Vision Flashcards
Describe visual processing in the retina:
Photoreceptors
- Light-sensitive cells that convert photons into electrical signals)
- rods are more numerous highly sensitive to light, enable night vision, but do not perceive colour
- cones are less numerous, concentrated at the fovea, responsible for colour vision and function best under bright lights
Bipolar cells - receive input from photoreceptor cells and modulate the signals before passing it to ganglion cells
Retinal ganglion cells:
- output neurones of the retina transmit visual information via the optic nerve to the brain
- process spatial, color, and motion cues before sending signals to the brain
- RGC axons in the retina aren’t myelinated but become myelinated in the optic nerve due to oligodendrocytes
- Signals travel via the optic nerve to the LGN, Superior Colliculus, and Pretectal Nucleus
Describe the different types of retinal ganglion cells:
Magnocellular:
- M cells/ Midget cells
- rods for movement and contrast
- low spatial resolution
Parvoceullar:
- Parasol cells
- M + L cones
- High spatial resolution, color vision, slower response
Koniocellular:
- Bistratified cells
- S cones (blue)
Intrinsic Photosensitive ganglion cells or ipRGCs - control of pupil diameter, circadian rhythm
Explain the Central (lower) visual pathways (ganglion cells to thalamus)
This pathway transmits raw visual information from the eye to the thalamus for initial processing
Retinal ganglion cells:
- photoreceptors detect light and convert it into electrical signals
- bipolar cells process these signals and send them to RCGs
- Ganglion cell axons form the optic nerve (cranial nerve II)
Optic nerve and chiasm:
- optic nerves from both eyes meet at the optic chiasm
- fibres from the nasal retina (inner half of each eye) cross to the opposite side
- fibres from the temporal retina (outer half) stay on the same side
- this crossing ensures that the left visual field is processed in the right hemisphere and vice versa
Optic tract and Lateral Geniculate Nucleus:
- optic tract carries visual information to the LGN in the thalamus
- LGN is a relay center in the thalamus that organizes and filters signals before sending them to the cortex
- has six layers, receiving input from both eyes but keeping the signals separate
- LGN distinguishes between motion, contrast, and color using different types of ganglion cells (magnocellular & parvocellular pathways)
Explain the Cortical (higher) visual pathways (from thalamus to cortex):
Optic radiations:
- neurones from the LGN project to the primary visual cortex (V1, located in the occipital lobe)
- 2 pathways carry visual information
- Meyer’s loop (Temporal) → upper visual field (lower retina)
- Parietal pathway → lower visual field (upper retina)
Primary Visual Cortex:
- organises the image based on features like edges, orientation, and contrast
- information is sent to higher visual areas (V2–V5) for complex processing
Higher visual pathways:
- V2 & V3: Shape recognition and depth perception.
- V4: Color processing.
- V5: Motion detection
Dorsal stream:
- where pathway, parietal lobe
- processes motion, spatial awareness, and object location
Ventral stream:
- “What” pathway, temporal lobe
- Processes object recognition, faces, and colors
- Involves the inferotemporal cortex
Describe the fovea and blind spot:
Fovea - Densely packed with cones, responsible for high-acuity vision
Blind spot - optic disk region lacks photoreceptors where the optic nerve exits the eye
Describe the dark phase of photo-transduction
cGMP formed in outer segment and opens cation channels in the OG membrane
Generates a dark current and outer segment depolarises
Depolarisation spreads to inner segment VGCCs open in presynaptic membrane
Vesicles containing glutamate fuse with pre-synaptic membrane, then released inti synaptic cleft
mGLUR6 receptors on the Bipolar cell activate
Gαo inhibits TRPM1 in ON bipolar cell membrane – hyperpolarisation – ON bipolar cell switched off
cGMP-gated Na+ channels remain open, maintaining depolarization
Describe the light phase of photo-transduction
Light causes opsin to isomerise retinal and activate transducin which then activates phosphodiesterase (PDE)
PDE breaks down cGMP
CNGAs close, reducing dark current and cell hyperpolarises
Hyperpolarisation spreads to the IS and VGCCs close in presynaptic membrane decreasing glutamate release
mGLUR6 receptors inactivate + Gαo stops inhibiting TRPM1
TRPM1 channels open – Na+ enters and ON bipolar cell depolarises
Describe the bipolar cell activity:
ON Bipolar cells:
- depolarise when light increases
- express metabotropic glutamate receptors (mGluR6), which cause depolarisation when glutamate levels drop
OFF Bipolar cells:
- depolarise when light decreases
- express ionotropic glutamate receptors (AMPA/kainate), which cause depolarsation when glutamate bind
play a key role in contrast enhancement and edge detection via center-surround receptive fields
Their graded response allows smooth modulation of visual signals before transmission to ganglion cells
his differential response system ensures efficient detection of brightness changes and enhances the visual system’s ability to process fine details and contrast
Describe the transmission of colour by bipolar cells
Colour perception arises from different cone-bipolar cell connections, forming opponent colour systems
Depending on the wavelength of light detected the type of signals from S, M or L cones + rods will activate the appropriate
Bipolar cell and switch off the opposing one
Describe the Pupillary responses and the Pretectal nucleus:
Pupillary light reflex controls the diameter of the pupil in response to light, regulating the amount of light entering the eye
The relfex is bilateral, light stimulation in one eye affects both pupils
Pathway:
Retina: Light activates retinal photoreceptors
Optic Nerve (CN II): Signal travels via optic nerve to the pretectal nucleus in the midbrain
Pretectal Nucleus: Sends bilateral projections to the Edinger-Westphal nucleus (EWN) of the oculomotor nerve (CN III)
Edinger-Westphal Nucleus: Provides parasympathetic innervation to the ciliary ganglion via CN III
Ciliary Ganglion: Sends postganglionic fibers to the sphincter pupillae muscle, causing pupil constriction
Pretectal Nucleus:
- Located in the midbrain, anterior to the superior colliculus
- Essential for coordinating pupillary responses
- stimulates both sides of EW Nucleus even though light was only received in 1 eye
Describe the Lateral geniculate nucleus & visual perception:
Function:
- Key relay centre in the thalamus for visual processing before it reaches the primary visual cortex
- acts as a filtering station that modulates sensory input from the retina
- Enhances contrast via center-surround receptive fields
Structure: 6 layers
Magnocellular - layers 1+2,process motion, brightness, and contrast
Parvocellular - layers 3-6, process colour, fine detail and texture
Koniocellular sublayers - between layers, colour perception
Input - retinal ganglion cells
Output - sends signals to primary visual cortex
Describe the bipolar cell to ganglion cell synaptic activity
All bipolar cells when stimulated by a photoreceptor transfer the graded signal to a neighbouring retinal ganglion cell via glutamate release
Postsynaptically iGLUR receptors in the retinal ganglion cell cause depolarisation
Action potential is stimulated when the threshold for Na+ channel opening is reached in a RCG axon
Describe the nervous connections to the visual cortex:
Retina → Optic Nerve (CN II) →
Optic Chiasm → Optic Tract
Lateral Geniculate Nucleus (LGN) of the Thalamus
Superior Colliculus (for reflex eye movements)
Pulvinar (for visual attention and integration with other sensory modalities)
Optic Radiations → Visual Cortex (V1 in the occipital lobe):
- Meyer’s loop (temporal pathway): Carries information from the upper visual field
- Parietal pathway: Carries information from the lower visual field
What are the different brain regions that retinal ganglion cells (RGCs) project to, and what functions are associated with these projections?”
M,P&K RGCs project to the Thalamus (Lateral Geniculate Nuclei) - essential for normal visual perception
But also other RGCs (eg intrinsically photosensitive or ipRGCs) project to the hypothalamus, superior colliculus & pretectum
Circadian rhythms, reflex orientation to visual stimuli, accommodation and control of pupil diameter
None of these functions are dependent on rods and cones but rather start in the RGC layer
Describe accommodation:
Accommodation is the process by which the eye changes optical power to maintain a clear focus on objects at varying distances
Lens shape:
- ciliary muscles contract or relax, changing the shape of the lens
- more curved lens (via contraction) increases refractive power for near vision
- flatter lens (via relaxation) decreases refractive power for distance vision
Ciliary Muscles:
- When focusing on a near object, the ciliary muscles contract
- this contraction reduces tension on the zonular fibers (suspensory ligaments), allowing the lens to become more rounded
Retinal image focus - adjusted lens refracts (bends) light rays properly so that they converge precisely on the retina
Neural control:
Afferent: Retinal input → Optic nerve → LGN → Visual cortex.
Efferent: Edinger-Westphal nucleus → Ciliary ganglion → Ciliary muscle & pupillary sphincter
describe some disorders of higher visual processing:
occur due to damage in the extrastriate cortex V1
Ventral pathways:
Cerebral achromatopsia - complete absence of colour perception
Cerebral dyschromatopsia - impaired color perception (hues and saturation)
Colour anomia or agnosia - can discriminate colours accurately but cannot name them
Visual agnosia no longer recognize previously familiar objects and cannot learn to identify new objects by sight alone
Prosopagnosia - impaired ability to recognize familiar faces or to learn to recognize new faces
Alexia - can write but cannot read well - disconnection of the visual input in both hemifields from language areas in the left hemisphere
Dorsal pathways:
Cerebral akinetopsia - a selective impairment in motion perception
Balint’s syndrome - a deficit in attention
Blindsight - impaired conscious awareness of visual stimuli
Describe the higher visual processing centres:
Primary Visual Cortex (V1, Striate Cortex): Basic visual processing.
Secondary Visual Areas (V2-V5): Complex processing like motion, depth, and color.
Dorsal Stream (“Where” Pathway, V1 → Parietal Lobe): Spatial awareness, motion detection.
Ventral Stream (“What” Pathway, V1 → Temporal Lobe): Object recognition, face perception.
Describe the photoreceptor structure:
Outer Segment (OS)
Contains light-sensitive photopigments (opsins) embedded in stacked membrane discs.
Rods → Contain rhodopsin (high sensitivity, night vision).
Cones → Contain cone opsins (red, green, blue for color vision).
Discs increase surface area for photon capture.
Connecting Cilium
Narrow structure linking the outer and inner segments.
Acts as a transport bridge for proteins and molecules from the inner segment to the outer segment.
Inner Segment (IS)
Contains mitochondria (high energy demand for phototransduction).
Houses ribosomes & Golgi apparatus → Synthesizes proteins (e.g., opsins).
Nucleus
Located in the outer nuclear layer (ONL) of the retina.
Contains the genetic material for photoreceptor maintenance and function.
Synaptic Terminal
Forms synapses with bipolar cells and horizontal cells in the retina.
Releases glutamate (the primary neurotransmitter) in a graded manner based on light intensity.
