I am studying Parkinson's by analyzing electroencephalogram (EEG) data. This post will describe what this data is.
EEG data is collected by placing many electrodes onto a person's scalp. These electrodes will then generate data (data which are typicall called "brain waves" though this an incorrect description!) by comparing the voltage they detect to a common or ground electrode. Every electrode compares its readings against the same ground electrode. This data is then collected over time to create a voltage vs. time graph of a person's brain activity.
Each line in an EEG graph represents the potential difference of the neurons under the electrode compared to the ground electrode. Thus, if the neurons are firing fast and often under an electrode, that electrode channel will exhibit a high frequency because the potential difference of those neurons is fluctuating very quickly.
This data can then be used to understand how the brains electrical activity changes depending on the task the brain is trying to accomplish. For instance, when a person is trying to manipulate an object with their hand, the electrodes over their motor cortex will display electrical activity.