Vision is THE most important sense for humans. Most of the information we get is through vision

• 90% vision
• 8% tactile, touch
• 3% hearing I have no idea how did they measure this aspect. This is true for humans, but for mice for example it is different, they have probably a 64x64 pixel resolution equivalent. For humans, visual data is more important, it is faster compared to speech and other senses.

Human vision is estimated to be about 576 Megapixels of data (3M snapshots patched together with saccades, that has that pixel image value), since it can distinguish 0.6arc-minutes (0.01 degrees). There is an estimate of about 60kk rods and 3kk cones.

Eye Anatomy

We can change the shape of the lens to focus on objects of different distance.

Saccades

To get the full view of the environment humans are used to do saccades. Brain directs it to look for the spaces that are important, the key features.

The Retina

Introduction to the Retina

Basic Anatomy

The retina is able to reshape its lens, to focus on close or far objects. This part is transparent, there are some cells that register the information and send it in the optical nerve. There is one spot in the retina that has no receptors, it is called blind spots. Density of receptors define the resolution of the image you get.

The Macula contains many Cones, usually good for color detection, it is important for sharp vision. The fovea is the center of the macula, it has very very high concentration of cone cells, exclusively those.

Neurons in the Retina

Spiking Ganglion Cells

Ganglion Cells are the output of the retina, it is where the information is sent to the brain, it takes about 10ms to process this information and exit the eye.

These cells do not spike, they continuously send the signal. Ganglion cells are the only cells that spike here (digital signals have long range without decay (see Neural mechanisms, while analog signals have this problem, for short distances they are great).

Photoreceptor Layer

In the photoreceptor layer, Cones and rods live. Their role is to capture the light, and process it locally before sending it to the brain.

• Rods: Sensitive to low light (scotopic vision); no color perception.
• Cones: Active in bright light (photopic vision); responsible for color and detail.

These rods have synapses, but they are very special. The optic disks contain opsins, which interact to light and are able to communicate this signal to the other parts.

Sensitivity to Light

In the disks of the Rods and Cones there are opsins, proteins that react to light.

• Activates transduction of g protein.
• This hydrolizes cGMP to GMP, activating an ion channel for sodium, which is now closed. This changes the membrane potential of the rod or cone.

In the dark, the channels are open, so you have influx and pumps for potassium, getting depolarized. In light, you are pushing out potassium, so the cell is getting more negative in membrane potential -> hyperpolarized.

The cones and rods are also specialized in some forms of wavelengths. Cones have different lengths to get different wavelents (three forms Short, Medium and Long), different humans have different sensitivity. This difference is also why humans and animals are sensitive to different wavelenghts.

There has been some research in genetic engineering cross animals and nanoantennaes to convert infrared to visible light.

Recycling the Retinol

In the disk, the cis-isomer is transformed into a trans-isomer, which are not usable anymore for the light, these are then recycled using some enzymes. Recycled in the retinal pigment epithelium. There is some transport to have it in the outer segment.

The 11-cis-retinal is a form of Vitamin A.

Differential responses of Rods and Cones

Rods -> scotopic vision, concentrated for black and white vision Cones -> photopic vision, concentrated for color and detail (you need a lot of light to activate cones).

Retinal Epithelium Recycling

When new disks are formed, old disks accumulate at the top of the rod, they get engulfed in pigment epithelium and separates from the rod.

Rods and cones are shed following the circadian rhythm.

Mainly used to recycle the components and remove the waste, produce the 11-cis-retinal molecule. Prevents disfunctional discs.

The disks provide several benefits:

• Light absorption
• Growth factors
• Phagocitosis?
• Visual Cycle
• And metabolism. I have no idea how.

On and Off center cells

Bipolar cells behave in opposite ways. They are connected to separate pathways of ganglion cells. Increase and decrease interleaved patterns, they flip their outputs, quite easily.

If you have surround, then they both spike, but a little bit less. This is probably useful to detect movements in the sides, probably good to detect edges of the image, useful for the V1 pathways in vision, see Architecture of the Brain, see paper from olshausen and Field, (Olshausen & Field 1996).

There is an experiment of Kuffler 1951 which is close to this. Hubel and Wiesel.

Pathways

The signals gets into Superior colliculus, that handles saccades and eye movements. Gets also to the lateral geniculate nucleus that then projects to the V1 (to check). The pretectum has reflex for pupil and lenses. See Architecture of the Brain.

Retinal Visual Projections

Projections to the Visual Cortices

Visual Pathways

References

[1] Olshausen & Field “Emergence of Simple-Cell Receptive Field Properties by Learning a Sparse Code for Natural Images” Vol. 381(6583), pp. 607--609 1996