Lecture 16 Vision I Flashcards
3 basic parts required for vision
Eyes, optic connections to brain, Brain
Describe the process of vision
Light passes through cornea & lens (transparent)
Focused on photoreceptors in retina
Retina send info as electrical signals by optic nerve to brain
Brain - process image - see
Parts of eye and function
1. Lens & cornea:
2. Lens:
3. Ciliary muscle:
4. Retina
- Bend light to focus on retina
- Flatten when object far, round up when near
Nutrients from aqueous & virtreous humour
Waste removed to aqueous humour
Haem pigments.- absorb light - Change lens shape
- Transducer signal into electrical impulse
How is image projected into retina
Convex lens surface -> light rays converge
Top light directed bottom. Bottom light directed top
Back appears front, front appears back
-> image upside down in retina
- What is the function of photoreceptors?
- List the two types of photoreceptors and their functions
- What is the function of inner retina cells
- To detect light
- Cone cells detect colours and rod cells detect black and white light.
- They process information
What are the layers of the retina from the outer side to the inner side? 2(POI)GO
- Pigment epithelium melanin, nutrition
- Photoreceptor layer light
- Outer nuclear layer
- Outer plexiform layer
- Inner nuclear layer
- Inner plexiform layer
- Ganglion cell layer output retinal cells
- Optic nerve layer axons
nuclear = cell bodies, plexiform = synaptic interaction
What are the characteristics of the first four layers of the retina from the outside
- Pigment epithelium: melanin - excess light absorption, nutrition provision for photo receptors
- Photoreceptors layer: light sensitive, convert light energy to electrical energy
- Outer nuclear layer: photoreceptor (rod & cone) cell bodies
- Outer plexiform layer: 1st synaptic transmission between photoreceptor & bipolar cells
What are the characteristics of the next 4 layers of the retina starting from the furthest from the inner retina
- Inner nuclear layer: amarine, bipolar, horizontal cell bodies
- Inner plexiform layer: 2nd synaptic transmission between bipolar cells & ganglion cells
- Ganglion cell layer: Output retinal cells (capable of firing AP)
- Optic nerve layer: ganglion cells axons
Describe retinal cells responses to darkness
- bipolar cells
-mGLuR
-AMPAR
- Center of receptive field: dark, surrounds: light
- Depolarisation (cone cell) - glutamate released
- ON-center bipolar cell (mGLuR) hyperpolarisation -> decreased transmitter released
OFF-center bipolar cell (AMPAR) depolarisation -> increased transmitter released - ON-center ganglion cell firing decreased
OFF-center ganglion cell firing increased
Describe retinal cells responses to light
- bipolar cells
-mGLuR
-AMPAR
- Receptive field center: light, surrounds: dark
- Hyperpolarisation (cone cell) - glutamate released decreased
- ON-center bipolar cell (mGLuR) depolarisation -> increased transmitter increased
OFF-center bipolar cell (AMPAR) hyperpolarisation -> decreased transmitter released - ON-center ganglion cell firing increased
OFF-center ganglion cell firing decreased
Receptive fields of ganglion cells have two regions: the centre and the surround.
What is the receptive field?
What do the both regions provide?
A retinal area where stimulation of a small spot of light creates a change in ganglion cell firing rate
Centre: direct input - photoreceptors to bipolar cells
Surround: indirect input - photoreceptors to bipolar cells through horizontal cells (accessory cell)
What happens when a spot of light is turned on in the centre of an ON-centre ganglion cell receptive field and an OFF-centre ganglion cell receptive field?
ON-centre ganglion cell: a burst of electrical activity produced
OFF-centre ganglion cell: firing decreased
In a receptive field, what is the relationship between the centre and its surround?
Always antagonistic
Cancel each others’ activity
Describe what happens when light is exposed to the centre or surround of a receptive field of an on-center cell
Centre exposed to light: on-center cell stimulated, depolarisation, maximum firing rate during stimulus
Surround exposed to light: on-center cell inhibited, hyperpolarisation, firing rate suppressed during stimulus
What is the firing rate of a ON-center/OFF-surround cell when
-no light exposed to centre & surround of receptive field
-light exposed to center/ no light exposed to surround of receptive field
-light exposed to centre & surround of receptive field
-no light exposed to centre/ light exposed to surround of receptive field
- fire at base-line rate
- fire at maximum rate during stimulus
- firing rate reduces close to baseline firing rate
- baseline firing rate suppressed during stimulus period
What is more effective in activating an ON-centre ganglion cell? Uniform illumination of visual field or light spot on the centre of cells receptive field?
What is luminance contrast?
Light spot on centre of cells receptive field
The ganglion cells organisation that makes them sensitive to illumination differences across the receptive field
The visual pathway (general -light)
1. transmit what
2. describe pathway
3. What are the two sides of the retina
4. Which nerves crossover and which don’t
5. Which side of the brain receives information
- Transmit sensation from retina to brain
- Optic nerve (monocular information) -> Optic chiasm (nasal fibres cross) -> LGN [thalamus] (axons synapse) -> optic radiation -> 1° visual cortex both hemispheres(output to other levels)
- Temple retina (ipsilateral), nasal retina (contralateral)
- Nasal retina (contralateral) crossover, temple retina (ipsilateral) don’t
- Left brain side receive right visual field information, right brain side receive left visual field information
Motion detection
Encoded by a unique group of retinal ganglion cells excited by starburst amacrine cells (subtype of amacrine cells)
Photoreceptors not sensitive to direction of movement of object
Directed selectivity ganglion cells
-AKA M (ganglion) cells (terminate in brain LGN magnocellular layer) , parasol cells
- Large
- neurons -> respond differentially to visual stimulus direction
- show a moving stimulus stimulates strong AP sinking in one direction but not opposite
- prefer & respond best -> motion one of these directions : dorsal, ventral, temporal, nasal direction (up, down, front, back)
- open radiating patterns
- Fast processing (high conduction velocity)
- Transient fashion response due to motion
- Lens inversion : object movement (visual scene) -> opposite direction movement in retinal plane
Visual pathway (motion)
Location in cortex
Inner layers of lateral geniculate nucleus (LGN)
Optic nerve (monocular information) -> optic chiasm (nasal fibres cross) -> LGN [thalamus] (magnocellular pathway - axons synapse to large magnocellular areas )
Dorsal pathway to parietal lobe [spatial processing : location, movement, spatial transformation & relations]
Two layers (M1 & M2) -> transmit movement perception related information to 1° visual cortex
Motion after effect
Visual illusion seen after looking at a moving visual stimulus and focusing on a stationary stimulus, resulting in the stationary stimulus moving in the opposite direction to original stimulus
What causes motion aftereffect? Describe it
Neural adaptation at cortical level
- brain neurons synapsing a movement in a particular direction become less sensitive to motion in that direction
- when look away, neurons detecting opposite motion -> more active compared to those neruons -> stationary object look like moving in opposite direction
Interocular transfer
Transfer of visual info from one eye to another
