The beat goes on: rhythmic modulation of cortical potentials by imagined tapping

Abstract

A frequency analysis was used to tag cortical activity from imagined rhythmic movements. Participants synchronized overt and imagined taps with brief visual stimuli presented at a constant rate, alternating between left and right index fingers. Brain potentials were recorded from across the scalp and topographic maps made of their power at the alternation frequency between left and right taps. Two prominent power foci occurred in each hemisphere for both overt and imagined taps, one over sensorimotor cortex and the other over posterior parietal cortex, with homologous foci in opposite hemispheres arising from oscillations 180 degrees out of phase. These findings demonstrate temporal isomorphism at a neural level between overt and imagined movements and illustrate a new approach to studying covert actions.

Figure 1

Trial-Types A and B. The sequence of events on Type A and B trials are shown respectively in the top and middle panels. Alternating taps with the left (L) and right (R) index fingers (real and imagined) were synchronized to stimuli presented every 840 ms. Taps with the same finger (L or R) were thus repeated every 2 × 840 = 1680 ms. The order of L and R taps was reversed in trial Types A and B, while the numeric stimuli remained the same. The bottom panel shows results of subtracting ERPs on trial Type B from those on Type A. Odd taps = L − R. Even taps = R − L. ERPs evoked by the numeric stimuli (same in A and B) cancel, and the remaining movement-related ERPs (L − R and R − L) cycle at 1680 ms = 0.6 Hz.

Figure 2

Distribution of intertap intervals (time between successive overt taps) for each participant. The middle line, inner pair of lines, outer pair of lines, and pair of dots represent respectively the median, 25th and 75th percentiles, 10th and 90th percentiles, and 5th and 95th percentiles. ISI = Interstimulus interval (time between onsets of successive synchronization signals).

Figure 3

Responses to overt and imagined taps at the cortical and muscular levels. Each panel shows results of the trial-Type A-B subtraction. Timing and identity of the synchronization signals presented on each trial are shown at the top. Panel A = EMG recorded from the left (LA) and right (RA) arms. Panel B = Lateral asymmetry in ERP at electrode sites over the left (C3) and right (C4) cortical hand areas (C3 − C4). Panel C = Difference between HEOG recorded near the left (LE) and right (RE) eyes (LE − RE). Panel D = Laplacian derivative of ERP recordings (LD-ERPs) from sites over the left (C3) and right (C4) cortical hand areas. Gray-hatched areas indicate intervals used to calculate amplitude spectra (see Figure 5). See text for further description of the measures.

Figure 4

LD-ERP at each electrode site. The traces were obtained by performing a Laplacian transform (see text) on the ERP difference waves resulting from the trial Type A - trial Type B subtraction.

Figure 5

Amplitude spectra of the LD-ERP at each electrode site during overt taps (top panel) and imagined taps (bottom panel). Each line shows the spectrum at an individual electrode site. 0.6 Hz = frequency of alternation between left and right taps. Note that the scale (uV) is twice as large for overt as for imagined movements.

Figure 6

Topography of LD-ERP amplitude at the frequency of alternation between left and right taps (0.6 Hz). Color indicates amplitude (red = more), concentric lines indicate regions of equivalent amplitude, and dots indicate electrode positions. (See text for explanation of circles around dots.) The central sulcus is slightly posterior to the central row of electrodes. Note that the scale (uV) is approximately twice as large for overt as for imagined movements.

Figure 7

Amplitude (top panel) and phase (bottom panel) of the tap-alternation frequency during overt and imagined movement. Each bar shows an average based on the six electrode sites surrounding one of the four power foci (circled dots in Figure 6). The values at individual electrode sites can be found in the Appendix.

Figure 8

Phase of the tap-alternation frequency at individual electrode sites during overt (abscissa) and imagined (ordinate) movements. Each point corresponds to one of the 24 (4 × 6) electrode sites surrounding the four power foci (circled dots in Figure 6). The numerical values can be found in the Appendix.

Figure 9

Single-trial classification of overt and imagined tap segments. Left and middle panels: Type-A (closed circles) and Type-B (open circles) segments for an individual participant; Phase = angle on plot; Trial # = distance from center. Right panel: Classification accuracy (PC) for each of the 18 participants (dot).