Perception & Cognition 1/2 Flashcards
Explain the timeline of understanding of vision
Democritus: atoms fro images pass through the eye into the body
Plato: No one will ever be able to explain vision as it is too complex
Alhazen: ‘light rays’ from the surface of an object are carried into your eye
Da Vinci: recognised depth in vision
Helmholtz: founded modern vision science and the ophthalmoscope
What does the ciliary body do in the eye?
makes aqueous humour
What is a receptive field?
An area of photoreceptors (rods&cones) attached to a single ganglion cell
What is lateral inhibition?
Where two stimulated neurones next to each other in a receptive field are activated, the one with stronger stimulation shuts down the response from the other
e.g. negative surround inhibits positive centre
What is myopia?
short-sightedness
What is hyperopia?
long-sightedness
What is the number one cause of blindness?
Age related Macular Degeneration
What is AMD?
Age related Macular Degeneration:
parts of the retina start peeling away causing light spots at the back of the eye and vision degeneration from the centre out
What is Glaucoma?
Aqueous humour cannot drain so pressure builds up crushing nerves in the optic disc, vision degenerates from outside in
What is diabetic retinopathy?
Blood vessels leak into the back of the eye disrupting photoreceptor function, vision loss patchy
What does lateral inhibition cause?
Enhanced contrast
Explain the timeline of understanding colour vision
Isaac Newton: Prism experiments discovered 3 primary colours of light
Young: Trichromatic theory of vision
Williams: Could look into the back of the eye using adaptive optic system and see proportions of different types of cones
What is the trichromatic theory of vision?
Humans posess only 3 types of photoreceptors - L, M & s that when stimulated in correct ratios mean we can see any colour
What are the 3 types of photoreceptors in humans and what light wavelengths do they prefer?
L=red cones (long wave sensitive)
M=green cones (medium wave sensitive)
S=blue cones (short wave sensitive) least common cone
Opsins exist inside cones to absorb different wavelengths.
How is colour blindness caused?
Missing or abnormal L or M opsin genes
Why are men more prone to colour blindness?
L and M opsin genes are carried on the X chromosome, since men only have one, if it is faulty they have no backup unlike women
Why L and M so close to each other on the absorption spectrum? How does this mean we can see colours outside of their spectra?
It is more efficient to send the difference between the signals from L and M rather than two signals (one from each receptor) to the brain
What happens to L and M cones in colourblindness?
M and L swap or L-M difference disappears causing red-green discrimination to become poorer
What is the opponent processing theory?
All receptors have a complex: red-green, blue-yellow, black-white. Therefore cells can only detect the presence of one of the colours in its complex at any one time as they oppose each other. Explains why we do not see ‘greenish-reds’
What part of the brain is concerned with object identity + form (colour)?
ventral stream
What part of the brain is concerned with motion, action & location (colourblind)?
dorsal stream
What is gain control?
How neurones control their sensitivity to light
How does an image get to the brain?
Light enters the eye
passes through the retina where luminance is detected
luminance turned into contrast by RFs
enters the LGN - eye of origin determined
enters the cortex - object determined
What sort of gain control does the retina do?
Short range, untuned, dependent on the eyes
What sort of gain control does the cortex do?
longe range, tuned, works on complex features (e.g.facial expressions)
When a response curve shifts in gain control where does the steep part of the curve go?
around the average to form a baseline for adaptation
What can happen if gain control fails?
can cause schizophrenia, epilepsy and Parkinson’s
Explain how motion detection occurs. What is this process called?
Reichardt detector:
Dark spot hits one RF which sends a signal which is delayed
dark spot moves across and hits an RF to the side of the first one sending another signal
these two signals together cause a neurone to fire IF the time it takes for the ds to move from RF1 to RF2 is equal to the delay of RF1’s signal
How does the brain process motion?
motion signal sent to VI then middle temporal and medial superior temporal
What is Helmoltz’s outflow theory?
His theory on how we compensate for eye movements (i.e. not everything is moving when we move our eyes)
An efferent signal from the brain moves the eye muscle
COPY of said signal is sent to the cortex
visual cortex knows about eye movement and cancels it out
What is Sherrington’s inflow theory?
His theory on how we compensate for eye movements
an efferent signal from the brain moves the eye muscle
EYE MUSCLE sends signal to the cortex
cortex aware of eye movement and cancels it out
Who is correct - Helmholtz or Sherrington - why?
Helmholtz
If you move your eye with your finger the world still stays still, therefore the signal cannot be sent from the eye muscle twitching
How does motion blindness occur?
damage to the middle temporal area
What is the motion after-effect?
Where neurones adapt to a movement
The ‘up’ neurone gets tired, ‘down’ neurone is still firing at a base level so no upwards motion is detected and there appears to be a downward movement instead
e.g. watching a waterfall after a while rocks at the side appear to move upwards
How does the eye determine how far away something is?
The visual system senses the degree of strain on the lens
What is a horopter?
The circle of vision that is the set of points in space that fall on the same place on the retina in both eyes
What is it called if an object is further away from the eye than the horopter?
uncrossed disparity - represented on the retina closer together than a point on the horopter
What is it called if an object is closer to the eye than the horopter?
crossed disparity - points on the retina are further away than those on the horopter
