We investigated the temporal evolution of changes in mu-alpha (7-14Hz), and mu-beta (15-29Hz) power in a localized dipole source in the right hand-area of SI after a cue to attend to tactile finger-stimulation to the contralateral left hand (attend in
condition), or to tactile toe-stimulation to the left foot (attend out
condition). shows example localizations in two subjects in the SI hand representation in the anterior bank of the postcentral gyrus (area 3b), confirmed by proximity to the omega shaped bend (marked in red bottom panel) in the central sulcus (Moore et al., 2000
). shows the corresponding average percent change from baseline (n=12 subjects (Ss)) in SI mu-alpha, and mu-beta power after the cue [−100, 1100]ms in attend in
and attend out
conditions. A significant difference across subjects was observed between the conditions in the mu-alpha band during the anticipatory post-cue time period from [500, 1100]ms post-cue (p<0.05 marked with asterisks, Wilcoxon sign-rank test). Significant differences in the mu-beta band were observed for a time window between [800, 850]ms.
Aligning trials to the tactile stimulus onset [−1000, 250]ms, rather than visual cue, also showed a dominant effect of cued attention on pre-stimulus mu-alpha activity (). Significant differences between attend in and attend out conditions are seen in the mu-alpha across the entire pre-stimulus time period, and in the [−200, 0]ms time window for mu-beta, with another period of significance around −800ms.
Next, we investigated attentional modulation of the broad-band SI evoked response to the visual cue and subsequent threshold-level tactile stimulus (). There was a rapid response in SI to the visual cue with an initial peak near 70ms (labeled pcM70, for “post-cue M70”, for reference in ) that was greater in the attend in
condition. Several other time points showed a significant difference in attend in
vs. attend out
conditions, and the timings of these differences was consistent with previously reported modulation of EEG measured event-related potentials (ERPs) during attention deployment in parietal, frontal and visual cortices (Kelly et al., 2009
). Most notable are the statistically significant differences at 200ms, 400ms and at several intervals between 500−900ms (see magenta asterisks in ). The 200ms difference is consistent with that seen in parietal cortices and typically referred to as an ‘early directing-attention negativity’ (EDAN), labeled pcMedan (for “post-cue MEG EDAN”) in . The difference near 400ms is consistent with the anterior directing-attention negativity (ADAN) observed in frontal cortices, and the later non-continuous differences between 500–900ms are in line with those seen over occipital cortex known as late directing-attention positivity (LDAP), labeled pcMadan and pcMldap, respectively.
shows the SI tactile ER from [−100,175]ms. Peaks in the waveform were consistent with previous reports using similar stimuli (Jones et al., 2007
; Jones et al., 2009
). Four peaks occurring at approximately 50ms (M50), 70ms (M70), 100ms (M100), and 135ms (M135), respectively, are labeled for visualization as in (Jones et al., 2007
; Jones et al., 2009
). A previous study showed that high pre-stimulus mu-alpha and mu-beta was correlated with an increase in the magnitude of the M50 peak and a subsequent trend toward decreased M70 and later response elements (Jones et al., 2009
). Here, we found that there was also a significant difference in the magnitude of the ER near the M50 peak between the attend in
versus attend out
conditions, such that the magnitude of the ER was greater in the attend out
conditions, when pre-stimulus mu-alpha and mu-beta were higher (, significant time-points marked with asterisks, p<0.05 Wilcoxon sign-rank test). There was also a significant difference in the ER near the M100 peak (p<0.05 Wilcoxon sign-rank).
Although a slow cue-locked fluctuation is apparent visually in the averaged cue ER in attend in and attend out conditions (), this did not bias the averaged tactile ER () since the tactile stimulus was jittered within the [1.1, 2.1]s post-cue time window (marked in ). However, it is possible this early ER was impacted by post-cue attentional modulation of the mu-alpha and mu-beta activity that is not phase-locked to the cue () (see Discussion).
We assessed trial-by-trial impact of pre-stimulus mu-alpha and mu-beta in our SI signal on tactile detection probabilities, using a second data set that employed analogous MEG and tactile detection methods, but with sustained attention to the finger, (Jones et al., 2007
; Jones et al., 2009
; Ziegler et al., 2010
). This previously collected data set gave greater statistical power than the current data, where the relevant hit and miss trials represented a statistically small sample (see Methods
). Following prior convention (Linkenkaer-Hansen et al., 2004
), on individual trials average pre-stimulus mu-alpha and mu-beta power was calculated (1s pre-stimulus) and sorted from high to low power into 10 equally sized percentile bins. Detection probabilities in each bin were calculated as the percent change in hit rate from the mean (see Methods
). We found a linear relationship between tactile detection probabilities and mu-alpha and mu-beta power (p<0.05, F-test, R2
=0.65 and R2
=0.85, respectively) such that the hit probability was greater during trials with lower pre-stimulus mu-alpha and mu-beta power ().