What does an electroencephalogram (EEG) measure?

An electroencephalogram (EEG) is a test that measures the electrical activity of the brain. This is done by placing electrodes on the scalp to detect and record the electrical impulses produced by neuronal activity. The EEG provides crucial information about brain function and is particularly useful in diagnosing conditions such as epilepsy, sleep disorders, and other neurological issues.

The other options refer to measurements of different physiological processes: heart rate pertains to the cardiovascular system, blood glucose levels relate to metabolic functions primarily involving the pancreas, and hormone secretion levels involve hormonal functions typically assessed through blood tests. These functions are unrelated to the specific electrical activity of the brain that an EEG assesses. Therefore, the correct answer highlights the EEG's unique role in monitoring and understanding brain activity, distinguishing it from the other choices.