  Recent advances in brain-computer interface technology have shown the
potential of imagined speech and visual imagery as a robust paradigm for
intuitive brain-computer interface communication. However, the internal
dynamics of the two paradigms along with their intrinsic features haven't been
revealed. In this paper, we investigated the functional connectivity of the two
paradigms, considering various frequency ranges. The dataset of sixteen
subjects performing thirteen-class imagined speech and visual imagery were used
for the analysis. The phase-locking value of imagined speech and visual imagery
was analyzed in seven cortical regions with four frequency ranges. We compared
the functional connectivity of imagined speech and visual imagery with the
resting state to investigate the brain alterations during the imagery. The
phase-locking value in the whole brain region exhibited a significant decrease
during both imagined speech and visual imagery. Broca and Wernicke's area along
with the auditory cortex mainly exhibited a significant decrease in the
imagined speech, and the prefrontal cortex and the auditory cortex have shown a
significant decrease in the visual imagery paradigm. Further investigation on
the brain connectivity along with the decoding performance of the two paradigms
may play a crucial role as a performance predictor.
