Cortical electrical activity during nonrapid eye movement (non-
REM) sleep is dominated by slow-wave activity (SWA). At larger
spatial scales ( approximately 2-30 cm), investigated by scalp
EEG recordings, SWA has been shown to propagate globally over
wide cortical regions as traveling waves, which has been
proposed to serve as a temporal framework for neural plasticity.
However, whether SWA dynamics at finer spatial scales also
reflects the orderly propagation has not previously been
investigated in humans. To reveal the local, finer spatial scale
( approximately 1-6 cm) patterns of SWA propagation during non-
REM sleep, electrocorticographic (ECoG) recordings were
conducted from subdurally implanted electrode grids and a
nonlinear correlation technique [mutual information (MI)] was
implemented. MI analysis revealed spatial maps of correlations
between cortical areas demonstrating SWA propagation directions,
speed, and association strength. Highest correlations,
indicating significant coupling, were detected during the
initial positive-going deflection of slow waves. SWA propagated
predominantly between adjacent cortical areas, albeit spatial
noncontinuities were also frequently observed. MI analysis
further uncovered significant convergence and divergence
patterns. Areas receiving the most convergent activity were
similar to those with high divergence rate, while reciprocal and
circular propagation of SWA was also frequent. We hypothesize
that SWA is characterized by distinct attributes depending on
the spatial scale observed. At larger spatial scales, the
orderly SWA propagation dominates; at the finer scale of the
ECoG recordings, non-REM sleep is characterized by complex SWA
propagation patterns.
