1
Q
What is the wavelength range of visible light that the human visual system can detect?
A
380-750 nanometers
2
Q
What are the three tissue layers that enclose the eye (from innermost to outermost)?
A
Retina (innermost), Uveal tract (choroid, ciliary body, iris), Sclera (outermost)
3
Q
What is the retina and how many photoreceptors does it contain?
A
The innermost layer of the eye, part of the CNS, containing more than 100 million specialized photoreceptive cells (photoreceptors)
4
Q
What is the function of the choroid?
A
A dense capillary bed that nourishes the outer retina; contains a pigmented epithelial layer rich in melanin that absorbs light
5
Q
What does the ciliary body contain and what are its functions?
A
Contains: 1) A muscular component that adjusts lens shape (accommodation), and 2) A vascular component that produces aqueous humor
6
Q
What is the function of the iris?
A
Contains two sets of muscles with opposing actions that regulate pupil diameter, thereby controlling the quantity of light entering the eye
7
Q
What is the main refractive element of the eye?
A
The cornea - a transparent tissue that provides most of the eye’s refractive power
8
Q
What is the aqueous humor and where is it located?
A
A clear, watery fluid in the anterior chamber (behind cornea, in front of lens) that supplies nutrients to the lens and cornea
9
Q
What causes glaucoma?
A
Dysfunction of aqueous humor drainage, leading to elevated intraocular pressure and loss of retinal ganglion cells
10
Q
What is the vitreous humor?
A
A thick, gelatinous fluid between the lens and retina, accounting for ~80% of eye volume; maintains eye shape and contains phagocytic cells that remove debris
11
Q
What are cataracts?
A
Clouding of the lens due to protein breakdown from aging or disease; accounts for roughly half the cases of blindness worldwide
12
Q
What is myopia (nearsightedness)?
A
Too much refractive power or an eyeball too long, causing light to focus in front of the retina; cannot focus on distant objects
13
Q
What is hyperopia (farsightedness)?
A
Weak refracting system or an eyeball too short, causing light to focus behind the retina; cannot focus on near objects
14
Q
What is astigmatism?
A
A condition that distorts or blurs vision due to defects in the curvature of the cornea (or lens)
15
Q
What is accommodation?
A
Dynamic changes in the shape of the lens to bring objects at various distances into sharp focus
16
Q
What is presbyopia?
A
Age-related loss of lens elasticity that impairs accommodation, making near-vision activities like reading difficult; typically occurs in middle age
17
Q
What is the pinhole effect?
A
Reducing pupil size prevents unfocused light from entering the eye, improving vision (explains why squinting improves vision)
18
Q
What is the macula lutea?
A
A circular region (~3mm diameter) near the center of the retina containing yellow pigment (xanthophyll) that supports high visual acuity
19
Q
What is the fovea?
A
A small depression at the center of the macula where visual acuity is greatest; has high cone density, specialized circuits, and an avascular zone
20
Q
What is the foveola?
A
The central region of the fovea that is rod-free
21
Q
What is the blind spot (scotoma)?
A
A region at the optic disk where retinal axons leave the eye; contains no photoreceptors
22
Q
How does light refraction affect image formation on the retina?
A
The cornea and lens invert and left-right reverse images as they fall on the retina
23
Q
What is the binocular visual field?
A
The overlapping region of visual space seen by both eyes, consisting of two symmetrical visual hemifields (left and right)
24
Q
Where do nasal retina ganglion cells project?
A
They cross at the optic chiasm to project to the contralateral hemisphere
25
Where do temporal retina ganglion cells project?
They remain ipsilateral (same side)
26
What are the five primary classes of neurons in the retina?
Photoreceptors, horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells
27
Where are photoreceptor cell bodies located?
In the outer nuclear layer
28
What is the three-neuron chain in the retina?
Photoreceptor → Bipolar cell → Ganglion cell (the most direct path from photoreceptors to optic nerve)
29
Where do horizontal cells receive and send synaptic connections?
In the outer plexiform layer; they provide lateral interactions at the photoreceptor-bipolar cell synapse
30
Where do amacrine cells have their processes?
In the inner plexiform layer; they are postsynaptic to bipolar cells and presynaptic to ganglion cells
31
What are the two types of photoreceptors?
Rods and cones
32
Where does phototransduction occur in photoreceptors?
In the outer segments, which contain membranous disks with light-sensitive photopigments
33
What is the function of the pigment epithelium?
Clears degenerating outer segment fragments, regenerates photopigments, and provides nourishment for photoreceptors
34
What is a photon?
The minimal amount of radiant energy capable of interacting with atoms and molecules; the minimal energy required for phototransduction
35
What is phototransduction?
The process by which radiant energy (light) is converted into electrical signals by specialized cells in the retina
36
How do rods and cones respond to light (unlike typical neurons)?
They hyperpolarize in response to light, producing graded potentials (not action potentials)
37
What are the components of photopigments?
An organic, light-absorbing chromophore (11-cis-retinal) coupled to a 7-pass transmembrane protein called opsin
38
What is rhodopsin?
The photopigment in rods (opsin specific to rods)
39
What happens when retinal absorbs a photon?
It changes from 11-cis isomer to all-trans isomer, triggering alterations in the opsin component
40
Describe the phototransduction cascade.
Light → 11-cis to all-trans retinal → activates transducin → activates phosphodiesterase (PDE) → hydrolyzes cGMP → closes cGMP-gated Na+/Ca2+ channels → hyperpolarization
41
What is the signal amplification in phototransduction?
1 photon → 1 rhodopsin → 800 transducin → 800 PDE → hydrolysis of 4800 cGMP → ~200 Na+ channels close → ~1 mV change
42
Why do photoreceptors release MORE neurotransmitter in the dark?
In darkness, high cGMP keeps cation channels open → cell is depolarized → more voltage-gated Ca2+ channels open → higher glutamate release
43
Why do photoreceptors release LESS neurotransmitter in light?
Light reduces cGMP → cation channels close → hyperpolarization → fewer Ca2+ channels open → less glutamate release
44
What are intrinsically photosensitive ganglion cells (ipRGCs)?
A small subset (~5%) of ganglion cells that express melanopsin and can be directly depolarized by light; regulate circadian rhythms
45
What photopigment do intrinsically photosensitive ganglion cells express?
Melanopsin
46
How many rods and cones are in the human retina?
~120 million rods and ~6 million cones
47
Compare the distribution of rods and cones in the retina.
Rods: high density in periphery, absent in foveola; Cones: low density in periphery, high density (peak) in fovea
48
Compare the convergence in rod vs cone pathways.
Rods: high convergence (15-30 rods → 1 rod bipolar cell); Cones in fovea: 1:1 (1 cone → 1 cone bipolar cell)
49
What is the benefit of convergence in the rod system?
Improves ability to detect small amounts of light by pooling signals from many rods
50
What is the cost of convergence in the rod system?
Reduces spatial resolution because the signal source could come from anywhere within a large retinal area
51
Compare the sensitivity of rods vs cones.
Rods: extremely sensitive, can respond to single photons; Cones: require >100 photons for comparable response
52
Compare the adaptation speed of rods vs cones.
Cones recover/adapt 4 times faster than rods
53
Compare reaction to light of rods vs cones.
Rods: slow reaction; Cones: fast reaction
54
Compare the amplification in rods vs cones.
Rods: lots of amplification; Cones: little amplification
55
What is scotopic vision?
Rod-mediated vision at the lowest levels of illumination; poor acuity, no color vision
56
What is photopic vision?
Cone-mediated vision in normal indoor lighting or sunlight; good color vision, best acuity
57
What is mesopic vision?
A transition stage between scotopic and photopic vision where both rods and cones are active
58
Why are we 'legally blind' without cone function?
Loss of cone function means loss of central/foveal vision, which is essential for detailed vision
59
What does loss of rod function cause?
Loss of peripheral vision and night blindness
60
Why is human vision called trichromatic?
The retina contains three types of cones, each expressing a unique photopigment sensitive to different wavelengths
61
What are the three types of cones and their peak sensitivities?
S-cones (short wavelength, 437 nm), M-cones (medium, 533 nm), L-cones (long, 564 nm)
62
How is color detected when wavelengths overlap cone sensitivities?
By comparing the activity in different classes of cones (e.g., green light activates M-cones more strongly than L-cones)
63
What are color opponent neurons?
Neurons sensitive to differences in activity between two cone types; critical for extracting color information
64
What is color contrast?
A patch returning the same spectrum can appear different depending on its surround
65
What is color constancy?
Test patches returning different spectra can appear the same color; allows recognizing objects regardless of illumination
66
What percentage of males vs females have color vision deficiency?
8% of males, less than 1% of females
67
Why is color blindness more common in males?
L- and M-wavelength pigment genes lie adjacent on the X chromosome
68
Where is the S-wavelength cone gene located?
Chromosome 7
69
What is dichromacy?
Color vision with only two functional cone types due to mutations
70
What is protanopia?
Dichromacy with impaired perception of long wavelengths (loss of L-cones); 1% of males
71
What is deuteranopia?
Dichromacy with impaired perception of medium wavelengths (loss of M-cones); 6% of males
72
What is the cone distribution in the retina?
64% L-cones, ~32% M-cones, 4% S-cones; few S-cones in fovea
73
What type of responses do bipolar cells have?
Graded responses (no action potentials)
74
What are the two types of bipolar cells?
ON bipolar cells and OFF bipolar cells
75
What glutamate receptor do ON bipolar cells express?
mGluR6 (metabotropic, G-protein coupled) - sign-inverting
76
What glutamate receptor do OFF bipolar cells express?
AMPA receptors (ionotropic) - sign-conserving
77
How do ON bipolar cells respond to light?
They depolarize when photoreceptors are activated by light (mGluR6 causes hyperpolarization by glutamate, so less glutamate → depolarization)
78
How do OFF bipolar cells respond to light?
They hyperpolarize when photoreceptors are activated by light (reduced glutamate removes depolarizing influence)
79
What type of signals do retinal ganglion cells produce?
Action potentials (unlike photoreceptors and bipolar cells which produce graded potentials)
80
Where do ganglion cell axons project?
Through the optic disk to the optic nerve, then to the LGN (90%), hypothalamus, pretectum, and superior colliculus
81
What is the receptive field of a neuron?
The region of visual space where light will alter the neuron's activity or firing pattern
82
What is an ON-center ganglion cell?
A cell that increases firing when light falls on the center of its receptive field and decreases when light falls on the surround
83
What is an OFF-center ganglion cell?
A cell that decreases firing when light falls on the center and increases when light falls on the surround
84
What is center-surround antagonism?
Stimulation of the surround has the opposite effect of stimulating the center of the receptive field
85
Why are ganglion cells more sensitive to contrast than absolute luminance?
Due to center-surround antagonism created by lateral inhibition from horizontal cells
86
How do horizontal cells create surround antagonism?
They receive glutamate from photoreceptors, are linked via gap junctions, and release GABA that hyperpolarizes photoreceptor terminals in the surround
87
What are the three major types of retinal ganglion cells?
M cells (Magno/Parasol), P cells (Parvo/Midget), and K cells (Konio)
88
What do M ganglion cells (Parasol) detect?
Luminance contrast, motion; have ON-center and OFF-center subtypes; input from rods
89
What do P ganglion cells (Midget) detect?
Color (contrast between two types of cones); shape
90
Compare M and P cell receptive fields.
M cells have larger receptive fields than P cells
91
Compare M and P cell temporal responses.
M cells respond transiently; P cells respond in sustained fashion
92
What is the parvocellular pathway important for?
Color discrimination and detecting fine details of objects
93
What is the magnocellular pathway important for?
Motion detection, high temporal resolution, detecting low-contrast stimuli
94
What percentage of ganglion cell axons go to the LGN?
~90%
95
What other structures receive retinal ganglion cell projections?
Hypothalamus (circadian rhythms), Pretectum (pupil reflex), Superior colliculus (eye/head reflexes)
96
What is the function of the suprachiasmatic nucleus?
Master regulator of circadian rhythms; uses light signals to photoentrain rhythms to day-night cycle
97
What is the function of the olivary pretectal nucleus?
Regulates the pupillary light reflex
98
What is the function of the superior colliculus?
Coordinates head and eye movements to visual targets
99
How many layers does the LGN have and what are they?
6 layers: 2 magnocellular layers (1,2), 4 parvocellular layers (3-6), plus koniocellular layers between them
100
How are eyes segregated in the LGN?
Cells are monocular; layers 2,3,5 receive ipsilateral eye input; layers 1,4,6 receive contralateral eye input
101
What is the optic chiasm?
The midline crossing point where nasal retina ganglion cell axons decussate to the contralateral hemisphere
102
What percentage of ganglion cell axons cross at the optic chiasm in humans?
About 60% decussate; 40% remain ipsilateral
103
What is a retinotopic map?
An orderly spatial representation of visual space in the brain, where adjacent ganglion cells innervate adjacent neurons
104
What is V1 (primary visual cortex) also called?
Striate cortex (due to the stripe appearance of dense layer 4 thalamocortical axons)
105
Where is primary visual cortex located?
In the occipital lobe of the cerebrum
106
How many cellular layers does the visual cortex have?
Six layers (layers 1-6)
107
Which layer of V1 receives the main input from the LGN?
Layer 4
108
What are pyramidal neurons and where are they found?
Excitatory glutamatergic neurons in layers 2,3,5,6; principal source of projections leaving the cortex
109
What are spiny stellate cells?
Excitatory neurons in layer 4 that receive LGN input and relay it to other cortical neurons
110
What percentage of V1 neurons are inhibitory interneurons?
About 20%; they use GABA
111
What is a cortical column?
A vertical domain where neurons share similar receptive field properties regardless of which layer they reside in
112
What are ocular dominance columns?
Alternating eye-specific columns in layer 4 where inputs from each eye remain segregated
113
Where does binocular convergence first occur in the visual pathway?
Outside layer 4 of V1; most neurons in other layers are binocular
114
What stimulus activates neurons in V1?
Light-dark bars or edges at specific orientations (not small spots of light like in retina/LGN)
115
What is orientation selectivity?
The property of cortical neurons to respond maximally to edges at a particular orientation
116
What is the preferred orientation?
The orientation angle that produces the peak response in an orientation-selective neuron
117
Who discovered orientation selectivity in V1?
David Hubel and Torsten Wiesel (1981 Nobel Prize)
118
How are simple cells in V1 created?
By convergent input from multiple LGN neurons with aligned center-surround receptive fields
119
What are the two visual processing streams beyond V1?
Dorsal stream ('where' pathway) and Ventral stream ('what' pathway)
120
What does the ventral stream process?
Object recognition - shape, color, texture; leads to temporal lobe (V4 → inferior temporal cortex)
121
What does the dorsal stream process?
Spatial vision - location, motion; leads to parietal lobe (MT/V5)
122
Which pathway is the magnocellular pathway associated with?
Dorsal stream
123
Which pathway is the parvocellular pathway associated with?
Ventral stream
124
What is MT (V5) specialized for?
Direction and speed of moving stimuli
125
What is cerebral akinetopsia?
Inability to perceive motion due to damage to MT; objects appear 'frozen'
126
What is cerebral achromatopsia?
Loss of ability to see color due to V4 damage; colors replaced by 'dirty' shades of gray
127
What is prosopagnosia?
Face-blindness due to lesions in the fusiform gyrus; cannot recognize faces
128
What is the fusiform face area (FFA)?
A region in the fusiform gyrus (inferior temporal cortex) precisely tuned to detect faces
129
What is stereopsis?
The perception of depth that arises from viewing objects with two eyes
130
What is binocular disparity?
The difference in retinal images between the two eyes due to their slightly different viewing angles
131
What is diplopia?
Double vision - when an object's image falls on non-corresponding points on the two retinas
132
What is the binding problem?
Different visual features are processed in different areas, yet we perceive a single unified percept
133
What is strabismus?
Misalignment of the eyes; if disparity is too large, fusion fails
134
What happens when a kitten's eye is sutured closed during development?
Ocular dominance columns fail to develop normally; neurons become dominated by the open eye
135
What is a critical period for visual development?
An early sensitive period during which visual experience is necessary for normal development of binocular cells
136
What is the pathway of the pupillary light reflex?
Retina → Olivary pretectal nucleus → Edinger-Westphal nucleus → Ciliary ganglion → Pupillary constrictor muscle
137
What causes pupil constriction (miosis)?
Parasympathetic activation via the oculomotor nerve (CN III) stimulating the circumferential constrictor muscle
138
What causes pupil dilation (mydriasis)?
Sympathetic activation stimulating the radially arranged dilator muscles
139
What is the direct vs consensual pupillary response?
Direct: constriction in stimulated eye; Consensual: constriction in unstimulated eye (due to bilateral connections)
140
What is the near response (accommodation reflex)?
Three linked events when viewing near objects: accommodation, convergence, and pupil constriction
141
What controls lens shape during accommodation?
Ciliary muscles via zonule fibers; contraction reduces tension on zonule fibers allowing lens to become rounder
142
What are the two types of eye movements for redirecting gaze?
Smooth pursuit (slow, continuous tracking) and saccadic (rapid, ballistic) eye movements
143
What is saccadic suppression?
The inability to detect visual percepts during saccadic eye movements; prevents motion blur perception
144
How much of V1 is dedicated to foveal (central) vision?
Almost 50%, despite the fovea making up only ~1% of the retinal surface
145
What is hemianopsia?
Loss of vision in half the visual field
146
What is homonymous hemianopsia?
Loss of corresponding visual field halves in both eyes (e.g., left visual field of both eyes)
147
What is bitemporal hemianopsia?
Loss of temporal visual fields in both eyes; often caused by pituitary tumors pressing on the optic chiasm
148
What is Meyer's loop?
A branch of the optic radiation in the temporal lobe carrying information from the superior visual field
149
What is macular sparing?
Loss of peripheral vision with preserved central (foveal) vision; common with cortical damage
150
What is quadrantanopsia?
Loss of vision in one quadrant of the visual field
Sensorimotor Systems 1.4
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