Topic 3: senses - vision Flashcards
Explain the historical context of sensation
- 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
How many of the sensory receptors of our body are in the eyes?
70%
What part of the brain is involved in perceiving + recognizing?
Cerebral cortex
Explain the visual stimulus of light
- 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
What is sclera?
- Tough outer coat = protect entire eyeball
- White outer wall
- Tough fibrous tissue
- Extends from cornea -> optic nerve
What is choroid?
- Vascular layer = contains blood vessels = nourish inner part of eye
What is cornea?
- Front portion of eye
- Convex bulging shape
- Light enters eye through here
What is iris?
- Located behind cornea
- Regulates amount of light entering eye by adjusting size of pupil
What is pupil?
- The hole in the middle of iris
- Size increases/decreases depending on light intensity
Explain when mydriasis takes place
- 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
Explain when miosis takes place
- 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
What is eye lens?
- 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
Define accommodation
Ability of eye to focus distant/nearby objects by changing focal length
What is aqueous humor?
- Viscous liquid in between cornea + eye lens
- Prevents eye from collapsing due to changes in atmospheric pressure
What is retina?
- 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
What are the 2 types of light-sensitive cells?
1) Rods = respond to intensity of light
2) Cones = respond to color of object
What is fovea?
- Dimple in the middle of retina
- Where vision is sharpest + most color perception
- Max number of cones present
What is optic disk?
- 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
Explain how light is processed
- 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
What part of the brain is associated with visual processing?
- Occipital lobe = contains visual cortex
- Primary visual cortex = Brodmann area 17
Describe the ventral stream in visual processing
- Involved in processing color + texture + detail + shape + size
- Damage impairs contrast sensitivity + color vision + depth perception + prosopagnosia
Describe the dorsal stream in visual processing
- 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
Describe function + sensitivity + location of cones
- Function = preception of detail + color
- Sensitivity = best under bright condition = high activation threshold
- Location = central region of retina = contributes to central vision
Describe function + sensitivity + location of rods
- 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
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
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
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
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
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