Week 6 - topic 3

Question 1

Role of retina in colour perception

Answer 1

The role of the retina in colour vision is explained by two theories:

• Trichromatic theory
• Opponent Colour System theory

Question 2

Trichromatic theory

Answer 2

Trichromatic Theory argues that the eye detects different colours because it contains three cones, each sensitive to a single hue
• Photoreceptors have photopigments that have different absorption characteristics.
• Absorption is determined by the protein (opsin) in the
photopigment
- Genetic differences in colour vision result from anomalies in one or more of the three types of cone

Question 3

Trichromatic theory - protanopia

Answer 3

• Protanopia = confuse red and green and the world in shades of blue and yellow
• Normal visual acuity
• Red cones are filled with green cone protein (opsin).

Question 4

Trichromatic theory - deuteranopia

Answer 4

Deuteranopia = confuse red and green
• Normal visual acuity
• Green cones are filled with red cone protein

Question 5

Trichromatic theory - tritanopia

Answer 5

Tritanopia = see the world in greens and reds
• Retina lacks blue cones
• Normal visual acuity

Question 6

Trichromatic theory - monochromatic vision

Answer 6

• Monochromatic Vision = do not perceive different hues

* Retina lacks all three cones

Question 7

Opponent colour system theory

Answer 7

• Ganglion cells use an opponent colour system with neurons responding to pairs of primary colours

Receptive Field of Ganglion Cells: When a portion of the receptive field is illuminated with the colour shown, the cell’s rate of firing increases. When a portion is illuminated with the complementary colour, the cell’s rate of firing decreases.

The response characteristics of retinal ganglion cells to light of different wavelengths are determined by the particular circuits that connect the three types of cones with the two types of ganglion cells.

Question 8

Opponent colour system theory and afterimages

Answer 8

• Negative afterimages can be explained via the opponent-colour system theory. Specifically, it is an adaptation of the firing rate of the ganglion cells that causes the afterimage to occur. When ganglion cells are excited or inhibited for a long amount of time, they later show a rebound effect - where they fire faster or slower than normal.
• For example, after staring at the green apple for a long time, the red-green cells are inhibited. Then, when you look to the white area, the red-green cells are no longer inhibited and fire faster than normal. This is causing a rebound effect - and you see the colour red

Question 9

Ganglion cells

Answer 9

• the ganglion cells in the retina process information about the different amounts of light falling in the centre and surrounding regions of their receptive fields.
• The information from the retinal ganglion cells is then sent off via the optic nerve to the LGN of the thalamus (in the forebrain).

Question 10

Colour processing and parvocellular, koniocellular and magnocellular divisions

Answer 10

• The parvocellular division of the LGN receives information from red and green cones.
• The koniocellular division receives information from blue cones.
• The magnocellular division is not involved in processing

Question 11

Ventral stream and the divisions

Answer 11

• Colour information from the parvocellular and koniocellular systems is conveyed along the ventral stream to the inferior temporal lobe, which is responsible for processing what an object is (including it’s colour).
• The ventral stream also receives magnocellular input

Question 12

Dorsal stream and the divisions

Answer 12

• the dorsal stream receives most information from the magnocellular division as it is not directly involved in the colour processing
• this input is light/dark contrast and movement

Question 13

Colour blindness from cortical damage

Answer 13

• It is possible to experience colour blindness following brain damage to specific regions of the extrastriate cortex.
• This is called cerebral achromatopsia, and often occurs following a stroke.
• To patients with cerebral achromatopsia, it appears as though the world is in black and white.
• > hemiachromatopsia - half of visual field is colour blind

Question 14

Visual agnosia and form perception

Answer 14

• Some individuals are unable to identify common items by sight, although visual acuity remains. It is thought to be caused by damage to parts of the ventral stream of the extrastriate cortex
• A region of the extrastriate called the Lateral Occipital Cortex (LOC) appears to respond to a wide variety of objects and shapes.
• There also appears to be a few regions that primarily process specific categories (faces, bodies, scenes - image on right)

Question 15

Prosopagnosia

Answer 15

• There are special face recognizing circuits in the fusiform face area (FFA) which is a region of the visual association cortex (extrastriate cortex) located in the fusiform gyrus on the base of the temporal lobe.
• People can experience prosopagnosia following damage to the FFA (aquired prosopagnosia), or they can experience prosopagnosia from birth (congenital prosopagnosia).
• Some research suggests that the anterior fusiform gyrus is smaller in those with congenital prosopagnosia

Question 16

Binocular disparity

Answer 16

• process of depth perception
• comparison of difference between retinal images
• used for distances, beyond 3 metres

Question 17

Convergence

Answer 17

• process of depth perception
• feedback from eye muscles, how inwardly they have turned determines how close the stimulus is
• best for objects within 3m of face

Question 18

Binocular disparity in the striate and extrastriate cortex

Answer 18

• Most neurons in the striate cortex are binocular, responding to visual stimulation from either eye, and some cells respond to binocular (retinal) disparity.
• Disparity sensitive neurons are found throughout the striate and extrastriate cortex:
• > Those found in the dorsal stream are involved in spatial perception and respond to large, extended visual surfaces
• > Those found in the ventral stream are involved in object perception and respond to the contours of 3D objects

Question 19

The parietal lobe and spatial location

Answer 19

• The parietal lobe is partly involved in the perception of spatial location and somatosensory perception.
• Damage to the parietal lobes disrupts performance on a variety of tasks that require perceiving and remembering the locations of objects and controlling movements of the eyes and the limbs.
• Finally, the dorsal stream of the visual association cortex terminates in the posterior parietal cortex

Question 20

Perception of orientation: the striate cortex

Answer 20

• Most neurons in the striate cortex (primary visual cortex) are sensitive to orientation. That means that the neurons in the primary visual cortex will respond only when a line is in a particular position (e.g., vertical, horizontal).
• The figure below shows the firing rate (remember, this is how often it sends an action potential) of a single neuron in V1 in response to a bar of light that is oriented in different ways.
• The neuron is sensitive to lines oriented vertically. When a horizontal line is shown, the cell does not fire, however, as the line is rotated closer to vertical, it begins to fire more

Question 21

Extrastriate cortex and motion perception - Area V5

Answer 21

• Area V5/MT of the extrastriate contains neurons that respond to movement
• Receives input directly from the striate cortex and other areas of the extrastriate,
• Also receives input from superior colliculus (involved in reflexes and eye movements)
-> Bilateral damage to V5 results in akinetopsia (motion blindness)

Question 22

Extrastriate cortex and motion perception - Area MST

Answer 22

• Area MST is adjacent from V5.
• Responds to complex patterns of movement
• Dorsolateral MST helps analyze optic flow

Question 23

Perception of form from motion

Answer 23

• Perception of movement can even help us to perceive 3D forms
• Involves different regions of the extrastriate cortex
• Assists in everyday activities (e.g. catching a ball)

Question 24

Summary table of visual area

Answer 24

*look up image