Noise reduction in physiological measurements (EEG/ECG)
You want to look into all the documentation on the ModularEEG Design that is here. This is an open source/hardware project to build an EEG and has a wealth of information

You also want to take a look at this app note by TI that discusses dealing with noise in ECG. 

Sidenote: Before you build anything taking bio-potential feedback please ensure that your circuit has sufficient isolation (via optocouplers, transformers or something else). Also try to design your circuit to run off two 9V batteries (+9V and -9V with current draw &lt;100mA on the rails). These are good precautions when starting out to make sure you don't hurt yourself. Nerves are very sensitive and can be damaged by low voltage high frequency pulses that you won't even feel as a shock.