This is a question that has neither a quick nor a simple answer. Spontaneously, I would not talk about the challenge but the reward system included. Maybe the addiction question ( and it is indeed an addiction for some people ) can be answered by analogy of a much simpler experiment using the “Candy Crush Saga” app that works extensively with audiovisual rewards. Even time.com is now writing about this app and identifies 9 key issues:
1. It Makes You Wait
2. We’re All Suckers for Sweet Talk
3. You Can Play With One Hand
4. There’s Always More
5. You Don’t Have to Pay – but if You Want to, It’s Easy
6. It Taps Into Our Inner Child
7. It’s Social
8. It’s an Escape
9. It Grows on You
Except for 3. – you don’t need any hand – all of these apply to science too, yea, yea.