Signal Error by Joel Webster

Far out past the edges of the solar system lies a smallish chunk of rock. This is the planet Rupert, and it is a monitoring station for the Grebulons. The goal of the Grebulons is unknown, even to them. All they know is that they're supposed to monitor something, so they have been secretly watching Earth for a long time, studying its inhabitants.

The Grebulons abduct earthlings and put their victims through strange tests. The Grebulons have hundreds of test subjects working their way through various tests in the devious labryinthian contraption code-named the 'Z-MACHINE' housed on planet Rupert. Most of these tests involve wiping the subject's memory and placing them in a nondescript room containing objects that aren't really there.

After a successful (or unsuccessful) test, the subject's memory is wiped again and they are returned to Earth, where they live out their lives oblivious to the mental agony they have been through.

But one previous test subject has other plans...