Summary Task 3 - EEG & ERP as dependent variables

TASK 3: EEG & ERP AS DEPENDENT
VARIABLES
NEURAL ORIGINS OF EEG

SINGLE-CELL RECORDINGS IN ANIMALS

 Enabled researchers to describe response characteristics of individual elements
 How does it work?
 Thin electrode inserted into animal’s brain
 When electrode is in vicinity of neuronal membrane, changes in electrical activity
can be measured
 Done extracellularly (electrode outside the neuron)
 Typical experiment
 Recordings obtained from series of cells in target area of interest
 Functional map can describe similarities & differences between neurons in cortical
region
 Visual areas
 Single cell not responsive to all visual stimuli
 Receptive fields – a limited region of space to which a specific neuron / cell responds
 Neighbouring cells have partially overlapping receptive fields
 Retinotopic – topographic representation in vision
 Multiunit recording – looking at pattern of activity over a group of neurons (today >400
cells simultaneously)

SINGLE-CELL RECORDINGS IN HUMANS

 Only when surgical procedure is required to treat patient (e.g., for epilepsy)
 Commonly placed in medial temporal lobe (MTL)
 MTL neurons respond selective to specific familiar images

ELECTROCORTOGRAM (ECOC)

 Similar to EEG BUT electrodes placed directly on surface of the brain (outside / beneath
dura)
 Appropriate only for people undergoing neurosurgical treatment
 Electrodes measure electrical signals before they pass through the scalp & skull 
less distortion
 Excellent spatial & temporal resolution
 Used to stimulate the brain & to map and localise cortical & subcortical neurologic
functions
 Limitation – experimental question often dictated by location of the ECoC grid

, THE NEURONAL SOURCE OF EEG

 EEG arises from synchronised synaptic activity in populations of cortical neurons
 Excitation of postsynaptic neurons  Dipole – extracellular voltage near neural
dendrites that is more negative than elsewhere along the neurons
 Source – region of positive charge separated from a region of negative charge
by some distance
 Sink – region of negative charge
 Electrodes detect sum of positive & negative charges in their vicinity
 Can only detect dipoles if electrode is closer to positive OR negative end of dipole


Dipoles  2 major types of dipoles
 Radial dipoles – oriented
perpendicular to the
surface (a)
 Tangential dipoles – oriented parallel to
the scalp surface (b)
 Dipoles have positive & negative side  produce both positive & negative
deflection at different regions of the scalp
 Dipoles from multiple neurons in a region will sum together


 In order to achieve a measurable (non-zero) signal, neurons
must be both
 Arranged in parallel fashion (a)
 Signals can sum to form larger signal
 Any other configuration (c) – individual dipoles’ positive &
negative ends will cancel each other out
 Synchronously active
 Yields a net charge on the scalp-facing side of the dipole
sheet (a) rather than charges cancelling each other out
(b)
 Signal large enough to be measured
 Polarity measured at scalp also depends on particular orientation of dipole
 Excitatory postsynaptic potential (EPSP)
 EPSP triggered at dendritic synapse  the local extracellular fluid becomes more
negative compared to intracellular fluid due to depolarising current in the
neuron
 Current flows elsewhere along neuron  more distributed extracellular
negativity