http://www.sciencedirect.com/science/article/pii/S1053811913000633
Abstract
Analyzing
single trial brain activity remains a challenging problem in the
neurosciences. We gain purchase on this problem by focusing on globally
synchronous fields in within-trial evoked brain activity, rather than on
localized peaks in the trial-averaged evoked response (ER). We analyzed
data from three measurement modalities, each with different spatial
resolution: magnetoencephalogram (MEG), electroencephalogram (EEG) and
electrocorticogram (ECoG). We first characterized the ER in terms of
summation of phase and amplitude components over trials. Both
contributed to the ER, as expected, but the ER topography was dominated
by the phase component. This means the ER topography is akin to an
interference pattern in phase across trials. Hence the observed
topography of cross-trial phase will not accurately reflect the phase
topography within trials. To assess the organization of within-trial
phase, traveling wave (TW) components were quantified by computing the
phase gradient. TWs were intermittent but ubiquitous in the within-trial
evoked brain activity. At most task-relevant times and frequencies, the
within-trial phase topography was described better by a TW than by the
trial-average of phase. The trial-average of the TW components also
reproduced the topography of the ER; we suggest that the ER topography
arises, in large part, as an average over TW behaviours. These findings
were consistent across the three measurement modalities. We conclude
that, while phase is critical to understanding the topography of
event-related activity, the preliminary step of collating cortical
signals across trials can obscure the TW components in brain activity
and lead to an underestimation of the coherent motion of cortical
fields.
No comments:
Post a Comment