Abstract
The human brain’s ongoing activity is characterized by intrinsic networks of coherent
fluctuations, measured for example with correlated functional magnetic resonance
imaging signals. So far, however, the brain processes underlying this ongoing blood
oxygenation level dependent (BOLD) signal orchestration and their direct relevance for
human behavior are not sufficiently understood. In this study, we address the question
of whether and how ongoing BOLD activity within intrinsic occipital networks impacts on
conscious visual perception. To this end, backwardly masked targets were presented
in participants’ left visual field only, leaving the ipsi-lateral occipital areas entirely free
from direct effects of task throughout the experiment. Signal time courses of ipsi-lateral
BOLD fluctuations in visual areas V1 and V2 were then used as proxies for the ongoing
contra-lateral BOLD activity within the bilateral networks. Magnitude and phase of these
fluctuations were compared in trials with and without conscious visual perception,
operationalized by means of subjective confidence ratings. Our results show that ipsilateral
BOLD magnitudes in V1 were significantly higher at times of peak response when
the target was perceived consciously. A significant difference between conscious and
non-conscious perception with regard to the pre-target phase of an intrinsic-frequency
regime suggests that ongoing V1 fluctuations exert a decisive impact on the access
to consciousness already before stimulation. Both effects were absent in V2. These
results thus support the notion that ongoing slow BOLD activity within intrinsic networks
covering V1 represents localized processes that modulate the degree of readiness for
the emergence of visual consciousness.