Vision Flashcards
What is sensation?
detection of external or internal stimuli through specialized receptors that are present in special senses
What is perception?
what your brain makes of these stimuli, for example, your experiences of colour, noise, the fragrance of perfumes, insect bite on the skin
What does the Old Lady illusion tells us about the difference between sensation and perception?
Everyone experiences the same stimulus. However,people have a different interpretation of what they first see. That is, they differ in their perception.
What are photons?
tiny particles that are emitted from light sources and travel in waves
What is electromagnetic energy? How is it measured?
Light energy is measured in wavelengths
How are wavelengths measured?
Wave lengths are measured in nanometers (10^-9m) (nm)
What is the electromagnetic spectrum?
range of possible wavelengths
What is the visible spectrum?
range of the electromagnetic spectrum that we can see (about from 400nm to 700nm)
What are photoreceptors?
receptors at the back of our eyes, that are transducers
What are transducers?
device that converts one form of energy into another (light energy into action potentials)
What controls the pupil and lens’ size?
The ciliary muscles that control zonule fibers
Where does the light concentrates once it penetrates the eye?
At the retina
What is the foeva?
pit at the center of the retina that picks up light and is concentrated in photoreceptors
What is the blind spot on the retina?
Where the optic nerve starts (called the optic disk)
What is the cornea?
protective tissue covering the eye
What is the aqueous humor?
watery fluid nourishing the lens and cornea
What is the vitreous humor?
fluid filling space between the lens and the retina
What is the sclera?
white protective layer of the eye
What is refraction? How does it happen in the eye?
change in direction of light rays as they travel from one medium to another
• Light is 1st refracted by lens and cornea, converging its rays in the middle of the retina
In what tissue are photoreceptors embedded?
• Photoreceptors are embedded in pigment epithelium; which nourishes them
What is object visualization?
humans visualize objects because the light reflected form the objects falls onto the retina
What is accomodation of the eye?
The ability of the eyes to keep objects in focus as they vary in distance
How does the lens adapts to close objects?
Lens become more convex, increasing its focusing power (ciliary muscles contract)
How does the lens adapts to far objects?
lens relaxes (as do ciliary muscles) and flattens
How are called the photoreceptors of the retina?
Rods and cones
What are horizontal cells?
Inhibitory neurons that release GABA (activated by photoreceptors)
What are amacrine cells?
spread activation form bipolar cells to ganglion cells
What are the 2 types of retinal ganglions cells? What are they responsible for?
Midget and parasol cells
Midget: colour vision
Parasol: dark and light contrasts
What are the characteristics of rods?
• Peripheral vision
• No rods at the foeva
• Responsible for dark adaptation
Low acuity
What are the characteristics of cones?
• Adapted for daylight vision
• Concentrated in the foeva
• High acuity
Colour vision
What are photopigments?
Light-absorbing molecules, made of the protein called opsin
contained in photoreceptors
Which colour is associated with S-Cones?
Blue
Which colour is associated with M-Cones?
Green
Which colour is associated with L-Cones?
Red
What is spectral sensitivity?
Each cone is not limited to their range of wavelengths
What type of photopigment do rods contain?
photopigment rhodopsin (around medium wavelengths) • There is only 1 type of rod, since it only perceives black/white
What is the process of bleaching? What does it implies for our vision?
when photopigments undergo an amount of degeneration reducing their ability to transduce light to adjust to the level of illumination when moving from bright to dark (Makes it hard to adapt to seeing when we turn off the lights)
What is phototransduction?***
Transducing of energy from light rays in the photoreceptors in the retina into nerve impulses
What is convergence? How is it different in rods and cones?
ratio of connectivity between photoreceptors and retinal ganglion cells
• More rods than cones converge to one ganglion cell
How is the convergence in rods? What does it implies?
1 ganglion = many rods (spatial summation)
• All these rods come from slightly different places from the retina, therefore having lower acuity but higher sensitivity to light
How is the convergence in cones? What does it implies?
1 cone = 1 ganglion cell (more precise than rods as there is only one input but less sensitivity to light because there is no spatial summation)
What are receptive fields?***
Area of the body where a stimulus can elicit a reflex
How is electrical activity induced in a retinal ganglion?***
It was found that electrical activity was induced in a retinal ganglion cell when a specific spot on the retina was illuminated
What are the receptive fields of parasol cells?***
Activated by achromatic light (dark or light only, no colour)
What are the 2 factors determining the size of receptive fields?***
Field’s distance from the foeva (further = larger field)
Type of retinal ganglion it is associated with (parasol = larger)
What are on-center receptive fields? What type of object do they prefer?***
- Make ganglion cells fire when the middle is illuminated
- When surroundings are illuminated there is no firing
- Prefer light objects surrounded by dark background
What are off-center receptive fields? What type of object do they prefer?***
- Illumination of center = inhibition of ganglion cells
- Illumination of surroundings = firing of ganglion cells
- Prefer dark objects surrounded by light background
Name 2 functions of receptive fields***
Perception of dark/light contrasts
Perception of edges (Mach bands illusion)
What is lateral inhibition?***
Lateral inhibition refers to how photoreceptors associated with the surround of on-center receptive fields form inhibitory connections with photoreceptors associated with the center of the same receptive fields.
What is white light?
all wavelengths combined
What is the trichromatic theory? How does it relates to cones?
Colour perception as a whole depends on the relative activity of each type of cone
Each cone has a colour that it maximally responds to (ex: S-Cones = blue), but they are not limited to this; they respond with less intensity to other variations of that colour.
How can we see colours?
We can see colours because certain materials absorb all wavelengths except one (red for example), therefore we se the material as the colour it reflects
What is the across-neuron response pattern theory?
a stimuli is not result of the response of only 1 neuron but a bunch of neurons altogether sending messages
What are the 3 categories of colour deficiencies?
Red-green
Blue-yellow
Complete colorblind
What is protanopia?
no L-Cones (red-green deficiency)
What is deuteranopia?
No M-cones (red-green deficiency)
What is tritanomaly?
Limited nbr of S-cones (blue-yellow deficiency)
What is tritanopia?
Absence of S-Cones (blue-yellow deficiency)
What is monochromacy?
Failure/absence of 2-3 types of cones (complete colorblind)
What is rod monochromacy?
Absence of all types of cones (complete colorblind)
What is color afterimage?
image of an object persisting after having stared at it for several seconds (appears in colours DIFFERENT from the original colours of the image)
What are the 2 zones of the retina? Where are their axons going?
Nasal retina (axons cross over) and temporal retina (axons stay same side)
What is the primary target of visual info from the retina?
a region of the thalamus known as the lateral geniculate nucleus (LGN)
The LGN has 6 layers. Those 3 layers contain 3 types of cells; what are they? What do they do?
Parvocellular (P-cells): Layers 3-4-5-6
• Receive colour info from the axons of midget ganglions
Magnocellular (M-cells): Layers 1-2
• Convey info about motion and depth
• Are sensitive to contrast
• Receive info from parasol ganglion cells
Koniocellular (K-cells): Layers 1 to 6
• Receive info from s-cones and from another type of retinal ganglion (bistratified retinal ganglion cells) that conveys info about spatial resolution
Retinal ganglion cells are activated by specific colours; how does this work?***
The colours that activate those cells are pairs (yellow/blue and red/green) - depending on the colour the cells will be activated or inhibited
Midget cells: colours (communicate with P-cells of LGN, which process colour)
P-cells can be Y+/B-, B+/Y-, R+/G-, G+/R-
Where does the info goes to from the LGN?
to V1 (primary visual cortex, or striate cortex)
What other parts of the cortex play a role in vision exccept V1? How are these areas called?
V2, V3, V4 and MT (V5) also play a role in vision (called higher order visual areas)
What is retinotopic mapping?
V1 to V5 contain a neural map of the retina (neurons are placed the same way one from another as in the retina)
How does V1 processes info? What are the 2 types of cells it uses?
Simple cells and complex cells
• Simple cells respond to different orientations (orientation selectivity)
• Complex cells respond to movement (directional selectivity)
○ Complex cells depend on the input of multiple simple cells
What is the 2 stream hypothesis of vision?
visual info has 2 pathways:
• 1 specialized in the identification of objects - “What” (dorsal)
• One specialized in processing their location - “Where” (ventral)
