Vibrotactile driving responses

Figure 6 illustrates how MP-based estimates of signal energy density (eq. 7) helped to resolve important details of spatiotemporal structure in EEG frequency-following or "driving" responses produced in somatosensory cortex by vibrotactile stimuli applied to the hand.

Figure 6: Energy distribution (eq. 7, proportional to shades of gray) of averaged responses to a 200 micron vibrotactile stimulus at 33Hz applied to the tip of right digit 2. Horizontal axis time in seconds, vertical--frequency between 29 and 37 Hz. The plots derive from a line of channels spanning the hand region at a spacing of 2.8 cm, with the top position anteromedial and the bottom posterolateral. The middle channel is at position C3 of the 10-20 system. Data and interpretation courtesy of prof. E. F. Kelly.

At the most anteromedial position (top) a small response becomes just visible between 0.5 and 1.5 s after stimulus onset. In the next two positions the main part of the response both enlarges and occurs later in the stimulus period, and in the latter it can also be seen forming just after stimulus onset. In the two most posterior and lateral recording positions the response is consistently much larger, but also breaks up progressively into two distinct phases--an early burst of activity lasting on the order of 0.5 s or less, and a later-sustained response which is sharply focused on the stimulus frequency and fills the entire second half of the stimulus period. These spatiotemporally diverse patterns are believed to reflect the differential activity of responding neural subpopulations occupying progressively shifted positions and orientations within areas 3b and 1 of primary somatosensory cortex [16].