Chapter 4 - Vision Flashcards
10 questions
What is light and how is it measured?
Electromagnetic radiation with a specific frequency that falls within the observable spectrum
It is measured in wavelengths and expressed in nanometers
Visible light = 400nm-700nm
Structure of the human eye
Where is the cornea and what is its purpose?
Light passes through the cornea. The cornea protects structures inside the eye, and refracts and bends the light rays. This happens because the cornea tissue is denser than air
What is the pupil?
An opening in the iris. It appears to be black because light that enters the eye is absorbed by the retina (when light is extremely bright, it may reflect back through the pupil → red pupils on pictures taken with flash). The size of the pupil (controlled by muscles in the iris) determines how much light enters the eye
What is the purpose of the lens?
After the light passes through the cornea, it is refracted/bent again by the lens (a fine-tuning; about 20% of refraction happens in lens, 80% happens in cornea) to focus light on the retina (achieved by ciliary muscles that bend or flatten the lens)
What is the inside lining of the eye called, and what is it comprised of?
The retina
It is comprised of ~130 million light-sensitive receptors that are responsible for signal transduction
What are the 2 types of receptors and how many are there of each?
Cones and rods
7 million cones, 123 million rods
What does light pass through before reaching cone and rod receptors?
Gangilion cells and bipolar cells
Cones vs Rods (location and responsibility)
Cones:
- are mostly packed in the fovea in the central part of the retina
- are responsible for perception of color and fine details (foveal vision is central and sharp
Rods:
- are mostly centered around the fovea and are sensitive to low-intensity light
- are colorblind and responsible perception of movement, peripheral vision and night vision
Where and how is compressed information sent from the cones and rods?
It is sent to the optic nerve via bipolar and ganglion cells
Why is there a blind spot and where is it located?
Information has to travel out from the eye, causing a blind spot. It is located where the optic nerve leaves the eye
How does our brain fill in the blind spot?
With the immediate surroundings
Hyeropia (Hypermetropia)
Eye is too short, focal plane (where light is focused) lies behind retina: the person is farsighted and cannot focus on close objects (corrected with convex lens [+powered])
Myopia
Eye is too long, focal plane lies before retina: the person is nearsighted and sees distant objects as blurred (corrected with concave lens [-powered])
Presbyopia
Type of farsightedness (like hyperopia) related to hardening/diminished elasticity of the lens as we get older (> 40 years). Corrected with reading glasses (+ powered convex lens)
Astigmatism
Imperfections in the spherical curvature of the cornea or the lens results in multiple focal points (blurry vision at all distances). Can be corrected with a cylinder
Cataract
Clouding of the lens (age, diabetes)
Macular degeneration
Retinal degeneration of the fovea. The focus is black and distorted
Glaucoma
Worsening/loss of peripheral vision (failure of nerve cells due to increased eye pressure)
Ganglion cells structure
On Center cells
Where do ganglion cells feed information to?
V1 (primary visual cortex)
Why is the projection of the left visual field in the right hemisphere of the brain?
The nasal part of the optic nerve crosses at the optic chiasm(a), the temporal part continues on the ipsilateral (i.e. ‘same’) side
Where are the vast majority of nerve fibers in the optic tract projected to?
Lateral Geniculate Nucleus (LGN)
The main relay station in the pathway to the primary visual cortex
What happens if the optic nerve is damaged or severed?
All vision is lost in that eye
What happens if the optic chism(a) is damaged or severed?
Outer part of the visual field is lost in both eyes
Visual pathway from LGN to V1 is damaged or severed?
Visual field is lost in both eyes
V1
The main site where input from the retina arrives
How many layers does V1 have, and in which layer does information from LGN arrive?
6 layers
Information from LGN arrives in layer 4
How are the cells in V1 organized?
the ~100 million cells have retinotopic organization
How do cells in V1 respond to lines?
They selectively respond to lines with a particular orientation within specific retinal location
What are the 2 main routes that cells project to?
- ventral pathway to temporal cortex (IT): WHAT (shape and identity of object)
- dorsal pathway to parietal areas: (perceiving spatial relations, aiming, reaching) – originally viewed as WHERE pathway, but HOW pathway seems to be more appropriate
3 properties of color perception
- Hue (which color? Wavelength)
- Brightness/Intensity (Wave amplitude)
- Saturation (Complexity of light)
What did Newton show regarding white light?
It is comprised of 7 colors: violet, indigo, blue, green, yellow, orange, red
What is additive mixing?
Red, green and blue light can be combined (wavelengths are added in the mix)
What is the trichromatic theory (Young-Helmholtz)
All colors can be obtained by mixing red-green-blue (the 3 primary colors) in different proportions (RGB color coding)
Trichromatic theory explains how different cone receptors detect different wavelengths at a receptor level
What are the 3 types of cones in the eye?
- S cones (~2% of cones in retina) respond most to blue spectrum (peak ~420 nm = violet)
- M cones (~33% of cones in retina) respond most to green-to-yellow spectrum (peak ~530 nm = yellowish green)
- L cones (majority of cones in retina) respond most to red spectrum (peak at ~560 nm = yellow)
What is subtractive mixing?
Mixing colored ink, paint etc. (wavelengths are removed in the mix)
CMYK color model
(cyan, magenta, yellow, key [black]): color coding model based on subtractive mixing (printers)
Which ganglion cells are sensitive to differences in wavelengths, which is reflected in their receptive fields?
Most parvocellular (small) ganglion cells
Opponent-process theory
Six primary color pairs (red-green, blue-yellow, black-white). The opponents inhibit each other on ganglion-level, which leads to perception of achromatic (colorless) light
Opponent process theory explains how cones connect to the ganglion cells that determine how we actually perceive a color (the neural level)
Why does colorblindess often go unnoticed?
- Unaffected colors can be discriminated just fine
- Lightness can be a cue for color
- The label attached to a color can be correct, even though it may not be precisely discriminated
- Knowledge about the world (e.g., when the spatial layout of a traffic light is understood, perceiving the correct color is not necessary)
after-images
afterimages can spread to areas that were not actually colored in the image a person is adapting to
the opponent processing theory can explain negative color afterimages
the classic color-afterimage is specific for the retinal position and eye
why cant I see colors at night?
cones need light with high intensity. at night, only rods are used (rods are colorblind).
patients with visual form on agnosia
- sensation is just fine
- but objects cannot be recognized
- patients with apperceptive agnosia can’t copy a line drawing
gestalt psychology
discovered many principles of perceptual grouping
we perceive complex objects rather than the individual features
simplicity
we perceive the simplest shape possible, even when it is made up of several shapes
closure
we fill in the missing elements (edges that are separated by gaps)
continuity
edges or contours with similar orientation provide ‘good continuation’
similarity
similar looking areas of a visual display are perceived as belonging to the same object
proximity
objects close together tend to be grouped together
common fate
elements that move together are part of the same moving object
recognizing objects: image-based-theories
an object you have seen before is stored in memory as a template which can be compared to the retinal representation
recognizing objects: distinctive features
an object has a distinctive feature
recognizing objects
features are organized into based subcomponents: about 36 Geons are needed to recognize all visual objects
context is extremely important for correct object recognition
current view
perception is checking expectations with sensory evidence
- doesn’t mean you always perceive correctly
perception of faces
faces seem to represent a special category of ‘objects’
upright faces are processed differently than upside-down faces
face recognition requires training/experience
other race effect
people are better at recognizing/discrimination faces from their own race compared to other races
perception of depth, size and form
a large object has a large retinal representation (also depends on distance)
monocular depth cues (also shadow casting and elevation)
familiar size is a particularly powerful and automatic cue
binocular depth cues
binocular disparity
Higher-order interpretation can contribute to 3-D interpretations
illusions
objects maintain their characteristics regardless of changes in lightness, form, and size of retinal representations. when object constancy across features clashes, this can produce optical illusion
perceiving motion
motion is an important depth cue because objects closer to observer seem to move faster than objects that are further away
change blindness
humans are bad at spotting changes in a visual scene when there is a mask (eye-blink, saccade)
only during the tracking of moving objects, the eyes make a single continuous motion
apparent motion
successive alternating signals induce a motion percept
brain mechanisms for higher order visual processing
after information arrived in V1, it is further processed in the visual association cortex/area
