Block 7: cognitive neuroscience Flashcards
Explain what BOLD signals are and how they are interpreted.
When groups of neurons in the brain become activated, neurovascular coupling is required to nourish them (increased blood flow to the region for oxygen). Oxygenated blood has different magnetic properties than de-oxygenated blood (Hb has magnetic properties, but Hb02 does not). This means that as the neurons consume the oxygen from Hb, the MR signal increases (known as the BOLD signal- blood-oxygen level dependent imaging). The BOLD response peaks after 5-6 seconds following an activity. Due to the slight delay, fMRI experiments are all based on a particular experimental design where stimulus is applied or task instructed for 10 seconds followed by 10 seconds rest.
The BOLD signal is quantifiable, meaning that it yields descriptive statistics which can be analysed. Activity of voxels (3D pixels) are matched with expected time-course activation- those which match are given a high activation score (positive), those with no correlation have no score, and those which show the opposite (deactivation) are given a negative score.
Explain what EEG and MEG measure.
Brain waves come from dipoles which are formed when action potentials are fired from pyramidal neurons. When the neuron fires, one side (axon terminal side) becomes more positively charged relative to the other side (soma), creating a dipole with measurable strength and direction of current flow. Since these neurons follow the folding of the brain, so do the dipoles. Radial dipoles are those which are perpendicular to the scalp surface; tangential dipoles are parallel to the surface of the scalp. EEG records both radial and tangential dipoles; MEG are only sensitive to tangential ones. As a result, depending on the brain region in question, you might better record EEG or MEG only, or use a combination of both. For instance, V1 is extremely folded, and you might miss deep radial dipoles if you only use MEG.
The electrical fields generated by individual pyramidal neurons are negligible, and must cross many layers (meninges, skull, scalp) before being received by the electrode. Therefore, electrodes are picking up signals from many millions of neurons at a time, and the amplitude of these signals depends on the number of pyramidal cells activated, as well as how synchronised their actions are (simultaneous excitation will have complementary oscillations (in phase); whereas unsynchronised excitation will generate irregular rhythm and will not be detected as much).
What are brain oscillations? How can they be categorised?
The brain exhibits oscillations between 0.05Hz and 500Hz- types of brain oscillation can be categorised based on their frequency:
1) Delta oscillations (0.05-4Hz) are slow, large amplitude brain waves which are hallmarks of deep sleep.
2) Theta (4-7Hz) are associated with both sleeping and waking states.
3) Alpha (8-13Hz) are largest over the occipital cortex, and are specific to relaxed waking state (also associated with motor and somatosensory cortex where it is known as mu rhythm).
4) Beta (14-30Hz) and Gamma (30-90Hz) are specific to the activated/attentive brain states.
Alpha, beta, and gamma signals are typically of wakefulness. Theta signals are typically of stage I sleep; K-complex signals (sleep spindles) are typical of stage II sleep; delta signals are typical of stage III and IV of sleep. Generalised absence seizures generate synchronised delta oscillations over the entire cortex for short bursts of time.
What are the mechanisms by which neurons produce synchronous rythms?
There are two mechanisms by which neurons produce synchronous rhythms:
1) Pacemakers exist in the brain, like the conductor of an orchestra, where some neurons are responsible for mediating the rhythm at which other neurons fire. The thalamus is the conductor in the brain, giving intrinsic properties to neurons in the cortex to sustain rhythmic patterns of discharge. This is governed by a fine interplay of excitatory and inhibitory connections within the thalamus. This is sent to the cortex by the thalamo-cortical projections. Few cells in the thalamus can quickly spread signals throughout the whole brain (for instance, in sleep- when delta generating nuclei project to the cortex, synchronising all neurons in the brain in delta waves.
2) The second mechanism by which groups of neurons could generate synchronised rhythms is through collective behaviour (cortical cells progressively become synchronised through cooperative interaction via excitatory and inhibitory interconnections). The pattern can either remain localised or can spread over several cortical regions. Specific frequencies of oscillations are responsible for local excitation, whereas in order to spread to other networks, different frequencies are required to synchronise different brain regions. This is known as cross-frequency coupling.
Explain the processes of information gating, communication through coherence, and multiplexing.
The roles of brain rhythms are not totally understood, however, the functions that we have identified include gating of information, communications through coherence (information transfer), coding of different components of a piece of information, and feature binding.
Not all neurons discharge continuously, they do it in bursts. The local field potential is an oscillation within which the neurons fire at certain phases. Certain oscillation parameters are important in the coding of information (information gating)- they are characterised by amplitude, phase, and frequency. Oscillations are therefore rhythmic modulations of local neuronal excitability (phases of low and subsequently high excitabilities). During the phases of high excitability, neurons will fire in bursts, transmitting information (this may be responsible for preventing the brain from being saturated with information). The shape of alpha oscillations shapes our visual perception- we do not continuously see, we see at a frame rate of 10Hz, sampling snapshots.
Neuronal populations may be coupled or decoupled, depending on whether their high excitability phases are aligned. Communication is facilitated when two populations are aligned in their synchrony. Transmission of information will be reduced when neuronal populations are asynchronised. EEG rhythms can be used to measure coherence between different brain rhythms, and thus can be used to determine an index of functional cortico-cortical coupling (whether two brain regions are interchanging information).
Oscillations tend to have very rapid oscillations embedded within them. It has been suggested that each of these two frequencies codes simultaneously different aspects of a piece of information (known as multiplexing). For instance, it has been found that visual perception of blurred shapes of faces alone is carried by low spatial frequency (LSF) oscillations (theta waves) in the parietal lobe, whereas the outline details alone (mouth, nose, eyes, etc) is carried by high spatial frequency (HSF) oscillations (beta waves).
Explain how EEG and MEG data are interpreted.
EEG/MEG signals recorded during task can be analysed as the evoked response to external stimuli (time-locked to stimulus presentation). Analysing evoked responses in the time domain is known as waveform analysis; in the frequency domain is known as spectral analysis. Application of stimulus can be added to the raw EEG signal, and this can be time-locked such that when the experiment is repeated many times, an average response can be determined. Repeating an experiment only a few times will result in a lot of noise in the signal, so these must usually be repeated many times for a clean result. Once a clean result is produced, this can be graphed as evoked potential or event-related potential (EP/ERP) against time after stimulus presentation. This response is labelled into different components. This is the average EEG response evoked by the stimulus (P=positive, N=negative, number symbolises the latency). Each component is an indicator of a particular processing stage (each code for a different component of the coding).
If you have two experimental condition, you can calculate the evoked response for each condition (for example, picture of a face compared to picture of a house). Differences between the timing or amplitude of different components of the evoked response can be compared between the different experimental conditions. This is for each individual electrode, but when you have many electrodes, you can do waveform analysis of all of them, and can compare between two experimental conditions using a paired T-test. If you have more than two you can use ANOVA with condition, time, and electrode position as factors. You are measuring differences in time windows of differences between conditions. Post-hoc T-tests can be used to test which specific electrodes were differently activated between different conditions.
Spatio-temporal analysis of ERPs involves superimposing all ERP waveforms. Digitising the position of electrodes in 3D space can be used to reconstruct the electrical field measured at the surface every millisecond (topographical map). The electrical field produced by the neurons over time stays stable over time before gradually changing configuration- this can be visualised with many topographical maps over time during the waveform.
Pattern-cluster analysis can be used generalise subsequent states during the ERP (categorise them into main dominant electrical field configurations. After identifying this, mathematical reconstruction/modelling can be used to determine the source of the electrical fields.
The other form of ERP analysis is spectral analysis, which looks at the frequency domain. This analysis begins the same way, with many repetitions which are averaged into an ERP. However, this time, the frequency of oscillations is plotted against time following the stimulus onset (time-locked to zero- stimulus). The second stimulus includes an illusory contour (illusion of a triangle due to the shapes), and so evoked a specific gamma frequency response.
What is transcranial magnetic stimulation? How can it be performed and to what end?
Transcranial magnetic stimulation (TMS) is a non-invasive method of brain stimulation which can produce a disruptive effect or a productive effect. A stimulator linked to a magnetic coil is placed on the scalp, and sends magnetic field pulses. Each pulse transiently disrupts brain functions in healthy participants under controlled conditions. TMS is also known as the virtual lesion approach.
TMS can be presented in different modes or protocols. For instance, a single pulse in principle will interfere with brain function through induction of electrical activity in a functional network (transient virtual lesion). Single-pulse TMS can be used to measure cortical excitability, has resolution of millisecond range, and can allow chronometry and mapping of brain functions. Alternatively, you can use repetitive TMS (rTMS- train of single pulses over a certain duration), the principle of which to interfere with brain function through cumulative effects which last beyond rTMS administration. Behaviour post-stimulation can be compared to behaviour pre-stimulation, looking for changes in cortical excitability. This protocol can be used for mapping of brain functions and plasticity (brain recovery after network disturbance), but loses the temporal resolution (has minute to minute resolution rather than millisecond). You can also use two-coil TMS to look at the functional connectivity between two regions of interest (but they have to be a certain distance apart since the coils are large).
Multi-modal neuroimaging (combining imaging techniques) can be done for almost all techniques (fMRI/EEG; fMRI/TMS; EEG/MEG; TMS/EEG), with the exception of TMS/MEG, because the electromagnetic pulse would disrupt the MEG signal, destroying the sensitivity and preventing it from working.
