Vision

Question 1

law of specific nerve energies

Answer 1

Each receptor is specialized to absorb one kind of energy and transduce it into an electrochemical pattern in the brain. Impulses in certain neurons indicate light and impulses in other neurons indicate sound, odours etc.

Question 2

Pupil:

Answer 2

An opening in the center of the iris (a band of tissue that gives our eyes their color) in which light enters the eye. The pupil is focused by the lens (adjustable) and cornea (not adjustable) and projected to the retina.

Question 3

Retina:

Answer 3

Rear surface of the eye, which is lined with visual receptors.

Question 4

bipolar cells

Answer 4

Within the vertebrate retina, receptors send their messages to bipolar cells (neurons located close to the center of the eye).

Question 5

ganglion cells

Answer 5

Bipolar cells send their message to ganglion cells (neurons located even closer to the center of the eye). Ganglion cell axons join together, and then loop around and travel back to the brain.

Question 6

optic nerve

Answer 6

The ganglion cells join together to form the optic nerve (or optic tract)

Question 7

Blind spot

Answer 7

The point at which the optic nerve leaves (also where the blood vessels enter and leave) the eye is known as the blind spot, because it has no visual receptors.

Question 8

Fovea:

Answer 8

Central portion of the macula specialized for acute, detailed vision. The fovea has the least impeded vision, as blood vessels and ganglion cells are almost absent.

• Further aiding the detailed vision of the fovea, each receptor connects to a single bipolar cell, which in turn connects to a single ganglion cell.
• Foveal vision has better acuity (sensitivity to detail) and the peripheral vision has better sensitivity to dim light (because of the density of bipolar and ganglion cells)

Question 9

Midget ganglion cells:

Answer 9

The ganglion cells in humans and other primates. These cells are small and each receives an input from a single cone.
- Each cone in the fovea has a direct line to the brain and can register the exact location of any point of light on the fovea.

Question 10

Two types of receptors exist in the vertebrate retina:

Answer 10

rods and cones

Question 11

Rods:

Answer 11

are abundant in the periphery of the retina; they are involved in both peripheral and night vision.

Question 12

Cones:

Answer 12

are found primarily in the fovea; they are involved in both visual acuity and color vision.
- In humans, the ratio of rods to cones is 20-to-1. Although cones provide 90% of the brain’s input

Question 13

photopigments

Answer 13

Rods and cones contain photopigments (chemicals that release energy when struck by light). Photopigments consist of 11-cis-retinal bound to proteins called opsins.

Question 14

Wavelengths:

Answer 14

In the human visual system, the shortest visible wavelengths (about 350 nm) are perceived as violet; progressively longer wavelengths are perceived as blue, green, yellow, and red near 700 nm.

Question 15

Two major interpretations of color vision were proposed in the 1800s:

Answer 15

the trichromatic theory and the opponent-process theory.

Question 16

The Trichromatic (Young-Helmholtz) Theory

Answer 16

a. Young first recognized that color was not understood by examining light, but instead through biology. He believed that there are a few types of receptors, and each was sensitive to a different range of wavelengths.
b. According to this theory of color vision, humans have three different types of cones, each sensitive to a different set of wavelengths. We discriminate among wavelengths by the ratio of activity across the three types of cones.
c. Individuals differ in regards to where the short, medium, and long-wavelength cones are distributed in the retina.
d. Visual Field: The part of your world that you see.
e. But this is an incomplete theory of colour vision

Question 17

The Opponent-Process Theory

Answer 17

a. Negative color afterimages: Visual phenomena that occur when you stare at a colored object under a bright light without moving your head and then look at a plain white surface. You would see a replacement of the red you had been staring at with green, green with red, yellow and blue with each other, and black and white with each other.
b. To explain negative color afterimages and other visual phenomena, the opponent-process theory was proposed. According to this theory, we perceive color in terms of paired opposites: white-black, red-green, and yellow-blue.
c. Opponent-process theory states that negative afterimages result from fatiguing a response by opponent-process cells (e.g., a cell that responds to green light becomes fatigued after prolonged stimulation, which results in a red afterimage when the green light is removed).
d. Also incomplete theory

Question 18

Retinex Theory

Answer 18

a. Color Constancy: The ability to recognize the color of objects despite changes in lighting. This ability is not explained by the trichromatic theory or the opponent-process theory.
b. Retinex Theory: Theory proposed to account for color constancy. When information from various parts of the retina reaches the cortex, the cortex compares each of the inputs to determine the brightness and color perception for each area.

Question 19

Color vision deficiency

Answer 19

is also sometimes known as color blindness, and is characterized by the inability to perceive color differences as most people do. Note that complete colorblindness (perception of only black and white) is rare.
c. Red-green color blindness is the most common form of this disorder (primarily seen in males as is carried on the X chromosome, men have it 8 X more than women).

Question 20

An Overview of the Mammalian Visual System

Answer 20

1. Rods and cones make synaptic connections with horizontal cells and bipolar cells. Horizontal cells make inhibitory contact onto bipolar cells, which in turn synapse with amacrine cells and ganglion cells. All these cells are in the eye.
2. Axons of the ganglion cells from each eye form the optic nerves. The optic nerves from the left and right eyes meet at the optic chiasm, where in humans half of the axons from each eye cross to the opposite side of the brain. Most of the ganglion cell axons go to the lateral geniculate nucleus (LGN) of the thalamus. Most axons from the LGN synapse in the visual areas of the cerebral cortex.
   * *see p. 60

Question 21

Lateral inhibition:

Answer 21

The reduction of activity in one neuron by activity in neighboring neurons); a technique of the retina to sharpen the boundaries of visual objects.

Question 22

Receptive Field:

Answer 22

The portion of the visual field that excites or inhibits a specific cell in the visual system of the brain.

Question 23

Most primate ganglion cells are either (3)

Answer 23

parvocellular neurons (small cell bodies located in or near the fovea), magnocellular neurons (larger cell bodies distributed evenly throughout the retina), or koniocellular neurons (similar in size to parvocellular neurons, but distributed throughout the retina).

Question 24

Parvocellular neurons

Answer 24

have small receptive fields and respond best to visual details and color. These cells synapse only onto cells of the LGN.

Question 25

Magnocellular neurons

Answer 25

have larger receptive fields and respond best to moving stimuli. Most of these cells synapse onto cells of the LGN, but a few connect to other areas of the thalamus

Question 26

Koniocellular neurons

Answer 26

have several different functions and their axons connect to the LGN, other areas of the thalamus, and the superior colliculus.

Question 27

Area V1 or the striate cortex (primary visual cortex):

Answer 27

located in the occipital cortex, responsible for the first stage of visual processing. People with damage to this area report no conscious vision or visual imagery, even in their dreams.

Question 28

Blindsight:

Answer 28

The ability to respond in some way to visual information after extensive damage to area V1. People with blindsight will respond to the stimuli but will report that they cannot see it.

• Research suggests two reasons for the possible existence of blindsight:
  1) Small islands of healthy tissue remain in an otherwise damaged visual cortex, but not large enough to provide conscious perception.
  2) The thalamus sends visual input to several other brain areas besides area V1, which are strengthened after V1 is damaged.

Question 29

David Hubel and Torsten Wiesel distinguished three categories of neurons in the visual cortex:

Answer 29

simple, complex, and end-stopped or hyper-complex cells.

Question 30

Simple cells:

Answer 30

Neurons with fixed excitatory and inhibitory zones in their receptive fields; these cells are found only in the primary visual cortex (V1). Most simple cells have bar-shaped or edge-shaped receptive fields.
- Responds to a stimulus in only one location.

Question 31

Complex cells:

Answer 31

Located in either V1 or V2, these neurons have receptive fields that respond to particular orientations of light but cannot be mapped into fixed excitatory and inhibitory zones. Complex cells receive their input from a combination of simple cells.
- Responds most to moving stimulus.

Question 32

End-stopped (hypercomplex) cells:

Answer 32

Resemble complex cells but also have a strong inhibitory area at one end of their bar-shaped receptive field.

Question 33

Feature Detectors:

Answer 33

Neurons whose responses indicate the presence of a particular feature. The fact that prolonged exposure to a given visual feature decreases sensitivity to that feature supports this concept.

b. For example, the waterfall illusion is when you stare at a waterfall for a minute or more then look away, the rocks and trees nearby look like they’re flowing upwards.
c. Although, optical illusions such as “Mooney faces” imply “top down” processing where other brain areas interpret the casual stimulus and send messages back to reorganize the activity in the primary visual cortex.
d. Therefore, excitation of feature detectors is not sufficient to explain all of vision.

Question 34

retinal disparity:

Answer 34

the discrepancy between what the left and right eyes see.

- necessary for stereoscopic depth perception

Question 35

Strabismus (or strabismic amblyopia), or lazy eye:

Answer 35

Condition in which the eyes do not point in the same direction. Individuals born with this disorder cannot perceive depth better with two eyes than with one.

e. This disorder is treated by putting a patch over the active eye, forcing the child to use the ignored eye. The patch is most effective if used early, as many children refuse to wear it later in life.
f. A promising therapy for the disorder is asking the child to play action video games, which require the attention of both eyes.

Question 36

Astigmatism:

Answer 36

A blurring of vision for lines in one direction; this disorder is caused by an asymmetric curvature of the eyes. Corrective lenses during early childhood (before age 3-4 years) improve visual capacity in adulthood.

Question 37

Impaired Infant Vision and Long-Term Consequences

Answer 37

a. People who are born with cataracts (cloudy lenses) but have them surgically repaired at age 2-6 months eventually develop nearly normal vision but have subtle problems processing some visual information.
b. Children who had their cataracts removed at 7 or later could see pictures but not understand them. This improved after a week without special training.
c. Many other aspects of vision improve over this time, except for motion perception and depth perception, which remain permanently impaired.
d. Vision is more impaired the older the person is when cataracts are removed and proves that visual expertise depends on practice.

Question 38

Ventral Stream:

Answer 38

These pathways are also called the “what” pathways because they are specialized for identifying and recognizing objects. Through the temporal cortex.
-Damage to the ventral stream sees where but not what. They can see where objects are and grab them, but cannot make sense of a television program because they have trouble identifying what things are.

Question 39

Dorsal Stream:

Answer 39

This pathway is the “where” or “how” pathway: It helps the motor system find objects, move toward them, grasp them, and so forth. Through the parietal cortex.
- Damage to the dorsal stream (parietal cortex) seems to have the most normal vision. They can read, recognize faces, and describe objects in detail. They know what things are but not where they are. They cannot accurately reach out to grab an object and bump into things when they walk.

Question 40

Cells in the inferior temporal cortex:

Answer 40

respond to identifiable objects.

b. Cells also respond to what the viewer perceives and not to what the stimulus is physically (figure versus background). Cells continue responding the same way despite changes in the shape’s position, size, and angle.
c. The area is important for shape constancy (the ability to recognize an object’s shape even as it approaches, retreats, or rotates).

Question 41

visual agnosia:

Answer 41

Damage to the shape pathway of the cortex leads to specialized deficits. An inability to recognize objects despite otherwise satisfactory vision is called visual agnosia (meaning “visual lack of knowledge”).
- It usually results from damage in the temporal cortex.

Question 42

fusiform gyrus

Answer 42

fMRI studies have shown that the fusiform gyrus of the inferior temporal cortex is largely specialized for face recognition. The fusiform gyrus is also activated when identifying car models, bird species, and so forth.
- One part of the parahippocampal cortex responds strongly to pictures of places, and not so strongly to anything else

Question 43

Recognizing Faces

Answer 43

a. Human newborns are predisposed to pay attention to faces more so than other stationary objects. However, infants show a strong preference for a realistic, right-side-up face over an upside-down face or a distorted face.
b. Precision of face recognition is best when those faces are familiar. It has been shown that people are better at recognizing faces of their own ethnicity.
c. Face recognition depends on several brain areas, including the occipital face area, the amygdala, and parts of the temporal cortex, including the fusiform gyrus, especially in the right hemisphere.
d. Prosopagnosia: The impaired ability to recognize faces without an overall loss of vision or memory. People with prosopagnosia can identify if a person is young or old, male or female, but they do not recognize who they are.

Question 44

Prosopagnosia:

Answer 44

The impaired ability to recognize faces without an overall loss of vision or memory. People with prosopagnosia can identify if a person is young or old, male or female, but they do not recognize who they are

Question 45

Colour Perception (V4)

Answer 45

1. Area V4 and other nearby brain regions are believed to be important for color constancy. Area V4 also contributes to visual attention.
2. Damage to area V4 would result in, for example, someone not being able to find a yellow highlighter if the overhead lighting changed.

Question 46

The Middle Temporal Cortex

Answer 46

a. Area MT (middle-temporal cortex, also known as area V5) and adjacent area MST (medial superior temporal cortex) are important for motion detection.
b. Cells in the MT respond selectively to a stimulus moving in a particular direction, acceleration, deceleration, and somewhat to still photographs that imply movement. Cells in the MST respond best to complex stimuli such as the expansion, contraction, or rotation of a large visual scene. For example, it responds to objects that move relative to their backgrounds.

Question 47

motion blindness

Answer 47

b. Damage to, or around, area MT and MST results in motion blindness (ability to see objects but impairment at seeing whether they are moving or, if so, which direction and how fast).
c. You do not see your own eyes move because area MT and parts of the parietal cortex decrease activity during voluntary eye movements, known as saccades.
d. The opposite of motion blindness also occurs: Some people are blind except for the ability to detect which direction something is moving.

Question 48

saccades:

Answer 48

You do not see your own eyes move because area MT and parts of the parietal cortex decrease activity during voluntary eye movements

Question 49

Major pathways in the visual system

Answer 49

picture on p.64