Topic 1: Psychophysiological RM Flashcards
Magnetic Resonance Imaging (MRI) - how does it work?
Structural MRI is used to examine brain anatomy and structure
–> measure of the magnetic resonance signal from hydrogen over areas of the brain
–> parts of the brain with MORE hydrogen = appear brighter than parts with less
–> contrasts between tissues depends on the density of hydrogen (good because body has lots of hydrogen)
–> Scanner is a strong magnetic field: protons align with the field
–> MRI technician then introduces a radiofrequency pulse that disrupts the proton (forces it 90/180 degrees against the field)
–> Pulse turns off - protons re-align with magnetic field, releasing electromagnetic energy
–> MRI detects this energy, differentiating between tissues based on the speech at which they release this energy
What are the +/-‘s of MRI
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High spatial resolution (detailed high-quality brain images)
Non-invasive (no radiation)
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No measure of brain activity
Expensive and time-consuming
Sensitive to movement
Claustrophobic and noisy
Limited temporal resolution (no info on when processes are occurring)
Blood Oxygenation Dependent (BOLD) fMRI - how does it work?
Haemoglobin is the molecule in the blood that carries oxygen to tissues including brain cells
- Oxygenated Haemoglobin: when oxygen is attached
- Deoxygenated Haemoglobin: when oxygen is not attached
–> Oxy-HB does NOT interfere much with the MR signal
–> Deoxy-HB REDUCES the MR signal because it distorts the magnetic field (its paramagnetic)
During Brain Activity: brain cells use more oxygen
- to support this: the body sends extra oxygenated blood to these areas
–> this increase is MORE than the amount being used by the cells
–> therefore: active cells have LESS DEOXYHB (since its being flushed out) - so have a stronger MR signal
the BOLD fMRI picks up on changes in the MR signal caused by differences in the level of de-oxyHB
Summary: more brain activity –> more oxygen-rich blood delivered –> less de-oxyHB –> stronger MR signal –> brighter BOLD signal
BOLD fMRI - what is the ‘reverse inference’ concept
in fMRI reverse inference: a type of reasoning used to INFER A MENTAL PROCESS from OBSERVED BRAIN ACTIVITY
–> if a specific brain area is known to be active when a certain psychological process happens,
–> then if we see that brain area active during an experiment we infer that the psychological process is happening
Is it always valid? Not necessarily - for this to be valid the brain area has to be selectively activated ONLY (or mostly) by that specific mental process
–> the more specific the brain region is to one psychological process, the stronger the reverse inference
What are the +/-‘s of BOLD fMRI
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It is the safest neuroimaging method compared to PET etc.
It has high spatial resolution
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Blood flow changes associated with neuronal activity are slow - hence it’s temporal resolution compared to EEG methods are INFERIOR
People with metal cannot be scanned
Scanner is very tight (claustrophobic)
Scanner is noisy - hard to use auditory stimuli
Problem of causality - it is difficult to be certain which areas are necessary for a given task or process (co-activations occur!!)
Transcranial Magnetic Stimulation (TMS) - what is the neurophysiology of TMS
A large current is briefly discharged into a coil of wire held on the subjects head
–> The current generates a rapidly increasing magnetic field around the coil - this field passes into the brain
–> In the cortex the magnetic field generates electric current through neurons’ membranes
What are the effects of TMS
Depending on the intensity and number of stimulation pulses - TMS can increase/reduce excitability (the ease in which neuronal activity is produced)
–> In the context of goal-directed behaviour - this almost inevitably results in disrupted neuronal activity patterns —-> impairing performance
–> Thus the effect is similar to a neurological lesion, only subtle and safe and reversible
–> Often referred to as a virtual lesion technique
What are the challenges of TMS
Associated with strong confounds (side effects) which are not easy to control
Pulse is associated with a loud sharp click - causes sensations on the scalp and often muscle twitches…can have effects on performance
To control for these effects: there is typically a control condition of stimulating over a brain area NOT involved in the given process
What are the +/-‘s of TMS
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Potential to determine causation in the link between brain and behaviour
Relatively high spatial resolution (much more anatomically selective than looking at direct damage)
Excellent TEMPORAL resolution due to brevity of TMS pulse - hence it is capable of chronometric inference
Reversible - can compare conditions within same group of subjects
- Effects limited to surface of brain (the cortex) - cannot safely stimulate subcortical regions
Electroencephalography (EEG) - what is it?
EEG is the change in voltage (electricity) recorded from sensors on the scalp
–> Electrodes on the scalp pick up neuron signals which are amplified and recorded
–> The data shows brain wave patterns
–> Because the skull and scalp blur the signals, EEG captures general activity from the cortical brain areas but not deep structures
What are Event Related Potentials (ERPs)
ERPs are segments of the EEG, time-locked to particular events stimuli
–> Averaging segments: different types of stimuli are separately averaged and then compared
–> eg. separating different conditions and then averaged together
–> average of multiple traces are time-locked to a particular stimulus
What are the +/-‘s of EEG/ERP
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EEG/ERP has HIGH TEMPORAL RESOLUTION - it can provided detailed temporal information about stimulus processing –> the time-course of a particular ERP peak + its scalp topography can be seen as a spatio-temporal signature of a certain process
- However, has limited spatial resolution (cannot localise activity with high precision) - because the brain and cerebrospinal fluid conduct electricity well - hence the activity could originate in multiple locations
Eye-Tracking - how does this work based on eye physiology
Eye-tracking is used to measure where and how long a person fixates on part of a visual scene
Fovea - part of the retina with the smallest receptive fields (hence, best acuity) and highest concentration of cone receptors (vision is SHARPEST)
–> Visual acuity (clarity and sharpness of vision) is highest in the fovea and drops off quickly in peripheral vision
–> Because of this we move our eyes (and thus our fovea) towards objects we want to see clearly
–> Eye-tracking essentially follows where the fovea is pointing
Cortical magnification: disproportionality lage area of the visual cortex dedicated to the centre of the visual field (corresponding in the fovea)
Resolution (visual acuity) FALLS OFF RAPIDLY along the retina as one departs from the fovea
Eye-Tracking: How are movements measured?
The most common method relies on:
- monitoring pupil position by emitting an infrared beam and detecting its reflection from the cornea - the reflection is weaker where the pupil is
- the sampling rate can be as high as 1000Hz allowing not only measures of fixations (their timing location) but also precise measures of saccades (path and velocity)
- we get a measurement of the gaze on the screen every MILLISECOND AND X/Y COORDINATES as to where the gaze is
- eye-trackers can be head-mounted (allowing for more head movement) and remote (less tiring for subjects, can be combined with EEG)
What are the +/-‘s of Eye Tracking
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Safely measures the position of the gaze with very high spatial and temporal resolution
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Gaze position provides a clear index of the main locus of spatial attention, but also can be allocated covertly without shifting the gaze - so one cannot interpret the absence of a gaze shift as evidence that the region has not be attended to
Challenging in individuals who have correct vision and must wear glasses
