Global brain activity Flashcards
What are alpha waves
- Alpha waves are neural oscillations in the frequency range of 7.5-12.5 Hz
- They arise from synchronous and coherent(in phase or constructive) electrical activity of thalamic pacemaker cells
- AKA berger’s wave
Where do alpha waves normally originate from
- Predominantly originate from the occipital lobe during wakeful relaxation with closed eyes
When are alpha waves reduced
- Are reduced with open eyes, drowsiness and sleep
how is an EEG signal detected
- It takes many thousands of underlying neurons, activated together, in sync, to generate an EEG signal big enough to see
What are the basic requirements for EEG signal detection
- A whole population of neurons must be active in synchrony to generate a large enough electrical field at the level of the scalp
- This population of neurons must be aligned in a parallel orientation so that they summate rather than cancel out
What are the four basic EEG phenomena
- The alpha rhythm, 8-13Hz, awake subjects with eyes closed
- The beta rhythm, 14-60Hz, indicates mental activity and attention, eyes open
- Theta waves, 4-7Hz, indicates drowsiness and sleep or a pathological condition
- Delta waves, <4Hz, as in 3
Where do the theta rhythms come from?
- Come from the hippocampus
What is a K complex and what is it caused by
- K complex is an EEG waveform that occurs during stage 2 of NREM sleep
- Caused by calcium spikes
What allows thalamic cells to generate rhythmic, self-sustaining discharge patterns, even in the absence of external inputs
- Thalamic cells have a set of voltage-gated ion channels that allow each cell to generate rhythmic, self-sustaining discharge patterns, even in the absence of external inputs
- The rhythmic activity of each thalamic pacemaker neuron then becomes synchronised with many other thalamic cells
What causes the bursts of action potentials in a neuron during stage II sleep
- Bursts of action potentials are evoked when the neuron is hyperpolarised enough to activate low threshold Ca2+ channels
- These bursts account for the spindle activity of the EEG recording
What causes the oscillatory activity to change into a tonically active mode
- depolarisation of the cell(by injecting current or stimulating the cholinergic reticular activation system) transforms the oscillatory activity into a tonically active mode
What re the functions of brain rhythms
- Sensory input –> thalamus –> cortex
- Activity coordination(binding) of different cortical regions(synchrony oscillations)
- Meaningless by product of feedback circuits and connections
- EEG rhythms - window of the functional states of the brain
What are the three functional states of sleep
- Awake
- Non-REM
- REM
What is the type of modulation during the waking state
- Activation is high
- Modulation aminergic(NE locus coeruleus), 5-HT(raphe nuclei), and the information source is external
- Histamine-containing neurons in the tuberomammilary nucleus(tmn) of the hypothalamus are also involved(drowsiness is a side effect of an antihistamines which suppress the TMN network)
Type of modulation during REM sleep
- Activation is also high
- Modulation is cholinergic
- Information source is internal
What are the functions of sleep and dreaming
- Conservation of metabolic energy
- Cognition
- Thermoregulation
- Neural maturation and mental health
What are two examples of structural imaging
- CT
- MRI
What are two examples of functional imaging
- PET
- fMRI
How do CT scans function
- Based on the amount of x-ray absorption in different types of tissue
- Bone absorbs the most(the skull appears white), CSF absorbs the least(the ventricles appear black) and the brain matter is intermediate(grey)
- Used in clinical settings eg to diagnose tumours or identify haemorrhaging or other gross brain anomalies
How does an MRI work
- Completely safe, so people can be scanned many times
- Provides a much better spatial resolution
- Provides better discrimination between white and grey matter
- Can be adapted for detecting the changes in blood oxygenation associated with neural activity(fMRI)
What is the sequence of events for the acquisition of an MRI scan
- Magnetic fields of protons initially random
- Add external magnetic field and some protons align
- Brief radio wave pulse orients them 90 degrees and produces a measurable MRI signal
- The protons return back(relax) and a new brain slice is scanned
What does a PET scan measure
- Measures change of blood flow to a region
- Based on blood volume
- Sensitive to the whole brain
- Involves radioactivity(signal depends on radioactive tracer)
- Participants scanned once or few times
What does an fMRI measure
- Is sensitive to the concentration of oxygen in the blood
- No radioactivity(signal depends on deoxyhaemoglobin levels)
- Participants scanned many times
- Some brain regions(eg near sinuses) are hard to image
What is the BOLD signal
BOLD signal - blood oxygen level dependent contrast, is the signal measured in fMRI that relates to the concentration of oxy- and deoxyhaemoglobin in the blood
What is HRF
- Haemodynamic response function, describes the changes of the BOLD signal over time
What does structural and functional imaging reveal
- Structural imaging reveals the static physical characteristics of the brain(useful in diagnosing disease), whereas functional imaging reveals dynamic changes in brain physiology(might correlate with cognitive function)
How does neural activity affect PET and fMRI scans
- Neural activity consumes oxygen from the blood which triggers an increase in blood flow to that region(PET)
- and a change in the amount of deoxyhaemoglobin in that region(fMRI)
