Mouse follower in specific area?

I’m not sure if I can explain this clearly enough because I’m not familiar with all of the terms but I will try.

I have created a flash banner to act as a meter. The ‘brain’ logo is to move up and down the mercury of the thermometer according to the placement of the mouse on the text a long side the banner.

Right now I have the ‘brain’ logo set as a tween in that area; however this is not what I want. I also have a ‘brain’ logo follower which is pointless that I only made trying to figure this stuff out.

If someone could please point me in the right direction, I would really appreciate it.

For an idea of what I’m talking about the flash banner is located here.
Keep in mind that it is only a concept right now.