  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.
