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.