The Eye And Spatial Vision Flashcards
(Visible) light
400-700 nm—where human eye can pick up there is info out there
a narrow band of electromagnetic radiation that is visible to the human eye
Form of electromagnetic radiation (NRG made from vibrations of electrically charged material)
can be conceptualized as a wave or a stream of photons—tiny particles that consist of one quantum of NRG
Dual nature of light
visual acuity
the smallest spatial detail that can be resolved at 100% contrast
eye doctors use 20/20
Photon
a particle representing a quantum of visible light (or other form of electromagnetic radiation)
Demonstrates both particle and wave properties
The smallest measureable unit of light
Wave
An oscillation that travels through a medium by transferring NRG from one particle to point to another without causing any permanent displacement of medium
Absorb
To take up something such as noice, light or energy— and not transmit
Scatter
To disperse something such as light— in a irregular fashion
Diffracted
Electromagnetic
Made up of NRG from different wavelengths
The waves are not colored, just our visual system that interprets the wave so we see it in color
Hue
Perceptual attribute of colors that let them be classified as similar to red, green or blue or something in between
Reflect
To redirect something that strikes a surface—esp. light, sound or heat, usually back towards point of origin
Transmit
To convey something from one place or thing to another
Light neither reflected or absorbed
Refract
To alter the course of a wave of NRG that passes into something from another medium
To measure the degree of refraction in a lens or eye
necessary to focus light rays onto the retina
Image
A picture or likeness
Cornea
The transparent “window” into the eyeball—because cornea is made of highly ordered arrangement of fibers and has no blood vessels or blood focusing power—blood and blood vessels would absorb light
Has rich supply of transparent sensory nerve endings—these force eye to close and make tears of cornea is scratched to preserve its transparency—external layers of cornea regenerate quick—if scratched usually heals in 24 hours
Where light first passes
Most powerful refracting surface in eye
Spherical
Is 2/3 of eyes
Transparent
Characteristic of a material that allows light to pas through with no interruption, objects on the other side can be clearly seen
Most light transmitted through
Sine wave in vision
a pattern for which variation in
a property, like brightness or colour, as a function of space, is a sine function
Can decompose into a different sine wave components
Cycles
One repetition of a black and white stripe
Wavelength
in vision, the space required for one
cycle of a repeating waveform
Spatial frequency
the number of cycles(# of oscillations) of a grating per degree of visual angle (usually specified in cycles per degree)
Cycles per degree
the number of pairs of dark and bright bars per degree of visual angle
Phase
in vision, the relative position of a grating
Amplitude
(or intensity): the magnitude of difference in luminance intensity
Fourier analysis
Any complex image can be broken down into a series of sine wave components using Fourier analysis
a mathematical procedure by
which any signal can be separated into component sine waves at different frequencies
Spatial filtering
Long wavelength - low spatial frequencies - colour
Short wavelength - high spatial frequencies - fine details
Imp. When we take in images in our heads we combine this
spatial frequency
Our brains seem to analyze stimuli in terms of their sine wave components
Lens
Enables eyes change of focus
has not blood supply, so it can be completely transparent
because of crystallins
class of proteins that make up lens
and they are packed densely
Shape is controlled by cillary muscles
Refraction is necessary to focus light rays onto the retina (light bending)
Light changes anytime it passes a different density
Retina
Light sensitive membrane in back of eye that has photoreceptors and other cell types that transduce light into electrochemical signals and transmit them to the brain via optic nerve
Only some light reaches here, most will be lost due to absorption and scattering
Detects light and tells the brain about aspects of light that are related to objects in the world
Where seeing really begins because light is turned into electrical neurosignals (transduction)
a light-sensitive membrane in the back of the eye that contains rods and cones
the lens focuses an image on the retina, which then sends signals to the brain, through the optic nerve
100m photoreceptors
Performs important functions:
Transduction, data compression, light adaption, wavelength encoding
Rods concentrate in periphery. cones concentrate in center of fovea—where we want visual info, get most amount of detail
periphery
mostly rods
high convergence
large receptive field
low acuity (detail)
high light sensitivity
Aqueous humor
The watery fluid in the anterior Chamber
Fluid derived from bloods, fills the space immediately behind the cornea and supplies oxygen and nutrients to and removes waste from both the cornea and lens
Pupil
the dark circular opening at the centre or the iris in the eye, where light enters the eye
Hole in iris plays role in image quality
Iris
the coloured part of the eye, a muscular diaphragm that regulates light entering the eye by expanding and contracting the pupil
Controls the size of pupil
Adjusts the amount of light that reaches retina
Crystalline lens
the lens inside the eye, which
focuses light onto the back of the eye
Vitreous humor
the transparent fluid that fills the
transparent large chamber in the posterior part of the eye
Space between lens and retina
Where light is refracted the fourth and final time
80% of internal volume of eye
Gel light and viscous, generally transparent
Focal distance
The distance between the lens(or mirror) and the viewed objects, in meters
(P)=I/F
Diopeter
A unit of measurement of the optical power of the lens
Cataracts
Opacities of then lens—will get if something interferes with the regularity of crystallins will result in loss of transparency
Can occur at any age—most after 50
Absorb and scatter more light than the regular lens
Cogenetal—present at birth
Accommodation
the process in which the lens
changes its shape, thus altering its refractive power
Change in focus
Accomplished by contraction of cilliary muscles—lens attached here via zonules or zinn (suspensory ligaments)
Ability declines with age starting at 8 years old 1d/5years till 30
Lens flat
Zonules stretched
Eye focused on distances far away
Lens constriction
Ciliary muscle contract
Zonules have reduced tension
Lens buldges
Fatter then lens, the closer you can focus
Refractive error
Common disorder in which the image of the world is not completely focused on the retina
Ie/ myopia, hyperopia, astigmatism and presbyopia
When eyeball is too long or short
Emmetropia
No refractive error
When refractive power of 4 optical components are perfectly matched to length of eyeball (cornea, aqueous humor, lens and vitreous humor)
Myopia
(Nearsightedness): when light is focused in front of the retina
Distant objects cannot be seen sharply
Can be corrected with (-) lens—diverge ray before it enters eye
Eye is too long or lens too fat
Common:22% of population
Common increase with eduction
Likely to be 50% in next 30 years
Hyperopia
(farsightedness): when light is focused behind the retina
Near objects cannot be seen sharply
Can be corrected with (+) lens—converge ray before it enters eye
Eye is too short or lens too thin
Most newborns are hyperopic
Astigmatism
unequal curving of one or more of the refractive surfaces on the eye, usually the cornea
Cornea shaped like football
Vertical lines may be focused slightly in front of retina, while horizontal lines focused slightly behind (or vise versa)
Causing light to come in differently
Presbyopia
“Old sight”—age related loss of accommodation—makes it harder to focus on near objects
Inevitable because lens becomes stiffer
Symptoms similar to farsightedness(hyperopia)
Difficulty seeing close—move things away to see—start 8+ we lose accommodation
Lens becomes harder, and capsule that encircles lens that enables it to change shape, loses it’s elasticity
Photoreceptors
cells in the retina that initially transduce light energy into neural energy
Captures light and initiates seeing by producing electrochemical signals
Have different distributions across the retina
Rods
Photoreceptors specialized for night vision
We have more of these (90mill/eye)
Respond well in low luminance
Functions best at dim(scotopic) illumination
Absent from center of fovea
Do not process colour
All rods have same photopigment
Give us sufficient enough info for navigation
More sensitive to light NRG scotopic
We have more trust in central vision, even in dark
Cones
Photoreceptors specialized for daytime vision, fine acuity, and colour
Require brighter(photopic) illumination
4-5mill/eye
Respond best in high luminance conditions
Most [ ] in center of fovea
Observe fine detail
Daytime vision 3 different cones
3 different photopigments that differ in wavelengths at which they absorb light most efficiently
Visual angle
The angle that an object subtends at the eye
The size of visual stimuli is measured by how large
an image appears on the retina, not by the physical size of the object (e.g., cm on the screen)
The standard way to measure retinal size is in terms of “degrees of visual angle”
The visual angle of an object is a function of both its actual size and distance from the observer
Angle formed by lines going from top and bottom of cycle on the page, passing through the center of the lens, ending on retina
Will tell us how big the object is
tanØ=O/A
Central vision works slower than peripheral
The blind spot
Under dim illumination the central 1 degree of fovea is effectively blind
processing in the retina
Light passes through several layers of cells before
reaching the rods and cones
▪ Light activates a photoreceptor, which signals the
horizontal and bipolar cells that synapse with it
▪ Bipolar cells are connected to amacrine cells and
ganglion cells
▪ Ganglion cells have axons that leave the retina
through the optic disc (blind spot
Dark and light adaption
We can see under a wide range of luminance levels
Four mechanisms for dark and light adaptation:
Pupil dilation—more light gets in(constriction allows less light in),
Photopigment regeneration,
Two different types of photoreceptors
Fundus
The back layer of the retina, what the eye doctor sees through an ophthalmoscope
Only place in body where you can see veins and arteries directly— helps doctor see wellbeing of vascular system
Optic disk
Point where arteries and veins that feed the retina enter the eye and where the axons and ganglion cells leave via the optic nerve
Portion of the eye contains no photoreceptors—blind spot
Photopigment regeneration
The amount of photopigment available in photoreceptors changes over time
The more light entering the retina, the faster the photopigments are used up, and the fewer photopigments there are to process more light
The less light entering the retina, the more slowly photopigments are used up, and the more photopigments there are to process what little light is there
More photopigment available in dim conditions
As light increases the number of photons start to overwhelm the system
Neural circuitry
Codes for the relative amount of stimulation in the center versus the surround of receptive field
Decrease firing rate if light on periphery
Increase firing rate if light on center
Most sensitive to differences in intensity of light in center and in surround of its receptive field(region where visual stimuli influence neurons firing rate)
Ganglion cells respond to the contrast between adjacent retinal regions, rather than absolute amount of light
The pattern of illumination (or contrast), not the overall light level, is the primary concern of the rest of
the visual system
Phototopic
Cones work best in photopic (high-illumination) situations
Phototopic system;
Photoreceptors;4-5 million cones
Location in retina;throughout retina, with highest concentration close to fovea
Acuity(detail);High
Sensitivity;low
Scototopic
Rods work best in scotopic (low-illumination) situations
Scotopic system;
Photoreceptors;90 million rods
Location in retina;outside of fovea
Acuity(detail);low
Sensitivity;high
Acuity
the smallest spatial detail that can be resolved
“20/20 vision”
Your distance/normal vision distance
The Snellen E test
Minimal visible acuity
the smallest object or feature that one can detect
0.00014 degrees
Example: smallest dot you can see on the screen
Not used clinically
A limit in the ability to discern small changes in contrast, rather than special limit per se
If I put something on the screen can you tell something is there
Minimum resolvable acuity
the smallest angular separation between neighbouring objects or features that one can resolve
Example: the finest black and white stripes that can be resolved
0.017 degrees
Example: the finest black and white stripes that can be resolved
Represents one of the fundamental limits of spatial vision: it is the finest high-contrast detail that can be
resolved
In foveal vision the limit is determined primarily by the spacing of photoreceptors in the retina
minimum recognizable acuity
the angular size of the smallest feature that one can recognize or identify
Approach still uses by eye doctors today
same acuity as resolvable acuity (0.017 degrees)
Example: Snellen E
Letters get smaller but ratio remains the same
The angular size of the smallest feature that one can recognize or identify
Our ability to discern the difference of two relative positions of two features
minimum discriminable acuity
the angular size of the smallest change in a feature we can discriminate
Our ability to discern the difference of two relative positions of two features
Amblyopia
A development disorder characterized by reduced spatial vision in an otherwise healthy eye, even with proper correction for refractive error
Also known as lazy eye
Vernier acuity
the smallest visible misalignment
that can be detected among line segments or gratings
Misalignments are often smaller than the diameter and spacing of photoreceptors (resolve differences 10x smaller than width of smallest
foveal cones!)
Hyperacuity
Appears to be cortically mediated
Macula
Pigmented region near center of retina
Yellow appearance
Fovea
Prominent feature of the fundus
Small pit located at the center of the macula
Has highest [ ] of cones, and no rods
Portion of retina producing highest visual acuity and serves as point of fixation
Center of retina
mostly cones
low convergence 1:1
small receptive field size
high acuity
low light sensitivity
Visual acuity and eccentricity
Visual acuity declines with eccentricity(distance between retinal image and fovea)—causes density to drop
Foveal representation in the cortex is highly magnified (cortical magnification)
Humans have duplex retina because they have both rods and cones
Can also be about where we are fixating
Visual crowding
the deleterious effect of clutter on
peripheral object recognition
Objects that can be easily identified in isolation are difficult to identify when presented in proximity to
other objects
Crowded objects don’t disappear but our ability to recognize them is impaired
Sets limit on object perception, eye and hand movements, visual search, reading
Impairs ability to respond when object in a clutter
Make eye movement to prevent
Contrast sensitivity function
a function describing how the sensitivity to contrast depends on the spatial frequency(size) of the stimulus
Contrast threshold
the smallest amount of contrast
required to detect a pattern
100%=contrast sensitivity of 1
Receptive Fields in Striate Cortex (V1)
Cells in striate cortex respond best to bars of light, rather than to spots of light (which is what the retina and LGN prefer)
Orientation selectivity
Cells are tuned to detect lines in a specific orientation
Simple cell
a cortical neuron with clearly defined excitatory and inhibitory regions of its receptive field
Phase sensitive
Responds only when stimulus in specific position in its receptive field
Complex cell
a cortical neuron with NO clearly defined excitatory and inhibitory regions of its receptive field
Phase insensitive
Responds to a stimulus anywhere within its receptive field
End stopping
Hyper complex cells—role in ability to detect luminance boundaries and discontinued
Cell in cortex increases firing rate as length of bar increases until bar fills up its receptive field, and then decreases its firing rate as the bar is lengthen further
neuron fires less if a bar does
not reach the outside edge of the receptive field or extends beyond the receptive field
Max fire for preferred length
Receptive field properties
Orientation
Width(spatial frequency)—number of grating cycles(changes in light and dark) /unit of visual angle in a given unit of space.
Measured in cycles/degree
Direction of movement
Colour
Ocular dominance
Cortical neurons respond to both eyes but have a preferred eye (respond more to one eye than
the other)
Stimulus (bar) length
Consequences of cortical magnification
Visual acuity declines in orderly fashion eccentricity
