Topic 3: senses - vision Flashcards
1
Q
Explain the historical context of sensation
A
- Democritus proposed we sense the external world in small copies = objects emit small replicas of themselves which then reach our senses allowing us to perceive and recognize them = what was believed until 18th century
- Johannes Muller theorized in 1825 that stimuli reached sense organs produced responses in sensory nerves + different nerve = different sensations = brain can differentiate between light + sound
- Muller’s idea remains as biological approach to sensation
2
Q
How many of the sensory receptors of our body are in the eyes?
A
70%
3
Q
What part of the brain is involved in perceiving + recognizing?
A
Cerebral cortex
4
Q
Explain the visual stimulus of light
A
- It is electromagnetic energy
- Spectrum ranges from gamma rays - radio waves
- Visible part is very small from 400nm-700nm
- Ozone layer filters out energy from sun = 4/5 solar radiation reaching earth is wavelengths withing visible spectrum
5
Q
What is sclera?
A
- Tough outer coat = protect entire eyeball
- White outer wall
- Tough fibrous tissue
- Extends from cornea -> optic nerve
6
Q
What is choroid?
A
- Vascular layer = contains blood vessels = nourish inner part of eye
7
Q
What is cornea?
A
- Front portion of eye
- Convex bulging shape
- Light enters eye through here
8
Q
What is iris?
A
- Located behind cornea
- Regulates amount of light entering eye by adjusting size of pupil
9
Q
What is pupil?
A
- The hole in the middle of iris
- Size increases/decreases depending on light intensity
10
Q
Explain when mydriasis takes place
A
- This is pupil dilation
- Low light conditions = pupil enlarge for more light to enter eye = improve vision in dim light
- Sympathetic NS activation = stress/excitement/fear lead to dilation due to fight or flight response
- Medications/drugs = atropine + amphetamine side effect
- Attraction/interest = studies suggest that seeing someone/thing that interests you causes dilation
- Pain
11
Q
Explain when miosis takes place
A
- This is pupil constriction
- Bright light conditions = pupil become smaller to reduce light entering to avoid damage to retina
- Parasympathetic NS activation = relaxed physiological state can lead to constriction
- Medication/drugs = opioids + pilocarpine side effects
- Age = aging causes the muscles controlling pupils to not work as efficiently = pupil generally smaller
- Accommodation reflex = constriction due to eye focusing on close object = due to reflex = changing of lens shape to focus on near objects
12
Q
What is eye lens?
A
- Transparent jelly-like substance made of proteins
- Held in place by ciliary muscles + suspensory ligaments
- Both adjust focal length of eye = see distant + nearby objects clearly
13
Q
Define accommodation
A
Ability of eye to focus distant/nearby objects by changing focal length
14
Q
What is aqueous humor?
A
- Viscous liquid in between cornea + eye lens
- Prevents eye from collapsing due to changes in atmospheric pressure
15
Q
What is retina?
A
- Delicate membrane at back of inner eye
- Has many light sensitive cells = layer of tissue works like film of camera
- Inverted real image is formed on retina = electrical signals generated by cell = sent to brain via optic nerve
16
Q
What are the 2 types of light-sensitive cells?
A
1) Rods = respond to intensity of light
2) Cones = respond to color of object
17
Q
What is fovea?
A
- Dimple in the middle of retina
- Where vision is sharpest + most color perception
- Max number of cones present
18
Q
What is optic disk?
A
- AKA blind spot
- All axons of ganglion cells exit retina = form optic nerve
- There a small area in retina where optic nerve enters eye = insensitive to light = blind spot
19
Q
Explain how light is processed
A
- Light hits posterior retina = spreads from photoreceptors -> bipolar cells -> innermost ganglion cells = generate APs
- Axons of ganglion cells = thick optic nerve = leaves back of eyeball = carries impulses to thalamus -> visual cortex of brain
- Thalamus = grey matter in forebrain = relays sensory signals
20
Q
What part of the brain is associated with visual processing?
A
- Occipital lobe = contains visual cortex
- Primary visual cortex = Brodmann area 17
21
Q
Describe the ventral stream in visual processing
A
- Involved in processing color + texture + detail + shape + size
- Damage impairs contrast sensitivity + color vision + depth perception + prosopagnosia
22
Q
Describe the dorsal stream in visual processing
A
- Projects from visual cortex -> posterior parietal cortex
- Involved in spatial processing = location + movement + spatial relations
- Damage causes spatial disorders =
simultanagnosia + optic ataxia + hemispatial
neglect + akinetopsia + apraxia
23
Q
Describe function + sensitivity + location of cones
A
- Function = preception of detail + color
- Sensitivity = best under bright condition = high activation threshold
- Location = central region of retina = contributes to central vision
24
Q
Describe function + sensitivity + location of rods
A
- Function = vision in low light conditions + detect movement + peripheral vision
- Sensitivity = highly sensitive to light allowing vision in dim conditions BUT can’t detect color ONLY variations of brightness = greyscale
- Location = periphery of retina = more effective peripheral vision
24
Describe cones
- Approx. 6-7 million in the eye
- Less sensitive to light than rods
- Perceive changes more rapidly due to faster response time to stimuli compared to rods
- 3 types = S/M/L cones = different pigment sensitive to specific wavelength range = short/medium/long wavelength
25
Why do we have trichromic vision?
- Humans have 3 kinds of cones with different photopsins = different responses to variation in color
- 3 pigments responsible for detecting light vary in chemical composition = genetic mutations = people have cones with different color sensitivity
26
Explain color blindness
- Inherited
- Due to destruction of cone cells
- People unable to differentiate color e.g. red/green
- Rare form = Daltonism = John Dalton found his grey jacket was red
- Rare form = monochromacy = see world in greyscale
27
What are some epidemiological stats on color vision?
- Red/green = 7-10% men + only 0.4% women
- Men get gene from mother's father = genetic mutation on X chromosome
- If women have on 1 X and not on other X it will overide
28
Explain tetrachromatic vision
- People with 4 or more types of cones
- Mostly female
- Color vision more intense
- 15-47% women + 8% men are potentially tetrachromatic
