Vision Flashcards
(227 cards)
1
Q
What is the sensitivity of the human eye?
A
It is sensitive to the electromagnetic spectrum between the wavelengths 400nm (violet) and 700 nm (red), with maximal sensitivity at 550 nm (green). The eye can respond to a single photon of light, 5-8 photons arriving within a short time are required to give the experience of a flash of light in the dark-adapted state.
2
Q
Why is it difficult to distinguish differences in intensity at high light levels ?
A
Intensity is encoded by the visual system logarithmically
3
Q
What is the definition of vision?
A
The process of discovering from images what is present in the world and where it is
4
Q
What is parallel processing?
A
The brain uses a 2-D shifting pattern of light intensity values on the two retinas to form a representation of the form of an object, its color, movement and position in a 3-D space. Each of these visual channels are handled simultaneously by distinct but interdependent pathways, to create a unified percept.
5
Q
What is serial processing
A
A task is segmented into several subroutines that are executed sequentially.
6
Q
What does visual processing give higher weight to?
A
Regions of the world that are changing in time (movement) and space (contrast) than those that are constant
7
Q
What does visual perception require?
A
The existence of internal representations of the visual world to allow the brain to make hypotheses about what the retinal image is. Internal representations allow for the fact that vision allows pattern completion and generalisation. Some internal representations are specified during development and are immutable, but most depend on early learning
8
Q
What is pattern completion?
A
Generating a complete percept even when the raw sensory data is incomplete or corrupted by noise
9
Q
What is generalization?
A
The ability to recognise objects from a wide variety of vantage points and contexts.
10
Q
What is a visual illusion?
A
Results from an unresolvable mismatch occurring between the sensory input and the internal representation.
11
Q
What is perceptual constancy, and why is it important?
A
Visual perception can be invariant over wide differences in the properties of the retinal image, for eg, color constancy preserves the colors of objects in the face of alterations in the wavelength composition of the light source. Perceptual constancy permits successful object recognition under a wide variety of ambient conditions.
12
Q
There are both monocular and binocular clues for depth perception, monocular clues are more important for distant objects, where binocular clues cannot be used, what are examples of these?
A
-Motion parallax- movement of the head causes an apparent movement of near objects with respect to distant ones.
-Geometric perspective- Parallel lines appear to converge with distance
-Relative sizes of objects of known dimensions
-Occultation- Distant objects can be partly hidden by nearer ones
-Extinction- Distant objects are faded and bluer d/t intervening haze
-Ability to resolve fine detail falls off with distance
-Accommodation
13
Q
What is accommodation?
A
Neural signals that correspond to how much the visual system has had to change the shape of the lens to keep the object in focus
14
Q
The binocular clue to depth perception is stereopsis, what is this?
A
The eyes are 6.3 cm apart, therefore the image of a nearby object falls onto different horizontal planes on the left and right retinas (retinal/binocular disparity). When the eyes converge to fixate on a nearby point the two retinal images of the point are perceived as fused in to a single point. All other points at the same distance are fused. Points in space that lie closer or further away than these, will form images at different corresponding retinal positions and hence different binocular disparities, the closer the object, the bigger the disparity, beyond these, two images are seen (diplopia) or info from one eye is completely rejected by the visual cortex
15
Q
What is binocular parallex?
A
Each eye has a slightly different image of the world. When viewing a scene first through one eye and then the other, when nearby objects appear to jump sideways.
16
Q
With regards to stereopsis, when an object is closer than the close point, images of these points might also fuse, when is the disparity too great for this to happen?
A
> 0.6 mm or 2 degrees
17
Q
What is required for stereopsis?
A
It is only possible for the field of view in which the two monocular visual fields overlap. The brain is able to compute depth from disparity simply by comparing where the same pattern lies on the left and right retinas. It does not require form, movement or colour.
18
Q
What is the basic definition of colour vision?
A
It permits boundaries to be seen between regions that have equal brightness, provided the spectrum of wavelengths they reflect is different, but it not just a matter of measuring all the wavelengths in the relected light
19
Q
What does the spectrum of light reflected from an object depend on?
A
On the wavelength composition of the illuminating light and the reflectance of the surface
20
Q
What is dichromatic color Vision?
A
It requires a minimum of 2 types of receptor, that respond over different wavelength ranges. With these receptors, the visual system can assign 2 brightness values for each pixel of the visual field. By comparing these values, colors may be perceived. If a pixel reflects more short- wavelength light it will appear brighter to a short- wavelength receptor than a long wavelength receptor and will be seen as blue. If a pixel reflects reflects more long- wavelength light it will be seen as red. If a pixel reflects equal amounts of short and long wavelength light - it will appear monochrome, either white or shades of grey depending on intensity of light
21
Q
Human color division is trichromatic, why is called this?
A
The human eye has 3 populations of receptors (cones) that function is daylight, each sensitive to a different range of wavelengths. The system abstracts 3 brightness values for an object and comparisons of these determine color
22
Q
How do cones transmit the color of objects?
A
The 3 types of cones have maximum absorptions corresponding approximately to violet, green and yellow light. The wavelength of the light does not affect the character response of the cone. A given cone simply has a higher probability of absorbing a photon which is closer to its peak wavelength. The visual system has no way of detecting the absolute wavelength composition of any light.
23
Q
What is perceptual cancellation?
A
Some colors in the same pixel of visual space perceptually mix to produce other colour categories, but complementary colors do not perceptually mix
24
Q
what is simultaneous color contrast?
A
Is the perceptual facilitation of complementary colors that occurs across boundaries eg a gray disk within a red background looks slightly green
25
What three layers enclose the contents of the eye?
The sclera, choroid and retina
26
What is the structure and function of the sclera?
A thick, stiff, outer layer of connective tissue. At the front of the eye it becomes the cornea, at the back it becomes the dura mater covering the optic nerve. It maintains the shape of the eyeball and provides attachment for the extraocular muscles
27
What is the structure of the cornea?
It refracts incoming light and provides most of the focusing power of the eye
28
What is the choroid?
It is a thin, highly vascular layer, dark brown in colour because of the presence of choroidal pigment cells. By absorbing light it limits total internal reflection within the eye. At the front the choroid becomes the ciliary body and the iris
29
What is the structure of the ciliary body?
The ciliary body gives rise to numerous, thin zonular fibers which attach to the capsule of the lens as the suspensory ligament. Inside the ciliary body lies the ciliary muscle composed of smooth muscle fibers arranged in radial and circular directions.
30
What is the iris?
A diaphragm surrounding a central hole, the pupil. It contains two intraocular muscles which act in concert to control the size of the pupil. Innermost is a flat ring of circularly arranged smooth muscle fibers, the pupillary sphincter. Surrounding the sphincter is a layer of radially organised myoepithelial cells which form the pupillary dilator.
31
What is the course of aqueous humor?
Actively secreted by the epithelium of the ciliary body into the posterior chamber. It percolates through the pupil to the anterior chamber from where it drains into the venous system via the canals of Schlemm located in the irideocorneal angle
32
What is the function of aqueous humor?
supplies metabolic substrates for the lens and cornea which have no blood supply , and maintains eyeball pressure.
33
What is vitreous humor?
Is a gel of extracellular fluid which contributes to refractive power.
34
What is the structure of the lens?
it is biconvex, has a diameter of 9mm. It is encapsulated within an elastic connective tissue membrane which is attached to the suspensory ligament.
35
How are the distinct type of neurons interconnected in the retina?
Light-sensitive photoreceptors, which lie at the back of the retina form synapses with retinal interneurons, bipolar cells, horizontal cells , and amacrine cells. Bipolar cells synapse with the output neurons of the retina, ganglion cells, the axons of which form the optic nerve (only become myelinated once they leave the eye via the optic disk.
36
Which are the only cells that are not neurons in the retina?
Melanin-containing Pigmented epithelial cells.
37
If only the ganglion cells of the retina can fire action potentials, how to photoreceptors and retinal interneurons signal?
By way of passively conducted synaptic potentials
38
Each retina has roughly 10^8 photoreceptors, but an output of only 10^6 optic nerve axons, what is the significance of this?
This is a massive convergence and shows that considerable processing of visual input is done by the retina to achieve this level of data compression.
39
What is the significance of the fovea?
The gaze of the eye is adjusted so that images are brought to focus at the fovea. This region of the retina (diameter 1.5mm) has the greatest visual acuity.
40
Why does the fovea have the greatest visual acuity?
-High density of photoreceptors
-The displacement of overlying layers of the retina so the side so that light hits the photoreceptors directly
-Lack of blood vessels
-Being at the optical axis of the eye so image distortion by optic is minimal
41
What accounts for the blind spot that occurs in the visual field?
About 4mm from the fovea towards the nose les the optic disk, where optic nerve fibers and retinal blood vessels pierce the retina. This region lacks photoreceptors.
42
Outside the fovea, light passes through the full thickness of the retina before striking photoreceptors, what offsets some of the disadvantages of this?
Muller glial cells- funnel shaped cells with their broad end at the retinal surface and extend long slender processed to the photoreceptor layer. They act as light guides. They channel light through the retinal layers to the receptors. They enhance the signal-to-noise ratio by channeling more efficiently the light brought to focus than for light with multiple reflections in the eye.
43
How do muller cells ensure radiation in the near IR and UV leak out to be absorbed by surrounding neurons rather than excite photoreceptors?
They are tuned to visible light
44
How do muller cells correct chromatic aberration?
They refract light so that blue light is brought to the same focus as red light
45
What occurs in the optic decussation?
The optic nerves meet in the midline at the optic chiasm. 53% of optic nerve fibers, those from the nasal halves of the retina, cross to the contralateral side. Axons from the temporal halves remain on the ipsilateral side
46
Axons leave the optic chiasm to enter the optic tracts, what are their three targets from here?
A small proportion go to the pretectum of the midbrain (controls pupil and accommodation reflexes). Others go to the superior colliculus in the tectum of the midbrain which organises several visual reflexes. A visual pathway runs to the hypothalamus to entrain circadian rhythms. the great majority of axons go to the lateral geniculate nucleus of thalamus.
47
Where do the optic radiations go to after leaving the lateral geniculate nucleus?
Optic radiation sweeps to the medial aspect of the pole of the occipital cortex, most axons terminating in layer IV of Brodmann area 17, the striate or primary visual cortex
48
The pupil light reflex controls the amount of light entering the eye by altering pupil size, what is the range if this size?
It ranges between 1.5 and 8 mm in diameter, being maximal in complete darkness.
49
Why is the pupil light reflex useful?
It allows a 30-fold change in light entry and it operates over the light levels typically encountered during daylight.
50
What does light shone in one eye produce in terms of the pupil light reflex?
Light shone in one eye produces pupil constriction of the same eye (the direct reflex) and of the contralateral eye (consensual reflex) because of reciprocal crossed connection in the midbrain
51
What is the pupil reflex pathway?
Optic nerve axons synapse in the pretectum which sends output to the pregamglionic parasympathetic fibers in the edinger-westphal (accessory) oculomotor nucleus. these autonomic fibers travel in the oculomotor nerve to the ciliary ganglion. Postganglionic fibers from there go to the pupillary sphincter. Light stimulation of the optic nerve fibers excited the parasympathetic terminals to release AcH, which contracts the sphincter
52
What is the purpose of the accommodation reflex?
For close objects, light rays are diverging as they enter the eye, and so a greater refraction is needed to bring them to focus at the fovea, the reflex achieves this.
53
What is the effect of the accommodation reflex?
Contraction of the ciliary muscles pulls the ciliary body forwards and inwards, easing the tension in the suspensory ligament and lens capsule, allowing the lens to become more spherical and reducing its focal length.
54
What is the effect of the accommodation reflex?
Contraction of the ciliary muscles pulls the ciliary body forwards and inwards, easing the tension in the suspensory ligament and lens capsule, allowing the lens to become more spherical and reducing its focal length. It occurs in both eyes
55
What is the pathway of the accommodation reflex?
the stimulation is the blurring of the retinal image (large retinal disparity). this is monitored by the visual cortex which projects to the pretectum via the corticobulbar pathway. Via connections between the pretectum and the Edinger-westphal nucleus, parasympathetic fibers are activated which contract the ciliary muscles.
56
What is the vergeance reflex, and what is its function?
Observing a close object also causes convergence of the visual axes of both eyes. It enables both eyes to fix their gaze on an object. In addition, the degree of convergence provides a cue for stereopsis, since the closer the object is, the greater the convergence must be.
57
What is the pathway for the vergence reflex?
It can be triggered by a blurred retinal image or by consciously altering gaze to a point at a different distance. The circuitry is from the visual cortex to the frontal eye fields in the frontal cortex concerned with planning and execution of eye movements
58
What are similarities in rod and cone structure?
They have diameters ranging from 1-4 um, being smaller at the fovea. The inner segment contains the nucleus and has an axon-like process connected to a synaptic terminal. The outer segment is continually regenerated from the base, whilst its apical tip is phagocytosed by pigment epithelial cells at the rate of 3–4 discs per hour. Photoreceptors are incapable of mitotic division.
59
What are the differences between rod and cone cells?
The outer segment in the cone cell has its plasma membrane invaginated into numerous closely packed parallel folds, forming discs. In rod cells the discs are pinched off the plasma membrane to become completely intracellular. The disc membrane is densely packed with visual pigment. In rod cells this is rhodopsin. Each type of cone cells has its characteristic cone opsin. Rod cells are 20-fold more numerous than cones, they are 1000 times more sensitive to light than cones
60
What is the resting potential of the photoreceptor plasma membrane in the dark?
-40mV
61
Light produces a hyperpolarizing receptor potential, the amplitude of which is related to the light intensity. How is the hyper polarisation produced?
When light pho- tons strike the outer segments a cascade of biochemical events is initiated which results in the closure of the cation channels, reducing the dark current, and hyperpolarizing the photoreceptor.
62
What is the normal, relatively depolarized, state of the photoreceptor caused by?
The flow of dark current. The cyclic nucleotide-gated cation channel allows Na+ and Ca2+ ions to flow into the outer segment in darkness. Na+ ions are actively extruded by the Na+-K+ ATPase in the inner segment. Ca2+ leaves the photoreceptor via a Na+–K+–Ca2+ transporter.
63
What does rhodopsin consist of?
a G-protein-coupled receptor, opsin, and a prosthetic group, retinal, synthesized by retinol dehydrogenase from retinol (vitamin A). Retinol cannot be synthesized de novo in mammals and hence must be supplied in the diet.
64
How does transduction by rod cells occur?
In the dark, retinal is present as the 11-cis-isomer. Light causes photo-isomerization to the all-trans isomer. This triggers a series of conformational changes in the rhodopsin to form photoexcited rhodopsin (R*). Photoexcited rhodopsin couples with a G protein, transducin (Gt) and exchange of GDP for GTP occurs. The GTP-bound form of the transducin alpha subunit activates a phosphodiesterase (PDE) which catalyzes the hydrolysis of 3¢,5¢-cyclic guanosine monophosphate (cGMP) to 5¢-GMP. This reduces the concentration of cGMP in the photoreceptor and so the cation channels, normally kept open by the cyclic nucleotide, close
65
What is the effect of a single photon in terms of amplification at the rod cells?
A single photon activates about 500 transducin molecules, closes hundreds of cation channels, blocking the influx of 106 Na+ ions to cause a hyperpolarization of about 1 mV.
66
What are mechanisms to terminate the cascade associated with rod transduction?
Transducin has an intrinsic GTPase which hydrolyze the bound GTP to GDP, stopping the activation of PDE.
Photoexcited rhodopsin is phosphorylated by rhodopsin kinase, and then binds arrestin which blocks the binding of transducin.
Within a few seconds the bond between retinal and opsin in photoexcited rhodopsin spontaneously hydrolyzes and the all-trans retinal diffuses away from the opsin.
67
What happens to rhodopsin in high levels of light?
most of the rhodopsin exists in this dissociated state in which it is described as being bleached and the rod said to be saturated
68
How is rhodopsin regenerated?
Regeneration of rhodopsin occurs in the dark: retinal isomerase catalyzes the isomerization of the all- trans isomer to the 11-cis isomer which then reassociates with the opsin. This process underlies dark adaptation.
69
Restoration of the dark site requires synthesis of cGMP, what is this catalyzed by?
Guanylate cyclase
70
Light adaptation, in which photoreceptors become less sensitive to light exposure, allows them to respond to levels of illumination that vary, what is the role of calcium in photoreceptor light adaptation?
Light-evoked closure of the cation channels reduces calcium influx so the calcium concentration in the outer segment falls, since calcium normally inhibits guanylate cyclase needed for cGMP synthesis-> increase in cGMP production, offsetting its destruction in the light
71
Due to their sensitivity, rod cells are used for scotopic (dim light) vision, how are they so sensitive?
It is partly because they integrate responses to incoming photons over a long period (~100m.s), this means they are unable to discern flickering light if the flicker rate is faster than about 12 Hz. It is also due to to great amplification of the effect of incoming photons
72
Why does scotopic vision have low visual acuity?
- the image formed on the peripheral retina is distorted. -Many rods converge onto a single bipolar cell, therefore this increases sensitivity but information about localisation is less precise.
73
Where are rod cells situated?
Distributed across the entire retina except for the fovea and optic disk
74
Where are cone receptors located?
They are most dense at the fovea and their numbers fall sharply beyond 5 degrees of it.
75
What are the cone photoreceptors used for?
They have low sensitivity to light and do not saturate except in very intense light, so are used for photopic (daylight) vision.
76
Why does photoptic vision have high visual acuity?
There is little or no convergence between cone cells and bipolar cells. Cone cells integrate responses over a short time and so are able to resolve a flicker frequency of less than about 55 Hz.
77
What is mesopic vision?
At low light levels, rod cells operate alongside cones, boosting the cone cell signal by means of electrical synapses to preserve colour protection
78
What are the three populations of cone cells ?
Short (blue), medium (green) and Long (red) wavelength cones. Their peak sensitivities are not described by these colours. The S cones are sensitive to wavelengths down to 315 nm, however, the normal eye does not see wavelengths shorter than 400 nm, because they are absorbed by the lens.
79
What is the mechanism of color vision?
Requires comparisons of the relative strengths of the S, M and L cones.
80
Why are S cones absent from the center of the fovea and only make up 5- 10 percent of total cones?
The lens suffers from chromatic aberration, in which short-wavelength light is not brought to focus at the same point as longer wavelengths, causing image blurring. Therefore, this would compromise high acuity vision and so color vision at the central fovea is dichromatic.
81
Other than the vision at the fovea being dichromatic, what other feature means that colour vision is coarse grained and cannot resolve fine detail?
M and L cones are distributed randomly leaving patches in which there is only one population of cone.
82
Why is scotopic vision achromatic?
All rod cells have the same spectral sensitivity curve. They are unable to distinguish between wavelengths on the rising and falling limbs of the spectrum that excite the cell to the same extent.
83
What is the difference in sensitivity of the eye under scotopic, photoptic?
Under scotopic vision, the sensitivity of the eye is determined by the rod cells and peaks at 500nm. Under photoptic conditions the wavelength sensitivity is governed by cones and is maximal at 550 nm.
84
What is the purkinje shift?
The shift in wavelength sensitivity during mesopic vision. It means that as dusk falls red fades first and the last colour to be lost is green.
85
What is dark adaptation?
When moving from bright to dim light the sensitivity of the retina to light increases a million-fold over a period of 30 mins or longer. It has two phases, the first is due to cone cells which increase sensitivity about 100 fold, the second longer phase is due to rod cells.
86
What is light adaptation?
It occurs when going from dim to brightly lit conditions. It is very much faster than dark adaptation.
87
What are the two types of bipolar cells distinguished on the basis of morphology and physiological responses?
Invaginating bipolar cells- Have processes that form triad ribbon synapses deep in the photoreceptor terminal. They depolarize in response to light striking the photoreceptor.
Flat bipolar cells- form superficial basal synapses with photoreceptors and hyperpolarize in response to light
88
Why are triad ribbon synapses called this?
They have three postsynaptic components- the bipolar cell dendrite and dendrites of the two horizontal cells.
89
What arrangement in the retina gives rise to two labelled lines (on and off channels) in the retina?
Cone cells form synapses with midget bipolar cells of either one type or the other. These synapse directly with ganglion cells which respond to light in the same sense as their bipolar cells. On channels: cone-depolarizing bipolar cells (on ganglion cells). Off channels: cone-hyperpolarizing bipolar cells (off ganglion cells).
90
What is the difference between on and off ganglion cells?
On ganglion cells are depolarized and increase their firing rate as a function of light intensity. Off ganglion cells are silenced by hyperpolarisation.
91
If all photoreceptors use glutamate as a transmitter, how do invaginating and flat bipolar cells have opposite responses?
They have different glutamate receptors. For invaginating bipolar cells, tonic release of glutamate from photoreceptors in the dark is inhibitory. When light hyperpolarizes the photoreceptor glutamate release is suppressed, inhibition is lifted. For flat bipolar cells the response to tonic glutamate release is excitatory and light reduces this excitation
92
How is the existence of separate on and off channels enhance the boundaries between regions that reflect different amounts of light (and respond preferentially to stimulus change )?
On channels respond with increased firing to light levels that are greater than the local average. Off channels slow increased firing in response to dark regions (light levels lower than local average).
93
What is the purpose of lateral inhibition and where is it seen?
Enhances contrast, can be seen in the receptive fields of both bipolar and ganglion cells.
94
How is lateral inhibition brought about?
Horizontal cells form reciprocal connections at triad synapses between neighbouring photoreceptors. In the dark they are excited by glutamate release from photoreceptors but they release GABA, which inhibit photoreceptors.
95
What is the exact sequence of lateral inhibition?
Light which hyperpolarizes surrounding receptors causes them to secrete less glutamate so reducing horizontal cell excitation. GABA from horizontal cells is lowered, central cone can depolarise somewhat, releases more glutamate. The final step depends on the type of bipolar cell the central cone synapses with. In each case, the bipolar cell response is the opposite for surround compared with central illumination.
96
What is the effect of lateral inhibition?
The RFs are round and divided into an inner center and an outer surround. Stimulation of these two regions separately produces opposite effects on the cells, eg for an On ganglion cell, background firing rate is increased by center illumination but silenced by light on the surround. When light fills the whole RF there is little change in firing rate.
97
What is the S space?
Horizontal cells are extensively interconnected via gap junctions forming a network which spans an area of retina termed the S Space. These horizontal cells provide the signal for surround inhibition and it is thought that this signal is a measure of the mean luminance over quite a wide area of retina
98
What are the three ganglion cell types?
Parvocellular- Small and the most numerous
-Magnocellular- large and number 100 000 per retina
-Koniocellular- small and resemble P cells physiologically
99
How do properties of P and M cells differ?
-P cells have smaller RF
-P cells have slower conduction velocity
-P cells often show sustained responses, M cells respond transiently to a prolonged visual stimulus.
-P cells are wavelength selective whereas M cells are not
-M cells are more sensitive than P cells to low-contrast stimuli
100
Where do M and P cells get their input from?
P cells get their input from single cones or from several cones with the same wavelength sensitivity (S,M, or L). M cells get input from M and L cones together (not S cones), and from rods.
101
What are the functions of P and M cells?
P cells (not M cells) are involved in color vision. Rapid transient responses of M cells make them adapted for motion detection. The Small RFs of P cells and their sustained responses are suitable for fine form discrimination.
102
How do P cells transmit color of objects?
they are color single opponent cells. They have RFs that are excited by one type of cone cell but inhibited by another
103
What are the two types of P cell that can be distinguished by nature of their RFs, what are they?
Most common are the red-green cells, in which the RF compares input from M and L cones. Blue-yellow cells do not have a center-surround pattern but are either excited by S cones and inhibited by combined M plus L (to give the sensation of yellow) cone signal or vice versa
104
How do red-green cells respond to different sizes of spots of light?
They respond differently. They are more wavelength selective for large stimuli than for small ones. For small spots they cannot distinguish red or green from white light, but they can signal brightness
105
Why are M cells called broad band cells?
They get combined input from two types of cones. Their RFs only measure brightness contrast
106
What are two types of K cells?
Some are blue-on cells, others are motion sensitive and have low spatial resolution
107
How do rod cells augment cone function in a partially-dark adapted eye (at dusk), what is the purpose?
Rod cells come on stream but signal through gap junctions to cone cells. This maintains acuity and color vision.
108
What happens to the rod signaling once it is very dark?
Rod cells have a greater influx of calcium, this closes the gap junction between rod and cone cells. Rod signaling is now relayed via depolarising bipolar cells which synapse with a population of amacrine cells.
109
What is the structure of amacrine cells?
They have no axons but their extensive neurites share properties of axons and dendrites. Morphologically a diverse group, and most neurotransmitters identified are used by one or the other of the 30 or so types of amacrine cell.
110
What is the function of amacrine cells?
Implicated in rod signalling (increase contrast sensitivity), surround inhibition and detecting the motion of an object across the visual field
111
How do amacrine cells produce surround inhibition?
Dopaminergic amacrine cells have long dendrites that interconnect, possibly via gap junctions to form a network. These cells get input from cone bipolar cells so the network is able to signal average illumination which is used to produce surround inhibition.
112
Why are ganglion cells more sensitive to light in the dark-adapted eye?
Dopamine surround inhibition is turned off
113
Some ganglion cells are sensitive to the direction of motion of a stimulus, what is direction sensitivity conferred by?
Amacrine cell circuits
114
Which side of the visual field does the left lateral geniculate nucleus represent?
The right, it carries axons from the left side of both retinas.
115
Where do the pathways for visual perception start?
With the retinogeniculate fibers, axons of the ganglion cells that end in the lateral geniculate nucleus
116
What are the 6 layers of the primate LGN?
The two most ventral are the magnocellular layers (receive input from the M ganglion cells that are concerned with motion. Dorsal to these are 4 parvocellular layers that are innervated by P cells and encode form and colour. Interleaved between these major layers are koniocellular layers containing small K cells- receives input from K ganglion cells and usually have small receptive fields
117
Where do the K cells relay information to?
There are several classes of K cells. Some get input from blue-on K ganglion cellls and relay to the primary visual cortex. Others are motion sensitive and bypass V1, projecting to the medial temporal cortex, which is also the major target of M cells conveying motion information
118
What are the features of the LGN receptive fields?
LGN cells have circular RFs with surround antagonism. They show little or no response to diffuse light covering the whole receptive field.
119
When is the input from both eyes combined in the visual pathway?
Each layer in the LGN gets input from only one eye and no cells show binocular responses (responses to both eyes). It is not until the visual cortex is reached that input from both eyes is combined.
120
Is there a form of mapping in the LGN?
There is a very precise topographic (retinotopic) mapping in perfect register onto each of the layers of the LGN, with the fovea taking up about half of the space
121
What allows the wide range of contrasts seen in the natural world to be accommodated by the limited dynamic range of neurons?
Responses of LGN cells are smaller than can be explained by classic center–surround processing. The responses are suppressed by the retina and possibly by inhibitory inter- neurons in the LGN itself.
122
What is LGN output via?
Geniculostriate neutrons that project to the primary visual cortex
123
What are the sources of the geniculostriate neurons?
Only about 10% of the synapses on these are from retinal ganglion cells. Other synapses are made by back projections from V1, the superior colliculus (to koniocellular layers), the parabrachial nucleus, and the thalamic reticular nucleus.
124
What are geniculostriate neurons involved in?
These are probably involved visual attention; that is, in selecting which retinal input is transmitted through to the Visual cortex
125
Where do the fibres of the optic tract terminate?
in the striate cortex (Brodmann’s area 17) on the medial surface of the tip of the occipital lobe. This region is the primary visual cortex (V1)
126
What mapping is maintained all way to the visual cortex?
Precise retinotopic mapping is maintained up to V1 with the fovea having a dispro- portionately large representation.
127
What are the three parallel streams of information that go into the visual cortex?
The movement sensitive M LGN cells input into layer 4Ca, the P LGN cells go to layer 4Cb, whereas the koniocellular layers of the LGN project to layers 2 and 3. These streams remain quasi-independent throughout the visual system.
128
What is the property of the receptive fields of majority of cells in v1?
have elongated receptive fields (RFs) with both inhibitory and excitatory regions, and respond to bars, slits, edges, and corners rather than spots of light. Most fall into two categories based on their RF properties: simple or complex cells. Both are orientation selective, in that they respond to linear features in only a narrow range of orientations.
129
What type of cells are simple cells and where are they found?
Pyramidal cells in layers 4 and 6
130
What stimulus do simple cells respond to and what is their inputs?
They are highly sensitive to the position of a stimulus on the retina. They have small oval RFs with center– surround antagonism. A simple cell gets its input from a linear array of LGN cells having the same RF properties,
131
Where are complex cells found?
Most abundant in layers 2, 3 and 5
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What are the properties of the RF of complex cells and what are their inputs?
They have larger RFs than simple cells and, lacking distinct inhibitory or excitatory regions, a stimulus of the appropri- ate orientation anywhere in the RF evokes a response. Many complex cells show a preference for movement at right angles to the long axis of the stimulus. Some complex cells receive their inputs from simple cells but others get their input directly from the LGN.
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What is the visual cortex divided into? What are they called?
primary visual cortex is divided into radial columns 30–100 μm across. In each of these, all cells respond preferentially to linear features with a given orientation so they are called orientation columns. adjacent columns have an orientation preference that differs by only about 15∞.
Columns which have the same orientation are arranged in stripes across the cortex.
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Cells in which layers in particular show binocular responses (driven by either eye)?
4B, 2 and 3
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What is ocular dominance?
A phenomenon where binocular cells show a preference for one eye.
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What are ocular dominance columns?
Cells which have the same ocular dominance, occupy ocular dominance columns that are 500 micrometers across. Columns representing ipsilateral and contralateral input alternate regularly over the cortex.
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How are the receptive fields of binocular cells arranged?
The receptive fields of binocularly driven cells resemble those of simple or complex cells, lie in corresponding positions in the two retinas, have identical orientation properties and have similar arrangements of excitatory and inhibitory regions.
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How are the binocular cells of V1 responsible for stereopsis?
Similar input from both eyes into arrays of binocular cells is needed for perception of a fused image. Inputs into these cells are unequal they measure retinal disparity.
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What is the higher- order modularity in the visual cortex?
Hypercolumns
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What is the structure of hypercolumns?
it represents a given corresponding position for both retinas, and maps every orientation for that position. It consists of a full thickness slab of cortex with an area of about 1 mm2 containing a complete set of orientation columns for both ipsilateral and contralateral ocular dominance.
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How does the retinoptic map occur in V1?
adjacent pixels of the retina map to adjacent columns in an orderly fashion.
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What are the three independent pathways that can be delineated in the primary visual cortex?
Magnocellular pathways and two parvocellular pathways
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What is the sequence of the magnocellular pathway?
M Ganglion cell > M LGN cells > spiny Stellate cells (excitatory inter neurons) in 4c(alpha) > pyramidal cells in layer 4B which show orientation and direction selectivity > axon collaterals to pyramidal cells in layers 5 and 6.
Layer 5 cells project to subcortical regions, the pulvinar (a thalamic nucleus involved in visual attention), the superior colliculus, and pons. Layer 6 pyramidal cells go to the extrastriate cortex.
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What is the function of the M pathway?
It is specialized for analysis of motion. Its outputs via layer 5 are important in visual attention and gaze reflexes. Some cells in the M pathway are binocular so it con- tributes to stereopsis.
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Why is the M pathway color blind?
Because it originates with ganglion cells which combine input from two classes of cone cell it is not wavelength selective
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What is the common sequence for the parvocellular pathways?
P Ganglion cells > P LGN cells > Spiny stellate cells in 4C(beta) > pyramidal cells in 4B > pyramidal cells in layers 2 and 3 > deep pyramidal cells in layer 5
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Where does segregation of the parvocellular pathways occur?
In layers 2 and 3
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What are blobs?
Pillars of high activity in layers 2 and 3 that show up when stained with mitochondrial enzyme cytochrome oxidase. Each blob is centred on an ocular dominance colum. Between the blobs lies an interblob region.
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What are characteristics of cells in the blobs?
They are wavelength selective, show poor orientation and selectivity and are monocular.
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What are the characteristics of blobs in the interblob region?
Orientation selective, binocularly driven, complex cells. They are not wavelength selective or motion selective. They are part of the parvocellular -interblob pathway which processes high resolution analysis of form in the visual world.
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What does the parvocellular blob pathway mediate?
Color vision. They get input from the blue-on K cells in the koniocellular LGN layers, function of this input is not yet understood.
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What are wavelength selective blob cells called, where are their RF properties derived from ?
Double opponent cells, with receptive field properties derived from their inputs, the single opponent parvocellular LGN cells. They have center surround antagonist information.
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What do double opponent cells signal and how are they categorized?
They signal color contrast and come in four classes categorized by their preferred stimuli. (red spot, green surround; green spot, red surround; blue spot, yellow surround, yellow spot, blue surround). They are unaffected by white stiumuli of any size, so they are more selective detectors of color contrast.
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How do double opponent cells assist with colour constancy?
A shift in the wavelength composition of light will produce equal but opposite effects on the responses of the center and surround of double opponent cells. There will be little effect overall on the RF of the cell which will continue to signal the same color.
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On the scale of the entire visual field, what does colour constancy involve?
comparing red–green brightness, blue–yellow brightness (from color single opponent cells), and total brightness (added outputs of S, M, and L cones) over large areas of retina
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How does perceptual cancellation occur?
It is explained by the way in which colour opponency happens to be organized as red (R) versus green (G) and yellow (R+G) versus blue channels. Since mutual antagonism occurs between red and green or between yellow and blue only one color in each pair can be seen at a single pixel of the retina at any time
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What explains simultaneous colour contrast and the complementary after-images that appear after staring at a uniform patch of color?
can both be accounted for by the properties of double opponent cells. For example, one opponent cell cannot discriminate between a green stimulus to its surround or a red stimulus to the center; the response is the same for both. So a gray disc viewed in a green background is interpreted as red.
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What is the extrastriate visual cortex?
A term applied to all the visual cortex except v1.It includes not only occipital cortex areas 18 and 19 but also areas of parietal and temporal cortex.
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How are the 30 regions of the extrastriate cortex differentiated?
can be differentiated on the grounds of cytoarchitecture, connections, and physiological properties. Most have a retinotopic map of some aspect of the visual world.
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Where do most outputs for V1 go?
V2, the secondary visual cortex, which occupies part of area 18. V2 shows a characteristic cytochrome oxidase staining pattern, alternating thick and thin stripes running at right angles to the V1/V2 border. Pathway tracing techniques and electrophysiological studies reveal how the magnocellular and parvocellular pathways continue into V2 and beyond.
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What are the properties of cells in the V2 thick stripes?
They are motion sensitive and binocular, being driven by a preferential retinal disparity.
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What is the input and output of the V2 thick stripe?
The thick stripe receives inputs from layer 4 of the interblob region of V1 and sends much of its output, via V3, to the medial temporal (MT) visual cortex, V5.
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What is the function of the V2 thick stripe–V3–V5 (MT) connection?
It is the extension of the magnocellular pathway and concerned with motion and depth perception.
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What is the input and output of V2 interstripe region?
gets its inputs from the V1 interblob regions (layers 2 and 3) and sends outputs to V3 and then to V4.
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What is the function of the V2 interstripe- V3-V4 connection?
Many cells in V3 and V4 are orientation selective.
Hence this route is a continuation of the PI parvocellular pathway concerned mainly with form perception
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What is the input and output of the v2 thin stripe and what is its function?
The blobs of V1 project to the thin stripe of V2 which in turn sends outputs to visual area V4. That this route is the extension of the PB parvocellular pathway for color vision
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The M, PI, and PB pathways are not completely independent. Why do reciprocal pathways exist between V3 and V4, and V5 and V4?
allow interactions between M and PI systems, both of which contribute to stereopsis. Interaction of motion and form analysis is probably required for the identification of moving objects.
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What is the evidence that there is no cross talk between M and PB pathways?
The M system is color blind and for equiluminant stimuli (those varying in color but not in brightness), which can only be perceived by the PB system, the perception of motion vanishes. Moreover, although the PI system uses color contrast to localize borders, form information is not available to the PB pathway: when looking at equiluminant blocks of color they appear to “jump around” because the PB system cannot localize boundaries.
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During saccades what happens to the P and M systems and what is the relevance of this?
the M, but not the P system, is shut down. Thus the M (motion) system is not confused by the rapid eye movements. The response times of the P system are suf- ficiently slow that they are unaffected by the shifting image.
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Parallel processing beyond V5 and V4 results from the segration of information into two streams, what are these?
The dorsal and ventral stream
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What is the pathway of the dorsal stream?
It is largely from the MT and goes to the medial superior temporal and posterior parietal cortex.
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What is the function of posterior parietal cells?
PP cells show selectivity for size and orientation of objects, their firing depends on where an animal is looking and they fire as an animal grasps an object. Lesions to the MST and PP results in optic ataxia
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What is optic ataxia?
Visuospatial tasks are profoundly affected, but object recognition is unaffected.
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What is the pathway of the ventral stream?
From V4 to the inferotemporal cortex
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What is the function of the ventral stream?
It is crucial for object recognition. Lesions of the IT cortex cause visual agnosia, in which animals fail at tasks requiring object recognition. Visuopatial tasks are unaffected
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What do inferotemporal cortex cells respond to?
IT cells are sensitive to form and colour but are unfussy about object size, retinal position or orientation. Many respond to specific objects such as hands or faces. The IT area has no retinotopic map.
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What does bilateral loss of V1 cause?
Loss of visual perception
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What is blindsight?
Primates/ humans with bilateral loss of V1 are able to avoid obstacles whilst moving through a space much better than by chance. Humans with this are completely unaware of the visual world and do not understand how they are able to navigate through the space.
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What is blindsight mediated by?
May be mediated by a pathway that goes directly from the magnocellular LGN to the thick stripe of V2 (input to the dorsal stream)
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What is the implication of blindsight?
V1 is required for conscious visual perception.
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What is the purpose of eye movements?
They either stabilize the gaze by keeping the eyes fixated on an object during head rotation, or shift gaze so that the fovea is brought to bear on an object, or track a moving object.
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How many eye movements are there?
5
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What is the mechanism of gaze stabilisation?
Rapid head rotation, detected by semi-circular ducts provides input for vestibulo-ocular reflexes, whereas optokinetic relexes depend on visual input to monitor slow head rotations. For both systems their output causes conjugate eye movements in the opposite direction to the head rotation so that the retinal images do not shift.
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Which three systems organise gaze shift?
Saccadic system, smooth pursuit system and the vergence system.
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What are the extraocular eye muscles and their insertion and origin?
Two pairs of rectus muscles (superior and inferior and medial and lateral) originate from a common annular tendon attached to the back of the orbit. These muscle insert into the sclera in front of the equator of the eyeball. The third pair are the oblique muscles (superior and inferior) which insert into the sclera behind the equator of the eyeball.
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What are the actions of the medial and lateral rectus muscles?
They cause the eye to rotate about the vertical axis so that the gaze moves horizontally. the medial rectus brings about rotation to the midline (adduction), while the lateral rectus causes lateral rotation (abduction).
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Other than the medial and lateral rectus muscles what do the movements produced by the muscles produce?
Produce rotations that have components along two of the principal axes, and the components change depending on the horizontal position of the eye.
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How do the eyes produce conjugate movements?
Eye muscles act in complementary fashion in the two eyes during conjugate movements, in which the two visual axes move in parallel.
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What is the pathway for the output of all 5 eye movement systems?
Brainstem reticular adn medial vestibular nuclei axons (running in the medial longitudinal fasciculus)--> motor neurons in the nuclei of oculomotor, trochlear or abducens cranial nerves.
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How do the eye motor neurons fire?
Both statically, in a matter relating to eye position and dynamically, reflecting eye velocity. to hold the eye steady in a given position requires tonic discharge by a particular set of motor neurons. This set will be different for different positions. Eye movements are brought about by pulses of action potentials with a firing frequency directly proportional to the velocity of the movement. This brings the eye to a new position, which needs the generation of a new position signal
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How do the eye motor neurons fire?
Both statically, in a matter relating to eye position and dynamically, reflecting eye velocity. to hold the eye steady in a given position requires tonic discharge by a particular set of motor neurons. This set will be different for different positions.
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How does the firing of neurons resulting in eye movements and new positions likely occur?
probably achieved by integration of the velocity signal of the vestibulocerebellum and the prepositus nucleus of the brainstem reticular system
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What is the vestibulo-ocular reflex?
head rotation (detected by semi-circular ducts) triggers equal and opposite rotation of both eyes. the horizontal semi-circular ducts are effectively wired to the medial and lateral rectus muscles to achieve this
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What is nystagmus?
usually results from large head rotations when the eyes cannot continue to rotate so must be reset to a central position by rapidly moving in the same direction as the head. Characterised by a slow phase that stabilize the retinal image and quick phase that resets the eyes (direction of nystagmus is the direction of the quick phase)
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What is the 'gain' of the vestibulo-ocular reflex?
The magnitude of the eye rotation divided by the magnitude of the head rotation - quite close to one for fast head rotation.
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What is the significance of the gain of VOR being one for fast head rotation?
It means there is a good match between eye and head movements, which makes for a stable retinal image.
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How does the VOR get modified by visual experience?
The cerebellum is required for this adaptation to occur, but not for it to be maintained once it is established. The instability of the retinal image acts as an error signal which is relayed to the cerebellum from the inferior olivary nucleus. The cerebellum learns to minimize the errors and alters its drive to the extraocular muscles
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What is the retinal slip?
Slow rotation of the head causes an apparent movement of the visual world in the opposite direction.
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What is the optokinetic reflex?
A retinal slip is detected by large, movement sensitive retinal ganglion cells and used to produce eye movements which are equal in speed but of opposite direction of the retinal slip. As in VOR, nystagmus occurs for big head rotations
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What do lesions of the vestibulocerebellum cause?
Impairs the ability to maintain steady gaze, causing inappropriate nystagmus
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What is saccades?
Fast conjugate movements that move the fovea to target a different point in visual space. This system uses visual, auditory and somatosensory input to determine the eye rotation required to realign the gaze.
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What is horizontal saccades controlled by?
Paramedian pontine reticular formation. Contains burst cells which code for the size and direction of the eye movement and are excitatory to oculomotor neurons.
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What is vertical saccades organized by?
Rostral interstitial nucleus which lies in the medial longitudinal fasciculus. Contains burst cells which code for the size and direction of the eye movement and are excitatory to oculomotor neurons.
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Where do the signals that trigger saccades come from?
The superior colliculus and the frontal eye fields of the frontal cortex. Each can generate saccades independently of the other.
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Where do the superficial, intermediate and deep layers of the superior colliculus get their input from?
The superficial layer gets information from the retina and visual cortex and has a map of the contralateral visual field. The deep layer gets auditory and somatosensory input and so has an auditory map of the location of sounds in space and a somatotopic map in which the body parts closest to the eyes gets the greatest representation. The intermediate layers have a motor map.
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What are cells in the intermediate layer of the superior colliculus called?
Neurons here are called collicular saccade-related burst neurons because they fire a high frequency burst of action potentials about 20 ms before a sac- cade. Each one has a movement field (the equivalent of a receptive field) that covers the sizes and directions of the saccades it participates in.
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How is the direction of any given saccade encoded ?
encoded by a population of neurons in exactly the way in which the primary motor cortex uses population coding to determine the direction of movements.
The superior colliculus turns sensory coordinates into motor coordinates. All of its four maps are in register so each point on the superior colliculus represents a specific location in sensory space and the saccades necessary to direct gaze towards it.
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Does visual input to the superficial layers automatically lead to firing of cells in the inter- mediate layer of the superior colliculus and why?
No, because superficial layer cells relay through the pulvinar of the thalamus and the visual cortex before influencing the intermediate layer cells. The relay may function to allow saccades only to stimuli with high salience and hence is a visual attention mechanism.
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Other than head rotation, what other function does the superior colliculus serve?
the superior colliculus causes head rotation, by way of the tectospinal tract, to neck muscle motor neurons. This allows orientation towards a stimulus, so called orienting responses
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How does the frontal eye field trigger saccades?
directly by stimulating the intermediate layer burst cells in the superior colliculus, and operates the oculomotor basal ganglia- thalamocortical circuit to select appropriate eye movements.
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How does recovery of saccades occur after damage to the superior colliculus?
Recovery occurs because the frontal eye fields can trigger saccades by its direct connections with the pons and midbrain
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What is the consequence of damage to the frontal eye fields?
Causes a transient paralysis of gaze towars the opposite side, but refelx saccades soon return, produced by the superior colliculus. However, intentional saccades is prevented.
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What does the smooth pursuit system do?
Intentional tracking of a moving object so that its image remains on the fovea.
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How does the smooth pursuit system differ from optokinetic reflexes?
It is voluntary, and attends to movement over a small part of the visual space. Optokinectic reflexes are involuntary and are responses to movement f the entire visual world.
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What is the pathway of signals relating to the velocity of smooth pursuit movements?
Generated by medial temporal cortex > Dorsolateral pontine nucleus (translates the target velocity into the motor commands for the pursuit movement) > vestibulocerebellum > medial vestibular nuclei > drives the smooth pursuit movements.
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What is vergeance, how is it produced?
The only disjunctive eye movements. Signals to produce vergeance include blurring of the retinal image by large degree of retinal disparity, the extent of accommodation or monocular clues to distance. They all require the visual cortex. Fast vergeance movements occur during saccades
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What is sensory attention?
What occurs when attention selectively filters out irrelevant sensory information in favour of the relevant, which may be consciously perceived.
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What is covert attention?
Attention to stimuli that is made without moving, by alterations to the internal state. Covert visual attention shifts independantly of eye movements
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What are attention mechanisms thought to involve?
Increasing the signal-to-noise ratio and increased gain (signal amplification) of those neurons that encode the selective representation. Neural correlates of this include increased firing frequency, greater synchronicity in firing or synaptic activity within neuron populations, reduced response variability, and transient modulation of receptive fields
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How is visual attention accompanied by alterations in the receptive field properties of some visual neurons?
During attentional tasks these cells become transiently biased to ignore distracter stimuli that would, in a non-attending state, elicit a response. RFs shrink to only accompany the target stimulus so that the cells responds to it alone. It raising of signal-to-noise ratio. Only cells with large enough RFs to respond to several objects are involved. Usually in regions responsible for late rather than early visual processing
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How do receptive field properties place contraints on visual attention?
Cells that respond to both movement and colour have never been identified and the visual system is presumably forced to analyse each object in turn for each of the stimulus features in order to identify the one with the target combination and this takes time.
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What is the proposed circuitry of visual attention?
Closely related to that involved in oculomotor control. Stimulation of the FEF, superior colliculus and areas of the parietal cortex enhanced performance in an attention task. Activation of FEF allows covert attention to the appropriate location in the visual field and selects appropriate saccades to fixate the target, earliest activity in attention tasks seen here and so it is the source of attention-control signals. Several cortical areas also generate attentional signals (V4, V2, V1, lateral prefrontal cortex).
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Which three thalamic nuclei are implicated in visual attention?
Pulvinar, Lateral geniculate nucleus and thalamic reticular nucleus.
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What is the input to the pulvinar
Superior colliculus, LGN and cortex, it has topographic maps of the visual world and responds selectively to various aspects of visual stimuli, including orientation, color and motion, presumably because of its cortical connections. Motion sensitive pulvinar neurons respond to motion in the real world but are silent for the apparent motion of a stimulus caused by eye movements as inputs from the superior colliculus allow them to subtract retinal signals produced by eye movements.
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What are the cortical connections of the pulvinar and what is the implication of this?
Connections with the cortex are widespread, include the visual cortex and are reciprocal. If 2 areas of a cortex are directly connected they are also indirectly connected via the pulvinar. Pulvinar is thought to modulate the efficacy of the direct inputs from one cortical area to another. this could be an attention mechanism.
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How does the pulvinar provide a solution to the binding problem?
Controls intrinsic oscillations of cortical neurons and hence synchronise firing of widely distributed cortical neurons representing different aspects of the same object. This could be how the brain integrates information in parallel visual channels to produce a unified percept.
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Visual attention enhances LGN responses to attended locations and attentuates those to ignored stimuli, what are the two routes by which this happens
The thalamic reticular nucleus has access to information from the prefrontal cortex, visual cortex and superior colliculus and makes retinoptic connections with the LGN.TRN is inhibitory to the LGN and attention > decrease in inhibition.
Back projections from the FEF via V4 and V1 are retinoptic and enhance LGN responses. Responsible for a later phase of attention (200ms). P, M and K streams are segregated to allow specific features to be selected for attention. (LGN is a gatekeeper selcting what retinal input is transmitted through to the visual cortex.
