ERP Boot Camp Tip: Stimulus Duration

Here's a really simple tip regarding the duration of visual stimuli in ERP experiments: In most cases, the duration of a visual stimulus should be either (a) between 100 and 200 ms or (b) longer than the time period that you will be showing in your ERP waveforms.  

Here's the rationale: If your stimulus is shorter than ~100 ms, it is effectively the same as a 100 ms stimulus with lower contrast (look into Bloch's Law if you're interested in the reason for this).  For example, if you present a stimulus for 1 ms, the visual system will see this as being essentially identical to a very dim 100-ms stimulus.  As a result, there is usually no point is presenting a visual stimulus for less than 100 ms (unless you are using masking).

Once a stimulus duration exceeds ~100 ms, it produces an offset response as well as an onset response.  For example, the image shown here illustrates what happens with a 500-ms stimulus duration: There is a positive bump at 600 ms that is the P1 elicited by the offset of the stimulus.  This isn't necessarily a problem, but it makes your waveforms look weird.  You don't want to waste words explaining this in a paper.  So, if you need a long duration, make it long enough that the offset response is after the end of the time period you'll be showing in your waveforms.

The offset response gets gradually larger as the duration exceeds 100 ms.  With a 200-ms duration, the offset response is negligible.  So, if you want to give your participants a little extra time for perceiving the stimulus (but you don't want a very long duration), 200 ms is fine.  We usually use 200 ms in our schizophrenia studies.

Also, if the stimulus is <=200 ms, there is not much opportunity for eye movements (unless the stimulus is lateralized).  If you present a complex stimulus for >200 ms, you will likely get eye movements.  This may or may not be a significant problem, depending on the nature of your study.