If in the context of the options present, I think you could slightly improve the clarity of what is meant in this sentence by inserting additional copies of

“either”: A test comprises an equal number of items concerned with either prospective or retrospective, either long-term or short-term, and either self-cued or environmentally-cued memory subscales.

I don’t think a sentences are better than bullet lists. In a tests, for example, I

understand correctly: A test comprises items concerned with various characteristics of memory subscales. These items should be equally split across:

• prospective and retrospective
• long-term and short-term
• self-cued and environmentally-cued.

These items should be equally split across: prospective and retrospective long-term and environmentally-cued and environmentally-cued.