When you say activation do you mean phasic/tonic?
no clue.. I've seen it a couple times.. haven't really put much effort in comparing those types of studies, usually just see them in passing.
from personal experience, I don't get much upper-chest 'development' (results/growth) or what I precieve as stimulation (soreness, a pump, 'feel', ect...) from incline presses... could be from my past sternum, clavicle, shoulder, and neck injuries, but I can't honestly say I've had clients improve their upperchest with incline work either.. the clients who've had good upper chests had good upper chests from the start, and the ones with 'weak' upperchests usually had weak overall chests, and when their overall chest development improved, so did the upper chest.
personally, I get a ton of stimulation in my anterior delts from incline work (bb, db, cable, ect...) and better development top to bottom in my chest with decline pressing (which for many many years I'd avoided like the plague out of fear that it was an unstable position for the shoulder joints).
consider this...
I remember that since AT LEAST 1995, there's been TONS of attention paid to prioritizing the upper chest.. guys advocating starting the chest workout with incline work.. guys NEVER doing a movement that wasn't an incline of some degree... I myself (and a couple former training partners, couple well-known dudes most here would know) went years without ever doing flat or decline work, just focusing on incline...... now, that's been nearly 20yrs of guys prioritizing upper chest, and I've yet to see one guy who's upper chest overpowers his 'lower' chest.
logic would tell me that if training flat or decline work makes the 'lower' pecs overpower the upper, then the opposite would be true if for the past 20yrs, guys have been focusing on their upper chest as much as guys in the 70's and 80's did on their lower.
but, it hasn't turned out that way.