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There is growing interest in organic semiconductor devices for light-mediated neuromodulation, such as for retinal stimulation. Here, the key working principles of these devices are discussed, as well as promising applications and outstanding challenges for the field., Organic semiconductors have generated substantial interest in neurotechnology and emerged as a promising approach for wireless neuromodulation in fundamental and applied research. Here, we summarise the range of applications that have been proposed so far, including retinal stimulation, excitation and inhibition of cultured neurons and regulation of biological processes in other non-excitable cells from animal and plant origins. We also discuss the key chemical and physical phenomena at the basis of the interaction between materials and cells. Finally, we provide an overview of future perspectives, exciting research opportunities and the remaining challenges hampering the translation of this blooming technology into the clinic and industry.

Biomedical engineering online 20(1), 102 (2021). doi:10.1186/s12938-021-00938-9, Published by BioMed Central, London

To compare retinal reflectance changes (RCs) elicited by transcorneal electrical stimulation (TES) to those elicited by electrical stimulation of the optic chiasma (OX).Two eyes of two cats were studied under general anesthesia. Biphasic electrical pulses at 20 Hz were applied for 5 ms in TES, and monophasic pulses were applied for 50 μs at 100 Hz in OX stimulation. Fundus images observed with near-infrared light (800-880 nm) were recorded every 25 ms for 26 s, beginning 2 s before and ending 20 s after the electrical stimulation. To improve the signal-to-noise ratio, the images of ten consecutive recordings were averaged. Two-dimensional topographic maps of the RCs were constructed by subtracting images before the stimulation from those after the stimulation. The electrically evoked potentials (EEPs) were recorded at the OX. The effect of an intravitreal injection of tetrodotoxin (TTX) on the RCs elicited by electrical stimulation was also determined.After electrical stimulation, RCs were observed at the optic disc, retinal arteries, and retinal veins. The two-dimensional maps of the RCs elicited by both TES and OX stimulation were similar. The latency of the RCs ranged from 2.0 to 4.0 s, and the peak occurred 6 to 9 s after the onset of the ES. The intensity of the RCs was correlated with the amplitude of EEP elicited by TES stimulation. The RCs disappeared after the TTX injection in both TES and OX stimulation.TES activates principally the retinal ganglion cells, and a change in the blood flow is initiated thereafter.

Background: To test the function of implantable devices for electrical stimulation of the retina, long-term registration tests of cortical-evoked potentials are required. Skin electrodes are not appropriate to provide representative recordings, due to the voluminous pneumatic frontal sinus of minipigs. Therefore, epidural electrodes were permanently implanted in minipigs and tested with visual and electrical retinal stimulation. The present study describes long-term recordings of electrically evoked cortical potentials in minipigs. Methods: Three-channel silver-silver chloride electrodes were fixed on the epidura dorsal to the primary visual cortex of one hemisphere in Göttinger minipigs. Repeated light stimulation was performed and platinum-polyimide film multielectrode arrays were implanted on the retina for electrical stimulation and were later removed. Cortical potentials were recorded after stimulation with short biphasic charge-balanced currents. Results: For up to 18 months, the implanted epidural electrodes allowed recording of visual cortex potentials evoked by visual or electrical retina stimulation. Small changes of response amplitudes in subsequent experiments indicated a stable location and recording properties of the cortex electrodes. Visual stimulation often yielded stronger responses for the contralateral eye. Superthreshold electrical retina stimulation evoked cortical responses with less delay compared to visual stimulation. We found threshold currents of 50 µA for charge-balanced biphasic trains of current impulses. Postoperative examination showed an attached and unchanged retina. Conclusions: The minipig model is appropriate for the implantation of epiretinal stimulation electrodes and for the long-term tests of retinal implants by recording of cortical responses with chronically implanted epidural electrodes.

The authors discuss problems of localized retinal stimulation including the influence of stray light in cases of electroperimetry by means of VECP. The present study shows that we can successfully perform electroperimetric investigation under certain conditions. This is confirmed by investigation of visual field defects in a case of Arachnitis optico chiasmatica.

The CFF was determined with three sectored disks, each of which corresponded to one of the three rings in a Helmholtz disk. The fusion frequency is identical for all three rings of a Helmholtz disk but with the three sectored disks it was found that the rate of stimulus occlusion is a major factor in determining the CFF. It was tentatively concluded that the effect was not a consequence of different stimulus wave-forms. The phenomenon appears to have been reported late in the last century.

Vertical fusional movements elicited by stimulation of the non-foveal retina cannot be distinguished from movements elicited by foveal stimulation, except for a difference in the ratio between the amplitude of the response and the amplitude of the stimulus. This ratio is dependent on target configuration and on the retinal area stimulated. In some circumstances, if the size and richness of contour balances against the object fixated at the fovea, bifoveal fixation can be disrupted. It can be assumed that the stimulus for the motor response is made up of foveal as well as peripheral retinal stimulation. it is plausible that there is a gradient over the retina: the influence of the stimulated retinal area decreases form the fovea towards the periphery.

In a series of experiments studying response times to unstructured visual stimuli it was found that visual stimulation going initially to the right hemisphere is responded to faster than stimulation going initially to the left hemisphere. A model is described to account for these results and also for consistent differences in rate of responding depending upon which of four possible response pathways is involved.

The location of a frame was varied horizontally within a brightly illuminated Ganzfeld, Changing the position of the frame systematically shifted the apparent median plane, even though the entire retina was stimulated. It was concluded that the subjective straight ahead tends to be localized near the middle of a cognitive frame of reference, as opposed to the center of visual stimulation.

Als Reaktion auf Reizung der lateralen Gesichtsfeldhalfte wurden evozierte Potentiale der Hirnrinde des Menschen registriert. Reizung der einen Gesichtshalfte erzeugte fruhzeitig ein Maximum der Potentiale der gegenseitigen Hemisphare sowie Phasenumkehr der Potentiale der gleichseitigen Hemisphare.