So, what exactly happens in the brain that causes faster reaction times for the expected (attended) pictures in the activity we described above? In humans, one powerful method used to examine what is going on inside the brain is to record the electrical activity produced by the brain when it is active. Brain cells (neurons) send information to one another using small electrical signals. These tiny electrical signals can flow up through the tissues and across the skull and scalp, where they can be recorded using electrodes (small metal disks) attached to the scalp. Electrical devices called amplifiers boost the tiny signals so we can observe them more easily. This method is called electroencephalography (EEG) and the recorded signals are called electroencephalograms (EEGs or EEG signals).
The EEG signal is produced by the activity of thousands or millions of neurons in the brain, and contains lots of different electrical signals coming from multiple different brain regions at any particular moment.

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