The ERP Boot Camp. What Are ERPs and What Are They Good For?. Overview. A bit of history Review : Basic neurophysiology & electricity Neural origins of ERPs Comparison of ERPs with other techniques. CNV. The Dawn of History. 1964: Gray Walter and the CNV. No Task: Click Only.
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What Are ERPs and What Are They Good For?
No Task: Click Only
No Task: Flashes Only
No Task: Click followed by flashes
Task: Press button when flashes start
Walter, W. G., Cooper, R., Aldridge, V. J., McCallum, W. C., & Winter, A. L. (1964). Contingent negative variation: An electric sign of sensorimotor association and expectancy in the human brain. Nature, 203, 380-384.
P300The Birth of the P300
Cue stimulus indicating whether click or flash was likely
Delay of 3-5 seconds: Subject guesses whether stimulus will be click or flash
Click or flash occurs
Sutton, S., Braren, M., Zubin, J., & John, E. R. (1965). Evoked potential correlates of stimulus uncertainty. Science, 150, 1187-1188.
Typical Task Alternatives:
Press only for oddballs
Different buttons for standard, oddball
Stimulus Onset Asynchrony
Overall R < lowest individual R
Overall R = sum of individual Rs
Outside of Skin
Inside of Skin
Measuring between E1 and E8 gives you the sum of E1 and E8; which impedance is high?
Measuring between E1–E7 (in parallel) and E8 gives you the sum of E8 and less than the lowest of E1–E7
Cortical pyramidal cell (basic input-output cell of cerebral cortex)
Excitatory transmitter released on apical dendrites causes positive charges to flow into dendrites
Net negative on outside of cell
Current flows through cell, completing the circuit
Polarity reverses with inhibitory transmitter
Polarity reverses with PSP on cell body and basal dendrites
Polarity at scalp also depends on orientation of the cortical surface and position of reference electrode
Equivalent Current Dipole
To be recorded at a distance, large numbers of neurons must have similar voltage fields
Scalp-recorded potentials are possible only for layered structures with consistent orientations
Primarily cerebral cortex
Possible role of radial glia
Local Field Potentials
Voltages spread through the brain by “volume conduction”
Nearly speed of light
Voltage everywhere except at positive-negative transition
Skull causes lateral spread (like spraying hose on cardboard)
Magnetic fields travel around electrical dipoles
The skull is transparent to magnetism -- less blurring
Deep and radial dipoles are invisible from outside the head
Voltage at an electrode at time t is a weighted sum of all components that are active at time t
There is no foolproof way to recover the underlying components from the observed waveforms
Sonny Boy Williamson
See Luck et al. (2011, Biological Psychiatry)