When you set up an experiment, with "defined" "stimuli", these are the stimuli in YOUR imagination and/or YOUR model.
BUT: very often it is a matter of representation (from long-term memory) of the circumstance(s)/setting(s), AND the stimuli can only be understood in THAT context -- the context of the content of developed representation of such circumstances/settings (think, for example, of problem-solving). The Subject, in most significant settings, has her/his representation of such circumstances/situations/settings. THAT actually more than helps to properly define the stimuli , for such is often the MAIN THING for defining (recall that it is the Subject (surrounding behavior patterns) very often _THAT_ MUST, in science, be what allows any empirical or true definition of stimuli).
All this is outlined by, and fully consistent with, Ethogram Theory (see my Profile and, from there, read A LOT-- I do provide guidance on readings order). The Theory itself is internally , and likely externally, consistent and it is strictly empirical (in the grounding/foundation of ALL concepts -- i.e. ALL clearly linked to directly observable overt behavior PATTERNS); and thus, given all those characteristics, there are hypotheses that are clearly verifiable/falsifiable .