Hi,

According to your description of this design, the ISI refers to the time between the stimuli and the ITI refers to the time between the 3-stimulus trials. If you are interested only in differences between the trials (A vs J), and not interested in differences between the stimuli, then there is no need to jitter the ISI, only the ITI. You could essentially treat the 3 stimuli as one complex stimulus. However, (and this is a BIG caveat), if the triplet of stimuli are close together in time then the responses (e.g. to A,B, andC) would overlap and hard to distinguish from one another. The result of this is that if you detect a difference between two triplets of stimuli, e.g. (A,B,C) vs (J,H,I), it would be difficult to determine which response to each of the three stimuli (A,B, or C) or (J, H, or I) in the triplet was different. If for example you want to know whether A vs J is significantly different, and also whether B vs J, or B vs H, was significantly different, then you would need to jitter both the ISI and the ITI.

Hopefully this adds some useful info.

-Rasmus