Dynamics
and topography of cortical activity during time estimation: MEG and
MEG-EEG
studies
As yet
controversial, the neural bases of time perception in the sub-second
range have
been widely explored through electroencephalography (EEG) or cerebral
blood
flow imaging techniques. Substantial improvement might be obtained by
using
whole-head magnetoencephalography (MEG), which provides a unique
combination of
both high temporal (millisecond) and spatial (millimeter) resolutions.
Our
study involved a duration discrimination task performed by human
subjects
monitored by either MEG or simultaneous MEG-EEG recordings. Subjects
had to
discriminate a previously memorized "standard" duration (700ms) among
randomly presented test-stimuli ranging from 490ms to 910ms. The
signals we
used were either auditory (440Hz tones) or visual (LED light) in order
to
separate sensory modality-dependent activities and supramodal
time-specific
processing. Surface magnetic fields parallel the time course of the
Contingent
Negative Variation (CNV) demonstrated by EEG studies. But, while CNV is
mainly
a frontocentral slow wave, the magnetic signal peaks on temporal,
temporo-parietal and parieto-occipital sensors. The time course and the
topography of cortical activity were determined by solving the
so-called
"inverse problem". Based on individual head-models calculated from
the segmented anatomical magnetic resonance imaging slices of each
subject, we
performed distributed source analyses with Minimum-Norm regularization.
By using
MEG-EEG fusion techniques, better focalization of electrophysiological
activity
maps was achieved. Our results show sustained activities in sensory
areas
respective to the signal's modality. The dynamics of these sources
reproduces
the duration of the stimuli. High inter-individual variability was
observed in
the topography of cortical activations correlative to the time
estimation encompassing
central premotor, intra-parietal, temporo-parieto-occipital and
prefrontal
areas.