Vertex Sharp Transients
The vertex sharp transient (VST) is a surface negative sharp wave that phase reverses at or near the apex of the head and occurs in drowsiness and non–rapid eye movement (NREM) sleep. The waveform typically is broadly distributed with extension into the frontal, parietal, and sometimes temporal regions. Although it usually appears monophasic or diphasic, it often is triphasic on closer inspection. The three phases comprise two small positive waves that precede and follow a larger negative sharp wave. The negative wave typically has a duration of 100 milliseconds in adulthood, and may be slightly longer in childhood. Combined with the other two phases, the total wave duration is approximately 200 milliseconds. Although the negative sharp wave may not be of high amplitude, it usually exceeds the amplitude of the surrounding background activity. Typically, its amplitude is approximately 90 μV (Yasoshima et al., 1984). The negative wave’s phase reversal, which is the center of the VST’s field, is most often at Cz, but it also commonly occurs at the adjacent electrodes C3 and C4. Occurrences at these electrodes have a shifting asymmetry that does not favor one side in normal recordings. VSTs occur individually or in trains of successive waves at up to about 3 per second, and these trains rarely last longer than several seconds. When trains occur, the smaller waves preceding and following the negative wave are less apparent, and the pattern may appear to be a run of repeating monophasic sharp waves.
During development, VSTs first occur with a clear waveform within surrounding activity at 5 to 6 months (Hughes, 1998). Although they are not as sharp as they become in early childhood, their amplitude during infancy and the toddler years is higher than at any other age and may reach 250 μV. During childhood, the amplitude gradually decreases, and this decline then continues through adulthood with more gradual reduction over time. By late adulthood, VSTs may be blunted and have low amplitude or completely absent. The localization of the wave’s phase reversal during early childhood may be at or near either the central or parietal midline, and the EEGs of children with parietal VSTs demonstrate a gradual migration of the VST to the central region over several years. Biparietal hump is an old term for the VST that does not refer to possible parietal localization during childhood. It refers to the appearance of VSTs when they are recorded without midline or central electrodes, as they were in the early years after EEG’s invention.