Human brainwave activity is too subtle to read unless the signal is amplified. These units available now usually connect through a USB port and transmit signals to the therapist computer. Older units use a serial interface.
Details of these amplifiers are documented in the EEGer4 Technical Manual, available here [PDF].
Precise and Accurate Four-Channel Amplifier in a Small Form Factor
A Robust and Reliable Amplifier Available at an Affordable Cost for New and Experienced...