Analysis of tagging latency when comparing event-related potentials

Event-related potentials (ERPs) are very small voltage produced by the brain in response to external stimulation. In order to detect and evaluate an ERP in an ongoing electroencephalogram (EEG), it is necessary to tag the EEG with the exact onset time of the stimulus. We define the latency as the delay between the time the tagging command is sent and the detection of the stimulus on the screen. Failing to control sequencing in the tagging pipeline causes problems when interpreting latency, in particular when comparing ERPs generated from stimuli displayed by different systems. In this work, we present number of technical aspects which can influence latency such as the refresh rate of the screen or the display of a stimulus at different screen location. A few propositions are suggested to estimate and correct this latency.