Semester 1 Flashcards
what does TMS do?
create virtual cortical “lesions” by depolarising neurons in a small, circumscribed area of cortex
what is the TMS “neural noise” approach?
uses single-pulse TMS to disrupt cognitive processing. if this interference has a functional impact this is a powerful demonstration of a certain brain region’s causal involvement in this function and also provides information about the timing of the brains reaction to the electronic current
what is the TMS “virtual lesion” approach?
using repetitive TMS to interrupt or enhance cognitive processing. It is also possible to inhibit cognitive functions for a longer period of time by applying repetitive TMS (rTMS)
It can then be measured whether (and for how long) a specific cognitive task is impaired (usually slowing instead of total loss of function)
what is the TMS “probing excitability” approach?
Instead of aiming at disrupting cognitive functions (and measuring the effect of TMS on performance), the measure of interest is how strongly the motor cortex “reacts” to the pulse itself. The excitability of the primary motor cortex can be measured by recording “motor evoked potentials” (MEPs) using the electromyogram (EMG), which is electrical activity of muscles
what is the TMS “paired pulse” approach?
Uses two pulses, delivered in brief succession – one is usually sub-threshold while the second one is supra-threshold. The question is how strongly the first pulse influences the effect of the second
what are a clinical applications of TMS?
treatment of depression on the basis that it is caused by hemispherical imbalance
what is the alpha level usually set at?
0.05
what is t equal to in a t-test?
t = M(sample mean)-m(pop mean)/standard error of the mean
between groups or independent measures tests are what? pros and cons
two groups, and the values come from different people (i.e. each person is providing one measure in one group only)
- pro: don’t have to worry about learning effects due to repeated exposure
- con: People in the different groups might be quite different in various ways (can be overcome with large groups or counterbalancing)
Repeated-measures tests are what? pros and cons
one group, and the values for both experimental conditions come from the same people (i.e. each person is providing two measures)
pro: we don’t have to think about differences in baseline, personality, IQ, motivation, etc., because this will always affect both conditions equally
con: order effects and effects of learning
single-sample test compare results to what?
the likelihood of chance results using the null hypothesis
standard error of the mean is equal to what?
standard deviation/square root of the sample size
the standard deviation is equal to?
the square root of the variance (sum of squared differences)
df equals?
n-1
When the critical value is the same as the empirical value is it statistically significant or not?
no, not significant
assumptions of a t-test
- The observations must be independent
- The populations from which the samples are drawn must be normal
- If comparing two populations (independent-measures t-test), the samples must have equal variances (if the variances are not homogenous, calculating the pooled variance becomes a problem)
Electroencephalography (EEG)
is a method of detecting neural activity by placing electrodes on the scalp. These electrodes pick up small fluctuations of electrical signals, originating from activity of (mostly cortical) neurons
pros and cons of EEG
pros: temporal resolution is great (timely reflection of brain activity)
cons: spatial resolution is not good (understanding of structures from output)
why is PET no longer used much?
involves large amounts of radiation
MRI studies the structure of what?
the brain
MRI functions based on the fact that 70% of the brain is made of what?
water
The axis along which the magnetization is build up in the scanner is called
the z-axis
MRI measures the what of protons
the relaxation of protons after the emission of a radio frequency
how does MRI image a brain?
The transversal magnetization of the protons decays with different speeds depending on the tissue so structural brain image depends on when signal is recorded during this process
what is the slice selecting axis?
the x-axis
what is the frequency encoding gradient axis?
the y-axis
how does fMRI measure brain activity?
Neural activity is also accompanied by a local oversupply in oxygenated blood and therefore a better Blood Oxygen-Level Dependent (BOLD) signal. These areas of enhanced activity can then be mapped onto a structural image of the brain. Significantly stronger activation in region X for task A compared to task B is interpreted as involvement of the region in task A
BOLD
- Is an indirect measure of brain activity
- Enhanced neural activity impacts on (in a complex way) how much oxygen is in the blood, how fast the blood flow is and how much oxygen can be extracted (i.e. when blood flow is faster, relatively less oxygen is extracted per unit time, but because there is more of it, in total more oxygen is extracted…)
- This means, we have to be careful when interpreting differences in BOLD signal
- There is also a substantial temporal lag between neural activity and the peak of the BOLD response – in the order of 8 seconds!
- The BOLD signal further needs ~16 seconds before reaching baseline again
- It is also not valid to compare signals between different regions of the brain because the signal change is different
- The measured response looks very similar across regions and is described by the Heamodynamic Response Function (HRF)
Limitations of BOLD fMRI:
- fMRI shows regions of activity but cant show the full networks involved in most brain processes - leading to a tip of the iceberg scenario where the functional units that actually matter aren’t mapped
- Poor temporal resolution due to the slow processing of the machine compared to the brain’s speed
- The spatial resolution is better than other methods but still not great (measured in voxels which are 3D units of 3x3x3mm and containing around 100,000 neurons)
- Need for multiple comparisons and use of t-tests that set a p-value less than .01. With 50,000 voxels in the brain, 500 false positives are expected (which a lot!). To overcome this the strictest correction is Bonferroni-correction: divide the significance level (e.g., 0.01) by the number of tests (= voxels), and use this new significance level for each test: 0.01 / 50,000 = p < .0000002 (corresponds to the overall risk of 1% to have one false positive)
- Circularity and double-dipping: use of regions of interest (ROIs) to avoid the multiple-comparison issue leads to measurement of areas twice - first to find the ROIs and then in more depth. This would be fine, but requires independent analyses which can be difficult because of the circular nature of the issue - in order to find ROIs they’re using a flawed test which has false positives so any subsequent results will also be flawed
- Overinterpretation of null results: we should always avoid concluding that brain regions are not involved in cognitive processes even if a null result suggests we fall back on the null hypothesis as we don’t know whether our method might just not be sensitive enough to detect small differences
Sohn et al. study with fMRI found that
Found that an area in the prefrontal cortex and the posterior parietal lobe were activated during task switching. Are these regions controlling the switching? All we have is a correlation not an understanding of the interactions or role
Kanwisheretal et al. study with fMRI found that
attempted to investigate how faces are represented in the brain. They presented their participants with images of faces and contrasted BOLD signals to when participants saw objects. An area in the fusiform gyrus responded more strongly to faces than to objects. To rule out that this result was simply due to using objects, they replicated the study with faces > scrambled faces and than again with houses and again with hands. The result was the same: the region in the fusiform gyrus showed again stronger activation for faces
Gauthier and co study with fMRI found that
the fusiform gyrus may actually just be involved with recognition expertise rather than specifically faces
Malach and co study with fMRI found that
argued that coding is driven by resolution needs – the FFA is good for everything that usually requires “high resolution” - We usually encounter faces in the centre of our vision, and not in the periphery. We usually need a high resolution to recognise faces, simply because we need to really see the details. The “module” for places/houses (the Parahippocampal place area, PPA) is in reality just very suited for processing the periphery, not houses per se – but that’s where places/houses usually are in our visual field
Poldrack and the Reverse Inference Problem:
- If a brain region is activated by many cognitive functions, we learn very little from observing activation in those areas
- We need to know how good th task actually is for understanding cognitive process X. If the tasks measures more than one cognitive function, we also don’t learn much
at rest the neuron has a _ charge
negative
an action potential is triggered when the charge becomes sufficiently _
positive
Ion channels
Ion channels act like a “gate” for ions which opens when a neurotransmitter binds to the receptor. These channels are often selective and only let some ions pass through
G-protein coupled receptors
work through secondary messengers i.e. when the neurotransmitter binds to the receptor it activates a second messenger system that can either open a channel or cause other things to change within the cell (e.g. DNA being transcribed and new proteins being made)
Neurotransmission
A chemical substance released from a neuron at an anatomically specialised junction (synapse), which diffuses across a narrow cleft to affect one or sometimes two postsynaptic neurons, a muscle cell, or another effector cell. Can be either inhibitory or excitatory
Neuromodulation
A chemical substance released from a neuron in the central nervous system, or in the periphery, that affects groups of neurons, or effector cells that have the appropriate receptors. It may not be released at synaptic sites, it often acts through second messengers and can produce long-lasting effects.
- Slower process that alter the subsequence responses of neurons
- Presynaptic: alters neurotransmitter release
- Postsynaptic: alters neurotransmitter action
- May cause changes in neural function or structure
Psychopharmacology
the study of drug induced changes in mood, sensation, thinking and behaviour
Cycle of Neurotransmitters (seven steps)
- Synthesis
- Release from synaptic vesicles
- Binds to receptors
- Positive or negative influence on the post synaptic neuron
- Broken down by enzymes
- Reuptake of transmitted
Formation and storage in synaptic vesicles
Non-Traditional Neurotransmitters include
- Peptides
- Gases
- Lipids
- Nucleosides
drugs can be either
agonists (increasing effects of neurotransmitters) or antagonists (decreasing effects of neurotransmitters)