Day 4: Visual Processing

Where are the photoreceptors located

• what are photoreceptors and what do they do?

  • What is a process that they do?

• in the retina

• They are light-sensitive neurons that convert light energy into electrical energy in cells.

• The conversion is called phototransduction.

Two types of photoreceptors in the retina

• how many of each

• What are they?

• How do they respond to stimuli (what does the photoreceptors do next right after receiving stimuli)

• Each retina contains ~6 million cones and 120 million rods.

• Rods and cones are neurons, though they do not fire action potentials, but instead respond to stimuli with graded membrane potentials.

Basic structure of cones and rods

• outer segment structure

• In the outer segment, the membrane folds into disk-like layers which contain the visual pigments that respond to light.

• In the inner segment are the nucleus and organelles for protein synthesis; and in a basal layer (base of the photoreceptor cell), a synapse that releases glutamate.

• Both receptor types point toward the back of the eye.

The retinal architecture of the inner layer of the eye

• what do we have in the inner layer of the eye that prevents us from seeing a particular region in out vision

  • Explain why do we have this

  • What does it not have in this region

• What are muller glial cells

  • What does it act like

  • Two functions it plays

• Remember we do have a blind spot — a region where the optic nerve exits the eye and there are no photoreceptors.

• Müller glial cells play a crucial optical role. They span the entire retina and actually act like optical fibers, guiding light through the neural layers and reducing light scatter

Photoreceptors detect light using membrane-bound visual pigments

• what happens when light hits the pigments

• What happens to glutamate

• When are photoreceptors more active

• Number of type of pigments in rod vs cone

• When light hits them, pigment molecules change shape, starting a chemical cascade that hyperpolarizes the cell, reducing its release of glutamate, i.e. photoreceptors are more active in darkness.

• Each photoreceptor contains millions of molecules of its pigment, but each type of photoreceptor has just one type of pigment: rhodopsin in rods, 3 other pigments in 3 types of cone.

Type of pigment in rod

Rhodopsin

Phototransduction- Turning Light Into Signals

• precursor for retinal

• Why is it important for pigment

• What is Nyctalopia

• Vitamin A is the precursor for retinal.

• Without it: rhodopsin cannot regenerate, rods remain inactive

• This leads to nyctalopia (night blindness)- impaired vision in low-light conditions.

Dark vs light

• when is cones used

• when is rods used

• Sensitivity level of both

• How are each one deactivation in their opposite conditions

• Cones are for bright light, rods for dim

• Cones are less sensitive than rods; they are responsible for vision in bright light and for distinguishing colors, but they don’t operate in dim conditions.

• Rods can detect single photons (more sensitive). But they operate only in low light: in daylight they are “bleached out” , i.e. their rhodopsin is broken down so they can’t sense light.

  • When the lights go dim the rods dark adapt, i.e. they rebuild their stores of rhodopsin over ~30 minutes.

How photoreceptors are distributed

• where are most of them found (both cones and rods)

  • The general area - name it

  • The very dense spot in that area - name it

• Blind spot - why does this exist

  • How com we dont notice it?

• They are most densely packed in the macula, a central disk 5.5 mm across, and especially in its central pit called the fovea, 1.5 mm across (2° of visual angle — about as wide as 2 fingers viewed at arm’s length).

• We use the fovea for detailed vision. 5° away from its center, acuity is quartered; at 20° it falls below the standard for legal blindness.

• There are no receptors in the blind spot — the hole where axons carrying visual information exit the eyeball to form the optic nerve.

  • Normally you don’t notice your blind spots because objects in one eye’s blind spot aren’t in the other eye’s, and because the brain fills in the gaps.

Cones and rods are distributed differently

• what does the fovea have more of

• what receptors does the rest of the macula have more of?

• The fovea contains almost exclusively cones. More-peripheral retina contains mainly rods.

• That is one reason your peripheral retina is more sensitive to light than your fovea is; e.g. you can see a dim star better by looking slightly away from it.

The 3 Layers of Neurons in the Retina

• Receptors

• Bipolar cells

• Ganglion cells

• Up to 45 photoreceptors may converge on a single bipolar cell. Bipolars in turn converge on ganglion cells, so in each eye the signal from 126 million receptors is condensed into 1 million ganglion cells.

• Convergence is greatest in the peripheral retina and least in the fovea, where some receptors project 1:1 to bipolars.

Two kinds of receptive fields for bipolar cells

• draw the two parts and name them

• Explain what on-center is and what off center cells is

  • Explain area of sensitivity and pathways that are affected (metabotropic vs ionotropic pathways and what receptor for each pathway - name them)

• Bipolar cells have center-surround receptive fields

• Every neuron in the visual system has a receptive field, also called its visual field — the region of the retina where light affects the cell’s activity, i.e. the set of photoreceptors which affect the cell.

• Bipolar cells have center-surround fields, with a round center region and a doughnut-shaped surround.

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• On-center cells are excited by light in the center of their field, and inhibited by light in the surround. So these cells respond most when a light spot fills their center and the surround is dark.

• Off-center cells are inhibited by light in the center, and excited by light in the surround. They respond best when a dark spot fills their center and the surround is light.

What type of light do bipolar cells react to?

• explain why they aren’t activated in some cases

• Explain how they activate when they react to ___

• Firing method

• React to contrast

• When the lighting is uniform, whether bright or dim, neither type of bipolar cell responds, because the effects of the center and surround cancel, leaving the cell at its resting level of activity (only weakly active).

• In other words, bipolar cells react strongly only when the lighting is not uniform, i.e. they respond to contrast.

• They respond with graded membrane potentials; they do not fire action potentials.

retinal ganglion cells

• type of firing of receptors

• similarity to bipolar cells

  • Two things

    • Type of cells

    • What they detect

• Fires action potential

• But like bipolar cells, most retinal ganglion cells also have center surround receptive fields, and these fields can also be on-center or off-center.

• Detect contrast

Ganglion cells in different parts of the retina have different-sized receptive fields

• near the fovea vs in the peripheral

  • number of bipolar cells converging

  • Level of sensitivity

  • level of spatial resolution

• A ganglion cell near the fovea gets input (via bipolars) from only a few photoreceptors, mostly cones. Farther out, each ganglion cell gets input from many receptors (up to 75,000), mostly rods.

• So in the periphery, each ganglion cell is very sensitive to light but poor at reporting spatial detail because it blends information from a wide swathe of receptors.

• And near the fovea, ganglion cells are less sensitive to light but have better spatial resolution because each one gets input from just a few densely packed cones.

Ganglion cells

• types (in terms of classification)

  • M cells - full name - what. it detects

  • P cells - full name - what it detects

  • Melaonpsin - what does it have - what do they do to the brain

  • Percentage of ganglion cells that are M cells, P cells, Melanopsin cells

• Large, magnocellular ganglion cells, or M cells, provide information that is used by the brain to infer the movement of objects. These cells are phasic. ~10% of retinal ganglion cells are M.

• Small, parvocellular ganglion cells, or P cells, provide information that is used to infer form and fine detail, such as texture. ~70% of retinal ganglion cells are P.

• ~1% of retinal ganglion cells are melanopsin ganglion cells, which are photoreceptors, with their own visual pigment, melanopsin. They project to the suprachiasmatic nucleus, a center for circadian rhythms.

Full visual field involves…

• explain each individual field

• two eyes

• left hemifield involves which part of each eye

• Right hemifield involves which part of each eye

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hint: think of the located toward the nose vs away from the face

• the section of each eye will be the same side

• left hemifield - right side of each eye

  • left eye: nasal retina - more closer to the nose

  • right eye: temporal retina - away from the nose

• Right hemifield - left side of each eye

  • Left eye: temporal retina - away from the nose

  • eight eye: nasal retina - closer to the nose

What field is the monocular visual field/central overlap

• draw location in the field of the general eyes

• what is it helpful for in vision

What field is the binocular visual field

• draw location in the field of the general eyes

• what is it helpful for in vision

• Central overlap/binocular visual field is essential for depth perception and 3D spatial awareness

• the monocular zones extend your peripheral awareness

Where doe the visual info from the retina reach what type of nerves

• two names for it

• Where is these kind of nerve located in the eye - what is this located called

• Visual information leaves the retinas in the optic nerves

• Each of the million ganglion cells in each retina sends its axon out the back of the eye through the blind spot.

• These million fibers from each eye form its optic nerve, also known as cranial nerve II.

Where in the nerve does half of the nerve fibers split in two separate ways (for left and right hemifield information)

• draw a diagram if needed

• In what cerebral hemisphere do each group go to (draw if needed)

• the optic chiasm

• In the eye, the right side of the scene (the right visual hemifield) projects onto the left side of each retina, i.e. onto the nasal side of the right retina and the temporal side of the leftretina.

• Because the nasal fibers cross, all the information from the right hemifield comes together in the left cerebral hemisphere, and vice versa.

• What are the optic tracts

  • When it splits where does it end up in the thalamus - name

    • After the thalamus where does it go to in the brain and how does it reach there (what is the type of signalling does it do to reach there?)

    • Difference in number of neurons for each destination (2 destinations)

  • Through the type of signalling, what two lobes can it reach and pass through?

    • For each what is the type of visual field processing is it?

• The nerve bundles emerging from the chiasm are called the optic tracts. They end in the 2 lateral geniculate nuclei (LGN) in the thalamus, which project via the optic radiations to primary visual cortex,V1.

• Optic radiations are going through either the parietal lobe carrying information from the lower visual field vs. one looping through the temporal lobe — called Meyer’s loop — carrying information from the upper visual field

• Together, the 2 LGN have ~2 million neurons — the same as the number of ganglion cells in the 2 retinas — whereas V1 has far more neurons, ~300 million.

In what lobes of the brain is the primary visual cortex located?

• What area (name) is most of the visual cortex concentrated in this lobe?

• in the occipital lobe (back of the brain)

• The calcarine sulcus is associated with the visual cortex. It is where the primary visual cortex (V1) is concentrated.

Organizing the visual areas in the brain in connection with what part of the eye?

• What is this called?

• Where is this arrangement organization found in which areas of the brain

• Give an example. Light hitting in the center vs peripheral

• visual areas connected with retina areas

  • That is, neurons close to each other in the brain get information from close-together parts of the retina.

• Called retinotopically organized

• This arrangement is found in the lateral geniculate nuclei, V1, and many higher visual processing areas.

• E.g. when light hits the central retina, shown in red, then the region shown red in V1 is active; light in more-peripheral retina activates more-peripheral parts of V1.

The retinotopic map

• which part of the eye gets more are in the brain for processing and why?

• The fovea, which covers only a small area of the retina (0.01% of the retina’s total surface area), projects to large areas in V1 (and in other cortical regions and LGN),

• i.e. a large proportion of visual cells in the brain receive and process data from foveal photoreceptors.

• The fovea gets a lot of space because it has many photoreceptors, bipolars, and ganglion cells, and so carries a lot of information.

Visual field deficits

• explain each type

  • Anopia

  • Scotoma

  • Monocular

  • Binocular

  • Hemianopia

  • Quandrantanopia

  • Homonymous

  • Bitemporal

  • Contralateral

• Anopia = General term for vision loss

• Scotoma = Focal area of lost vision

• Monocular = Affecting one eye

• Binocular = Affecting both eyes

• Hemianopia = Loss of half the visual field

• Quadrantanopia = Loss of one-quarter of the visual field

• Homonymous = Same side lost in both eyes

• Bitemporal = Outer halves of both eyes lost

• Contralateral = Lesion is on the opposite side of the lost visual field