Optics of Vision, The Retina & the Retinal Visual Field

What anatomical feature determines the visual field?

• The head morphology

• Cat has larger binocular field due to more forward facing eyes

• Horse have a very small binocular field of vision with extended monocular vision due to eyes being on the side of the head

What does vision require to be detected?

• Vision requires detection of various light wavelengths

• Humans, apes and Old world monkeys have enhanced visual acuity, distance perception and colour range compared to other mammals.

• Birds, are superior and have the most advanced visual sense of all animal kingdom. Photons (light particles) contain more energy in lower (i.e. violet) range and less in the higher (i.e. red) spectrum range.

  • High energy is detrimental to sensory cells. Infrared waves are usually not detected by retinal photoreceptors, many animals absorb heat energy for body processes

What is the fovea centralis?

Region of the retina which lacks photoreceptors

What gland has the main role of keeping the lens clean within the eye?

• The lacrimal gland

  • Lacrimal gland secrets saline tear onto the eye through ducts

• Fluid cleans and lubricates the front of the eye during a blink

  • Prevents cornea from drying out and frost injury during cold weather

• Tears are empty into the nasal cavity and function to maintain mucous membrane within the cavity

• Fluid contains Ivsozvme and immunoglobulin A to protect against infections

Describe the two axis of the eye involved in light detection. 

• Optical Axis - Maximizes the visual field presented to the back of the eye, giving optically clear image (Crosses the cornea, lens dead center)

• Visual Axis - visual axis to fovea gives the best color vision

How are images refracted through the eye or lens?

• Light 'bends' (i.e. refraction) when passing from across the cornea, lens and vitreous humor

  • Leads to an inverted image in the back of the eye

• Light information through a convergent lens are refracted and focused at a focal point in the back of the eye

• Distance from the focal point behind the lens is dependent on the distance to the image

• Divergent lens does not produce a focused image at a focal point; distributed leads to a distributed production of that image

What is the accommodation ability of the lens?

Ability of lens to change power - through ciliary muscles

• Focal length is fixed but increased power increases ability of lens to refract divergent light (ie near objects) and achieve focus

• Ciliary body relaxes = expansion of the lens

• Ciliary body contracts = decreased in elongated structure

Describe the species variabilities in accomodation.

Horse lacks full accommodation ability. Shape of retina means it can see near and far by moving head/eyes

• Distant objects - head depressed

• Close objects - head raised

  • to maximize acuity

Describe the neural response to light. 

Light enters - Binding to photoreceptors at the back of the retina (Either cones or rods)

• Conveyed back to front via the bipolar cells and retinal ganglion cells which integrate information and send it along the optic nerve

  • Ganglion cells also express photoreceptors called melanopsins

Describe the difference in light detection between the cones and rods.

Rods

• Have a number of discs which have plasma membranes (Photoreceptors like rhodopsin are embedded), these are involved in light detection and irradiance

• Rhodopsin contains retinal - involved in light detection, when it is detected the protein retinal becomes a trans isomer (Isomerisation)

• To become responsive to light again, enzymes must transform it back into cis isomer or retinal

Describe the intracellular phototransduction of rhodopsin,

1. Photon hits rhodopsin (R)

2. Isomerisation of retinal

3. Activation of alpha subunit of transducin

4. Activation of phosphodiesterase (PDE)

5. Decreases GMP levels and closes ion channels

Hyperpolarises the receptor cell, leading to the release of NTs

How does the distribution of photoreceptors change across the retina?

• Ratio of receptors to nerves also changes across retina.

• At fovea nerve number = receptors, at periphery ~300 receptor/afferent nerve

Describe the light sensitivity between the rods and cones within the retina.

Rods are much more sensitive to light (irradiance).

• One rod responds to one photon

• There are multiple rods per bipolar cell

• Rods work in the scotopic region

• Rods are poor for spatial detail

Cones provide more detailed information (including colour)

• But they need more light (photopic region), more photons required to activate obne cone receptor

• The ratio of cones to ganglion cells is 1:1

What role do the ganglion cells have in their relationship to the photoreceptors?

Ganglion cells integrate multiple photoreceptor signals

• Some ganglion show higher activation when light is shown in the center of the field

• Some show higher activation when light is shown in the periphery

  • Depending on receptors, determines the frequency of action potentials

Ganglion cells show ____ ___________.

intrinsic photoresponsiveness

What is melanopsin?

A photoreceptor that responds to irradiance information, the signals control the daily rhythms

• Melanopsin is very sensitive to light in blue light range

What is the clinical relevance of the pupillary light reflex?

• Pupillary reflexes indicate functional state of the afferent and efferent that control the pupil

• For example, if a light stimulus directed to the left eye elicits a consensual constriction in the right eye, but not a direct one in the left eye, then the afferent limb of the reflex (i.e. optic nerve is intact; but the efferent limb to the left eye is damaged)

• Possible reason: damaged oculomotor nerve

• If shining light and there is a response in both eyes, optic nerve is intact

• If there is a lack of response, there is damage to ciliary ganglion or the preganglionic parasympathetic fiber

What is the dazzle reflex?

Subcortical mediated brainstem response

• Palpebral fissure closes in response (e.g. blink, head movement) in response to sudden intense illumination of the eye

• Ipsilateral response is greater than the contralateral response

• Absence of a response indicates blindness

Retina recieves light (CN II(Optic Nerve)) into rostral colliculi (Midbrain) through CNVII (Facial Nerve) → Response = blink, sudden movement, blindness

• Tests CNII and CNVII

Describe the neural circuit in the eyes from the retina to the lateral geniculate nucleus.

Lateral geniculate nucleus located in the thalamus

• LGN, maintains a retinotopic (i.e. topography)

  • 6 layers of the 'retinal map' consists of parvocellular (1-4) and magnocellular (5,6) cells

  • The lateral geniculate nucleus (LGN) is called a retinal map because it preserves the spatial layout of the retina — neighboring points on the retina project to neighboring points in the LGN. This retinotopic organization keeps the visual scene’s structure intact as it’s relayed to the visual cortex.

• Parvocellular cells are small, integrate signals from cones and necessary for colour and form

• Magnocellular cells are large, integrate signals from rods, involved in movement, depth and Irradiance

From the LGN → where do the cells project?

• The layers of cells project into the primary visual cortex or occipital lobe (Located in the back of the brain)

• The primary visual cortex integrates multiple different LGN cells, in an attempt to formulate visual perception

What three cells are found in the primary visual cortex?

Simple cells - respond to stimulus with correct orientation

• Simple stimuli move in a particular direction, received by the complex cells

Complex cells - respond to moving stimulus of correct orientation

Hyper-complex cells - shows end-stopping - responds to objects of a particular size.

• Particular light stimulus of a certain size has an end direction

How do simple cells, complex cells and hypercomplex cells react to light?

🟦 Simple cells

• Respond to: a bar or edge of light at a specific orientation and position in their receptive field.

• How they react: they fire most when the light bar is at the right angle and in the right place.

• Example: a vertical bar in the center of the field excites them; moving it slightly or rotating it reduces firing.

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🟩 Complex cells

• Respond to: a bar or edge of light at a specific orientation, and the direction of movement.

• How they react: they are less sensitive to the exact position — they respond as long as the oriented edge moves through their area (often motion-sensitive).

• Example: a vertical line moving across the field keeps them firing.

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🟥 Hypercomplex (end-stopped) cells

• Respond to: a bar or edge of a specific length or corner, often ending within their receptive field.

• How they react: they fire when the line or edge is the right orientation and length, but stop firing if it extends too far into the off region. 

• Example: best response to a short vertical bar, weaker or no response to a longer one.

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In short:

Simple cells detect edges, complex cells detect moving edges, and hypercomplex cells detect line ends or corners.

Summarize the neural circuit for vision.

• 🟡 Photoreceptors (rods and cones) – in the retina, convert light into electrical signals.

• 🟢 Bipolar cells – relay signals from photoreceptors to ganglion cells.

• 🔵 Ganglion cells – their axons form the optic nerve, carrying visual information to the brain.

• ⚪ Optic chiasm – where fibers from the nasal half of each retina cross to the opposite side, ensuring that each hemisphere processes the opposite visual field.

• 🔴 Lateral Geniculate Nucleus (LGN) – in the thalamus; acts as a relay and maintains the retinotopic map.

• 🟣 Primary Visual Cortex (V1) – processes basic features like orientation, edges, and movement.

• 🔶 Higher visual areas (V2, V3, V4, MT, etc.) – integrate information for object recognition, color, depth, and motion.

  • Separation into dorsal and ventral streams:

Describe the dorsal stream and ventral streams.

• Dorsal stream (Or the action stream) flows from V1 to V5/V3A

  • Guides movements such as the hand postures for grasping a mug or pen

  • damage to dorsal stream results in optic ataxia

• Ventral Stream (Or the recognition stream), helps us recognize what is in the environment

  • Damage to the ventral stream prevents identification of objects

  • Reaching movements directed toward individual body (e.g. tactile or proprincentive feedhack) remains intact

What is the association cortex and grandmother cells?

The association cortex interprets and combines sensory information for recognition and meaning, while “grandmother cells” are a (mostly theoretical) idea of single neurons that respond to one very specific stimulus

• Within the temporal cortex → direction of movement integrated at lower levels is integrated in temporal cortex, giving rise to species specific responses

What are some of the non-image forming pathways responsible for biological rhythms?

• Circadian Rhythm and the SCN

  • Melanopsin-containing retinal ganglion cells detect light levels (especially blue light) and send signals through the optic nerve to the suprachiasmatic nucleus (SCN) in the hypothalamus.

  • The SCN acts as the body’s master clock, regulating daily (circadian) rhythms like sleep, wakefulness, hormone release, immune function, and behavior.

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  🧬 Molecular Clock Mechanism (in SCN neurons)

  1. During the day, the genes BMAL1 and CLOCK are active.

  2. These genes produce proteins (BMAL1 and CLOCK) that form a dimer, which activates other genes — Period (Per1, Per2) and Cryptochrome (Cry1, Cry2).

  3. PER and CRY proteins build up in the cytoplasm, pair up (dimerize), and move back into the nucleus.

  4. There, they inhibit BMAL1 and CLOCK, stopping their own production.

  5. As PER and CRY levels fall overnight, BMAL1 and CLOCK become active again — restarting the cycle.

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  ⏰ Result

  • This feedback loop takes about 24 hours, creating a daily rhythm that synchronizes the body’s internal clocks with the external light–dark cycle.

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  In short:

  Light → Melanopsin cells → SCN → activates BMAL1 & CLOCK → produce PER & CRY → inhibit BMAL1 & CLOCK → cycle repeats every 24 hours, setting the body’s circadian rhythm.

Summarize the four man visual and non-visual pathways in the brain. 

Visual Field = 1

Blink/Head movements = 2

Pupillary Light reflex = 3

Control of rhythms = 4

What is visual adaptation or afteraffects?

• Visual adaptation means your eyes and brain adjust to constant stimulation.

• When you look at something for a while, the neurons responding to it get tired (their response decreases).

• When you look away, you can get an aftereffect — you briefly see the opposite pattern or motion.

What are the main photoreceptors that underlie color vision?

The cones

• 3 Main types: red, green, blue

• Each responds to a different wavelength

What are the three main theories of color vision?

Trichromatic theory - colour vision that is based on the coding of the three basic colours: red, green and blue

Opponent-process theory - colour vision that emphasizes the importance of the opposition of pairs of colours: red versus green and blue versus yellow.

• Test of OPT - stare at cross for 30 seconds; then look at a blank screen

The detection of color starts within the ganglion cells.

Multiple colour opsins information is integrated by gangiolonic cells; then transmit information to LGN parvocellular cells

Which pathway is dependent on color vision?

Which is not?

dorsal - no color vision

ventral - color vision

Describe the organization of the primary visual cortex.

1. Ocular dominance columns

   • Stripes of neurons in V1 that prefer input from one eye (left or right).

   • Help the brain combine signals from both eyes for depth perception.

2. Orientation columns / pathways

   • Neurons in V1 are arranged so that neighboring cells prefer similar line orientations (vertical, horizontal, diagonal).

   • This allows the brain to detect edges, shapes, and contours.

3. Blobs

   • Interspersed within these columns, blobs are color-sensitive neurons.

   • Mostly respond to wavelength (color) rather than orientation or motion.

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In short:

V1 is organized into ocular dominance columns (eye preference), orientation columns (edge detection), and blobs (color), so different features of vision are processed in specialized, interwoven areas.