Retinal and Cortical Contributions to Phosphenes During Transcranial Electrical Current Stimulation

Abstract

It is generally believed that the phosphenes induced by transcranial electric current stimulation (tECS) are a product of retinal activation, even when electrode placement is directly over the primary visual cortex. However, the origins of these tECS-induced phosphenes have not yet been conclusively determined. In this study, phosphene detection thresholds using an FPz-Oz montage were compared with those from (i) an Oz-Cz montage to determine whether prefrontal regions, such as the retina, contribute to phosphenes and (ii) an FPz-Cz montage to determine whether the visual cortex in the occipital lobe contributes to phosphenes. Twenty-two participants received transcranial current stimulation with each of these montages (as well as a T3-T4 montage included for exploratory purposes) at 6, 10, 16, 20, 24, 28, and 32 Hz. To estimate differences in current density at the retina and occipital lobe across montages, modeling of current density at phosphene thresholds was measured across 20 head models. Consistent with the proposal that tECS-induced phosphenes are generated in the retina, increasing current density near the retina (FPz-Oz relative to Oz-Cz montage) reduced phosphene thresholds. However, increasing current density near the occipital cortex (FPz-Oz relative to FPz-Cz montage) also reduced phosphene thresholds while also requiring less current density at the retina according to the modeling estimates. This suggests that tECS of this occipital cortex also contributed to phosphene perception. Bioelectromagnetics.

Evaluating Current Density Modeling of Non-Invasive Eye and Brain Electrical Stimulation Using Phosphene Thresholds

“…by using montages close to the eyes like Fp2-So or by improving the subjects’ ability to report phosphenes, is an issue deserving further studies. Of note, we are not proposing that the origin of phosphenes is only retinal but phosphenes can also be elicited by cortical activation [ 43 ], and there are montages studied by Laakso and Hirata [ 15 ] that help prevent unintended retinal stimulation. Yet, in agreement with their computation modelling results we showed in normal subjects that phosphene perception correlates indeed with current densities modelling in the retina and brain.…”

Effect of Ambient Lighting on Frequency Dependence in Transcranial Electrical Stimulation-Induced Phosphenes

“…1a), resulting in luminance at the eye measured at 0.6 ± 0.05 cd/m 2 . This lighting level was chosen in order to match that of previous studies [18,19] and thus to enable replication of their results. No light entered the testing room during the dark condition, resulting in zero cd/m 2 .…”

“…However, recent research suggests that under mesopic (dim) lighting conditions, tECS phosphenes are induced with considerably less current using 16 than 20 Hz stimulations [18,19], which suggests that 20 Hz does not provide the lowest stimulation level required to induce phosphenes. This raises concerns about the relative veracity of these new reports.…”

“…This makes it di cult to draw conclusions from such a comparison. In contrast to Schwarz's [20] research, more recent studies [18,19] used considerably larger samples (either 24 or 22 participants in each of these studies) with tightly controlled lighting (consistent 0.6 cd/m 2 lighting across the entire eld of view). Furthermore, the consistency of the initial [18] and replication [19] study suggests that their results are indeed reliable.…”

“…In contrast to Schwarz's [20] research, more recent studies [18,19] used considerably larger samples (either 24 or 22 participants in each of these studies) with tightly controlled lighting (consistent 0.6 cd/m 2 lighting across the entire eld of view). Furthermore, the consistency of the initial [18] and replication [19] study suggests that their results are indeed reliable. This conclusion, however, would appear (at face value) to contradict the view that the greatest sensitivity to tECS occurs at the stimulation frequency that matches the dominant cortical oscillation frequency.…”