6.1 Visual prosthetics: retinal and cortical implants

Visual prosthetics offer hope for those with severe vision loss. Retinal implants stimulate remaining functional cells, while cortical implants bypass the entire visual pathway. Both use electrode arrays to deliver electrical stimulation based on captured visual information.

Surgical procedures for retinal implants involve placing electrodes on or under the retina, while cortical implants require brain surgery. Future advancements aim to improve resolution, field of view, and integrate new technologies like optogenetics and stem cell therapy.

Retinal and Cortical Visual Prosthetics

Principles of visual prosthetics

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Top images from around the web for Principles of visual prosthetics

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  Figures and data in Probing the functional impact of sub-retinal prosthesis | eLife View original

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  Improvement in the resolution of Retinal Prostheses - Electronics-Lab.com View original

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  Frontiers | Stimulation Strategies for Improving the Resolution of Retinal Prostheses View original

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  Figures and data in Probing the functional impact of sub-retinal prosthesis | eLife View original

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• Retinal implants stimulate remaining functional retinal cells, typically in the inner nuclear layer or ganglion cell layer to bypass damaged photoreceptors (rods and cones) and provide visual perception
  • Suitable for conditions like retinitis pigmentosa or age-related macular degeneration
• Cortical implants directly stimulate the visual cortex, bypassing the entire visual pathway making them suitable for conditions causing complete blindness (glaucoma or optic nerve damage)
• Both implants use electrode arrays to deliver electrical stimulation based on captured visual information from external components (camera, image processing unit, and wireless transmitter)
• Implanted components include a receiver, stimulator, and electrode array that work together to provide visual perception

Retinal vs cortical implants

• Retinal implants have advantages of utilizing remaining functional retinal circuitry for more natural visual perception and requiring less invasive surgery compared to cortical implants resulting in lower risk of complications
  • Limitations include limited applicability as some functional retinal cells are required and limited visual acuity and field of view due to constraints on electrode array size and placement
• Cortical implants have advantages of being applicable to a wider range of visual impairments, including complete blindness and potential for higher visual acuity and larger field of view due to direct stimulation of the visual cortex
  • Limitations include highly invasive surgery with increased risk of complications and less natural visual perception due to bypassing the entire visual pathway requiring more complex image processing and stimulation patterns

Surgical Procedures and Future Advancements

Surgical procedures for prosthetics

• Retinal implant surgery typically involves:
  1. Vitrectomy (removal of vitreous humor)
  2. Creating a small incision in the sclera
  3. Placing electrode array either subretinally (between photoreceptors and retinal pigment epithelium) or epiretinally (on the surface of the retina)
  • Risks and complications include retinal detachment, inflammation, and device failure
• Cortical implant surgery requires:
  1. A craniotomy to expose the occipital lobe of the brain
  2. Placing electrode array on the surface of the visual cortex (subdural) or inserted into the cortex (intracortical)
  • Risks and complications include brain injury, seizures, infection, and device failure

Future of visual prosthetics

• Current state shows several retinal implant systems have received regulatory approval (Argus II, Alpha IMS) while cortical implants are still in research and development with limited human trials
  • Visual acuity and field of view remain limited compared to natural vision
• Future advancements aim to improve electrode array designs for higher resolution and wider field of view, develop more sophisticated image processing algorithms and stimulation patterns, and integrate advanced technologies (optogenetics and stem cell therapy)
• Challenges include developing more biocompatible and long-lasting implant materials, addressing the complexity of the visual system and the brain's plasticity, ensuring safety and efficacy through long-term studies, and improving affordability and accessibility of visual prosthetic systems