Actually scientific experiments show that the human brain can detect flicker up to kiloherz frequencies even though the test subjects aren't aware of it.
It causes stress, headaches and concentration problems. Common cause of trouble are fluorescent lights that are common in offices. With them, I often see severe flickering even though the monitor is displaying 85hz or more. It's caused by the harmonic effect between the monitor and fluorescent light flickering.
The only way to remove the problem is to use conventional or halogen lamps or to buy electronic switching devices for the fluorescent tubes (they raise the flicker to 30 000 hz which is not detectable by brain at present knowledge.)