Event-related potential · Lobeworks/17

An event-related potential is the small, stereotyped voltage change that the brain produces at a fixed delay after an event (a flash, a sound, a word), and it is what lets an EEG, too noisy to show much in a single trial, report reliably that something was noticed, expected or surprising.

Event-related potential. An event-related potential is the small, stereotyped voltage change that the brain produces at a fixed delay after an event (a flash, a sound, a word), and it is what lets an EEG, too noisy to show much in a single trial, report reliably that something was noticed, expected or surprising.

A single response is buried in the ongoing activity, so the potential is found by averaging many recordings aligned on the event: what is locked to the event adds up, and the rest cancels out. The result is a sequence of peaks named by sign and delay. The best known is the P300, a positive wave about 300 milliseconds after a rare or relevant event, larger when the event matters to the person and when it was less expected.

It made the first spelling interface possible. Farwell and Donchin flashed the rows and columns of a grid of letters while a person attended to the one they wanted; only the flashes that lit that letter evoked a P300, so the letter was found where the responses crossed. The brain never had to encode a letter. The screen turned an intention into a response that is easy to tell apart.

It reads attention and expectation, never content. A P300 says that a stimulus mattered to the person, not what they thought about it, which is why its uses are selection among options the interface offers and tests of recognition.

Its speed is set by the averaging. Each choice needs several repetitions to rise above the noise, so spellers of this kind reach a few letters a minute, traded against accuracy by how many repetitions they wait for.

Questions: How can a flashing grid let someone spell without reading a letter from the brain? The interface never looks for the letter in the brain. It flashes the rows and columns of a grid while the person attends to the letter they want, and only the flashes that light that letter carry a P300, the brain's response to an event that matters. The row and the column that evoked it meet at the letter. The design moves the hard part from decoding to the screen: an intention that no recording could read directly becomes a choice among options the interface offers, each answered with a response that is easy to tell apart.