Course 1 Flashcards
What are the effects of measuring from a distance?
Trying to measure a signal from far away and try to infer what it might be, reconstructing something from a signal what we measure
Even if we could easily measure specific neurons, it wouldnt be easy to see frmo that what the brain is doing as a whole so we record from outside the brain and try to infer from the signals we get
In MRIs we use voxels to reconstruct the brain image but in EEGs with electrodes on the outside its harder to reconstruct because we need to figure out where the signals came from
What is the history of brain imaging?
1930: EEG
1970: radiographic methods
1980-1990: positron emission tomography and functional magnetic resonance imaging
1990-2000: decade of the brain = an evolution not without criticism
What are 2 examples of what brain scans are used for?
Brain scans as evidence in court of law
Personality assessment
Control of actions
Lie detection
State of consciousness tests
Patients in persistent vegetative state or locked-in syndrome
Ask to answer yes/no questions by imagining two very
different events
Does neuroimaging lead to objective diagnosis?
Several neurological syndromes
But less progress for psychiatric and mental syndromes
Depression, autism spectrum disorder, schizophrenia, …
Differences on a group level
But not large and consistent enough to allow diagnosis of an
individual subject
What is the basis of neural signals?
Neurons, with cell bodies in grey matter of cerebral cortex and subcortical structures; white matter contains axons. Without input (at rest), cell membrane of neuron has an electrical potential difference between in- and outside of -70 mV Post-synaptic potential is determined by integrating input of many synapses at the dendrites. It can hyper- and depolarize Action potential travels along the axon to all terminals.
What is neural communication?
Input neurons: action potentials over time
–> Membrane potential of postsynaptic neuron depolarizes or
hyperpolarizes
Over time membrane potential of post-synaptic neuron changes in function of input it receives = signal
Signal is the sum of signals that one neuron is receiving (excitatory minus inhibitory)
How to describe a signal?
Simplest signal = sinusoidal oscillation
Frequency: rate of change of signal, e.g., in the time dimension
1 Hz= completing a full cycle (going up & down) in one second
Biological signals never contain just one frequency (happens in artificial signals,
e.g., pure tone)
What are frequency components?
Complex signal can be decomposed into frequency components
Each has a particular frequency (e.g., 1 Hz, 2 Hz, 3 Hz, …)
Amplitude: how much it goes up and down; More neurones are recruted, larger the ampli is (if exci)
Phase: when it goes up and down; where the peaks and troughs happen in time
We learn about latency (how fast signal happens (when brain activated) so we can compare to ctrl and see if ex demyelinated disease) so signal phase will be diff
What is the frequency spectrum?
Highest frequency
Limited by sampling frequency
½* sampling frequency (Nyquist sampling theorem)
Imp for EEG+ MEG because thers broad band of freq, we cant measure much non invasively faster than 100 Hz
Lowest frequency
Limited by how long the signal is measured
1 / number of seconds measured
Filtering: attenuating or excluding certain part of measured frequency spectrum (low-pass, high-pass or band-pass)
If you get lots of gamma signal, we know its activation of a local patch of neurones (30 Hz), if we get much slower we know its neurones farther away from each other comm and coming back
What is a spectrogram?
Signal components and their strength can be shown in a spectrogram
Time frequency decomposition
If you record EEG occipital cortex, you would expect to see when they blink because signals from muscles are an order of magnitude longer than brain signals
What are molecular and hemodynamic signals?
Electrophysiological changes are connected to other kind of changes…
… At a smaller scale: movement of chemical substances and molecules
E.g., depolarization: influx of Na+, repolarization: outward current of K+
E.g., calcium concentration high in electrically active neurons –> two-photon calcium imaging
… At a larger scale: hemodynamics
Blood supply is adjusted to current energy needs
What is energy consumption like in the brain?
Electrophysiological events require energy
Amplitude of potential changes not necessarily the best predictor of energy consumption
Action potential = passive chain of events that does not consume much energy
Restoring resting potential requires energy è extra reason why energy consumption could correlate with number of action potentials
Pre- and post-synaptic factors (e.g., neurotransmitter release) also require energy
What is clustering?
Noninvasive methods cannot achieve single neuron resolution
Methods with highest spatial resolution still average signal from many neurons
Neurons of similar functional properties are clustered together
The more clustering, the more the averaged signal from many neurons corresponds to the signal of the individual neurons
–> Sensitivity of a noninvasive imaging technique depends upon amount of clustering present
Clustering on different spatial scales
What is temporal and spatial resolution?
Temporal resolution: the smallest unit of time that can be differentiated by a method
Spatial resolution: the smallest unit of space which can be resolved
How can we measure brain structure? (2)
Histology
Structural magnetic resonance imaging (MRI)
Investigation of anatomy in individuals
Anatomical localization of functional findings
Relate anatomical structure to differences between
participants in e.g., behavior
