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
What are the steps involved in the perceptual process?
Look at L1 notes
How does sensation differ to perception?
Sensation = Detecting elemental properties of a stimulus & Perception = a higher order process to offer interpretation of a stimulus
What is the light that we are able to perceive?
- Visible light = The energy within electromagnetic spectrum that humans can perceive
- Different wavelengths, appear different colours (Short = Blue & Long = Yellow)
What is the goal of perceptual research?
Understand the steps of the perceptual process that lead from the stimulus to behavioural responses of perception, recognition and action
How do we study the perceptual process?
- Study three relationships via the oblique effect (horizontal or vertical lines seen better than oblique)
- Stimulus-Perception
- Relates stimuli (Distal and Proximal) to behavioural responses
- Oblique effect shows behavioural demonstration as acuity is best for gratings orientation vertically or horizontally (perceiving)
- Stimulus-Physiological
- Relationships between stimuli and physiological responses (Receptor processes and Neural processing)
- Found horizontal/vertical orientations = Larger brain responses than oblique orientations
- Physiology-Perception
- Related physiological responses and behavioural
- Could detect horizontal and vertical 0 orientations at smaller light differences than for oblique orientations
- Stimulus-Perception
What are the cognitive influences on perception?
- Top-Down Processing - Knowledge based processing
- Bottom-Up Processing - Data based processing
What is neural convergence and how does it work in perception?
- Convergence → Number of neurons synapse onto a single neuron
- Signals generated in receptors → Travel to bipolar cells à Ganglion Cells
- Receptors & bipolar cells = Smaller axons
- Convergence causes rods to be more sensitive than cones (rods have more receptors connected to one cell = easier to reach threshold)
- Lack of convergence = Cones have better acuity as 1:1
