Why are most evoked potentials (EPs) considered far-field responses?

Most evoked potentials (EPs) are considered far-field responses primarily because their generator sites are typically located deep within anatomical structures. This means that the electrical activities generated by these deep structures must travel through various layers of tissue (including the scalp, skull, and cerebrospinal fluid) before they can be detected on the surface of the scalp. This travel distance and the number of intervening layers contribute to the classification of EPs as far-field responses, where the recorded signal is influenced by multiple factors along the path to the recording electrodes.

In contrast, responses considered nearby or near-field would originate from sites closer to the surface or directly beneath the electrodes. The depth of the generators in evoked potentials also affects how the signals propagate, resulting in a blended recording that reflects contributions from a broader area rather than from a discrete surface location.

Therefore, the link between the depth of the generator sites in the brain and the far-field nature of the recordings helps distill why option B accurately describes the reason for classifying most evoked potentials as such.