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 Event Related Potentials (ERPs) are averaged EEG waveforms elicited by exposing research participants to external stimuli. Typical external stimuli might be sounds, shapes, or words, though the type of stimulus is limited only by the experimenter’s resourcefulness.

EGI offers a combined two-system approach to ERP studies: the Geodesic EEG System (GES) for collecting participant EEG during an experimental study, and the Experiment Control Computer (ECC), for handling and presenting the experimental stimuli to the participant via speakers or a monitor. The ECC also establishes or controls the stimulus timing and communicates the timing information, either by way of an Ethernet cable to the EEG Data Acquisition Computer, or by way of sending Digital Inputs (DINs) directly to the amplifier via a DIN patch cable.  Net Station then can register this data as Experimental Control Interface (ECI) events or Digital Input (DIN) events. 

EEG data and ECI and DIN events are indexed with respect to time, an essential feature if the ERPs are to be correctly associated with their causal stimuli. Errors in the indexing of either the EEG or the event stream result in misalignment between stimulus and EEG.

Misalignment can result from a lack of synchronization, programming errors, or inherent timing tolerances in software or hardware. Misalignment can manifest as drift, offset, or variability.

Effect on a Single Recording

For a single trial, misalignment will skew the EEG and the stimulus by some number of milliseconds, producing uncertainty of what is evoked EEG and what is background EEG.

Effect on Averaged ERPs

For an averaged ERP, misalignment between event and EEG has the effect of degrading the average. 

Averaged ERPs help determine the constant to a given stimulus for a single subject and also across subjects. One way researchers create averaged ERPs is by selecting regions of EEG around a given stimulus type (segmentation), aligning all the EEG segments so that their causal events coincide, and then averaging the pieces together to result in a smoothed ERP waveform, with little of the subject's background EEG left: 

If timing offsets between the event and EEG vary from segment to segment, the individual EEG segments are misaligned when indexed by causal event:

The result is an averaged ERP that is blurred temporally and less distinct than it would be with proper indexing. In extreme cases the averaged ERP may vanish entirely because of event / EEG misalignment.

Therefore, a Timing Test is essential for validating experiment timing and that an experiment is programmed correctly.