Vision

The Retinex Theory

-The Trichromatic theory and the opponent-process theory cannot explain color constancy, the ability to recognize colors despite change in lighting.

- If we change the color of the lighting or wear color-tinted glasses, we still identify certain objects as their original color. This is because our brain compares the color of one object with the color of another, and subtracts a certain amount of the extra color. Though, certain wavelengths of light can appear as different colors depending on the background.

-Edwin Land proposed the retinex theory, which is a concept that the cortex compares information from various parts of the retina to determine brightness and color for each area.

The Opponent-Process Theory

The Trichromatic Theory

Although, the trichromatic theory is a good theory, it does not fully explain color vision. One reason psychologists believe this is because after staring at certain images for a long time in bright light, then switching to a plain white surface a negative color after image will occur.

Color Vision Deficiency

Thomas Young (1773-1829) was the first person to help our understanding of color. Young believed a biological explanation was needed to understand color. He proposed we perceive color by comparing the responses across a few type of receptors, each which are sensitive to different wavelengths.

Ewald Hering, a 19th century physiologist proposed the opponent-process theory to explain this phenomena.

A Negative Color After image is an image with the replacement of red with green, blue with yellow, black with white, and vice versa.

- One of the first discoveries in psychology was color blindness, better known as color vision deficiency.

This theory was later modified by Hermann Von Helmholtz and became the trichromatic theory, which is the theory that color is perceived through the relative rates of response by three kinds of cones, each one maximally sensitive to a different set of wavelengths.

- Before the 1600s, people assumed that everyone sees the same way, but today we know that some people see color better than others.

The opponent process theory states that we perceive color in terms of opposites. Which means the brain has a mechanism that perceives color on a continuum from red to green, another from yellow to blue, and another from white to black.

- After staring at one color in one location for a long time, you fatigue that response and swing to the opposite.

- Psychologists determined that color is in our brain not in the light of the object itself. Color defency results when people with certain genes fail to develop one type of cone, or develop an abnormal type of cone.

There are three different types of cones:

-A short cones which is associated with blue light.

-A medium cone associated with green light, which has a medium wavelength.

-A long cone associated with red light, which has a long wavelength.

- Also, while we have three types of cones, some birds and some fish can have up to five types of cones.

-Both opponent process theory and trichromatic theory are correct they just work on different parts of the body.

-The trichromatic theory explains how light is received by the rods and cones in the eyes.

-The opponent process theory explains how the color is perceived in the mind.

Proteins for seeing?

Photoreceptor cells contain photopigments made of opsin(protein) and a cofactor which helps it work.

Photopigments change shape when they detect light, triggering chemical reactions and sending a signal to the visual cortex, telling it how much light there is on each point of the retina.( This is what creates a picture of the outside world.)

Route within the Retina

A Few Consequences

Fun Fact!

An octopus does not have a blind spot because The photoreceptor in the octopus retina are located in the inner portion of the eye and the cells that carry information to the brain are located in the outer portion of the retina.

• The message received by the retina is sent closer to the center of the eye to bipolar cells.
• Bipolar cells send their message even closer to the center of the eye to the ganglion cells.
• The axons of the ganglion cells make up the optic never which travels out of the back of the eye to the brain

1) The light has to pass through the ganglion, amacrine, and bipolar cells to get to the receptors. This is not a major problem because these cells are transparent

2) Where the optic nerve leaves in the back of your eye there are no receptors creating a blind spot. This does not affect us daily because our eyes are constantly moving, we receive images from both eyes, and our brain fills in the image

How does the eye connect to the Brain?

1) Light enters into the opening in the iris known as the pupil

2) The image is focused my the lens (adjustable) and cornea (not adjustable)

3) This image is projected onto the back of the eye known as the retina. The retina is composed of rods and cones that act as visual receptors

Note: light that enters through the left hits the right side of the retina, from above hits the bottom etc

Rods and Cones

"Color is the code that your eye generates when a spectrum of light hits it."

Light is electromagnetic radiation within the range from less than 400 nanometers, or to more than 700 nm.

Shortest wavelengths-Violet

Longest wavelengths-Red

Key Terms:

How do our eyes perceive detail?

Blood vessels and ganglion cell axons are not found in the fovea.

Closely packed receptors help clearness of detail.

Foveal vision>excitation from 1 cone>relays info to a single midget ganglion>heads directly to the brain

Peripheral vision>input from many rods

converge onto each bipolar cell

Foveal vision-better sensitivity to detail

Peripheral vision-better sensitivity to dim light.

References

Pupil- opening in the center of the eye that light enters through

Retina- the rear surface of the eye lined with visual receptors

Biopolar cells- located closer to the center of the eye. Receives input directly from the receptors

Ganglion cells- axons join together and travel back to the brain

Optic Nerve- axons of ganglion that exit the back of the eye

5.1 Visual Coding

Blind spot- the point at which the optic nerve leaves because there are no receptors

Fovea- a tiny, dimpled area of the retina specialized for acute, detailed vision.

Midget ganglion cells-ganglion cells in the fovea of humans and other primates.

Rods- Type of retinal receptor(specialized nerve) that detects brightness of light.

Cones-type of retinal receptor that contributes to color perception

Photopigments- chemicals contained in rods and cones that release energy when struck by light

Trichromatic theory- Theory that states that we perceive color through the relative rates of response by three kinds of cones, each one maximally sensitive to a different wavelength.

Visual Field- area of the world that an individual can see at any time.

Negative Color Afterimage - result of staring at a colored object for a prolonged length of time and then looking at a white surface, the image is seen as a negative image, with a replacement of red with green, green with red, yellow and blue with each other, and black and white with each other

Kalat, W. J. (2016). Biological Psychology.

Blind Spot. (n.d.). Retrieved from https://faculty.washington.edu/chudler/chvision.html

C. (2015). Vision: Crash Course A&P #18. Retrieved from https://www/youtube.com/watch?v= o0DYP-u1rNM.

Visual System. (2016, March 29). Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Visual_system

Aspects of Vision

• Anatomists have identified at least nearly a hundred brain areas that contribute to vision in various ways
• location, shape, faces, color, and movement
• The visual system works better because visual areas specialize in a particular task without trying to do everything

The Inferior Temporal Cortex

Who Are They?

Color Perception

• Color consistency- Identifying colors even with differences in lighting.
• Color Agnosia- inability to identify colors
• Can be present from birth or be caused by a brain injury

Facial Recognition

Vocabulary

portion of the cortex where neurons are highly sensitive to complex aspects of the shape of visual stimuli within very large receptive fields

Facial Recognition

Ventral and Dorsal Paths

Facial Recognition

• Responds and Perceives

The primary visual cortex sends information to the secondary visual cortex

Vision

Motion Perception

• Ventral Stream- specialized for identifying and recognizing objects (What)
• Dorsal Steam- helps the motor system locate objects (Where)

• MT (middle temporal) tracks movement
• MST (medial superior temporal) is responsible for complex stimuli such as expansion, contraction and rotation.
• Help distinguish between eye movement and movements in the environment.
• Motion Blindness- the inability to process movement in the environment.

5.3 Parallel Processing in The Visual Cortex

• Lateral Geniculate Nucleus: Thalamic nucleus that receives incoming visual information
• Receptive Field: Area in visual space that excites or inhibits a neuron
• Parvocellular Neurons: Small cell bodies with small receptive fields, located in or near the fovea
• Magnocellular Neurons: Larger cell bodies with larger receptive fields, distributed evenly throughout retina
• Koniocellular Neurons: Small cell bodies that occur throughout ganglion cell
• Primary Visual Cortex: Area of visual cortex that is responsible for the first stage of visual processing (aka Area V1, or Striate Cortex)
• Binocular: Receiving stimulation from both eyes
• Sensitive Period: Time early in development when experiences have particularly strong and enduring influence; ends with onset of certain chemicals that stabilize synapses and inhibit axonal sprouting

RyleeAnne Leavitt

Bree Pasefika

Anice Richter

Raye Smith

Whitney Woidtke

Alexia Savenok

Eric Winfield

Optic Chiasm

• Optic chiasm: Where the optic nerves meet
• Roughly 50% of the axons from each eye cross to opposite side of the brain
• Nasal portion --> Contralateral
• Temporal portion --> Ipsilateral
• Most axons then travel to lateral geniculate nucleus of the thalamus
• Sends axons to other parts of thalamus & visual cortex

Types of (Primate) Ganglion Cells

• Parvocellular Neurons: Focus on fine visual details, respond to color (located in fovea with lots of cones)
• Magnocellular Neurons: Respond to movement and large overall patterns; color insensitive
• Koniocellular Neurons: Various functions; axons terminate in several locations

Receptive Fields

• Receptive Field: Area in visual space that excites or inhibits any neuron
• Receptive field of a rod or cone = point in space from which light strikes the cell
• Receptive field of other visual cells is derived from the connections they receive
• Rods/cones connect to bipolar cell (supervisor), which connects to ganglion cell, etc.

Lateral Inhibition

5.2 How the Brain Processes Visual Information

• Lateral Inhibition: Reduction of activity in one neuron by activity in neighboring neurons; the retina's way of emphasizing object borders
• 2 types of cells involved
• Bipolar Cell
• Horizontal Cell
• Gets excitatory input from receptor
• Inhibits bipolar cell
• Light hits receptor --> EXCITES both bipolar and horizontal cell
• Horizontal cell INHIBITS the SAME bipolar cell AND surrounding bipolar cells
• Result: heightened contrast between illuminated area and darker background

Primary Visual Cortex

• PVC (Area V1) is responsible for the first stage of visual processing
• Imperative for conscious visual perception
• Damage to V1 = Blindsight: Ability to respond (in a limited way) to visual information without conscious perception

V1 Cell Types

• Simple Cell: Has a bar-shaped receptive field with fixed excitatory and inhibitory zones
• Complex Cell: Responds to light patterns in specific orientations anywhere within large receptive field (no inhibitory zone)
• Responds most strongly to moving stimuli
• Hypercomplex: Same as complex cell, but with a strong (but small) inhibitory zone

• Cells in a given column have similar properties
• Might respond to one eye, or the other, or both about equally
• V1 cells are highly responsive to spatial frequencies

Impaired Infant Vision & Long-Term Consequences

• Deprivation of Visual Stimuli
• One eye
• Synapses from active eye inhibit synapses of the other
• Both eyes
• At first: Cortex remains responsive to visual input; begins to favor one eye or the other
• Prolonged deprivation: Sluggish, undefined cortical responses; V1 begins responding more to auditory/touch stimuli
• Uncorrelated stimulation of both eyes:
• Strabismus (lazy eye)-- treatments?
• Early Exposure to Limited Patterns
• Astigmatism: Decreased responsiveness to one kind of line or another, which occurs when the eye is not quite spherical

• Understanding of visual concepts improves over time
• EXCEPTION: Motion and depth perception
• Permanently impaired
• Cataracts --> Inability to understand visual objects presented after feeling an object

Chapter 5: Vision