L1 - Retina & Blind Spots Flashcards
What are the reasons the retina is not perfect?
- Retina is backwards
- Blood vessels and other tissue in the way
- Blind spots
- No rods (night vision) or S (blue cones) in central fovea
- Cones widely spaced in periphery
- Floaters
Why is the retina being backwards an issue?
- Multiple layers of neurons before photoreceptors = Blocks some light signals from photoreceptors
- Not a uniform blockage, depends on the structure - such as axons in the way
How are blood vessels and tissue a potential limitation of the retina?
- Retinal cells required energy, so need blood (explains red eye in photos) - do they actually get in a way?
- Not an issue in cephalopods as there is no machinery between their photoreceptors and outside world (all behind photoreceptors) = Forwards
Why are blind spots an issue with the retina?
- Placed at the optic nerve
- Where blood vessels and ganglion cells exit so there is no visual perception in that area (neurons are pushed out the way)
Why is having no rods or S cones a limitation of the retina?
- Fovea = Area of greatest visual acuity (a pit) where blood vessels are pushed out of the way
- Fovea = Only has cones
Why is having cones widely spaced in the periphery a limitation of the retina?
Fewer receptors in your peripheral vision
Poorer vision in periphery
How do floaters show the retina isn’t perfect?
- Seeing little molecules or bubbles float around in your eye from the environment
- Only flaw that we consciously notice
Why do we not notice the flaws of our retina?
- The process of perceptual filling-in = Fills-in what we expect to see in areas such as our blind spot
- Brain has to fill in the gaps as we have multiple situations where there is a lack of information
- Blindspot
- 94% of photoreceptors are only used at night
- S-cone scotoma (blindspot)
- Blood vessels
- Cone coverage irregular in periphery
- Weak signals from motion capture or noise
- Multiple 3d interpretations of 2D shape
- Multiple possible sizes for same retinal size
- Brain has to fill in the gaps as we have multiple situations where there is a lack of information
- Troxler fading – Lack of change in a scene will stop photoreceptors signalling
- Change bias – why we don’t see blood vessels
What are the two kinds of photoreceptors that we have?
- Rods
- For vision in the dark
- Lower threshold
- Contains opsin protein that changes shape when it is struck by a photon
- Cones
- Vision in the daytime
- Three types: S cone (blue), M-Cones (green), L-Cones (red)
- Also contains opsin protein
- Centre is specialised for high-resolution
Why is our peripheral vision extra light-sensitive but with poor acuity?
- Inherited from nocturnal animals
- Compromise between detailed vision and wide field
What issues do we not face as humans with our eyes compared to issues that cameras face?
- Don’t experience exposure, pixelation and motion blur
- If we experience exposure as a human it is a brief overexposure before we become adjusted
- Each photoreceptor has their own exposure setting = Changes in response to light
What are photons?
- Light is comprised of photons that carry energy and travel at the speed of light
- High energy photons = Perceived as blue
- Low energy photons = Perceived as red
- When photons collide with matter = Either rerouted or absorbed
- e.g. Water absorbs low energy photons so appears blue
- Dark objects absorb nearly all photons
- Photodiodes = Photon-absorbing objects
How do photons of light transpire into image in the retina?
- Photon hits retinal molecule (protein in photoreceptor)
- Retinal molecule twists and reconfigures
- Proteins morph, causes cell membranes to shut etc. and signals sent
How does motion impact our visual system?
- Filling-in prevents movement from appearing blurred despite our ability to be the same as cameras In motion contexts
- Different neurons react to different directions/orientations etc
How and why do we recognise faces?
- Seek out faces in a scene due to survival instincts
- Visual signal leaves retina to thalamus, which goes to the temporal cortex to identify faces
