Oscillatory Transcranial Electrical Stimulation and the Quantitative Features of Phosphene Perception

Background Introduction

Phosphenes refer to the phenomenon of perceiving light points without any external visual stimuli. This phenomenon holds significant importance in visual neuroscience and consciousness studies, as it helps us understand how the brain links neural activity to perceptual content. Previous research has shown that phosphenes can be induced through direct electrical stimulation of the visual cortex or transcranial magnetic stimulation (TMS). In recent years, transcranial alternating current stimulation (TACS) has also been demonstrated to elicit phosphenes, but the underlying mechanisms remain unclear. TACS involves rhythmic changes in the electric field and alternating polarity (excitatory vs. inhibitory phases), making it difficult to clarify the precise mechanisms behind phosphene perception.

To disentangle the effects of rhythmic changes in the electric field from those of alternating polarity, this study employed oscillatory transcranial direct current stimulation (oTDCS). Unlike TACS, oTDCS confines current oscillations to a single polarity (positive or negative), thereby eliminating the influence of polarity switching. By comparing the effects of TACS and oTDCS on phosphene perception, this study aimed to reveal the role of current oscillations in phosphene perception and explore the impact of amplitude modulation (AM) frequency on phosphene perception.

Source of the Paper

This paper was co-authored by Che-Yi Hsu, Tzu-Ling Liu, and Chi-Hung Juan, affiliated with the Institute of Cognitive Neuroscience and the Cognitive Intelligence and Precision Healthcare Research Center at National Central University, Taiwan. The paper was published in 2025 in the European Journal of Neuroscience under the title Oscillatory Transcranial Electrical Stimulation and the Amplitude-Modulated Frequency Dictate the Quantitative Features of Phosphenes.

Research Process and Experimental Design

1. Participants and Design

This study recruited 37 participants with normal or corrected-to-normal vision, excluding individuals with a history of neurological diseases or epilepsy. Ultimately, 25 participants (13 males, 12 females, aged 20 to 45) completed the experiment. A within-subject design was employed, with each participant visiting the laboratory three times, spaced one week apart. During each visit, participants received stimulation of a single polarity (anodal oTDCS, cathodal oTDCS, or TACS) across four stimulation blocks: threshold-level sinusoidal wave (18 Hz), threshold-level AM wave (2 Hz amplitude-modulated 18 Hz), suprathreshold sinusoidal wave, and suprathreshold AM wave.

2. Experimental Apparatus and Stimulation Parameters

The experiment was conducted in a dimly lit room, with participants seated 60 cm away from a 24-inch LCD monitor. Stimulation was delivered via a 128-channel elastic cap (Geodesic Transcranial Electrical Neuromodulation, GTEN), with 20 electrodes positioned over the posterior scalp selected for current delivery. The stimulation intensity for oTDCS ranged from 0 to 2000 μA, while TACS was delivered as a sinusoidal or AM waveform with an amplitude between -1000 and 1000 μA.

3. Experimental Procedure

For each condition, the initial tested intensity was set at the maximum output of the equipment, 2000 μA. Participants who could not perceive any phosphene at this intensity were excluded from the experiment. Threshold intensities were determined using the Modified Binary Search (MOBS) method, and the suprathreshold intensity was set at 120% of the validated threshold. The formal experiment included both threshold and suprathreshold intensities, with 10 trials per condition in each block. In each trial, participants were stimulated for 5 seconds and asked to press the space key when they perceived a phosphene to record their response time. They then used a mouse to draw the phosphene pattern on the screen and reported brightness, flash rate, and confidence level after the stimulation.

Key Findings

1. Threshold Intensity

The study found that the phosphene threshold for AM stimulation was significantly higher than that for sinusoidal stimulation (1284.33 ± 86.78 μA vs. 1079.47 ± 42.62 μA). However, polarity (anodal oTDCS, cathodal oTDCS, and TACS) had no significant effect on the threshold.

2. Response Time

Response time was influenced by stimulation polarity, AM condition, and intensity. Response times for TACS were significantly faster than those for cathodal oTDCS but not significantly different from anodal oTDCS. Response times were also significantly shorter for sinusoidal stimulation compared to AM stimulation and for suprathreshold intensity compared to threshold intensity.

3. Brightness Ratings

Phosphenes induced by anodal oTDCS were perceived as significantly brighter than those induced by cathodal oTDCS and TACS. Brightness ratings were also significantly higher for suprathreshold intensity compared to threshold intensity.

4. Flash Rate Scoring

Flash rate scores were significantly lower for AM stimulation compared to sinusoidal stimulation. Suprathreshold intensity significantly increased flash rate scores only in the anodal AM oTDCS condition.

5. Confidence Levels

Confidence levels were significantly higher for suprathreshold intensity compared to threshold intensity, particularly in the anodal AM oTDCS and TACS sinusoidal conditions.

6. Phosphene Size

Phosphenes induced by sinusoidal stimulation were significantly larger in area than those induced by AM stimulation. Phosphenes generated at suprathreshold intensities were also significantly larger than those at threshold intensities.

Conclusions and Significance

This study demonstrates that oscillatory electrical stimulation (both TACS and oTDCS) can effectively induce phosphene perception. Current oscillation is a key factor in phosphene generation, while polarity influences the perceptual quality of phosphenes (e.g., brightness and response time). Additionally, AM frequency plays a dominant role in phosphene flash rate perception, independent of the carrier frequency. These findings highlight the importance of neural oscillations in visual perception and provide new insights for future research on visual prosthetic technologies.

Research Highlights

1. Innovative Experimental Design: By comparing TACS and oTDCS, this study successfully disentangled the effects of rhythmic changes in the electric field from those of alternating polarity on phosphene perception.
2. Dominant Role of AM Frequency: The study found that AM frequency plays a dominant role in phosphene flash rate perception, offering a new perspective on temporal frequency encoding in visual perception.
3. Impact of Polarity on Perceptual Quality: Phosphenes induced by anodal oTDCS were perceived as significantly brighter than those induced by cathodal oTDCS and TACS, indicating that polarity can independently influence the perceptual quality of phosphenes, regardless of threshold intensity.

Additional Valuable Information

Limitations of this study include the relatively small number of trials and the block design, which may have led to predictive judgments. Future research could enhance the robustness of the findings by increasing the number of trials and adopting an interleaved trial design. Additionally, the study recommends avoiding high-contrast fixation points in future phosphene research to minimize interference with phosphene perception.

Through this research, we have deepened our understanding of the mechanisms underlying phosphene perception and provided important theoretical support for the development of neural oscillation-based visual prosthetic technologies.