You've probably heard of transcranial magnetic stimulation (TMS). With TMS, brain-hacking gadgeteers and doctors both use extremely strong pulsed magnetic fields to make big changes to how the brain's working... instantly. Some obscure research suggests this works the other way around, too.
Instead of a really strong field applied for an extremely short period of time, a weak one that lasts much longer may also have a powerful effect... and weak fields are a lot easier for you to make at home.
In short, a weak magnetic field that pulses at the same frequency for a long period of time may cause the brain to "sync up" to the frequency at which it's pulsing. (http://www.elfis.net/elfol8/e8elfeeg2.htm)
If so, then there's an easy way to overclock that grey matter. You don't even need flashing-light glasses or loud tones throbbing in your ears, just a ten-minute project with a 555 and a relay coil (or any other electromagnet) and a variable resistor.
Check out the schematic. It's not the usual way of doing a 555, it's simpler. You can forget S1 and R2 if you want, though you lose the extra "setting." All the schemtic does is get the 555 to produce a totally symmetric square wave across L1. L1 can be any coil on an iron core, but ideally it has a DC resistance of about 15K.
Why? The magnetic field can be incredibly weak and still produce an effect, so using a low-impedance electromagnet just wastes batteries.
The schematic also includes a Zener-biased BC547, putting out a few mV of white noise into the metal case around the device. You can also use twenty or thirty centimeters of hookup wire as an antenna. What does this do? The science is a little less clear, so consider it optional. However, if you accept the claims that other forms of electromagnetic interference can have physiological effects, this part might help to "jam" them.
Some scientists have found the brain responds best to magnetic pulses, instead of continuous waves. Therefore, if you want more of an effect, try re-buiding the coil.
Get a piece of ferrite rod and wind the coil around it. Now take a magnet and use it to bias the ferrite rod just below saturation. The result is something called a "magnetic pulse compressor," which creates a low-power magnetic pulse with an extremely fast rise time (faster than you could easily generate by normal means).
Even better are a few strips of Mu-Metal sheeting for a core. The Mu-Metal's incredible permeability makes for a really strong magnetic pulse, while the nearly instant saturation of the material takes care of the pulse compression part.
You can go even further by making a relaxation oscilator, instead of relying on the 555. A unijunction or programmable unijunction transistor, and a small handful of external parts, is all you need.
Have the oscillator dump the charge from the capacitor through the coil around the core. When it does, it'll create a "flyback" magnetic pulse as soon as the transistor turns off... this is even more powerful and faster than what you get from the 555.