Retinal and Cortical Implants. Stat 19 SEM Talk 4. The brain is a network of nerve cell, neurons. The neurons communicate by electrical and chemical signals.

Light to Electricity. The input to the visual system is light rays/photons. The input to the visual system is light rays/photons. These are received at the retina and converted into electrical signals. These are received at the retina and converted into electrical signals. These signals are transmitted and processed by networks of neurons. These signals are transmitted and processed by networks of neurons. Idea: Input the same information directly by using electrodes to stimulate the neurons. Idea: Input the same information directly by using electrodes to stimulate the neurons.

Retinal & Cortical Implants. First attempt in First attempt in Excitement and hype. Stevie Wonder. Excitement and hype. Stevie Wonder. Currently – cautious long term optimism. Currently – cautious long term optimism. “To impart a coarse level of vision that would expand a blind person’s autonomy is an ambitious but plausible goal”. “To impart a coarse level of vision that would expand a blind person’s autonomy is an ambitious but plausible goal”. John Wyatt (MIT). John Wyatt (MIT).

Retinal and Cortical Implants. Where to Implant? Where to Implant? Visual Information received at Retina. Visual Information received at Retina. Information sent by the Optic Nerve. Information sent by the Optic Nerve. Information decoded in the Visual Cortex. Information decoded in the Visual Cortex. All three places have been tried.

Implants in the Visual Cortex. First attempts were in the visual cortex. First attempts were in the visual cortex. But: But: (I) The retina acts like a sophisticated camera. (I) The retina acts like a sophisticated camera. (II) The cortex is involved in processing information. (II) The cortex is involved in processing information. The cortex is more complicated and less understood. The cortex is more complicated and less understood. “Easy to implant electrodes and stimulate an epileptic fit”. “Easy to implant electrodes and stimulate an epileptic fit”.

Visual Cortex. Complexity of the Cortex. Complexity of the Cortex.

Retinal Implants. Currently: most attempts are made to stimulate the retina. Currently: most attempts are made to stimulate the retina. Two alternative types: Two alternative types: (i) Epiretinal (MIT, USC,…) (i) Epiretinal (MIT, USC,…) (ii) Subretinal (Chicago, Tubingen). (ii) Subretinal (Chicago, Tubingen). Light perception, but can patients process images (e.g. read a letter)? Light perception, but can patients process images (e.g. read a letter)? USC far more optimistic than MIT. USC far more optimistic than MIT.

Retina. Background. Light enters the Pupil. It is received and converted to electricity in the retina. The signal is transmitted down the Optic Nerve to The Cortex.

Effect of Eye Diseases. Normal. Normal. Glaucoma Glaucoma Retina Pigmentosis, Retina Pigmentosis, Macular Degeneration. Macular Degeneration.

Retinal Pathway. Retina is organized backwards. Retina is organized backwards. 1. Rods and Cones. 2. Horizontal Cells. 3. Bipolar Cells. 4. Amacrine Cells. 5. Ganglion Cells. 6. Optic Nerve.

Photoreceptors. Structure of a photoreceptor. Structure of a photoreceptor.

Photoreceptors: Light to Electricity.

Processing in Retinal Pathway. Processing in the Retinal Pathway is fairly well Understood.

Two Types of Implants. I. Subretinal Implants (Chicago, Tubingen) replace damaged rods and cones by I. Subretinal Implants (Chicago, Tubingen) replace damaged rods and cones by Silicon plate carrying 1000’s of light-sensitive microphotodiodes Silicon plate carrying 1000’s of light-sensitive microphotodiodes Each with a stimulation electrode. Each with a stimulation electrode. Light from image activates the microphotodiodes, the electrodes inject currents into the neural cells (horizontal, bipolar, etc.) Light from image activates the microphotodiodes, the electrodes inject currents into the neural cells (horizontal, bipolar, etc.)

Two Types of Input. II. Epiretinal Implants (MIT, USC). II. Epiretinal Implants (MIT, USC). The implants receive input from a camera and processing unit. (E.g. on glasses). The implants receive input from a camera and processing unit. (E.g. on glasses). Electrodes from the implants electrically stimulate the ganglion cells and axons at the start of the optic nerve. Electrodes from the implants electrically stimulate the ganglion cells and axons at the start of the optic nerve.

Two Types: Tradeoffs. Subretinal Implant uses all of the retina (except the rods/cones). Subretinal Implant uses all of the retina (except the rods/cones). Epiretinal Implant does not, it must replace the function of entire retina and convert light to neural code. Epiretinal Implant does not, it must replace the function of entire retina and convert light to neural code. But the input to the Epiretinal Implant is more easily controlled (external camera). But the input to the Epiretinal Implant is more easily controlled (external camera). Fixing the Subretinal Implant is comparatively easier. Fixing the Subretinal Implant is comparatively easier.

Can Patients see with Implants? USC and MIT groups attached Epiretinal Implants to human patients. USC and MIT groups attached Epiretinal Implants to human patients. Patients perceived light patterns. Patients perceived light patterns. Only rarely did patients perceive geometric patterns. Only rarely did patients perceive geometric patterns. Chicago group attached Subretinal implants to patients. No results reported yet. Chicago group attached Subretinal implants to patients. No results reported yet.

Will Implants be Stable? “The eye (and brain) is full of water. electronic devices do not like water”. “The eye (and brain) is full of water. electronic devices do not like water”. Implants in animals can function for at least one year. The electronics can be encased in inert polymers. Implants in animals can function for at least one year. The electronics can be encased in inert polymers. Long term stability is unclear. Implants held in place by cellular contacts, microtacks. Long term stability is unclear. Implants held in place by cellular contacts, microtacks.

Will Resolution be High Enough? The image resolution will depend on the density of electrodes. The image resolution will depend on the density of electrodes. Ideally one electrode to stimulate each neuron. Ideally one electrode to stimulate each neuron. Current Epiretinal Implants are much lower than this (many orders of magnitude). Current Epiretinal Implants are much lower than this (many orders of magnitude).

MIT Presentation. MIT MITMovie.

Summary. Cautious Optimism for partial sight. Cautious Optimism for partial sight. Unknowns: Resolution, Long term stability of implants. Unknowns: Resolution, Long term stability of implants. Practicality – probably only for patients who had fully functioning visual systems before disease (Glaucoma). Practicality – probably only for patients who had fully functioning visual systems before disease (Glaucoma). Readings: (i) Cohen. “Mix of Hype and Hope”. (ii) Zrenner. “Will Retinal Implants Restore Vision”? Readings: (i) Cohen. “Mix of Hype and Hope”. (ii) Zrenner. “Will Retinal Implants Restore Vision”? Webpage: yuille/teaching/. Webpage: yuille/teaching/.