Cognitive And Clinical Neuropsychology Of Vision (Year 3)

Question 1

Define sensory transduction

Answer 1

Conversion of one energy form to another e.g light energy into electrical energy

Question 2

Briefly outline some arguments for vision being an indirect process

Answer 2

1. Sensory transduction: Light has to be processed into electrical energy, and then processed again to form visual images
2. Inversion/reflection: Crossover of information from left/right top/bottom (info in the right/left visual fields are projected to the opposite cerebral hemisphere)
3. Cortical magnification - PVC gets most of its info from the fovea, however this info is magnified (fovea is disproportionately represented in early visual areas but we do not see the world that way
4. We can perceive illusions - visual experience goes beyond the sensory input

Question 3

Give some characteristics of vision

Answer 3

1. Innate
2. Indirect
3. Influenced by context
4. Actively reconstructed

Question 4

Define an ischemic stroke

Answer 4

Strokes caused by a blockage/blood clot

Question 5

Give three parallel pathways in vision

Answer 5

Magno, parvo and koniocellular pathways

Question 6

Where does information cross over?

Answer 6

The optic chiasm

Question 7

Explain how Visual information crosses over at the optic chiasm

Answer 7

Left visual fields from both eyes-> right side of brain
Right visual fields from both eyes -> left side of brain

Question 8

Describe the role of the cornea

Answer 8

• Focuses light onto the retina
• Fixed

Question 9

Describe the role of the pupil

Answer 9

-The aperture in the iris
- Changes size according to light levels

Question 10

Describe the role of the lens

Answer 10

• Focuses light
• Adjustable (accommodates light levels)

Question 11

Describe the role of the retina

Answer 11

• Rods and cones in retina absorb light and convert it into electrical signals

Question 12

Describe the path of a signal from the optic nerve to the rods and cones

Question 13

Give the number of rods and cones per eye

Answer 13

125 million rods per eye
6 million cones per eye

Question 14

Give the number of nerve fibres per eye

Answer 14

800,000 per eye

Question 15

Explain the function of nerve fibres

Answer 15

Pool signals from multiple rod or cone receptors

Question 16

Describe the function of rods

Answer 16

• Achromatic night vision
• 1 Type

Question 17

Describe the function of cones

Answer 17

• Daytime, achromatic and chromatic vision
• 3 types

Question 18

Give the three types of cones

Answer 18

• Long-wavelength sensitive (red cone)
• Middle-wavelength sensitive (green cone)
• Short wavelength sensitive (blue cone)

Question 19

Give the wavelength of visible light

Answer 19

350-700nm

Question 20

Describe the role of wavelength sensitive cones in the retina

Answer 20

• Short, middle and long wavelength sensitive cones are sensitive to a different range of wavelengths
• L and M are strongly overlapping, s is sensitive to bluish light

Question 21

define mesopic range

Answer 21

Rods and cones

Question 22

Define phototopic range

Answer 22

Cones only

Question 23

Give the maximum and minimum pupil

Answer 23

maximum: 7.9mm
minimum: 2.0mm

Question 24

Describe the function of the fovea

Answer 24

• High acuity area - outputs from only a few cone cells are pooled together
• Cones are used in normal lighting conditions and provide us with high acuity and colour vision. We move our eyes to bring important things onto the fovea.

Question 25

Describe the distribution of rods and cones

Answer 25

Rods are mostly in the periphery, outside the fovea - rods only function under low light conditions

Question 26

Explain why there is low acuity in the periphery

Answer 26

Outputs from hundreds of rod cells are pooled together

Question 27

Explain what the blind spot is and why we don't see a black spot

Answer 27

Where the optic nerve leaves the retina so no rod or cone cells to detect light Brain 'fills in the gap'

Question 28

Where does parallel processing start?

Answer 28

in the retina

Question 29

Give the three types of retinal ganglion cells and what wavelengths they process

Answer 29

Midget - red-green Parasol - light-dark Bistratified - yellowish-purple

Question 30

Give the properties of Midget ganglion cells

Answer 30

70-80% incidence rate Photopic luminance only Spatial response: opponent Spectral response: L vs M (central) L + M (peripheral) Temporal response: sustained Projection - Parvo LGN

Question 31

Give the properties of parasol ganglion cells

Answer 31

8-10% incidence Phototopic/scotopic Spatial response: opponent Spectral response: L+M Temporal response: Transient Projection: Magno LGN

Question 32

Give the properties of bistratifired ganglion cells

Answer 32

Below 10% incidence Photopic only Spatial response: nonopponent Spectral response: S vs (L+M) Projection: Konio LGN

Question 33

Describe the structure of the lateral geniculate nucleus

Answer 33

layered structure layer 1,4,6 input from contralateral eye layer 2,3,5 input from ipsilateral eye Layers 1,2: Magnocellular (from Parasol cells) Layers 3,4,5,6: parvocellular layers (from Midget cells) ventral to each layer, numerous tiny neurons are found, the koniocellular layers (Konio, greek: dust; input from Bistratifed cells) Retinotopic map – topographically arranged receptive fields

Question 34

Describe the different functions of the P,M,K layers in the LGN

Answer 34

• P,M,K layers have different functions M cells: large receptive fields, larger cell bodies, respond to transient information P cells: small cell bodies, sustained response K cells: very small cell bodies; implicated in colour vision

Question 35

Describe the structure of the primary visual cortex

Answer 35

Primary visual cortex: anatomically distinct layers (1-6) Layer 4 divided into 4 sublayers (4A,4B,4Calpha, 4Cbeta) Parvo, magno, and konio divisions of the LGN project selectively into different layers. Most of the input goes to Layer 4C: LGN magnocellular layers project to 4C alpha LGN parvocellular layers project to 4Cbeta Konio cells project to superficial layers 2 and 3 Information is kept separate in visual cortex!

Question 36

Describe the structure and function of parvo and magnocellular pathways

Answer 36

- ‘luminance/achromatic/light-dark pathway’ and ‘red-green’ pathways have bee extensively characterised - Majority of photoceptors feed into the P and M pathway and subserve most visual functions: spatial vision including high acuity vision; motion vision; form perception - Function of koniocellular pathway is not fully understood; phylogenetically the oldest pathway

Question 37

Describe the function of the Koniocellular pathways

Answer 37

Koniocellular pathway gets input from the nonP, nonM retinal ganglion cells and is probably involved colour perception

Question 38

Describe the function differ ences between the parvo and magnocellular pathways

Answer 38

- Response to contrast - Different sensitivity to spatial frequencies 1. Different spectral tuning: Parvo: ‘L-M’; Magno:’ L+M’ Parvocellular neurones: respond best to red-green modulations Magnocellular neurones: respond best to achromatic modulations 2. M and P neurones have different contrast gains 3. Different sensitivity to different spatial frequencies (SF)

Question 39

Describe how gratings in different colour directions can be used to isolate the different pathways

Question 40

Describe how we see colours

Answer 40

An image of the world is projected by the cornea and lens onto the rear surface of the eye: the retina. The back of the retina is carpeted by a layer of light- sensitive photoreceptors (rods and cones).

Question 41

Describe the role of photopigments

Answer 41

The rods all have the same photopigment, rhodopsin. But there are three different types of cones in the human retina, each with a slightly different photopigment. (L, M,S cones) The 3 photopigments absorb and reflect different wavelengths of light, giving rise to the different colors in the picture.

Question 42

Describe how human cone mosaics

Answer 42

The three different types of cones are arranged randomly in the retina (shown as red, green, and blue in this in the picture). These are the retinas of two different subjects (AN and JW). It is remarkable how different the number and the distribution of the L and M cones are. Yet, these differences in the retinal make-up do not result in different colour experiences.

Question 43

Define trichromacy

Answer 43

Trichromacy: colour is encoded by the relative outputs of the three cone classes

Question 44

Explain colour-opponent theory

Answer 44

Hering was the first to notice that some pairs of colours, namely red and green, and yellow and blue, cannot be perceived at the same time. He named these pairs of colours the opponent colours (red-green, yellow-blue) since they are mutually exclusive colours. The perceptual ‘red-green’ colour direction is not aligned with the LGN L-M direction. The perceptual yellow-blue colour direction is not aligned with the LGN S-(L+M) direction. Therefore, the colour signals are transformed again somewhere in visual cortex (or beyond) to provide us with the percept of ‘red’, ‘green’, ‘yellow’, ‘blue’ etc.

Question 45

Describe early chromatic pathways in LGN

Answer 45

Three cone classes supply three "channels" The achromatic channel receives nonspectrually opponent input from L- and M cone classes The two chromatic channels receive spectrally opponent inputs to create the L-M or red-green channel and S-(L and M) or blue yellow channel

Question 46

Explain our perceptual representation of colour

Answer 46

Perceptual representation of colour: Hue is what we could loosely refer to as colour, as in red, yellow, blue etc. Hueless or achromatic colours include black, gray, white. Saturation refers to purity, or how much grey there is in a colour. Brightness goes from complete dark to white.

Question 47

Explain why physically different lights can look identical to us

Answer 47

Just three cone receptors underlie colour vision

Question 48

Describe the evolutionary advantage of seperate L and M cones

Answer 48

Most mammals are dichromats (short and medium- or long- wave). Primates because of the advantages of detecting red/orange fruit against a green background, evolved a third pigment (L and M)

Question 49

Describe the two kinds of colour deficits

Answer 49

1. Congenital (Retinal) (one or more of the three kinds of cones is missing) Protanopia: L cones are missing Deuteranopia: M cones are missing Tritanopia: S cones are missing 2. Cerebral: the damage is to the cortex, often in the extrastriate area V4 (Zeki, 1973). Cerebral Achromatopsia Cerebral Hemiachromatopsia

Question 50

Describe how colour deficiencies are diagnosed

Answer 50

Ishihara plates Farnsworth-Munsell D-15 test

Question 51

Give the location of the striate visual cortex and extra striate visual cortex

Answer 51

V1 - SVC V2, V3, V4, etc - EVC

Question 52

Desccribe Lueck et al, findings

Answer 52

Hypothesis: Neurones in colour-selective brain areas will respond more vigorously to the chromatic Mondrian compared to the achromatic (black-white) one. Neural activity is obtained by measuring cerebral blood flow. Expected colour-selective area: V4 (‘colour area’) Change in CBF in different brain areas: Changes in responses a coloured Mondrian are shown in black; Changes in response to a black-white Mondrian are shown in grey. Striate cortex (V1) responds well to both colour & achromatic stimuli V2 also showed similar activation, but resolution was not high enough to reliablu distinguish V2 from V1. ‘Colourarea’ (V4): responds more to colour than achromatic stimuli ~ 12-14% A small increase in the Temporo-occipto- parietal area (V5) ~3-5%

Question 53

Desccribe Cowey and Heywood's findings

Answer 53

Damage aroud fusiform gyrus associated with only partial achromatopsia