Vision Flashcards
1
Q
What is sensation?
A
- how cells of the nervous system detect stimuli in the environment
- how they transduce (convert) these signals into a change in membrane potential and neurotransmitter release
2
Q
What is perception?
A
- the conscious experience and interpretation of sensory information
3
Q
What are sensory neurons?
A
- specialized cells that detect a specific category of physical events
4
Q
What are the categories of physical events sensory neurons detect?
A
- the presence of specific molecules
- the presence or absence of physical pressure
- the temperature
- the pH of a liquid (whether it is acidic or alkaline)
- electromagnetic radiation (light)
5
Q
What do sensory neurons do?
A
- have specialized receptors that transduce sensory stimuli into a change in membrane potential
- many don’t have axons or action potentials
- they all release neurotransmitter
6
Q
How do sensory neurons that don’t have action potentials release neurotransmitter?
A
- in a graded fashion, dependent on their membrane potential
- more depolarized they are, the more neurotransmitter they release
- more hyperpolarized, less neurotransmitter release
7
Q
What are photoreceptor cells?
A
- sensory neurons responsible for vision
- transduce the electromagnetic energy of visible light into a change in membrane potential
- affects how much neurotransmitter they release
- do not have action potentials
8
Q
What are opsins?
A
- light-sensitive proteins
- in photoreceptor cells
- metabotropic receptors
- sensitive to light because they bind a molecule of retinal, which changes shape in response to light
- change in the shape of retinal is what activates this metabotropic receptor
9
Q
What is retinal?
A
- small molecule
- attaches to the opsin proteins in the photoreceptor cells in our eyes
- absorbs the electromagnetic energy of visible light that allows us to see
10
Q
What happens when retinal absorbs light?
A
- molecule changes shape
- activates the opsin protein
- this launches an intracellular g protein signaling cascade that changes the membrane potential of the photoreceptor cell
- affecting how much neurotransmitter it releases
11
Q
What are the 2 configurations of the retinal molecule?
A
- bent (not activated)
- straight (activated by light)
12
Q
How many photoreceptor cells contribute to our conscious perception of vision?
A
4
13
Q
What are the 4 types of photoreceptor cells that contribute to our conscious perception of vision?
A
- red cone cells
- green cone cells
- blue cone cells
- rod cells
14
Q
What type of metabotropic opsin protein do red cone cells express?
A
- red cone opsin
15
Q
What type of metabotropic opsin protein do green cone cells express?
A
- green cone opsin
16
Q
What type of metabotropic opsin protein do blue cone cells express?
A
- blue cone opsin
17
Q
What type of metabotropic opsin protein do rod cells express?
A
- rhodopsin opsin
18
Q
Which photoreceptor cells were the last to evolve?
A
- rod cells
19
Q
Which photoreceptor cells are more sensitive to light?
A
- rod cells
- 100 times more sensitive to light than the cone cells
20
Q
What is visible light (the visible spectrum)?
A
- electromagnetic energy that has a wavelength between 380 and 760 nm
- detect this light using 1 rod cell and 3 cone cells
21
Q
What is the order of the electromagnetic spectrum from high energy/small wavelength to low energy/long wavelengths?
A
- gamma rays
- x rays
- ultraviolet rays
- visible light spectrum
- infrared rays
- radar
- tv and radio broadcast bands
22
Q
What order did the cone cells evolve?
A
- red
- blue
- green
23
Q
What are blue cone opsins?
A
- most sensitive to short wavelengths of light
- 430 nm
24
Q
What are green cone opsins?
A
- most sensitive to medium wavelengths of light
- 535 nm
25
What are green red opsins?
- most sensitive to long wavelengths of light
- 575 nm
26
What is orange?
- 610 nm
- activates green cones 13% of their maximum level
- activates red cones 75% of their maximum level
27
What is colour perception?
- a function of the relative rates of activity across the three types of cone cells
- colours are discriminated by the ratio of activity across these cells
- key consideration for our brain for identifying colour is how much each type of cone cell is activated relative to its maximum level
28
Which cone opsins are more sensitive to light in general (order)?
- green
- red
- blue
29
What are the primary colours of light?
- red
- green
- blue
30
What colour do we get when we mix red and blue light?
- magenta
31
What colour do we get when we mix red and green light?
- yellow
32
What colour do we get when we mix green and blue light?
- cyan
33
What do we get when we mix all coloured light?
- white light (sunlight)
34
What are the 3 dimensions of our perception of light and colour?
- brightness
- saturation
- hue
35
What is brightness?
- intensity (luminance, amount)
- how much light we have
- 0-100% (bottom to top)
36
What is saturation?
- purity (in terms of wavelength mixture)
- 0-100% (middle to side)
37
What is hue?
- dominant wavelength (colour)
- 0-360 degrees
38
What happens when brightness is 0?
- the image is completely black
- hue and saturation have no impact without brightness
39
What happens when there is bright light and saturation is 0?
- middle of the colour cone where there is an equal contribution from all visible wavelengths
- grayscale (black and white), because all wavelengths are present in equal amounts
40
What happens when saturation is more than 0?
- the hue indicates the colour that the light is saturated with
41
What are the types of colour vision deficiency?
- protanopia
- deuteranopia
- tritanopia
- achromatopsia
42
What is protanopia?
- absence of the red cone opsin
- trouble distinguishing colours in the green-yellow-red spectrum
- visual acuity is normal
- red cone cells switch to using the green cone opsin
43
What happens when there are mutations in the red cone opsin?
- simple mutations in the red cone opsin produce less pronounced deficits in color vision
- hinder color vision if they make it act more like the green opsin (in terms of what light it can detect)
- 1% of males
44
What percent of the population has protanopia?
- 1% of males
45
What is deuteranopia?
- absence of the green cone opsin
- trouble distinguishing colors in the green-yellow-red spectrum
- visual acuity is normal
- green cone cells switch to using the red cone opsin
46
What happens when there are mutations in the green cone opsin?
- simple mutations in of the green cone opsin produce less pronounced deficits in color vision
- 6% of males
47
What percent of the population has deuteranopia?
- 1% of males
48
What is tritanopia?
- absence of the blue cone opsin
- blue cone cells do not compensate for this in any way
- blue cone opsin is not that sensitive to light anyway
- visual acuity is not noticeably affected
- no mutation
49
What percent of the population has tritanopia?
- 1% of the population
50
What is achromatopsia?
- true colour blindness
- caused by mutations in the g protein signaling cascade that is used by all the cone opsins
51
Why are cone cells useful?
- there are three of them, and they are sensitive to different wavelengths, which is necessary for color vision
52
What are the parts of the eye?
- conjunctiva
- sclera
- cornea
- iris
- pupil
- lens
- vitreous humor
- retina
- fovea
- optic disk
53
What is the sclera?
- opaque and does not permit entry of light
- the tough, outer white of the eye
53
What is the conjunctiva?
- a mucous membrane that lines the eyelid
54
What is the cornea?
- the outer, front layer of the eye
- focuses incoming light a fixed amount
- bends light and focuses it
- can get thicker or thinner
- surrounds eye and holds into place
- transparent
55
What is the iris?
- a ring of muscle
- the contraction and relaxation of this muscle determines the size of the pupil
56
What is the pupil?
- determines how much light enters the eye
57
What happens when the iris contracts?
- pupil small
- little light in
58
What happens when the iris relaxes?
- pupil bigger (dilates)
- lot of light in
59
What is the lens?
- consists of several transparent layers
- we change the shape of this lens to focus near versus far
60
What is accommodation?
- when the lens changes shape to focus near or far
61
What is the retina?
- the interior lining (furthest back part) of the eye
- photoreceptor cells are located in the furthest back layer of the retina
- periphery of the retina only contains rod cells
62
What is the vitreous humor?
- a clear, gelatinous fluid
- light passed through the lens and crosses the vitreous humor
63
What is the fovea?
- central region of the retina
- primarily contains cone cells
64
What is the optic disk?
- where blood vessels enter and leave the eye
- where the optic nerve exits the eye, carrying visual information to the brain
- no photoreceptors in this spot
- blind spot
65
What are orbits?
- bony sockets in the front of the skull where the eyes are suspended
66
How does the eye move?
- 6 extraocular muscles are attached to the sclera
- these muscles rotate the eye and hold it in place
67
When do our eyes move?
- our eyes never sit still for very long
68
What type of movements do our eyes do?
- saccadic eye movements
- pursuit movements
69
What are saccadic eye movements?
- rapid, jerky shifts in gaze from one point to another
- how our eyes scan a scene
- every 150 milliseconds
- 5-6 things every second
70
What are pursuit movements?
- maintain focus on an object that is moving (relative to us)
- only time are eyes appear to calm down and move smoothly, slowly
71
Where does visual information propagate (neural activity?
- from photoreceptor cells --> bipolar cells --> retinal ganglion cells --> brain
- when light enters our eyes, it must pass through each of the cell layers in the retina before it can reach the opsin proteins in photoreceptor cells
72
What is the organization of the retina?
- ganglion cell layer
- bipolar cell layer
- photoreceptor later
73
What is in the fovea?
- an equal number of photoreceptor cells, bipolar cells, and retinal ganglion cells
- no compression of information
74
What is the only part of our retina where visual acuity is good enough to read text?
- the fovea
75
What is the fovea good for?
- visual acuity (reading)
- high resolution (no compression)
- colour vision
76
How does the fovea support colour vision?
- the photoreceptor cells in our fovea are mostly cone cells
77
What is in the periphery of the retina?
- massive compression (averaging) of information (low resolution)
- 100x more photoreceptor cells than retinal ganglion cells
- high density of rod cells
78
How is visual acuity in peripheral vision?
- 20/200
- blurry
- make out general shapes but not any details
79
What is the periphery good for?
- grayscale
- easily detect dim light and movements of light
80
Why can we easily detect dim light and movements of light in the periohery?
- because rod cells are very sensitive to light
81
What is the location and response characteristics of cones?
- most prevalent in the central retina; found in the fovea
- sensitive to moderate to high levels of light
- provide information about hue
- provide excellent acuity
82
What is the location and response characteristics of rods?
- most prevalent in the peripheral retina
- sensitive to low levels of light
- provide only monochromatic information
- provide poor acuity
83
What are the neurons in the retina?
- photoreceptor cells
- bipolar cells
- retinal ganglion cells
84
What are photoreceptor cells in the retina?
- located in the furthest back part of the retina
- they express the opsin proteins that transduce light
- synapse on bipolar cells
85
What are bipolar cells?
- relay information from photoreceptor cells to retinal ganglion cells
- do not have action potentials
- release glutamate in a graded manner dependent on their membrane potential
86
What are retinal ganglion cells?
- the only cells that send information out of the eye
- their axons form the optic nerve, which exits the retina through the optic disc
- have action potentials, unlike most other cells in the retina
- typical neurons
- express normal excitatory ionotropic glutamate receptors
87
What are horizontal and amacrine cells?
- interconnect cells within each layer, which gives rise to complex interactions between neighbouring cells (within a layer)
88
What are the visual information pathways?
Retinal ganglion cells axons go to 3 places"
- thalamus
- midbrain
- hypothalamus
89
What is the thalamus pathway?
- retina --> thalamus --> V1 pathway
- lateral geniculate nucleus (in thalamus)
- projects to primary visual cortex (area V1) in the occipital lobe where visual information enters consciousness
- creates an internal (mental) representation of your entire visual space: the objects in it, their position, and their attentional value
- main pathway for conscious vision
90
What is the midbrain pathway?
- superior colliculi (in midbrain)
- visual information is used here to control fast visually-guided reflexive movements
- midbrain doesn’t know what you are looking at, but it can draw attention to unexpected visual events
- turn head toward
91
What is the hypothalamus pathway?
- visual information is used here to control circadian rhythms such as sleep-wake cycles
- hypothalamus doesn’t know what you are looking at, but it knows how much light is present in your environment
92
What is the visual cortex wiring diagram of visual information processing?
- 1970s
- oversimplification because there are actually tons more pathways and most of them are bidirectional (bottom-up and top-down)
93
What is the predictive coding theory?
- each node in the network tries to predict what its ascending inputs will look like in the next moment, based on previous experience
- top-down (descending) activity represents sensory predictions that neutralize (cancel out) any correctly predicted bottom-up ascending signals
- what propagates up through the network may only be prediction error signals, which inform the brain of how the current moment differs from what was expected
- the prediction error signals that ascend through the network would cause learning to improve future predictions
94
What is the receptive field of a neuron?
- a description of the (external) stimuli that activate it
- where in space can put some stimuli to activate it and what do properties of stimuli need to be to have most activity in neuron/biggest response
95
Where is the receptive field for a neuron involved in visual processing?
- where light must be in visual space and what properties it must have to change the activity of the cell
- an area of visual space relative to a fixation point
96
How do we identify the receptive field of a cell involved in visual processing?
- we record the cell’s activity as the animal maintains focus on one spot on a computer screen
- shine light in different areas of the monitor to determine where in visual space the presence of light influences the activity of the cell
- determine if the cell responds differently to different colors or patterns of light in that location
97
What do photoreceptor cells do?
- do not have action potentials
- release glutamate in a graded fashion dependent on their membrane potential
- more depolarized they are, the more glutamate they release
98
What is a photoreceptor cell at rest?
- sit at -40 mV
- resting membrane potential
- continuously release glutamate
99
What happens when photoreceptor cells are activated by light?
- hyperpolarize to -70 mV
- stop releasing glutamate
100
What is the dark current?
- photoreceptor cells express an uncommon “leak” sodium ion channel that sits open at baseline (in the dark)
- the influx of sodium ions through these ion channels (the dark current) causes photoreceptor cells to sit at -40 mV, where they continuously release glutamate
101
What happens when an opsin protein absorbs light during the dark current?
- it launches an intracellular g-protein
signaling cascade that closes the open sodium ion channels
- the closing of these ion channels causes the membrane to hyperpolarize to -70 mV, at which it point the photoreceptor cell stops releasing glutamate
102
What are all the opsin proteins responsible for our conscious perception of vision?
- inhibitory metabotropic receptors
- when activated by light, they cause membrane hyperpolarization, which stops the photoreceptor cell from releasing glutamate
103
What are the 2 types of bipolar cells?
- OFF bipolar cells
- ON bipolar cells
104
What are OFF bipolar cells?
- express normal excitatory ionotropic glutamate receptors
- their activity patterns follow that the photoreceptor cells that connect to them
105
What happens to OFF bipolar cells in the dark?
- when photoreceptors are depolarized (-40mV) and releasing glutamate
- OFF bipolar cells will also be depolarized and releasing glutamate
106
What happens to OFF bipolar cells in the presence of light?
- when photoreceptors are hyperpolarized (-70mV) and not releasing glutamate
- OFF bipolar cells will also be hyperpolarized and not releasing glutamate
107
What are ON bipolar cells?
- express inhibitory (metabotropic) glutamate receptors
- their activity patterns are the opposite of the photoreceptor cells that connect to them
108
What happens to ON bipolar cells in the dark?
- when photoreceptors are depolarized (-40mV) and releasing glutamate
- ON bipolar cells will be hyperpolarized and not releasing glutamate
109
What happens to ON bipolar cells in the presence of light?
- when photoreceptors are hyperpolarized (-70mV) and not releasing glutamate
- ON bipolar cells depolarize and release glutamate
110
What are horizontal cells?
- do not have action potentials
- release glutamate in a graded manner dependent on their membrane potential
- interconnect neighbouring photoreceptors cells
- regulate the amount of glutamate that is released from photoreceptors cells based on the activity of their neighbors
- compare the activity of neighbouring photoreceptor cells
- accentuate the difference by counteracting the light in the center cell
111
What do horizontal cells do when the center photoreceptor cell senses dim light and neighboring photoreceptor cells are activated by bright light?
- compare the activity of neighboring photoreceptor cells
- recognize that the center photoreceptor cell is getting less light than its neighbors
- accentuate this difference by counteracting the small light-induced hyperpolarization in the dimly lit cell
- horizontal cells depolarize the “axon terminals” of photoreceptor cells according to how brightly lit the neighboring photoreceptor cells are
- don't let the center cell hyperpolarize because relative to neighbour, it's dark
112
What is the response in an ON bipolar cell when an upstream photoreceptor cell detects light?
- depolarize (> -60 mV)
- more neurotransmitter release
113
What is the response in an ON bipolar cell when upstream photoreceptor cell is in darkness, but neighboring cells are brightly lit?
- hyperpolarize (< -60 mV)
- less neurotransmitter release
114
What is the response in an OFF bipolar cell when upstream photoreceptor cell is in darkness, but neighboring cells are brightly lit?
- depolarize (> -60 mV)
- more neurotransmitter release
115
What is the response in an OFF bipolar cell when an upstream photoreceptor cell detects light?
- hyperpolarize (< -60 mV)
- less neurotransmitter release
116
What is the receptive fields of bipolar cells?
- the influence of horizontal cells creates a “center-surround” organization in the receptive fields
117
What is the response when the center of a bipolar cell receptive field is illuminated?
- photoreceptor hyperpolarized
- OFF bipolar cell hyperpolarized
- ON bipolar cell depolarized
118
What is the response when the surround of a bipolar cell receptive field is illuminated?
- neighbouring photoreceptors hyperpolarized
- center photoreceptor depolarized
- horizontal cell hyperpolarized (keep center photoreceptor depolarized cause darker than neighbours)
- ON bipolar cell hyperpolarized
- OFF bipolar cell depolarized
119
What are the receptive fields of retinal ganglion cells?
- inherit their receptive fields from bipolar cells
- “center-surround” organization
120
What are the 2 types of retinal ganglion cells?
- ON retinal ganglion cells
- OFF retinal ganglion cells
121
What are ON retinal ganglion cells?
- increase their rate of spiking when light is in the center of their receptive field
- decrease their rate of spiking when light is brighter in the surround area of the receptive field
122
What are OFF retinal ganglion cells?
- decrease their rate of spiking when light is in the center of the receptive field
- increase their rate of spiking when light is in the surround area
123
What is the response in bipolar and ganglion cells when on-center, off-surround receptive field is center illuminated?
- bipolar cell depolarization, release glutamate
- ganglion cells many action potentials
124
What is the response in bipolar and ganglion cells when on-center, off-surround receptive field is surround illuminated?
- bipolar cell hyperpolarization, stop releasing glutamate
- ganglion cells few action potentials
125
What is the response in bipolar and ganglion cells when on-center, off-surround receptive field is entirely illuminated?
- bipolar cells, slight depolarization
- ganglion cells moderate action potentials (more than baseline)
126
What is the response in bipolar and ganglion cells when off-center, on-surround receptive field is center illuminated?
- bipolar cell hyperpolarization, stop releasing glutamate
- ganglion cells few action potentials
127
What is the response in bipolar and ganglion cells when off-center, on-surround receptive field is surround illuminated?
- bipolar cell depolarization, release glutamate
- ganglion cells many action potentials
128
What is the response in bipolar and ganglion cells when off-center, on-surround receptive field is entirely illuminated?
- bipolar cells, slight hyperpolarization
- ganglion cells moderate action potentials (more than baseline)
129
What do retinal ganglion cells in the fovea process?
- colour information
- integrate information from many bipolar cells
130
What are the types of receptive fields of retinal ganglion cells in the fovea?
- red on, green off
- green on, red off
- yellow on, blue off
- blue on, yellow off
131
summary of photoreceptor cells receptive fields
- sensitive to different wavelengths of light
- all hyperpolarize and release less glutamate when the appropriate wavelength of light is in their receptive field
- their receptive fields are generally quite simple, defined by a location in space and a wavelength of light
132
summary of bipolar cells receptive fields
- sum of the receptive fields of the cells they receive input from
- receive synaptic input photoreceptor cells directly and indirectly (via horizontal cells)
- their receptive fields are larger than the receptive field of an individual photoreceptor cell
- defined by a location in space, a wavelength of light, and whether they exhibit ON or OFF responses to light in the center of their receptive field
133
summary of bipolar cells in the fovea receptive fields
- bipolar cells in the fovea receive direct synaptic input from only one photoreceptor cell, so the center of their receptive fields are the same size as one photoreceptor cell
134
summary of bipolar cells outside the fovea receptive fields
- bipolar cells outside the fovea receive direct synaptic inputs from many photoreceptor cells, so the center of their receptive fields are quite large (the sum of the receptive fields of all the photoreceptor cells that connect to them)
135
summary of retinal ganglion cells receptive fields
- similar to that of the bipolar cells
- the fovea they also show colour opponency
136
What are the receptive fields of thalamic (LGN) neurons?
- similar to that of the retinal ganglion cells
- center-surrond
- on and off
- circular
137
What are the receptive fields of neurons in V1 (primary visual cortex)?
- the sum of many LGN neurons
- center-surround
- line/oval shape
- simple cell
138
What are simple cells in V1?
- sensitive to lines of light
- their receptive fields are typically organized in a center-surround fashion
139
When do neurons in V1 spike?
- when there is a line of light in a particular orientation in their receptive field
- some neurons respond best to vertical lines, some to horizontal lines, and some to lines oriented somewhere in between
140
What are the cortical columns in V1?
- every spot in your visual field is rigorously analyzed by a cortical column in V1
- all neurons within a cortical column analyze the same area of visual space
- together, they analyze the orientation of light in the associated receptive field
- location of sharp transitions in the contrast/color of light reveals borders, edges, and corners
141
What do neurons downstream of V1 do?
- put all the information from cortical columns together to identify whole objects and their position in space
142
How much of the cerebral cortex is dedicated to processing visual information?
- more than 25%
143
What is visual association cortex?
- all of the occipital lobe that is not primary visual cortex
- extends into the parietal and temporal lobes, forming respectively the dorsal and ventral streams of visual information processing
144
What is the dorsal stream of visual information?
- starts in primary visual cortex and ends in posterior parietal lobe
- involved in identifying spatial location
- encodes where objects are, if they are moving, and how you should move to interact with them or avoid them
145
What is the ventral stream of visual information?
- starts in primary visual cortex and ends in inferior temporal lobe
- involved in identifying form (shape)
- encodes what the object is and its color
146
What is monocular vision?
- some V1 neurons respond to visual input from just one eye
147
What is binocular vision?
- most V1 neurons respond to visual input from both eyes
148
What is depth perception?
- many monocular cues that can be used to estimate depth
- relative size, amount of detail, relative movement as we move our eyes, etc
- only one eye is required to perceive depth with monocular cues
- requires the dorsal stream "where" pathway
149
What is stereopsis?
- the perception of depth that emerges from the fusion of two slightly different projections of an image on the two retinas
150
What is retinal disparity?
- the difference between the images from the two eyes in stereopsis
- results from the horizontal separation of the two eyes
- improves the precision of depth perception, especially for moving objects
151
What is an agnosia?
- deficit (problem) in the ability to recognize or comprehend certain sensory information
- the specific sense is not defective nor is there any significant memory loss
- relates to a problem in some sensory association cortex (typically in a single sensory modality) - not to problems that relate to the sensory neurons themselves or to the primary sensory areas
152
What are visual agnosia related to damage in the dorsal stream?
- akinetopsia
153
What is akinetopsia?
- a type of visual agnosia
- a deficit in the ability to perceive movement
- caused by damage to the dorsal visual stream in the parietal lobe
154
What are visual agnosia related to damage in the ventral stream?
- cerebral achromatopsia
- prosopagnosia
155
What is cerebral achromatopsia?
- visual agnosia
- caused by damage to the cerebral cortex in the ventral visual stream
- deny having any perception of color
- everything looks dull or drab, and that it is all just “shades of grey”
156
What is prosopagnosia?
- visual agnosia
- caused by damage to the fusiform gyrus (fusiform face area) in the ventral visual stream
- failure to recognize particular people by sight of their faces
