4.1 The current oscillation is the key to phosphene occurrence
Following our prediction and Evans et al.’s finding (Evans et al.,
2021), participants reported phosphene occurrences in both anodal and
cathodal otDCS, with no significant polarity effect on threshold
intensity. This result suggests two key points: (1) current oscillation
could be necessary for phosphene generation, and (2) the occurrence of
phosphene is independent of cortical excitability, typically modulated
by tDCS. In other words, the occurrence of phosphene relies on therelative change rather than the absolute level of the
neural electromagnetic field.
This result is plausible as the phosphene percept essentially involves
the detection of changes in brightness. By discerning spatial or
temporal changes in brightness, individuals develop perceptions of
contours or flashes. This perspective finds support in prosthesis
research, where it has been observed that only dynamic stimulation on
the visual cortex can elicit meaningful perceptions in individuals who
are blind (Beauchamp et al., 2020). The importance of relative change in
phosphene perception aligns with the well-established predictive coding
theory (Rao & Ballard, 1999; Schultz et al., 1997). This theory posits
that the system makes prior predictions about what should be perceived
in a top-down manner. Within this framework, certain neurons carry
information about the ”difference” between the predicted and perceived
information (Koster-Hale et al., 2013). Only unexpected inputs are
signaled for further processing (Koch & Poggio, 1999), or the awareness
of information would be suppressed if it is ”explained away” (Seth et
al., 2011).
Our results underscore the significance of neural oscillation in
perception generation. As tACS applied to human participants (less than
2000 μA) is considered insufficient to induce neural spiking in the
visual cortex, the source of phosphene has remained a puzzle for the
past decade. While the prevailing explanation attributes tACS-induced
phosphene to the activation of retinal cells through current leakage
over the scalp (Laakso & Hirata, 2013), there are lines of evidence
suggesting a contribution from cortical areas, as seen in (Evans et al.,
2019, 2021). Despite the inherent challenges in measuring the cortical
activation of phosphene perception in vivo in humans, the possibility of
cortical involvement cannot be ruled out. In fact, the electromagnetic
field of the neural system has been proposed as the basis of
consciousness (McFadden, 2021). The global resonance theory posits that
brain electromagnetic field oscillations could serve as the ”seat” of
consciousness (Hunt, 2020; Hunt et al., 2022; Hunt & Jones, 2023).
According to this theory, synchronous firing, or neural oscillation,
signifies a shared electric field wherein local and global oscillatory
couplings may represent complex information transfer across different
brain areas or levels (Buzsáki, 2006). Supporting this perspective, an
increasing number of studies have suggested the importance of neural
oscillations and their functional connectivity in conscious function, as
seen in (Engel & Fries, 2016; Modolo et al., 2020; Plosnić et al.,
2023). Therefore, our results suggest the crucial role of the neural
electromagnetic field in perception, as it causally induces visual
perception through external current stimulations.