I scored 4082 in easy mode, but I wasn’t able to make much progress in normal mode. It feels like there’s a gap between the difficulty levels.
As for the fun factor, I’ll be honest, after a while it started to feel a bit repetitive. Maybe you could introduce some variations as the player progresses. For example, you could occasionally change the arrow’s color, and that would mean the player has to press the opposite direction, or adding visual elements that slightly disrupt focus.
Just ideas to keep things evolving.
Regarding intuitiveness, it took me a little while to understand exactly what I was supposed to focus on. I eventually figured it out, but I think many players might not take that time. My suggestion would be to add a very easy first level where, for example, each correct input advances progress more noticeably (like 2 neurons in a single pump). That way the player immediately understands the mechanic.
Also, when showing the mouse input buttons, they’re displayed on opposite sides of the screen. It might be clearer if they were placed next to each other.
I hope this feedback helps, nice job on your game 🙂