**Core Concept**
An electroencephalogram (EEG) is a neuroimaging technique that records the electrical activity of the brain. The EEG signal is generated by the synchronized electrical discharges of neurons in the cerebral cortex.

**Why the Correct Answer is Right**
The EEG signal is generated by the postsynaptic potentials (PSPs) and action potentials (APs) of neurons. The PSPs and APs are produced by the flow of ions across the neuronal membrane, which is regulated by voltage-gated ion channels. The synchronized electrical discharges of neurons are detected by electrodes placed on the scalp, which record the electrical activity as a waveform.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the EEG signal is not generated by the heart or cardiovascular system.
* **Option B:** This option is incorrect because the EEG signal is not generated by the muscles or skeletal system.
* **Option C:** This option is incorrect because the EEG signal is not generated by the eyes or visual system.

**Clinical Pearl / High-Yield Fact**
The EEG is a valuable tool for diagnosing and monitoring neurological disorders, such as epilepsy and encephalopathy. It can also be used to assess the level of consciousness and detect abnormalities in brain function.

**Correct Answer: D. Cerebral cortex (correctly stated as the source of EEG)**