Role of retina in colour perception
The role of the retina in colour vision is explained by two theories:
- Trichromatic theory
- Opponent Colour System theory
Trichromatic theory
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
Trichromatic theory - protanopia
- 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).
Trichromatic theory - deuteranopia
Deuteranopia = confuse red and green
• Normal visual acuity
• Green cones are filled with red cone protein
Trichromatic theory - tritanopia
Tritanopia = see the world in greens and reds
• Retina lacks blue cones
• Normal visual acuity
Trichromatic theory - monochromatic vision
- Monochromatic Vision = do not perceive different hues
* Retina lacks all three cones
Opponent colour system theory
• 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.
Opponent colour system theory and afterimages
- 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
Ganglion cells
- 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).
Colour processing and parvocellular, koniocellular and magnocellular divisions
- 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
Ventral stream and the divisions
- 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
Dorsal stream and the divisions
- 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
Colour blindness from cortical damage
- 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
Visual agnosia and form perception
- 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)
Prosopagnosia
- 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
Binocular disparity
- process of depth perception
- comparison of difference between retinal images
- used for distances, beyond 3 metres
Convergence
- 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
Binocular disparity in the striate and extrastriate cortex
- 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
The parietal lobe and spatial location
- 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
Perception of orientation: the striate cortex
- 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
Extrastriate cortex and motion perception - Area V5
• 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)
Extrastriate cortex and motion perception - Area MST
- Area MST is adjacent from V5.
- Responds to complex patterns of movement
- Dorsolateral MST helps analyze optic flow
Perception of form from motion
- 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)
Summary table of visual area
*look up image
-
Week 113
-
Week 2 - topic 128
-
Week 2 - topic 219
-
Week 2 - topic 323
-
Week 3 - topic 116
-
Week 3 - topic 235
-
Week 3 - topic 312
-
Week 4 - topic 122
-
Week 4 - topic 217
-
Week 4 - topic 340
-
Week 5 - topic 117
-
Week 5 - topic 216
-
Week 5 - topic 310
-
Week 6 - topic 131
-
Week 6 - topic 26
-
Week 6 - topic 324
-
Week 7 - topic 150
-
Week 8 - topic 1 and 225
-
Week 9 - topic 126
-
Week 9 - topic 321
-
Week 10 - topic 142
-
Week 10 - topic 320
-
Week 1131
-
Week 11 - topic 233
