The Effects of an Ocular Direct Electrical Stimulation on Pattern-Reversal Electroretinogram

Validation of eyes-closed resting alpha amplitude predicting neurofeedback learning of upregulation alpha activity

Neurofeedback training (NFT) enables users to learn self-control of EEG activity of interest and then to create many benefits on cognitive function. A considerable number of nonresponders who fail to achieve successful NFT have often been reported in the within-session prediction. This study aimed to investigate successful EEG NFT of upregulation alpha activity in terms of trainability, independence, and between-session predictability validation. Forty-six participants completed 12 training sessions. Spectrotemporal analysis revealed the upregulation success on brain activity of 8-12 Hz exclusively to demonstrate trainability and independence of alpha NFT. Three learning indices of between-session changes exhibited significant correlations with eyes-closed resting state (ECRS) alpha amplitude before the training exclusively. Through a stepwise linear discriminant analysis, the prediction model of ECRS's alpha frequency band amplitude exhibited the best accuracy (89.1%) validation regarding the learning index of increased alpha amplitude on average. This study performed a systematic analysis on NFT success, the performance of the 3 between-session learning indices, and the validation of ECRS alpha activity for responder prediction. The findings would assist researchers in obtaining insight into the training efficacy of individuals and then attempting to adapt an efficient strategy in NFT success.

Ocular direct current stimulation affects retinal ganglion cells

Ocular current stimulation (oCS) with weak current intensities (a few mA) has shown positive effects on retinal nerve cells, which indicates that neurodegenerative ocular diseases could be treated with current stimulation of the eye. During oCS, a significant polarity-independent reduction in the characteristic P50 amplitude of a pattern-reversal electroretinogram was found, while no current stimulation effect was found for a full field electroretinogram (ffERG). The ffERG data indicated a trend for a polarity-dependent influence during oCS on the photopic negative response (PhNR) wave, which represents the sum activity of the retinal ganglion cells. Therefore, an ffERG with adjusted parameters for the standardized measurement of the PhNR wave was combined with simultaneous oCS to study the potential effects of direct oCS on cumulative ganglion cell activity. Compared with that measured before oCS, the PhNR amplitude in the cathodal group increased significantly during current stimulation, while in the anodal and sham groups, no effect was visible (α = 0.05, p_cathodal = 0.006*). Furthermore, repeated-measures ANOVA revealed a significant difference in PhNR amplitude between the anodal and cathodal groups as well as between the cathodal and sham groups (p* ≤ 0.0167, p_{cathodal − anodal} = 0.002*, p_{cathodal − sham} = 0.011*).

Effects of Ocular Direct Current Stimulation on Full Field Electroretinogram

Studies on weak current stimulation (1-2 mA) examine effects on neuronal cells for the treatment of neurological diseases, like depression. Ocular current stimulation showed positive effects on retinal nerve cells which indicate that neurodegenerative ocular diseases, e.g., glaucoma, can be treated with current stimulation of the eye. However, up to now it remains unclear which exact retinal cells can be influenced. During an ocular direct current stimulation, a significant reduction of the characteristic P50 amplitude of a pattern-reversal electroretinogram (PERG) was found for an anodal and a cathodal stimulation. This current stimulation effect could originate from the modulation of pre-ganglion cell activity or by changes in local ON and OFF responses of ganglion cells. For clarification, we investigate acute direct current stimulation effects on a full field electroretinogram (ERG), which represents the activity of pre-ganglion cells (specifically cones and bipolar cells). The ERG from 15 subjects was evaluated before (ERG 1) and during (ERG 2) an ocular direct current stimulation with 800 μA over 5 min. The current was applied through a ring rubber electrode placed around the eye and a 25 cm2 rubber electrode placed at the ipsilateral temple. For ERG measurements, sintered Ag/AgCl skin-electrodes were positioned on the lower eyelid (active), the earlobe (reference), and the forehead (ground). The volunteers were stimulated in three independent sessions, each with a different current application (randomized order): cathodal polarity, anodal polarity (referred to the electrode around the eye), or sham stimulation. The changes between the two ERG measurements of the characteristic full field ERG amplitudes, a-wave, b-wave, and b'-wave (b-wave measured from zero line) were tested with the Wilcoxon signed-rank test (α = 0.05). Comparing before to during the current stimulation for all applications, the ERG waves showed no effects on amplitudes or latencies. Furthermore, no significant difference between the cathodal, anodal, and sham stimulation could be found by a Friedman test. These results indicate an unlikely contribution of pre-ganglion cells to the previously reported stimulation effect on PERG signals.