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Retina: Anatomy and modeling

Hassan NasserNeuromathcomp - INRIA

Context

• Presentation of two articles:– T. Gollisch, M. Meister: Eye Smarter than Scientists

Believed: Neural Computations in Circuits of the Retina (2009).

– Helga Kolb: How the Retina Works (2003).• Discussion in the scope of my PhD: How to

reproduce the retinal responses.

Visual pathways (2 min) Eye (2 min) Retina (5 min)

Interaction between cells (7 min)Model (3 min)

Discussion

3 min

What is a Visual pathway?- It’s a set of cells bodies and nerves conducting impulses from the eye to the cerebral cortex

- Reception of external images and transport of visual information

Retina Visual CortexV1, V2…

Optic nerve

light (ganglion cells)

Three different layers:

1- The external layer, formed by the sclera, pupil and cornea.

2- The intermediate layer, divided into two parts: anterior (iris, lens and ciliary body) and posterior (choroid).

3- The internal layer, or the sensory part of the eye, the retina.

Eye

-The retina is approximately 0.5 mm thick and lines the back of the eye.- Optic nerve contains: RGC axons + Blood vessels

Retina

RetinaAbsorption of light

Light direction

All vertebrate retinas are composed of three layers of nerve cell bodies and two layers of synapses. The retina is designed according to the life style!. EX: Fish, Frog, Turtle have 5 types of cones Colored vision.

• Layers of nerve cells:– Outer nuclear layer

contains cell bodies of the rods and cones.

– Inner nuclear layer contains cell bodies of the bipolar, horizontal and amacrine cells.

– Ganglion cell layer contains cell bodies of ganglion cells and displaced amacrine cells

• Layers of synapses:– Outer plexiform layer (OPL)

where connections between rod and cones

– Inner plexiform layer (IPL), and it functions as a relay station for the vertical-information-carrying nerve cells, the bipolar cells, to connect to ganglion cells.

About retinal cells

• Photoreceptors.• Bipolar cells.• Horizontal Cells.• Amacrine cells.• Ganglion cells.

Photoreceptors

• 2-3 types of cones – Cones are concentrated in the fovea centralis.– 6-7 millions ("red" cones (64%), "green" cones (32%),

and "blue" cones (2%))– Sensitive to colors.– Visual acuity.

• 1 types of Rods.– ~ 120 millions.– More sensitive (1000 times) than cones but not

sensitive to the colors Night vision.

Day Night

Cones & Rods

When light is shone onto a photoreceptor, the photoreceptor hyperpolarizes and reduces the release of glutamate.

Bipolar - Horizontal – Amacrine - Ganglion

PHB

A

G

Retina smarter than scientists believed!More than 50 types of connected cells.

The retina is:

-A photo transducer that transforms photons to current?

-The retina is a processor to detect image features (motion, contrast, edges)?

-The retina is a sophisticated processor that exceed the last two expectations?

Light

FM Signal

Connections between retinal cells

• Introduction:– Synapse.– Action potential.– Graded action potential.

• Connection types:– Chemical synapses.– Electrical synapses.

Chemical Synapse

-The action potential arrives in the Pre synaptic cells (Indicator: Increasing of calcium concentration inside)-Releasing of neurotransmitters from their vesicles in the synaptic cleft.-Fixation of neurotransmitters on the receptor of postsynaptic cleft.-Triggering of the action potential in the post synaptic cell.

(All-or-none principle)

In the retina: Only RGC obeys to All-or-none principle. Others shows graded behavior (the postsynaptic potential is proportional to the presynaptic potential)

Electrical connections – Gap junctions

-Allow direct and faster connection between two cells.-Resistance for current depends on several parameters (Acidity degree, Calcium concentration, temperature, …).-Simple model = Resistance. Complicated model: Current transfer taking into account the majority of parameters to produce biological plausibility.

Let’s see how retina works:

Connections roles in the information transport and parallel processing.

Mouse retina

Synaptic connection

Gap junctions

Functional role of Gap Junction

- Gap junctions exists between cells of every type in the retina.

Stewart A et al. 2009

… Between Photoreceptors

• Blurring of visual signal.• Reduction in acuity.

• Coupling between photoreceptors sums the correlated visual signal and attenuates the asynchronous noise which comes from photo-chemical transduction (improve the SNR by 80%).

What are then the benefits?

Stewart A et al. 2009

… Between Ganglion cells

• Ganglion-Ganglion gap junctions (fast excitation < 2ms narrow spiking frequency distribution).

• Ganglion-Amacrine gap junctions (slower connections … broad distribution).

• Synchronization of spike outputs.

Benefits?

The aim of my PhD is to improve the existent retina model (VirtualRetina) in order to produce the retinal synchronized

output. How? … In the discussion.

Circuit modeling of retinal function:How to represent the retinal function in simple electrical circuits?

• Light detection.• Differential motion.

Light detection

At the post-synaptic gate of photoreceptor, we have: Signal + noise.The synaptic connection between the photoreceptors and the bipolar cell isequivalent to: A temporal filter followed by a threshold operation.The filtered signals are given to the bipolar cell each through a dendrite connection:The potential in the bipolar cell will be the summation of all these signals.

Differential motion

- Coding: Making the difference between the periphery signal (fb) and the center signal.- Same signal same motion. Different signal differential motion.

Virtual Retina

-Large scale simulation (> 10000 cells).-Implementation of functional retinal events.-A compromise between computational cost and biological plausibility.-Spike outputs are not synchronized.

Gollisch et al. 2009

How to implement the synchronization?

• Adding connection/Gap-Junctions between Ganglion-Ganglion cells and Ganglion-Amacrine cells.

• How to add these connection? Study the statistics of real acquisition for vertebrate retinas.

Adding connection holds two possibilities

• Black box Add connections at the level of ganglion cells without take into account the connection between Ganglion and Amacrine?

• Adding both GG and GA physical connections?

Black Box

Rods-ConesHorizontalAmacrine

RGC

Adding connection between RGC: These connection take (in a hidden way) the connections between the RGC and Amacrine.

Amacrine-RGC

Rods-ConesHorizontalAmacrine

RGC

Adding connection between RGC: These connection take (in a hidden way) the connections between the RGC and Amacrine.

THANK YOU!

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Center Surround Antagonism

PR

B

RGC

http://news.softpedia.com/news/Connections-Between-Retinal-Cell-Population-Mapped-159743.shtml

• “Nobody has ever seen the entire input-output transformation performed by complete circuits in the retina at single-cell resolution,” explains expert E.J. Chichilnisky, PhD.

“We think these data will allow us to more deeply understand neuronal computations in the visual system and ultimately may help us construct better retinal implants,” adds the scientist.