Techniques Flashcards
Lecture 1 & 2
What is cognitive neuroscience
The study of the biological processes underlying cognition
A highly interdisciplinary intersection of psychology and neuroscience
What neuroscience methods are used to observe the structure and function of the brain
Neuroanatomy
Invasive neurophysiology (from inside the brain)
Non-invasive methods (from outside the brain)
Computer simulators
What is temporal resolution
The precision of measurement or brain activity in time
What is spatial resolution
The precision of measurement of brain activity with respect to space.
What is the function of the brain stem
It connects the cerebrum with the spinal cord, carrying signals between the brain and the rest of the body
What is the function of the cerebellum
Sensorimotor control and learning; as well as other cognitive functions such as language
What is the function of the hypothalamus
It regulates functions that are essential for maintaining the normal state of the body (homeostasis) and reproduction. It controls hormone release
What is the function of the thalamus
It is the relay station in the pathways from sensory receptors to the cortex
What is the function of the cerebral cortex
It integrates information from across the brain and is the seat of most cognitive functions
It is involved in awareness, perception, memory, attention, planning, langugage, emotions, consciousness
What are the cerebral hemispheres and their different parts
They form the largest region of the human brain.
The longitudinal fissure separates the two hemispheres
The corpus callosum connects the two hemispheres
What are the four cortical lobes per hemisphere
Frontal
Parietal
Occipital
Temporal
What is Computed Tomography (measurement of brain structure)
Provides a 3D reconstruction computed from multiple 2D X-ray images. It has relatively low resolution (around 1cm)
What is Magnetic Resonance Imaging (MRI) (measurement of brain structure)
It measures the proton making up the nucleus of the hydrogen atom in H2O
Combination of strong magnetic fields and radio pulses which leads to protons emitting radio signals.
MRI picks up these signals
What is Diffusion Tensor Imaging (DTI) (measurement of brain structure)
A variant of MRI, whereby MRI equipment is ‘tuned’ to detect diffusion of water.
Diffusion in the brain is anisotropic (restricted)
Myelin sheath surrounding axon produces a tight lipid (i.e. fatty) boundary
Water diffusion is greatest in axons
Pros of lesion studies
Artificial ablations are precise (usually in animals)
Unequivocal evidence that damaged area participates in certain cognitive functions
Cons of lesion studies
Spatially imprecise when cause by injury or disease
Does not reveal fully how or where function takes place
What are single and multi cell recordings (measurement of brain activity)
Extracellular recordings of action potentials from one or many cells
Usually in animals: visual and auditory processing
In humans, occasionally: electrodes implanted in epilepsy patients
What is Positron Emission Tomography (PET) (measurement of brain activity)
It measures changes in blood flow
It is usually between the control and experimental condition
Radioactive isotope is injected into the blood
As the isotope decays, it generates gamma rays (photons) which the PET instrument detects
What is functional Magnetic Resonance Imaging (fMRI) (measurement of brain activity)
It is an indirect measure; measures blood flow with same principle as MRI
Generates a Blood Oxygenation Level Dependent (BOLD) signal
Sluggish: response peaks several seconds after stimulus onset.
Pros and PET and fMRI (measurement of brain activity)
Good spatial resolution (PET; ~1 cm voxels, fMRI; 3-6 mm)
Access to all parts of the brain
Cons of PET and fMRI (measurement of brain activity)
Poor temporal resolution (~ 1 min for PET, 5 secs for fMRI)
Indirect: measures blood flow rather than enaural activity directly
What does the electric circuit of pyramidal cells consist of
Synaptic currents: positive ions flowing into dendrite
Primary current: current flowing along dendrite
Leak current: current leaks out along the dendrite
Secondary (volume) current: completes the circuit
How are cortical cells organised
Cortical cells are organised into columns, with multiple aligned apical dendrites, perpendicular to cortical surface.
Signal entering a column activates all the cells in the column.
Cells within a column are synchronously active.
What are the summed currents of cortex
Nerve cells seldom become active alone
Nearby cells and columns tend to become synchronously active.
A large number of aligned pyramidal neurons in columns sums up to large primary and volume currents.
EEG/MEG reflect these currents and the activity of neural masses.
What is the EED field pattern
Volume currents ‘suck’ ions from one region and ‘push’ them into another region.
This builds an electric potential between different scalp locations.
EEG measures the potential difference between two electrodes.
What are some techniques closely related to EEG
Electrocorticography (ECoG) - electric field potential measurements from the cortical surface.
Local field potential (LEP) - electric fields potential measured from inside cortex.
How is MEG sensitive to primary currents in cortical sulci
MEG picks up cortical activity, but very little from deeper in the brain
MEG is ‘blind’ to to activity in gyri.
It is sensitive to primary currents in sulci.
What is the oscillatory activity in MEG and EEG
Both MEG and EEG show oscillatory activity in different frequency bands.
Cognitive and arousal states are associated with characteristic oscillations.
Correlations between oscillations in different cortical regions reveal functional connectivity maps of the brain.
Can you measure localising activity with EEG and MEG
No - because EEG measures field patterns from the surface of the scalp and MEG measures beyond (i.e far away from brain activity).
What are the solutions to localising activity with EEG and MEG
Simplifying assumptions: assume the primary current is a single point (dipole)
Constrain solutions to the cortex on the basis of MRI
Constrain solutions to activity map given by fMRI.
Pros of EEG and MEG
Very good temporal resolution (1 ms)
Quite good spatial resolution
Can be combined with other techniques
Cons of EEG and MEG
MEG is sensitive to current orientation
MEG doesn’t see deep sources
Localisation is difficult and uncertain
EEG has worse spatial resolution than MEG
What are invasive perturbations
When direct stimulation is used to mapping speech areas for epilepsy patients, to avoid these in operations to remove parts of cortex.
Drug research is medically useful, but can be ethically problematic.
What are non-invasive perturbations
Non-invasive methods that are safe to use on normal subjects, these are:
Transcranial magnetic stimulation (TMS)
Transcranial direct-current stimulation (tDCS)
What is transcranial magnetic stimulation (TMS)
An electric current in coil generates a magnetic field.
Magnetic field generates focal electric field in brain.
This field depolarises neurons locally.
This interferes with cognitive processing.
Pros of TMS
Good temporal (10 ms) and spatial (10 mm) resolution
Harmless - acts like temporary lesion
Subjects act as their own controls
Cons of TMS
Cortical effect: doesn’t reach deeper brain areas
Neurophysiological effect not clear
What is transcranial direct-current stimulation (tDCS)
A positive low-level current is induced between two electrodes: the anode and the cathode.
This results in depolarisation and action potentials under the anode.
Hyperpolarisation (inhibited activity) under the cathode.
Pros of tDCS
Cheap, portable and safe.
Subjects act as their own controls.
Clinical applications (e.g. depression, Parkinson’s)
Cons of tDCS
Poor spatial and temporal resolution.
Cortical effect: doesn’t reach deeper brain areas.
Neurophysiological effect not clear.
