Are you supposed to disclose all stimuli that your participants have been exposed to?
Psychology is not my field, but your concerns seem valid to me. As a reader of your work I'd want a fairly complete picture of the experiment that was done. 

However, you don't need to discuss everything at the same level of detail. A synopsis of the parts you don't focus on is likely enough to satisfy your concerns without distracting readers from what may be more important to your purpose. 

In other words, the overall description can be broad, but not deep. Then focus on what you want to say. Arguably, the fuller description could come in an appendix/annex to the paper. Or if a fuller description of the experiment has been published you could just cite that. 

If I think that the "unexplained" parts are more important to the outcome then I, at least, have a basis for discussion, rather than having just been misled. 

But, again, this is advice from an outsider.