Source: Module 4, Chapter 14 | Course: A Brain-Mind Odyssey (UC Berkeley)
Tags: retina, fovea, blind spot, rods, cones, rhodopsin, cone opsin, retinal, photoisomerisation, GPCR cascade, cGMP, bipolar cells, ganglion cells, optic nerve, lateral geniculate nucleus, LGN, visual cortex, V1, V4, V5, scotoma, hemianopia, achromatopsia, prosopagnosia, agnosia, receptive field
Vision begins at the retina, where rod and cone photoreceptor cells convert light into neural signals. Rods handle dim light using rhodopsin; cones handle colour vision at higher light levels using three types of cone opsin. A photon triggers photoisomerisation of retinal (cis to trans), which activates a GPCR cascade with enormous signal amplification. Retinal signals flow through bipolar and ganglion cells to the optic nerve, then primarily through the lateral geniculate nucleus (LGN) to visual cortex areas V1–V5 in the occipital lobe. Different cortical areas specialise in edges, colour, and motion, and lesions in these areas produce specific deficits such as scotoma, achromatopsia, motion blindness, and prosopagnosia.
Retina
A layer of light-sensitive photoreceptor cells plus several layers of interconnected nerve cells, heavily supplied by blood vessels (photoreceptors need glucose and oxygen). From Latin rete, net.
Fovea
The centre of the retina where the lens focuses light. Has the highest density of photoreceptor cells (especially cones), so visual acuity is best for whatever you are looking directly at.
Blind spot
A small region of the retina with no photoreceptors. This is where axons from retinal neurons bundle together to form the optic nerve and exit the eyeball. Light falling here is not detected. Each eye's blind spot is in a different location.
Rods
Photoreceptor cells that are rod-shaped, very numerous (~100 million per retina), distributed throughout most of the retina, and sensitive to very dim light. Contain rhodopsin.
Cones
Photoreceptor cells that are cone-shaped, mostly concentrated in the fovea (~5 million per retina), and respond to higher light intensities. Three types, each responding to a different range of wavelengths, enabling colour vision.
Rhodopsin
The GPCR photoreceptor protein in rod cells, responsible for sensing dim light. Each rod cell contains about 100 million rhodopsin molecules, embedded in lipid bilayer membrane disks in the outer segment.
Cone opsin
GPCR photoreceptor proteins in cone cells, embedded in highly infolded cell membrane of the outer segment. Three types exist, each tuned to a different wavelength range.
Retinal
A molecule derived from vitamin A (retinol) and carotenoids (e.g. beta-carotene). Cannot be synthesised from scratch; must come from dietary sources. Attached to opsin proteins via a covalent bond to a lysine residue. Exists in 11-cis form (kinked) before light absorption.
Retinol (vitamin A)
Differs from retinal by the addition of hydrogen to the oxygen at the end of the carbon chain (aldehyde to alcohol conversion).
Beta-carotene
A carotenoid widespread in plants (gives carrots their orange colour). One molecule of beta-carotene yields two molecules of retinal.
Photoisomerisation of retinal (cis to trans)
When a photon is absorbed by 11-cis retinal (bound to rhodopsin or cone opsin), the kink in the carbon chain straightens as the molecule rotates around a double bond, converting to all-trans retinal. This shape change activates the opsin protein.
Retinal achromatopsia
A genetic or developmental condition resulting in loss of all functional cone cells. The person sees only in shades of black, white, and grey.
Bipolar cells
Retinal neurons that receive synaptic input from rods and cones.
Ganglion cells
Retinal neurons that receive synaptic input from bipolar cells. Their axons bundle together to form the optic nerve.
Horizontal cells and amacrine cells
Two additional retinal cell types present in the same layer as bipolar cells, also wired into the flow of visual information to the brain.
Receptive field
The specific region of visual space to which a given neural cell in the visual system responds. Determined by the light-focusing properties of the eye.
Optic nerve
The bundle of ganglion cell axons that exits the eye at the blind spot and carries visual information to the brain.
Superior colliculus
A midbrain region receiving about 10% of optic nerve axons. Involved in rapid, non-conscious responses to sensory stimuli.
Lateral geniculate nucleus (LGN)
A pair of thalamic structures receiving nearly 90% of optic nerve axons. Relays visual information to visual cortex in the posterior occipital lobe.
Visual area 1 (V1)
Primary visual cortex in the posterior occipital lobe. Neurons here respond to edges of contrast at specific orientations, sometimes moving in specific directions.
V4
A visual cortical area where cells respond to specific colours and are less influenced by shape and movement.
V5
A visual cortical area where cells respond to movement, its speed, and direction, and are less influenced by shape and colour.
Scotoma
A blind spot in a specific region of visual space caused by a lesion in V1.
Hemianopia
Loss of vision in one half of visual space. Caused by a large lesion that damages all of V1 in one hemisphere.
Cortical achromatopsia
Loss of colour awareness caused by a lesion in V4 (as opposed to retinal achromatopsia, which is a retinal condition).
Motion blindness (akinetopsia)
Unawareness of movement in some region or all of visual space; the world appears as a series of snapshots. Caused by a lesion in V5.
Prosopagnosia
Difficulty or complete inability to recognise faces. Caused by a lesion in the inferior and medial temporal lobe, a region containing cells that respond selectively to face images.
Agnosia
A general neurological syndrome (Greek a, without; gnosis, knowledge). Visual agnosia involves difficulty recognising visual objects despite intact ability to perceive details. Often associated with lesions where the occipital, temporal, and parietal lobes converge.
The retina lines the back of the eye and contains photoreceptor cells (rods and cones), bipolar cells, ganglion cells, horizontal cells, and amacrine cells
The fovea is the central region with the highest cone density and the sharpest visual acuity
The blind spot is where ganglion cell axons exit as the optic nerve; no photoreceptors here
Each eye's blind spot is in a different location, so binocular vision fills the gap
Rods: ~100 million per retina, distributed broadly, sensitive to dim light, contain rhodopsin
Cones: ~5 million per retina, concentrated at the fovea, require brighter light, three types for colour vision
Colour perception is best at the fovea (where cones are densest) and when light is sufficiently bright
Outer segment of rod cells: stacks of lipid bilayer membrane disks, each packed with rhodopsin molecules
Outer segment of cone cells: highly infolded membrane containing cone opsins
Opsin proteins: ~350 amino acids, span the lipid bilayer seven times (seven transmembrane alpha-helices), with hydrophobic amino acids in the membrane-crossing regions
Retinal is covalently attached to a lysine residue within each opsin protein
Retinal cannot be made from scratch; it comes from dietary vitamin A (retinol) and carotenoids
Retinol differs from retinal by having an alcohol group instead of an aldehyde at the chain end
Beta-carotene (gives carrots their colour) yields two retinal molecules when cleaved
The sequence of events when a photon hits rhodopsin:
11-cis retinal absorbs a photon and isomerises to all-trans retinal (straightening the kink)
This shape change activates the opsin protein
Activated opsin binds and activates intracellular G-proteins (up to ~100 per activated rhodopsin)
Each activated G-protein activates cGMP phosphodiesterase
Phosphodiesterase hydrolyses cGMP to non-cyclic GMP (hundreds of molecules per enzyme)
Reduced cGMP concentration causes ion channels to close
Changed membrane potential alters neurotransmitter release at the synapse
The amplification is enormous: one photon can reduce intracellular cGMP by more than 10,000 molecules in roughly one second, closing many ion channels. This enables detection of extremely dim light.
Photoreceptors synapse with bipolar cells
Bipolar cells synapse with ganglion cells
Ganglion cell axons form the optic nerve
Horizontal cells and amacrine cells modulate signals laterally within the retina
Fibres from each eye split so that information from the left visual field goes to the right brain hemisphere, and vice versa
~10% of optic nerve axons go to the superior colliculus (rapid, non-conscious visual responses)
~90% go to the LGN in the thalamus, which projects to V1 in the posterior occipital lobe
V1 responds to edges and orientations; sends to V2, V3, V4, V5
V4 specialises in colour
V5 specialises in motion
Lesion in V1: scotoma (localised blind spot) or hemianopia (loss of one visual hemifield if damage is extensive)
Lesion in V4: cortical achromatopsia (loss of colour perception)
Lesion in V5: motion blindness/akinetopsia (world appears as a series of snapshots)
Lesion in inferior/medial temporal lobe: prosopagnosia (inability to recognise faces)
Lesion at occipital-temporal-parietal junction: visual agnosia (inability to integrate details into recognised objects)
GPCR amplification cascade (rod cell):
1 photon → 1 activated rhodopsin → ~100 activated G-proteins → ~100 activated phosphodiesterases → >10,000 cGMP molecules hydrolysed → many ion channels close → altered neurotransmitter release
⚠️ The GPCR amplification cascade is a centrepiece of this chapter. Be able to walk through each step and explain why the amplification enables dim-light detection.
⚠️ Know the difference between retinal achromatopsia (loss of cone cells, retinal origin) and cortical achromatopsia (V4 lesion, cortical origin). Same symptom, different cause.
⚠️ Match each visual cortex area to its specialisation: V1 = edges/orientation, V4 = colour, V5 = motion.
⚠️ Match each clinical deficit to its lesion location: scotoma = V1, cortical achromatopsia = V4, akinetopsia = V5, prosopagnosia = inferior/medial temporal lobe.
⚠️ Retinal comes from diet (vitamin A, beta-carotene). One beta-carotene yields two retinals. This is commonly tested.
⚠️ ~90% of optic nerve axons go to LGN (conscious vision), ~10% to superior colliculus (rapid reflexive responses).
Q: Describe the amplification cascade that occurs when a single photon activates rhodopsin in a rod cell.
A: The photon causes 11-cis retinal to isomerise to all-trans retinal, activating the opsin. One activated rhodopsin can bind and activate up to ~100 G-proteins. Each G-protein activates a cGMP phosphodiesterase, which hydrolyses hundreds of cGMP molecules. The resulting drop in cGMP closes ion channels, changing the membrane potential and altering neurotransmitter release. A single photon can cause the hydrolysis of more than 10,000 cGMP molecules in about one second.
Q: What is the difference between rods and cones in terms of location, number, and function?
A: Rods (~100 million) are distributed broadly across the retina and are sensitive to dim light. Cones (~5 million) are concentrated at the fovea and respond to brighter light; three types of cones enable colour vision.
Q: A patient loses colour awareness but can still perceive shapes and motion. Where is the likely lesion?
A: V4. This is cortical achromatopsia.
Q: Why is colour perception best when you look directly at an object?
A: Because looking directly at an object focuses its image on the fovea, where cone cells (responsible for colour vision) are most densely concentrated.
Q: What causes the blind spot, and why don't we usually notice it?
A: The blind spot is where ganglion cell axons exit the eye as the optic nerve, leaving no photoreceptors at that point. We don't usually notice it because each eye's blind spot is in a different location, so binocular vision compensates.
Q: What is prosopagnosia, and where is the lesion?
A: Prosopagnosia is difficulty or complete inability to recognise faces. It results from a lesion in the inferior and medial temporal lobe, a region containing cells that respond selectively to faces.
retina, fovea, blind spot, optic nerve, rods, cones, rhodopsin, cone opsin, photoreceptor, outer segment, inner segment, lipid bilayer, retinal, retinol, vitamin A, beta-carotene, carotenoid, photoisomerisation, 11-cis retinal, all-trans retinal, GPCR cascade, G-protein, cGMP, cGMP phosphodiesterase, signal amplification, bipolar cells, ganglion cells, horizontal cells, amacrine cells, receptive field, superior colliculus, lateral geniculate nucleus, LGN, V1, V2, V3, V4, V5, visual cortex, occipital lobe, scotoma, hemianopia, retinal achromatopsia, cortical achromatopsia, akinetopsia, motion blindness, prosopagnosia, agnosia, visual agnosia