To determine scalp projection of the cortical median SEPs we used the differential mode of scalp recording. The principal advantage is recording of the potential changes generated in a limited cortical area to the adjacent pair of scalp electrodes.Twenty-nine subjects were examined.An array of five pairs of surface electrodes were attached along a line connecting F4 with P4. In the frontal and prerolandic recordings no response was shown in 11 subjects (38 %). In 14 subjects the late phase reversal developed within 34–52 ms. The responses at prerolandic, rolandic anterior, rolandic posterior and parietal points showed increased amplitudes in the anterior-posterior direction (0.99 µV frontal and 2.48 µV rolandic posterior) and gradually longer peak latencies (by 1.18 ms–7.01 ms, mean 3.90 ms). In 21 subjects, the phase reversal developed at the rolandic level within 19–24 ms, interspersed with one or two hybrid responses. The importance of volume conduction is discussed. High resistance of bone and volume conduction of soft tissues between the cortex and the skin surface causes widespread and interindividually different projection of the diminished cortical response over the scalp. The differential scalp recording mode copies the cortical activity relatively faithfully and shows only one set of cortical somatosensory generators that is localized in the rolandic/peri-rolandic region.

        Key words: median nerve SEPs, differential scalp recordings, volume conduction