Cognitive And Clinical Neuropsychology Of Vision (Year 3) Flashcards
Define sensory transduction
Conversion of one energy form to another e.g light energy into electrical energy
Briefly outline some arguments for vision being an indirect process
- Sensory transduction: Light has to be processed into electrical energy, and then processed again to form visual images
- Inversion/reflection: Crossover of information from left/right top/bottom (info in the right/left visual fields are projected to the opposite cerebral hemisphere)
- 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
- We can perceive illusions - visual experience goes beyond the sensory input
Give some characteristics of vision
- Innate
- Indirect
- Influenced by context
- Actively reconstructed
Define an ischemic stroke
Strokes caused by a blockage/blood clot
Give three parallel pathways in vision
Magno, parvo and koniocellular pathways
Where does information cross over?
The optic chiasm
Explain how Visual information crosses over at the optic chiasm
Left visual fields from both eyes-> right side of brain
Right visual fields from both eyes -> left side of brain
Describe the role of the cornea
- Focuses light onto the retina
- Fixed
Describe the role of the pupil
-The aperture in the iris
- Changes size according to light levels
Describe the role of the lens
- Focuses light
- Adjustable (accommodates light levels)
Describe the role of the retina
- Rods and cones in retina absorb light and convert it into electrical signals
Describe the path of a signal from the optic nerve to the rods and cones
Give the number of rods and cones per eye
125 million rods per eye
6 million cones per eye
Give the number of nerve fibres per eye
800,000 per eye
Explain the function of nerve fibres
Pool signals from multiple rod or cone receptors
Describe the function of rods
- Achromatic night vision
- 1 Type
Describe the function of cones
- Daytime, achromatic and chromatic vision
- 3 types
Give the three types of cones
- Long-wavelength sensitive (red cone)
- Middle-wavelength sensitive (green cone)
- Short wavelength sensitive (blue cone)
Give the wavelength of visible light
350-700nm
Describe the role of wavelength sensitive cones in the retina
- 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
define mesopic range
Rods and cones
Define phototopic range
Cones only
Give the maximum and minimum pupil
maximum: 7.9mm
minimum: 2.0mm
Describe the function of the fovea
- 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.
Describe the distribution of rods and cones
Rods are mostly in the periphery, outside the fovea - rods only function under low light conditions
Explain why there is low acuity in the periphery
Outputs from hundreds of rod cells are pooled together
Explain what the blind spot is and why we don’t see a black spot
Where the optic nerve leaves the retina so no rod or cone cells to detect light
Brain ‘fills in the gap’
Where does parallel processing start?
in the retina
Give the three types of retinal ganglion cells and what wavelengths they process
Midget - red-green
Parasol - light-dark
Bistratified - yellowish-purple
Give the properties of Midget ganglion cells
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
Give the properties of parasol ganglion cells
8-10% incidence
Phototopic/scotopic
Spatial response: opponent
Spectral response: L+M
Temporal response: Transient
Projection: Magno LGN
Give the properties of bistratifired ganglion cells
Below 10% incidence
Photopic only
Spatial response: nonopponent
Spectral response: S vs (L+M)
Projection: Konio LGN
Describe the structure of the lateral geniculate nucleus
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
Describe the different functions of the P,M,K layers in the LGN
• 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
Describe the structure of the primary visual cortex
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!
Describe the structure and function of parvo and magnocellular pathways
- ‘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
Describe the function of the Koniocellular pathways
Koniocellular pathway gets input from the nonP, nonM retinal ganglion
cells and is probably involved colour perception
Describe the function differ ences between the parvo and magnocellular pathways
- Response to contrast
- Different sensitivity to spatial frequencies
- Different spectral tuning: Parvo: ‘L-M’; Magno:’ L+M’
Parvocellular neurones: respond best to red-green modulations
Magnocellular neurones: respond best to achromatic modulations - M and P neurones have different contrast gains
- Different sensitivity to different spatial frequencies (SF)
Describe how gratings in different colour directions can be used to isolate the different pathways
Describe how we see colours
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).
Describe the role of photopigments
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.
Describe how human cone mosaics
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.
Define trichromacy
Trichromacy: colour is encoded by the relative outputs of the three cone classes
Explain colour-opponent theory
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.
Describe early chromatic pathways in LGN
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
Explain our perceptual representation of colour
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.
Explain why physically different lights can look identical to us
Just three cone receptors underlie colour vision
Describe the evolutionary advantage of seperate L and M cones
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)
Describe the two kinds of colour deficits
- 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 - Cerebral: the damage is to the cortex, often in the extrastriate area V4 (Zeki,
1973).
Cerebral Achromatopsia
Cerebral Hemiachromatopsia
Describe how colour deficiencies are diagnosed
Ishihara plates
Farnsworth-Munsell D-15 test
Give the location of the striate visual cortex and extra striate visual cortex
V1 - SVC
V2, V3, V4, etc - EVC
Desccribe Lueck et al, findings
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%
Desccribe Cowey and Heywood’s findings
Damage aroud fusiform gyrus associated with only partial achromatopsia
