  Electrocorticogram (ECoG)-based brain computer interfaces (BCI) can
potentially control upper extremity prostheses to restore independent function
to paralyzed individuals. However, current research is mostly restricted to the
offline decoding of finger or 2D arm movement trajectories, and these results
are modest. This study seeks to improve the fundamental understanding of the
ECoG signal features underlying upper extremity movements to guide better BCI
design. Subjects undergoing ECoG electrode implantation performed a series of
elementary upper extremity movements in an intermittent flexion and extension
manner. It was found that movement velocity, $\dot\theta$, had a high positive
(negative) correlation with the instantaneous power of the ECoG high-$\gamma$
band (80-160 Hz) during flexion (extension). Also, the correlation was low
during idling epochs. Visual inspection of the ECoG high-$\gamma$ band revealed
power bursts during flexion/extension events that have a waveform that strongly
resembles the corresponding flexion/extension event as seen on $\dot\theta$.
These high-$\gamma$ bursts were present in all elementary movements, and were
spatially distributed in a somatotopic fashion. Thus, it can be concluded that
the high-$\gamma$ power of ECoG strongly encodes for movement trajectories, and
can be used as an input feature in future BCIs.
