A visual prosthesis on high-density resolution brain stimulation
- Lara Merken
- Sep 13, 2024
- 2 min read
Updated: Feb 11
Abstract
A visual prosthesis can offer a new form of sight for the blind by electrically stimulating the primary visual cortex. Since the 19th century, multiple research groups have used different multi-electrode arrays to create this new form of artificial vision, both with intracortical and surface stimulation. Intracortical stimulation offers safer and lower stimulation intensities to induce dots of lights or phosphenes. However, the previously used implants are made from rigid materials which are less compliant with the soft-moving brain tissue and can cause more scar tissue formation. To increase biocompatibility and implant lifetime, the goal of this doctoral thesis was to test newly designed flexible multi-electrode arrays implanted in the macaque primary visual cortex. In the first study, we investigated the most optimal electrode size and tested the electrophysiological recordings of the flexible arrays (Chapter 2). We optimized the implantation procedure, showed the MRI compatibility of the arrays, and were able to record from single neurons (single unit activity, SUA) and neuronal populations (multi-unit activity, MUA) for up to a year (updated to 2.5 years after publication). In a second study, we hypothesized that combining a non-invasive method of stimulation, epicranial Direct Current Stimulation (eDCS), with electrical microstimulation could reduce the required current intensity and ensure safer stimulation limits (Chapter 3). Therefore, we first investigated the effect of eDCS on the neuronal level. We observed a range of effects where anodal and cathodal eDCS could induce excitations and inhibitions of neurons in V1. Additionally, eDCS could enhance the response to a preferred orientation and even shift the preference in orientation tuning. In a final study, we tested the feasibility of applying electrical microstimulation with these new flexible arrays (Chapter 4). We were able to reliably induce phosphenes in one monkey and observed activations in areas of the ventral stream during microstimulation-fMRI. In contrast, another monkey was only able to report phosphenes on one channel. During electrophysiological recordings in this animal, we observed strong activations in V1 and inhibitions in V4, whereas fMRI during microstimulation showed activations in V2 and V3 and deactivations of areas in the ventral stream. Comparing monopolar and bipolar stimulation in this monkey showed that bipolar stimulation could shift the effect of the neuronal response from inhibitions to activations.
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