Vision : part 1 Flashcards
Light
A narrow band of electromagnetic radiation that can be conceptualized as a wave or a stream of photons.
Photon
A quantum of visible light (or other form of electromagnetic radiation) demonstrating both particle and wave properties
Light is a waveform when it travels in…
Space
Light is a stream of photons when…
It hits the retina and is transduced in neuronal activity
Frequencies of light and their corresponding color(s)
Shorter wavelengths of light : blue
Medium wavelengths : green or yellow
higher wavelengths : purple or red
Vertically polarized light
When the electric field’s oscillation is restricted to the vertical plane
Every potential angle of polarity is ______ present in most light sources
Equally, so light is unpolarized
Polarizing glasses that only let light pass with a certain ______ can be used to reduce the total amount of light that reaches the retina, as with sunglasses, or to present slightly different visual information to the left and right eye, as with modern 3D glasses.
Polarity
Electromagnetic wave
- The electrical radiation is oscillating in one direction or polarity
- The magnetic field oscillation has a perpendicular polarity to the electrical radiation
Light is usually made up of different rays with different ______
Polarities
Light Scattering
The redirection (deviation from original path) of light as it interacts with particles or molecules in a medium
3 factors in light scattering
1) size
2) shape
3) composition of the particles relative to the wavelength of the light
Rayleigh Scattering
The scattering of light by particles much smaller than the wavelength of light, typically in gases. It causes shorter wavelengths (e.g., blue) to scatter more than longer wavelengths, explaining the blue color of the sky.
Mie Scattering
The scattering of light by particles that are the size of the wavelength of light, like water droplets or dust (larger molecules). Unlike Rayleigh scattering, it affects all wavelengths more equally and often results in a white or grayish appearance, such as in clouds, smog.
Non-Selective Scattering
The scattering of light by particles much larger than the wavelength of light, such as large water droplets in fog. It affects all wavelengths equally, creating a uniform, white appearance.
Light Absorption
The process by which light energy is taken up by a material, converting it into other forms of energy, such as heat (e.g. when the sun hits our skin, it ceases to exist as light and we no longer see it), rather than being transmitted or reflected.
Light Reflection
The process by which light bounces off the surface of a material. Some light get absorbed, some gets reflected.
Specular Reflection
When light reflects off a smooth surface in a single, predictable direction (all the same direction), e.g. still water, mirror
Diffuse Reflection
Where light reflects off a rough surface, scattering in many directions. e.g. desk, clothes
Transmission
The passage of light through a material, where the light continues to propagate without being absorbed or reflected.
Refraction
The bending of light as it passes from one medium to another with a different refractive index, changing its speed and direction.
Cornea
- transparent, dome-shaped outer layer at the front of the eye that helps focus light onto the retina.
- first structure that light encounters as it enters the eye
Anterior Chamber
The fluid-filled space between the cornea and the iris, containing a liquid : aqueous humor, which nourishes the cornea and lens.
Pupil
Circular opening in the center of the iris (colored part of our eye) that regulates the amount of light entering the eye.
Iris
- acts as a diaphragm and is attached to the ciliary muscle via zonular fibers.
- what enlarges or shrinks the size of the pupil.
Lens
Structure behind the pupil that will refract the light and redirect it on the retina and its fovea
Vitreous Humor
- clear, gel-like substance that fills the space between the lens and the retina
- helps maintain the eye’s shape
- transmits light to the retina.
Retina
- thin, light-sensitive tissue with nerve fibers, lining the back of the eye
- where photoreceptor cells (rods and cones) convert light into neural signals for visual processing.
Choroid
- vascular layer between the retina and sclera
- provides oxygen and nutrients to the outer layers of the retina.
Sclera
- tough, white, outer layer of the eye
- provides structural support and protection.
Optical infinity
Objects that are at a distance of 20 feet (~6 meters) or more from the eye
Emmetropia
When light rays that are coming from objects located at optical infinity are naturally focused on the retina without any anomalies
- No accomodation needed
- Parallel light rays
Accommodation
The process by which the eye’s lens changes shape, gets contracted by the ciliary muscles, to focus and refract light on the retina for clear vision at different distances.
Focal Length (f’)
- The distance between the optical center of a lens and its focal point (F’)
- where parallel rays of light converge or appear to diverge (light gets focused)
- determines the lens’s magnifying power and ability to focus on objects at various distances
Presbyopia
Age-related condition where the eye gradually loses the ability to focus on nearby objects (accommodate)
Cause of presbyopia
Reduction in elasticity of the lens and weakening of the ciliary muscles (more frequent around 40 years old)
With presbyopia, light ends up being focused ____before/behind the retina
Focal point behind the retina
Hyperopia (Farsightedness)
Refractive error where distant objects appear clearer than near ones
- light focuses behind the retina, often due to a shorter eyeball.
Myopia (Nearsightedness)
Refractive error where near objects appear clearer than distant ones
- light focuses in front of the retina, often due to a longer eyeball.
Astigmatism
Refractive error caused by an irregularly shaped cornea or lens, leading to distorted or blurred vision at all distances because light is focused unevenly on the retina.
- There will be many focal points
Hermann von Helmholtz (1821 –1894)
- In agreement with Kant (intuition that our mind is actively engaged in perception, has to process it)
- Perception is “unconscious inference” : the eye is a poor optical device
Blind spot
- A little to the right center of our visual field area where the optic nerve exits the eye,
- With the rule of good continuation, the brain guesses what will be behind the blind spot based on the environment around it
Fundus
- back, interior surface of the eye, visible through an ophthalmoscope, which includes the retina, optic disc, macula, and blood vessels
Optic Disc
- where the blood vessels that feed the retina enter the eye and where the axons of the retinal ganglion cells merge into the optic nerve
Macula
- central area of the retina responsible for detailed central vision, critical for tasks like reading and recognizing faces
- contains a high density of photoreceptors and almost no blood vessels
Fovea
- small, central depression within the macula
- High density of cones (more accurate vision)
- Photoreceptors are at the back of the retina , then bipolar cells, ganglion cells
Central vision
- Tasks that require high visual acuity, like reading, are limited to a visual angle of 2-3°, i.e. no more than the width of a thumbnail when viewed at arm’s length (57 cm).
- The rest is not well seen : only eye movement allows to see with great acuity
Visual Acuity
Clarity or sharpness of vision, measured by the ability to discern fine details, typically evaluated using standardized eye charts.
Visual Angle
Angle formed by an object at the eye, determined by the object’s size and distance from the observer
- used to quantify how large an object appears in the field of view
Ganglion Cells
-in the retina
- receive visual information from bipolar cells and transmit it to the brain via their axons, which form the optic nerve.
Bipolar Cells
Intermediate neurons in the retina that connect photoreceptors (rods and cones) to ganglion cells with synapses, transmitting visual signals from the outer to the inner retina.
Photoreceptors (Rods and Cones)
Specialized cells in the retina that detect light and convert it into electrical signals
Rods
Sensitive to low light levels and responsible for vision in dim lighting (scotopic vision). Imprecise.
Cones
Active in bright light and responsible for color vision and fine detail (photopic vision).
Phototransduction
Process by which photoreceptor cells in the retina convert light into electrical signals that can be processed by the brain.
Photopigment
- Light-sensitive molecule in photoreceptors composed of opsin (a protein sensitive to specific wavelengths of light) and a chromophore (a light-absorbing molecule).
Opsin
A protein component of photopigments that determines the light wavelength sensitivity of the photoreceptor, enabling color and light detection.
Chromophore
The light-absorbing molecule (reacts to light) within photopigments; in mammals, this is 11-cis retinal, which changes shape upon light absorption
Photoisomerization
- When light hits the photopigment, the chromophore changes shape into an all trans retinal : once in its trans form, the photopigment is bleached
- In a trans retinal state, the photopigment cannot react to light nor absorb it
Photopigment regeneration
When the photopigment comes back to its original state, and we can see again
Short wavelength-specific cones (S-cones) are responsible for our perception of the color ____
Blue
Medium (M-cones) are responsible for perception of the color ____
Green
Long wavelength-specific cones (L-cones) allow the perception of the color ___
Red
Some photoreceptors contain _____ (unconscious perception of daylight cycle that goes to the hypothalamus) and can monitor ambient light levels so as to influence our circadian rhythm
Melanopsin
Rods contain _______
Rhodopsin
Photopic vision
Vision under well-lit conditions, primarily mediated by cones, enabling color perception and high visual acuity.
Scotopic vision
Vision in low-light conditions such as night, primarily mediated by rods, providing high sensitivity to light but no color perception.
Dark adaptation
The process by which the eyes adjust to low-light conditions, involving increased sensitivity of rods and a shift from cone-dominated to rod-dominated vision. Can take up to 30 minutes
Pupil dilation
Allows the entry of more light in dim light
Fast, but not most effective
Gain in light sensitivity of photoreceptors
A dark environment will be associated with more regeneration than bleaching, rendering photoreceptors more receptive to light.
Time of maximum cone and rod sensitivity
cones: 5-8 minutes
rods: around 25 minutes
Rod-cone break: after 7-8 minutes, vision becomes dominated by rods.
Why can rods become more sensitive with time ?
Their percentage of available photopigments increases until surpassing that of the cons
Normalization through Lateral inhibition
A neural mechanism in which active neurons suppress the activity of neighboring neurons, enhancing contrast and improving the detection of edges and fine details.
General principle of Normalization through Lateral inhibition
Subtracting and dividing by the average luminance
Light is measured in ___ (brightness)
Lux
The goal of normalization is to discount the ______: the light source of the scene in front of us; to look at relative contrasts
Luminance
If visual system was perceiving the absolute intensity of light
Everything would be quite indistinct and the light would change a lot from inside to outside
3 mechanisms that allow us to see in low light
- Pupil dilation
- Gain in light sensitivity of photoreceptors
- Normalization through Lateral inhibition
Receptive field
Region on the retina in which stimuli influence a neuron’s firing rate.
ON-center/OFF-surround ganglion cells
Activated by light in the center and inhibited by light in the surround.
OFF-center/ON-surround ganglion cells
Opposite response—activated by light in the surround and inhibited by light in the center.
Ganglion cells are designed to detect dots of colors that will later on be assembled into ______
Objects
Once photoactivation (from light) starts, photoreceptors become _______hyperpolarized/depolarized
Hyperpolarized
When in _____darkness/light , photoreceptors release neurotransmitters
Darkness
Changes in photoreceptor activation are communicated to the bipolar cells in the form of ….
Graded potentials (vary continuously in their amplitude)
Bipolar cells synapse with retinal ganglion cells, which fire in an ______ fashion
All-or-none fashion rather than in graded potentials.
___off/on-center bipolar cells reverse the sign of cone
ON
Dark _____hyperpolarizes/depolarizes the surround cones of an OFF surround photoreceptor
Depolarizes
Depolarization of surround cones activates ______ cells which in turn inhibits all the cones.
Horizontal
Because the ____ cone is placed in the middle of two horizontal cells, it receives more inhibition (hyperpolarization) than surround cones in an OFF surround photoreceptor
Centre
More inhibition of the central cone will be transformed into more excitation of …
The ON bipolar cell and ganglion cells
If there is light in the center AND the surround of an ON center-OFF surround photoreceptor, all cones would be _______hyperpolarized/depolarized, and the horizontal cells would not be activated
Hyperpolarized
___OFF/ON-center bipolar cells don’t reverse the sign of the photoreceptor.
OFF
Why does black-on-white «stand out» as much as white-on-black ?
- ON-center bipolar cells reverse the sign of the photoreceptor
- OFF-center bipolar cells don’t reverse the sign of the photoreceptor.
Mach bands
Gray bands are perceived as more contrastive when closer together
How much ON center OFF surround ganglion cells fire depending on environment
- Highest ratio of light in the center and darkness in the surround
- Complete darkness
- Opposite pattern : light in surround, darkness in center
Neurons have receptive fields that when located at the junction of 2 bounds will enhance the ______ because of their off on surround center organization
Contrast
In foveal or central vision, the receptive field of ganglion neurons is ____smaller/larger
Smaller
In peripheral vision, the bigger retinal ganglion cells receptive fields have ____ in their surround and ____ in their center in the Hermann grid illusion
Black surround
White center
The peripheral vision retinal ganglion cells receptive fields will be ____less/more activated at the intersection because there will be less black in their surround
Less, which explains why the intersections appear darker
Impact of foveal vision receptive fields in Hermann Grid illuson
The retinal ganglion cells receptive fields are too small to make a difference : all white
Visual Acuity
Smallest spatial detail that can be resolved/perceived
2 components of visual acuity
- Contrast : difference in brightness
- Frequency : how fast pattern repeats
- peak in acuity is in medium contrast and frequency
20/20 vision visual acuity measurement
Your distance/Normal vision distance
20/20 : when placed at 20 feet from the letters, the smallest letter that you can read is the smallest letter that the average person can read at 20 feet
Why does 20/15 mean higher visual acuity than the average person ?
When placed at 20 feet from the letters, the smallest letter that you can read is the smallest letter that the average person can read at 15 feet
20/40
When placed at 20 feet from the letters, the smallest letter that you can read is the smallest letter that the average person can read at 40 feet.
- Over 20 is visual acuity impairment
Not enough frequency means losing …
Lateral inhibition
Visual acuity is determined by the _____ of photoreceptors in the retina.
Spacing
cones in the fovea have an average center-to-center separation of …
0.5 arc minute, or 1/120 degree
The distance between the leftmost and rightmost cones in a line of 3 cones is …
2 x 0.5 = 1 arc minute, or 1/60 degree
If the ______ of the grating is smaller than 1 arc minute, we see gray because each cone receives a mixture of black and white.
Frequency
In ____low/high contrast environments, visual acuity is strongly dependent on spatial frequency and is optimal at ~7 cycles/degree.
Low
The frequency of a certain pattern is defined by the number of ____ within one degree of visual angle.
Cycles
Sine waves represent the _____ pattern that can repeat
Simplest
____ waves can be decomposed into sine waves
Square
When looking at a complex image, ____low/high frequencies allow us to see the general picture
Low
____low/high frequencies allow the perception of details
High
How to add up sine waves to make a square wave
Add up high amplitude low frequency + higher frequency lower amplitude until it looks like a square wave
___low/high convergence in periphery vision
High
Convergence in periphery vision
Multiple rods will synapse on diffuse bipolar cells. Multiple bipolar cells will synapse on a parasol ganglion cell (larger receptive field).
____low/high convergence in the fovea
Low
Low convergence in the fovea
Single cone cells will synapse on single midget bipolar cells, which will synapse on single midget ganglion cells.
Parasol ganglion cells may also be called …
Magnocellular (or M) ganglion cells.
Midget ganglion cells may also be called …
Parvocellular (or P) ganglion cells.
Impact of high convergence on light sensitivity of the ganglion cell
The parasol ganglion cell in the periphery will be very sensitive to light because it won’t matter which specific photoreceptor the light is activating. However, it cannot identify the precise location of light.
Impact of low convergence on light sensitivity of the ganglion cell
The midget retinal ganglion cell in the fovea is not as sensitive : it waits to get light right to its receptive field to have an action potential.
_____midget/parasol retinal ganglion cell will react more strongly to light shone all over the receptors
Parasol (in periphery)
Link between convergence, sensitivity and response
- High convergence = high sensitivity : more response when more light
- Low convergence = less sensitivity : less response
Why low convergence means high acuity
Low convergence means midget cells of the fovea can detect precise patterns of light (pinpoint which cells receive light signal)
Link between sensitivity and acuity
- High sensitivity = lower acuity
- Low sensitivity = higher acuity
Optic Chiasm Structure
- Located in front of the hypothalamus.
- Site where half of the optic nerve fibers from each eye intersect to the contralateral brain hemisphere
Role of the optic chiasm
- Enables visual cortices to process information from the contralateral visual field (not the contralateral eye).
Temporal Retina
Oriented towards the contralateral visual field; no intersection required (already on right side)
Nasal Retina
Oriented towards the ipsilateral (same side) visual field; intersects to transmit visual information to the contralateral (opposite side) hemisphere; the one that is looking to the left on the right and vice versa
Optic Tract
Retinal projections beyond the optic chiasm
Approximately 90% of optic tract nerve fibers synapse in the …
Lateral geniculate nuclei (LGN) of the thalamus.
Hemianopia
loss of perception in one hemifield in BOTH eyes (nasal part ), e.g. each eye only sees the right visual field
Left eye anopia
Blind in left eye
Tunnel vision
Left eye sees right hemifield, right eye sees left hemifield only; losing periphery
Lateral Geniculate Nucleus (LGN)
Located in the thalamus; anatomically divided into six layers.
Magnocellular Layers (1 & 2)
- Input from parasol retinal ganglion cells (largest due to high convergence)
- Process peripheral vision.
- Important for perceiving movement
Parvocellular Layers (3–6)
- Input from midget retinal ganglion cells.
- Process central vision.
- Their input stay separate
Koniocellular Layers
- Located in intralaminar regions of the LGN.
- Specialized for color perception due to sensitivity to wavelength and low light sensitivity.
