Around a high tower and ladder to a wall parapet between the non-human (near dwarven smith) section of town and the craftmen's section of town are two "scientists" who wish Geralt to take part in an experiment.

In one play I did it -- never found any effect and in another I refused it with the same result.

Does anyone know what this is(was) about?

It's just that you get a critical mutagen effect for free (if you agree to participate). Nothing else.

Another example of this kind of thing is a soldier in option 1 of prologue who gives you money for something Geralt is supposed to have done some other time prior to TW2

Thanks, I already had some mutagens so that may be why I did not notice in the first play.

True, one of the issues in game is that the items you get flashes by so fast you don't see it. You see, theoretically, you are supposed to see on the left side of the screen what NPCs give you.

Seriously?! I have never noticed this. Wow.

Yeah, all the items you acquire are added to a list on the left-hand side of the screen; and as far as I can tell, the problem isn't that they flash by too fast, but that they are actually overwritten by other messages, such as quest updates. That's why you never have time to see what you are given as a quest reward when finishing quests - the notification appears, but only for a split second before the quest update cancels it.

Man. I noticed the ones when you pick things up, of course, but I never noticed it also lists items that characters give you (although that makes perfect sense, since it's added to the inventory). They should seriously fix that.

Agreed. It sounds like an oversight; as if the two messages (loot acquired and quest updates, for instance) unintentionally share the same box, or something. Nowadays I kind of know all the quest rewards by memory, but I remember how weird it was to notice that you suddenly had a new sword or piece of armour in your inventory, and you had no idea when you got it, or where you got it from. It doesn't help that the NPC's rarely include the reward in their dialogue, either - it just appears, without a word. It adds much more flavour if the NPC's actually talk about the item (briefly) before giving it to you. That way you won't need the loot update, either, since you're told through the game event that you've received it.