How do EEG instruments typically represent electrical activity?

EEG instruments represent electrical activity by plotting voltage against time. This method captures the fluctuating electrical potentials generated by the brain's neurotransmissions and synaptic activity. Brain activity is reflected in the EEG as waves, which can vary in amplitude and frequency. By creating a time-based plot, EEGs provide a visual representation of the electrical activity of the brain, allowing for the analysis of brain states, abnormalities, and responses to stimuli.

Other methods listed, such as measuring mechanical vibrations, detecting heat changes, or monitoring sound frequencies, do not relate to how EEG captures and interprets the electrical activity of the brain. EEG is specifically focused on the electrical signals, making the plotting of voltage against time the correct approach for the representation of brain function.