Week 7 (How do we see) Flashcards
Vision: from cornea to retina
-As light enters, it’s ‘bent’ by the cornea, travels through the pupil, and ‘bent’ again by the lens.
-Curvature of cornea is fixed
-Muscles adjust curvature of lens to focus on near/far objects (accommodation)
-Creates an inverted. backwards image. (Brain compensates)
What is accomodation
The changing of the lens shape to focus on close or far objects.
What is astigmatism
Cornea or lens is rugby ball shaped, not rounded. This prevents part of it from focusing light onto the retina, results in blurred vision.
What is the emmetropic eye?
Normal vision
What is the hypermetropic eye? (Hyperopia)
Long sighted vision
-When the lens is too weak, or the eyeball is too short.
-Or cones excessively fat
-Corrected with a converging lens
What is the myopic eye? (Myopia)
Short sighted vision
-When the lens is too strong, or the eyeball is too long
-Or cones excessively curved
-Corrected with a diverging lens.
What is the presbyopic eye?
Old peoples vision
-The lens is no longer flexible enough to accommodate (change shape), can no longer focus on close and far objects.
Light at the retina?
-Light passes straight to the photoreceptors at the back of the retina
-Neurons are transparent so light can pass through
-Neurons and receptors translate light into action potentials
-Discriminate wavelengths and operate at different light intensities.
Fovea (centre of retina)
0.3mm in diameter
-A small dimple in the retina
-Denser receptors at centre
-Sharpest vision, densest colour receptors (better at centre than periphery)
-Makes reading possible
What is the blind spot?
-Small area of the retina where blood vessels enter/exit eye.
-Where fibres leading from retinal neuron from optic nerve that goes to brain.
-Has no photoreceptors (blind spot)
-Different position in each eye
Photoreceptors
-Light triggers chemical reactions - change membrane potential.
-Two types of receptors.
-Cones - tapered at end, 6-7 million
-Rods - longer, cylindrical shape at one end 120 millions.
Light vs colour photoreceptors
Receptors for colour
-Small, densely packed, distinguish colours in bright light
Receptors for light (black/white)
-Larger and more scattered
-Have 2 mile sensitivity to a single slight source
Rods & Cones
Rods
-Very sensitive to low luminescence
-Works in broader spectrum of conditions
-Night vision: slow dark adaptation
-‘Starts at periphery’
-Low acuity
-None in fovea
-Has only 1 colour receptor rhodopsin.
Cones
Don’t respond to dim light but very responsive to bright light
They detect colour and help us see fine detail: high acuity
-Acuity drops dramatically at periphery /sharper at centre
-Highly concentrated in fovea
-Has 3 different colour receptors (R, G, B)
4 types of Retinal Neurons
-bipolar
-horizontal
-amacrine
-ganglion
Two categories of Retinal ganglion
Magnocells (M-cells) and Parvocells (P-cells)
Magnocells
-Sensitive to movement and low contrast, but not very sensitive to colour or high detail.
-Present throughout retina (including periphery)
-Axons (mostly) project to magnocellular level of lateral geniculate nucleus.
Parvocells
-Smaller
-Get info mainly from cones
-More sensitive to colour
-Found mostly in fovea
-Sensitive to fine detail, weak response to changes in contrast.
-Axons project to parvocellular level of lateral geniculate nucleus.
What is the optic chiasm
Where the pathways from each eye cross
Where does the medial (nasal) path go
Crosses to the opposite side
Where does the lateral (temporal) path go
Goes straight back (stays on ipsilateral side)
What are the two brain routes for vision
Geniculostriate & Tectopulvinar
Geniculostriate brain route
-All p ganglion and some m ganglion form this pathway
-Goes from the retina to the lateral geniculate nucleus of thalamus & via optic radiations to layer 4 of primary visual cortex (striate cortex) in the occipital lobe.
-Visual cortex takes info about colour, form, & motion.
Tectopulvinar brain route
-Other path made up of remixing M ganglion cells
-Sends axons to superior colliculus (midbrain structure also known as tectum) where sends connections to pulvinar region of thalamus.
-Pulvinar sends info to parietal and temporal lobes.
What does retinotopic mean
Contains a complete map of the visual field covered by the eyes.
V1 Striate (The primary visual area)
-First stage of cortical processing of visual info
-Located in and around the calcarine fissure in the occipital lobe.
-Retinotopic
-V1 neurons are selective for basic visual properties: position, orientation, spatial and temporal frequency.
V2 (Extrastriate)
Cells are turned to similar properties as V1, a spatial map.
V3 (Extrastriate)
Receives input from V1 and V2 , dorsal part projects to parietal cortex, ventral part to inferior temporal cortex.
V4 (Extrastriate)
Receives inout from V1 and V2, strong projects to inferior temporal cortex. Colour vision and simple shapes.
V5 (Extrastriate)(Mid-temporal visual area, MT)
Receives input from V1, V2 and V3. Major role in the perception of motion.
The two visual streams
Ventral pathway (for form and colour) “What” pathway
Dorsal pathway (for space and motion) “Where” pathway
Ventral pathway
Retinal P-cells —> Parvocellular LGN —> V1—> V2 —> V4 —> Inferior temporal cortex
Dorsal pathway
Retinal M cells —> Magnocellular LGN —> V1 —> V2 —-> V3 —> V5 —> Posterior parietal cortex
Functions along the what pathway (temporal lobe)
-Fusiform face area (specialised for processing faces)
-Parahippocampal Place area (specialised for processing places)
Functions along the where pathway (parietal lobe)
-Intraparietal sulcus areas involved in controlling eye movements, and visual control of grasping
-Parietal region involved in reaching
