3. Early Visual Processing Flashcards
Issac Newton theory of light
light acts like a particle
James Clerk Maxwell theory of light
light has wavelike properties (produces diffraction patterns)
(Visible) Light
- light is electromagnetic radiation (like gamma rays, radio, radar, etc.)
- visible from ~380 to ~760 nm (billionths of a metre)
- the eye transduces light energy → neural impulses
Ḥasan Ibn al-Haytham
- called the “father of optics” and a “pioneer of modern optics”
- wrote Book of Optics (1011-1021):
- vision produced by light reflecting from surfaces into the eye
- visual perception occurs in the brain-not just the eye
- perception is subjective and affected by individual experience
- laid the foundation for the scientific method
How light passes through the eye
- light first strikes the cornea: concentrates light rays
- passes through aqueous humour
- passes through pupil (hole in centre of the iris)
- pupil dilates (gets larger) in the dark to let in more light
- contracts in bright light to protect the eye
- passes through crystalline lens
- passes through vitreous humour to retina
C, A, P, L, V, R
What is the pupil and what does it do?
A hole in the center of the iris, the iris being a muscle that controls the size of the pupil
- pupil dilates (gets larger) in the dark to let in more light
- contracts in bright light to protect the eye
- sunglasses should have UV protection to guard against retinal and corneal damage
e.g., iggak (caribou antler goggles) worn by the Inuit protect against snow blindness (sunburned corneas)
The Crystalline Lens and accommodation
accomodation: ciliary muscles change shape of the lens, altering its focal length, which keeps image focused on retina
elasticity reduces with age, making near point (minimum distance at which you can focus) move farther away: presbyopia
the retina
- receptors (rods and cones) point to the back of the eye
- synapse with bipolar cells (have two long extensions)
- which connect to ganglion cells (2 types): P-cells and M-cells
they also have horizonal cells and amacrine cells
horizontal cells
make lateral connections among receptors and bipolar cells
amacrine cells
laterally connect among bipolar and ganglion cells
duplex retina theory
(Schultze, 1866):
- observed that retinas of nocturnal animals (e.g., owls) only contained rods
- diurnal animals (e.g., pigeons) only contained cones
- animals active during day and night had both rods and cones
duplicity theory
(von Kries, 1896):
- related rods and cones to scotopic (dark) and photopic (light) vision
Rods
we have 120-125 million of them
only located in the periphery
high sensitivity
scotopic(dark vision)
black and white vision
Cones
we have 5-6 million of them
located mostly in the fovea but the amount of them decreases as you get further and further away from it
low sensitivity
photoptic(light vision)
colour/day vison
fovea centralis
used for directed looking
- densest concentration of receptors in the eye
- only has cones (peripheral retina contains rods & cones)
explain the dark adaptation curve
different pigments in rods and cones
when going from light to dark environments, the rods need time to adjust so for the first 7ish minutes you are using cones. Once the rods have recovered they regain sensitivity and we switch over to them
- Boll (1876) found photosensitive pigment in rods: bleached in the light and regenerated in the dark
- rhodopsin comprised of retinal and opsin
- when hit by light, retinal changes shape (isomerization), causing a chain of events that culminates in a neural signal
rod monochromats
due to a genetic defect, have only rods on their retinas
isomerization
when hit by light, retinal changes shape causing a chain of events that culminates in a neural signal
rhodopsin splits into retinal and opsin
what is the absolute threshold for light?
one photon of light is the minimum to change the shape of a pigment molecule
Hecht, Shlaer, & Pirenne (1942): measured absolute threshold
pigment regeneration
Rushton (1961): measured using retinal densitometry
- shone thin, dim beam of light onto the retina
- some bounces off the back of the eye and is reflected out
- receptor pigment absorbs light–until it bleaches out, causing more light to be reflected out
- measured amount of reflected light over time: indicates time for pigment to regenerate
- result: cones take 6 min., rods take 30 min.
- pigment is re-formed, with the help of (beta carotene →) vitamin A + enzymes
Snellen Chart
measures foveal acuity only, not an absolute measurement
this is the typical eye exam chart we know
- normal is 20/20 vision (what you can see at 20 feet vs. distance for normal person to see)
- 20/200 (or worse) is legally blind, at 20 feet away they have the visibility of someone 200 feet away
diopters
used by optometrists to measure the reciprocal of focal length (m) of corrective lens
- negative = concave lens (for nearsightedness)
- positive = convex lens (for farsightedness)
visual angle
measurement of size of retinal image in degrees
tan (α) = size ÷ distance = 2.4 cm ÷ 70 cm = 0.034, therefore α ≈ 2° (a quarter at arms length)
- note: 1° = 60’ (minutes of arc), and 1’ = 60” (seconds of arc)
- with 20/20 vision, details of 1’ can be resolved (size of a quarter at the distance of a football field)
Visual acuity
refers to the clarity or sharpness of vision, and it is a measure of the eye’s ability to distinguish fine details. It specifically relates to how well someone can resolve two points or objects as separate from one another. Higher visual acuity means better detail perception.
Visual acuity depends on several factors:
-The sharpness of the focus on the retina.
-The health and function of the retina, particularly the
cone cells in the fovea, which are responsible for
detailed central vision.
-The brain’s ability to process visual information.
hyperacuity
the ability of the visual system to detect spatial differences or misalignments that are smaller than the diameter of individual photoreceptors in the retina. This means that even though the photoreceptors (rods and cones) have a certain physical limit in terms of their resolution, the brain can process visual information at a much finer level.
An example of hyperacuity is vernier acuity, which is the ability to detect slight misalignments between two lines. Even if the displacement is smaller than the width of the photoreceptor cells, the brain can detect this difference with remarkable precision, often up to 10 times more finely than the resolution limit of the retina.
Hyperacuity highlights the role of the brain in interpreting and refining the information gathered from the eyes, allowing us to perceive spatial details that surpass the physical limits of our sensory receptors.
resolution of details of 10” or less of vernier gratings (exceeds resolution of receptors):
- cone spacing in fovea = 12” (1 μm) since those 2 cones are next to each other wo would only perceive it as one stimulus
- expected resolution = 24” (theoretical limit) the closest perceivable separation of stimuli, one deactivated cone between the 2 activated ones->perception of 2 stimuli
retinal position
fovea has greatest acuity, it’s harder to pick up details in the periphery
- high (cone) receptor density
- low spatial summation (convergence of a number of receptors to a single neuron), in the fovea, receptors have one neuron per receptor, while in the periphery several receptors all lead to one neuron
cortical magnification factor
refers to the amount of brain area (specifically in the primary visual cortex, or V1) devoted to processing visual information from a given part of the visual field. It is a concept that explains how the brain disproportionately allocates more cortical space to process information from certain areas of the retina, particularly the fovea, than from the peripheral retina.
gives millimetres of cortex per degree of visual angle, as a function of retinal eccentricity
M = (1 + 0.36E + 0.000048E3)-1 M0
M0 = foveal cortical magnification factor
E = eccentricity
myopia
image focused in middle of the eyeball (nearsightedness)
long eyeball, convergence point not reaching the back of the eye
hyperopia
image focused behind retina (farsightedness)
short eyeball, focal point is behind the back of the eye
astigmatism
cornea is not spherical, but asymmetrically curved (like a football), causing multiple focal points
chromatic aberration
different wavelengths of colour focus at different points:
spherical abberation
light rays focus at different points depending on how far from centre they pass through a lens, causes fuzziness in vision
this can be minimized by a smaller pupil that blocks the edges
diffraction
light waves bend around obstacles in their path or through a slit; affects different wavelengths to different extents
- larger pupil minimizes this
- optimum pupil trade-off size = 2.4 mm
Application: how far should I sit from an HDTV?
- main issue used to be a trade-off: too close and you see each pixel; too far and you’ve paid for resolution you can’t see
- however, Ultra HDTV (4K or 8K) probably exceeds human visual acuity
- current debate centres on field of view: the amount of visual angle presented by a display
- Society of Motion Picture & Television Engineers recommends that a screen should fill 30 to 40° of your field of view for an immersive experience
- formula: size of screen × 1.6264 = distance for 30° field of view, optimal fov
e.g., a 55-inch screen × 1.6264 = 7.5 feet
the framing effect
Joor et al. (2009): top-down processing
- one group told they were watching HDTV clip
- other group told they were watching digital DVD clip
- HDTV group reported sharper, more detailed images
- but both groups watched the same (low) quality DVD clip
- shows the cognitive bias of the framing effect: top-down processing can influence even basic visual perception
what can improve visual acuity?
- being a pilot, then flying in a realistic flight simulator: 42% improved
- practice, then doing eye exercises: 18% improved
- motivation: 10% improved
- physical fitness, then doing jumping jacks: 38% improved
- reversing a Snellen chart so that you start with the smallest letters
photometry
measurement of light
radience
radiant power from a light source
- unit: lumen = light produced by a standard candle (“candela”)
e.g., 1 lm = 1.46 mW
illuminance/illumination
amount of light falling on a surface
- unit: lux = 1 lumen per square metre of area (lm/m2)
e.g., daylight = 10,000 lux, full moon = 0.1 lux
luminance
amount of light reflected from a surface
- unit: nit = 1 candela per square metre of area (cd/m2)
e.g., LCD monitor = 260 nits, CRT monitor = 150 nits
reflectance
proportion of light reflected from a surface
- unit: percent (%) or “albedo” = (luminance/illuminance) × 100
e.g., white paper = 90%, black paper = 10%
brightness
perceptual impression of intensity of light source; psychological counterpart to radiance
lightness
perceptual impression of surface “greyness”; psychological counterpart to reflectance
perceiving things that don’t emit light, just reflects light
Factors in Brightness Perception:
- dark/light adaptation
- retinal locus: threshold lower in the periphery (due to greater rod convergence):
- wavelength
- time and area: brightness affected by duration and retinal size of stimulus
wavelength
- match standard colour with comparisons based on brightness; get absorption spectrum. ask people to match the brightness of 2 colours
- repeat under different illuminations (photopic vs. scotopic)
- during dark adaptation, we shift from using cones to rods
- result is purkinje shift (1825): peak sensitivity changes to shorter (bluer) wavelengths
The Purkinje shift
a phenomenon where color perception changes as lighting conditions shift from bright to dim. In bright light, cone cells dominate, making reds appear brighter. In dim light, rod cells take over, and blues and greens appear brighter while reds seem darker. This shift occurs because rods, which are more sensitive in low light, are tuned to shorter wavelengths (blues/greens), whereas cones, used in daylight, are more sensitive to longer wavelengths (reds).
optic nerve
axons of retinal ganglion cells
- exits back of the eye where there are no receptors (optic disc), resulting in a blind spot
- retina begins processing visual information: ~126 million receptors; but only 1 million nerve fibres
receptive field
area on the retina that, in response to a stimulus, influences the firing of a neuron
this is a concept, not a physical thing
- typical (ganglion cell) receptive field is centre-surround: the center/surround is excitatory (light shining on that area is more likely to fire the neuron)
inverse relationship, inhibitory center means an excitatory surround and vice versa
- this formation is due to a pattern of connectivity between many receptors and a single ganglion cell
lateral inhibition
is a process in the visual system where neighboring photoreceptor cells in the retina inhibit each other’s activity. This enhances contrast at edges, making borders between light and dark areas appear sharper.
When a photoreceptor is stimulated by light, it reduces the activity of surrounding receptors through inhibitory neurons. As a result, areas of high light intensity seem even brighter compared to adjacent darker areas. This mechanism helps the brain detect edges and fine details, contributing to clearer and more defined visual perception.
some cells, when activated (e.g., by the presence of a stimulus), decrease the activity of adjacent cells
- due to the release of inhibitory neurotransmitter
Chevreul illusion (1861)
each band is uniform shade of grey, but seems to darken near a lighter band, and lighten near a darker band
- adjacent receptors believed to inhibit neighbouring receptors
-the difference in luminance at the border is affected by lateral inhibition
Simultaneous contrast
where there are 2 spots that are the same shade of gray but the surroundings make it appear darker/lighter
explained by lateral inhibition: receptors activated by larger surrounding square inhibit receptors in smaller central square
Benary cross (1924)
both triangles should receive equal lateral inhibition, but seem different shades
with the one inside the cross appearing lighter
White’s illusion (1979):
- rectangle A should receive little lateral inhibition (and seem lighter), but it seems darker
- rectangle B should receive a lot of lateral inhibition (and seem darker), but it seems lighter
white’s illusion and the benary cross can be explained in terms of….
explained in terms of “belongingness”: appearance of an areas is influenced by the surroundings to which it seems to belong (Gilchrist & coworkers, 1999)
- triangle B belongs to the dark cross, which makes it seem lighter in contrast
- rectangle B belongs to the black bars, which makes it seem lighter in contrast
- suggests higher-order (top-down) processing instead of retinal mechanism
Spatial Frequency Approach
(Blakemore & Campbell, 1969; Shapley & Lennie, 1985)
suggests that our visual system processes images in terms of their spatial frequencies—how often elements like light and dark patterns (or edges) alternate in a given space. Low spatial frequencies correspond to large, broad features (like general shapes or blobs), while high spatial frequencies relate to fine details (like edges and textures).
This approach highlights that different spatial frequencies carry different types of visual information, with the brain combining these frequencies to form a complete image. It explains how we perceive both the overall structure and fine details of a scene.
spacial frequency
how a stimulus changes over space (cycles per degree of visual angle)
Spatial frequency refers to the level of detail or frequency of patterns in a visual image, expressed as the number of cycles of light and dark (or other features) per unit of visual angle.
Low spatial frequency corresponds to large, broad patterns (e.g., overall shapes or general contrasts).
High spatial frequency relates to fine details (e.g., edges, textures, or intricate features).
- one cycle = one dark bar + one light bar:
- higher intensity regions produce peaks, lower intensity areas correspond to troughs
Fourier analysis
- simplified scene description in terms of a set of sine waves
result: mathematical expression describing the visual scene in terms of sine waves:
- better than taking inventory of the activity of all ~126 million receptors!
- allows us to investigate commonalities in the processing of visual information
it provides a mathematical framework for decomposing complex visual images into their constituent spatial frequencies. This technique helps to understand how the visual system interprets and processes visual information.
contrast sensitivity function CSF
describes ability of a system to preserve contrast and spatial frequency information after it has been encoded
quantifies how well an observer can detect differences in luminance (contrast) across various spatial frequencies. It describes the relationship between contrast levels and the spatial frequencies of visual stimuli, essentially indicating the range of contrasts at which different patterns can be perceived.
e.g., a spatial frequency of 100,000 black & white lines/degree of visual angle cannot be encoded by the visual system; perceived as a uniform grey field
- contrast ratio = (Lmax - Lmin) / (Lmax + Lmin)
- based on physical measures of light (L = luminance)
- however, apparent contrast is affected by spatial frequency:
e.g., wide black bars appear darker than narrower bars; wide white bars appear lighter than narrower bars
Creating CSF:
- present observer with a grating of black & white bars of a certain spatial frequency
- change contrast between the black and white bars until observer no longer perceives stimulus as lines
- change spatial frequency and repeat:
- describes contrast sensitivity and acuity
is CSF a monolithic function, or comprised of “channels”?
- adapt to a spatial frequency of 7.5 cycles/degree
- remeasure the contrast sensitivity function:
- suggests contrast sensitivity function is comprised of a series of spatial frequency channels (De Valois & De Valois, 1990):
Maffei & Fiorentini (1973):
- each simple cell responds best to a narrow range of spatial frequencies:
- adaptation effects are caused by neural fatigue
What is light? Why is it considered both a wave and a particle?
Light is electromagnetic radiation that is visible to the human eye. It exhibits dual properties, behaving as both a wave and a particle (photon). This concept is known as wave-particle duality. As a wave, light demonstrates properties like interference and diffraction, while as a particle, it can be described in terms of photons, which are discrete packets of energy. This duality is fundamental to quantum mechanics.
What are the roles of the cornea, iris, pupil, and lens in human vision?
Cornea: The transparent outer layer that helps focus light onto the retina by refracting incoming light.
Iris: The colored part of the eye that regulates the amount of light entering the eye by adjusting the size of the pupil.
Pupil: The opening in the center of the iris that changes size in response to light intensity.
Lens: A transparent structure that further focuses light onto the retina, allowing for adjustments in focus based on distance (accommodation).
What is accommodation? Explain how it produces a focused image on the retina.
Accommodation is the process by which the eye adjusts the focal length of the lens to focus on objects at varying distances. When viewing a nearby object, the ciliary muscles contract, allowing the lens to become thicker and increase its refractive power, resulting in a focused image on the retina. For distant objects, the ciliary muscles relax, making the lens thinner and less powerful.
What are the different classes and subclasses of rods?
Function: Rods are responsible for vision in low-light conditions (scotopic vision) and are highly sensitive to light. They do not detect color and are more concentrated in the peripheral regions of the retina.
Subclass: There is essentially one type of rod photoreceptor, but they can be functionally grouped based on their distribution across different species.
What are the different classes and subclasses of cones?
Function: Cones are responsible for color vision and visual acuity in well-lit conditions (photopic vision). They are concentrated in the fovea, the central region of the retina.
Subclasses:
S-cones (Short-wavelength cones): Sensitive to short wavelengths (blue light, approximately 420 nm).
M-cones (Medium-wavelength cones): Sensitive to medium wavelengths (green light, approximately 530 nm).
L-cones (Long-wavelength cones): Sensitive to long wavelengths (red light, approximately 560 nm).
How do rods and cones convert light into a neural signal?
Absorption of Light:
Light enters the eye and is absorbed by photopigments in photoreceptors (e.g., rhodopsin in rods). This absorption triggers a chemical change in the photopigment.
Hyperpolarization:
In darkness, photoreceptors are depolarized and release the neurotransmitter glutamate. When light is absorbed, the photoreceptor hyperpolarizes (becomes more negatively charged) and reduces the release of glutamate.
Signal Transmission:
The decrease in glutamate affects the activity of bipolar cells. ON bipolar cells depolarize in response to reduced glutamate, while OFF bipolar cells hyperpolarize, allowing for the encoding of light and dark information.
Ganglion Cell Activation:
Bipolar cells synapse with retinal ganglion cells, which integrate signals from multiple photoreceptors. Ganglion cells generate action potentials and transmit visual information to the brain via the optic nerve.
What is the difference between photopic vision and scotopic vision?
Photopic vision occurs in well-lit conditions and relies on cone photoreceptors, allowing for color perception and high visual acuity. Scotopic vision takes place in low-light conditions and relies on rod photoreceptors, providing sensitivity to light but no color perception. The physiological mechanism involves the different types of photoreceptors and their responses to varying light levels.
What is the Purkinje shift? What does it tell us about photopic vision and scotopic vision? How does it relate to dark adaptation?
The Purkinje shift describes the change in color perception from bright to dim lighting, where red colors appear darker and blue/green colors appear brighter in low light. It indicates that the visual system shifts from cone (photopic) to rod (scotopic) function as light decreases. This shift is also related to dark adaptation, the process by which the eye becomes more sensitive to light after being in the dark for a period.
What are retinal ganglion cells? What do they do in visual processing?
Retinal ganglion cells are neurons located in the retina that receive input from photoreceptors (rods and cones) via bipolar cells. They process visual information by integrating signals and transmitting them through their axons to the brain via the optic nerve. They play a crucial role in encoding visual information, including aspects of contrast, motion, and spatial patterns.
What are center-surround receptive fields? What is the difference between on-center and off-center center-surround receptive fields? How do these explain lateral inhibition?
Center-surround receptive fields are arrangements of retinal ganglion cells where the center responds differently to light than the surrounding area:
On-center cells are activated by light in the center and inhibited by light in the surrounding area.
Off-center cells are inhibited by light in the center and activated by light in the surrounding area. This arrangement enhances contrast and edge detection through lateral inhibition, where activated cells inhibit their neighbors, sharpening the perception of boundaries and details in the visual field.
Distinguish between myopia, presbyopia, and hyperopia.
Myopia (Nearsightedness): Difficulty seeing distant objects clearly because the eye is too long, causing light to focus in front of the retina.
Hyperopia (Farsightedness): Difficulty seeing close objects clearly because the eye is too short, causing light to focus behind the retina.
Presbyopia: Age-related loss of the eye’s ability to focus on near objects due to decreased lens elasticity.
What are the differences between macular degeneration and retinitis pigmentosa?
Macular degeneration: A condition that affects the central part of the retina (macula), leading to loss of central vision.
Retinitis pigmentosa: A group of inherited disorders that cause progressive degeneration of photoreceptors, leading to peripheral vision loss and, eventually, blindness.
What are some of the ways that eyes vary across different animals?
Nocturnal animals often have larger eyes with more rod cells for better night vision.
Predatory animals may have forward-facing eyes for better depth perception and binocular vision.
Prey animals typically have eyes on the sides of their heads for a wider field of view to detect predators.
Insects have compound eyes, which provide a different visual experience, allowing them to detect motion and polarized light.
What are vision prostheses? How do they restore vision to people with retinal disease?
Vision prostheses are devices designed to restore partial vision to individuals with retinal diseases, such as retinitis pigmentosa. These devices typically consist of a camera that captures images and a microelectrode array implanted in the retina. The camera sends visual information to the array, which stimulates the remaining functional retinal cells to create a perception of visual images. While not restoring normal vision, these prostheses can help users detect light, shapes, and movement.
anterior chamber
the fluid-filled space between the cornea and the iris
cataracts
a condition that results from a darkening of the lens
Center-surround receptive field:
a receptive field in which the center of the receptive field responds opposite to how the surround of the receptive field responds; if the center responds with an increase of activity to light in its area, the surround responds with a decrease in activity to light in its area
Dark adaptation
he process in the visual system whereby its sensitivity to low light levels is increased
Edge detection
the process of distinguishing where one object ends and the next begins, making edges as clear as possible
Electromagnetic energy
a form of energy that includes light that is simultaneously both a wave and a particle
Electromagnetic spectrum
the complete range of wavelengths of light and other electromagnetic energy
Field of view
the part of the world you can see without eye movements
fovea
an area on the retina that is dense in cones but lacks rods; when we look directly at an object, its image falls on the fovea
Frequency
the number of waves per unit of time; frequency is the inverse of wavelength
Hyperpolarization:
a change in the voltage of a neuron whereby the inside of the cell becomes more negative than it is in its resting state
intensity
when referring to waves, the height of a wave
light adaptation
the process whereby the visual system’s sensitivity is reduced so that it can operate in higher light levels
Macula
the center of the retina; the macula includes the fovea but is larger than it
Macular degeneration
a disease that destroys the macula, including the fovea and the area around it
Near point
the closest distance at which an eye can focus
Neurotransmitter
a chemical substance neural cells release to communicate with other neural cells
Off-center receptive fields
retinal ganglion cells that decrease their firing rate (inhibition) when light is presented in the middle of the receptive field and increase their firing rate (excitation) when light is presented in the outside or surround of the receptive field
On-center receptive fields
retinal ganglion cells that increase their firing rate (excitation) when light is presented in the middle of the receptive field and decrease their firing rate (inhibition) when light is presented in the outside or surround of the receptive field
Opsin
the protein portion of a photopigment that captures the photon of light and begins the process of transduction; the variation in opsin determines the type of visual receptor
photon
a single particle of light
Photopigment
a molecule that absorbs light and by doing so releases an electric potential by altering the voltage in the cell
Posterior chamber
the space between the iris and the lens; it is filled with fluid known as aqueous humor
Pupillary reflex
an automatic process by which the iris contracts or relaxes in response to the amount of light entering the eye; the reflex controls the size of the pupil
Receptive field
a region of adjacent receptors that will alter the firing rate of a cell that is higher up in the sensory system; the term can also apply to the region of space in the world to which a particular neuron responds
retina
the paper-thin layer of cells at the back of the eye where transduction takes place
retinal
a derivative of vitamin A that is part of a photopigment
retinal image
the light projected onto the retina
Retinitis pigmentosa
an inherited progressive degenerative disease of the retina that may lead to blindness
Sclera
the outside surface of the eye; it is a protective membrane covering the eye that gives the eye its characteristic white appearance
wavelength
the distance between two adjacent peaks in a repeating wave; different forms of electromagnetic energy are classified by their wavelengths
Zonule fibers
fibers that connect the lens to the choroid membrane
