Vision Flashcards
What is the sensitivity of the human eye?
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.
Why is it difficult to distinguish differences in intensity at high light levels ?
Intensity is encoded by the visual system logarithmically
What is the definition of vision?
The process of discovering from images what is present in the world and where it is
What is parallel processing?
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.
What is serial processing
A task is segmented into several subroutines that are executed sequentially.
What does visual processing give higher weight to?
Regions of the world that are changing in time (movement) and space (contrast) than those that are constant
What does visual perception require?
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
What is pattern completion?
Generating a complete percept even when the raw sensory data is incomplete or corrupted by noise
What is generalization?
The ability to recognise objects from a wide variety of vantage points and contexts.
What is a visual illusion?
Results from an unresolvable mismatch occurring between the sensory input and the internal representation.
What is perceptual constancy, and why is it important?
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.
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?
-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
What is accommodation?
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
The binocular clue to depth perception is stereopsis, what is this?
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
What is binocular parallex?
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.
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?
> 0.6 mm or 2 degrees
What is required for stereopsis?
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.
What is the basic definition of colour vision?
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
What does the spectrum of light reflected from an object depend on?
On the wavelength composition of the illuminating light and the reflectance of the surface
What is dichromatic color Vision?
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
Human color division is trichromatic, why is called this?
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
How do cones transmit the color of objects?
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.
What is perceptual cancellation?
Some colors in the same pixel of visual space perceptually mix to produce other colour categories, but complementary colors do not perceptually mix
what is simultaneous color contrast?
Is the perceptual facilitation of complementary colors that occurs across boundaries eg a gray disk within a red background looks slightly green
What three layers enclose the contents of the eye?
The sclera, choroid and retina
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
What is the structure of the cornea?
It refracts incoming light and provides most of the focusing power of the eye
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
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.
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.
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
What is the function of aqueous humor?
supplies metabolic substrates for the lens and cornea which have no blood supply , and maintains eyeball pressure.
What is vitreous humor?
Is a gel of extracellular fluid which contributes to refractive power.
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.
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.
Which are the only cells that are not neurons in the retina?
Melanin-containing Pigmented epithelial cells.
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
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.
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.
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
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.
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.
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
How do muller cells correct chromatic aberration?
They refract light so that blue light is brought to the same focus as red light
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
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.
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
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.
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.
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
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
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.
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.
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
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.
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.
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
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.
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
What is the resting potential of the photoreceptor plasma membrane in the dark?
-40mV
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.
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.
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.
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
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.
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.
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
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.
Restoration of the dark site requires synthesis of cGMP, what is this catalyzed by?
Guanylate cyclase
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
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
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.
Where are rod cells situated?
Distributed across the entire retina except for the fovea and optic disk
Where are cone receptors located?
They are most dense at the fovea and their numbers fall sharply beyond 5 degrees of it.
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.
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.
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
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.
What is the mechanism of color vision?
Requires comparisons of the relative strengths of the S, M and L cones.
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.
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.
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.
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.
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.
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.
What is light adaptation?
It occurs when going from dim to brightly lit conditions. It is very much faster than dark adaptation.
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
Why are triad ribbon synapses called this?
They have three postsynaptic components- the bipolar cell dendrite and dendrites of the two horizontal cells.
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).
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.
