Our recent analysis showed that this hierarchical activation from the sensory posterior areas to the anterior semantic systems propagates continuously along the ventral visual and semantic processing streams, ending in the ventrolateral prefrontal cortex of the Broca’s speech area and the frontal pole (Kucewicz et al., 2019). Theta power was also reported in the same areas preceding the gamma activation (Burke et al., 2013). This posterior-to-anterior sequence of high gamma power inspired a two-stage model of memory encoding (Burke et al., 2014a) to distinguish the early sensory and the late semantic phases of processing words. Task-induced power in the high gamma band is known to arrange into a hi hierarchical sequence of brain regions activated first in the posterior sensory areas and then in the more anterior associational temporal and pre-frontal cortex (Kucewicz et al., 2019, 2014,). Theta frequency bands were also found to show a profile of spatiotemporal changes that was distinct from the gamma activities. Hence, the spatiotemporal pattern of spectral power confirmed distinct roles played by the theta and gamma, as well as the alpha and beta activities. These were observed in anatomical areas that were overlapping with the gamma and theta power induced during word presentation. Our results confirm the differ- ent timing of alpha and beta power induction, occurring mainly before and after the period of word presentation on the screen. Compared with post-stimulus gamma activities related to sensory processing, alpha and beta activities were reported before stimulus onset and modulated by attention (van Ede et al., 2014 Bauer et al., 2014). Oscillations in these frequency bands were previously implicated with anticipatory or even inhibitory processes preceding task activations (Spitzer and Haegens, 2017 Engel and Fries, 2010). Even though cortical task activation was typically associated with spectral changes in the theta and gamma frequency bands (Miller et al., 2014 Greenberg et al., 2015 Osipova et al., 2006 Solomon et al., 2017 Burke et al., 2013 Kucewicz et al., 2014), we found the same or higher number of active electrode sites in the alpha and beta bands. The six frequency bands were similarly represented in the cortex,as quantified by the relative proportion of electrode sites recording theta, alpha/beta, and gamma activities. Multiple macro- and micro-electrodes implanted in individual subjects (Kucewicz, Berry, Worrell chapter in (Lhatoo et al., 2019)) would be required to address these questions further and elucidate the neuronal activities underlying our reported mosaic of spectral fingerprints. Our study was limited to multiple macro-contacts that were grouped from different subjects - high-density recordings from
One could go even further to ask how resolved this spectral distribution is and whether it can still be observed on the level of micro-electrode sites.
This broadband tilt in the spectral power can be resolved into specific spectral fingerprints at the level of individual electrode sites. While there was a significant laterality effect in at least one frequency band of every ROI (Fig.3A t-statistic, p 30 Hz) is increased upon activation of the cortical area in a given task. We investigated this laterality effect by estimating the temporal profiles of the mean power change in each ROI. Contact: Audrey Hu general, the total induced power was significantly different be- tween the left and the right hemisphere.