain areas be nudged toward functioning in a healthier way all by themselves
2016 study with a cadaver - electrodes placed inside the skull showed that around 90% of the electricity applied through tDCS did not get through at all (this makes sense as live tissue is more conductive)
Evaluation of tES:
Advantages:
very cheap technique and this can be advantageous when using it in a clinical setting - it can also be used at home using portable devices, making it accessible as a treatment for people with mental illness
Can be used potentially to help depression and stroke victims
Disadvantages:
Does not record brain activity and requires constant brain activity
tES has poorer spatial resolution than TMS and is also limited to cortical structures
Can be dangerous for epileptic individuals, as stimulation would increase brain activity and possibly cause seizures
Key concepts:
spatial resolution - refers to the smallest feature or measurement that a technique can detect - if SR is high, we can measure activity localised to particular regions of the brain
Temporal resolution - refers to the precision of a measurement with respect to time - when temporal resolution is low, there are less measurements for a given amount of time than when the temporal resolution is very high
Invasive v non-invasve - a procedure is described as invasive when it breaks the skin in some way
Causation v correlation - correlation indicates a relationship between two or more variables, but does not mean that a change in one causes a change in another
Causation indicates that one event is the result of the occurrence of the other event
