Jared

EEG picks up some small electrical events in neurons, yet mostly misses the largest. Why is this true? An action potential is the largest electrical event a single neuron can produce. It swings a neuron’s voltage by about 100 millivolts in roughly a millisecond. A single excitatory postsynaptic potential (EPSP, the small step a neuron receives from one synapse) moves the neuron’s voltage by about 1 millivolt, often less, over about 20 milliseconds. The two are linked, because an action potential in one neuron triggers EPSPs in the next. Yet a scalp recording is built mostly from those synaptic currents, with EPSPs as the textbook case. Two things make the difference. 1. The currents themselves differ: sodium in, then potassium out, along the axon for an action potential, positive ions in at a dendrite for an EPSP. The timings of action potentials vs EPSPs differ. EPSPs last about 20 times longer than action potentials, so they overlap and sum, and in parallel pyramidal neurons they run the full length from dendrite to cell body. Thus, EPSP fields line up! 2. Action potential currents run short loops that oppose each other and cancel before they reach the scalp. Whether we are talking about EPSPs or action potentials, either current needs a large patch of cortex acting in sync to be picked up by an electrode on the outside of the head. That is why alpha, about 10 hertz and strongest when a person’s eyes are closed, shows up so clearly: it is what synchronous synaptic input looks like from the outside. I made six images to show what EEG is a good and a poor fit for, with the voltages drawn to their true proportions. EPSPs are small in the neuron, yet dominant at the scalp! – at Foster City, CA