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Cognitive Psychology 2 > The Visual Pathway > Flashcards

The Visual Pathway Flashcards

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1
Q

what is the wavelength?

A

distance between corresponding points of two consecutive waves

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2
Q

what is amplitude?

A

the distance from the centre line (or the still position) to the top of a crest close crestThe topmost part of a wave

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3
Q

what is the visual field?

A

how much of the outside world the retina can see

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4
Q

what are degrees of visual angle?

A

the proportion of the retina taken up by an object

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5
Q

what is the anterior chamber?

A

filled with aqueous humour- a watery fluid

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6
Q

What is the cornea?

A
  • transparent membrane where light can enter the eye
  • lacks blood vessels and is nourished by aqueous humour that’s behind it
  • protects the eye and responsible for some refraction (bending of light)
  • ensures visual images aren’t blurred
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7
Q

what is the pupil?

A
  • opening in the eye
  • allows light to enter and reach the bottom of the eyeball
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8
Q

What is the iris?

A
  • a diaphragm
  • coloured muscle that regulates the light entry of the eye
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9
Q

what is the lens?

A
  • responsible for most of the refraction
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10
Q

what are the ciliary bodies?

A
  • hold the lens in place
  • help change the shape of the lens
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11
Q

what is vitreous humour?

A
  • jelly-like fluid
  • maintains the structure and shape of the eye
  • allows the movement of vitamins throughout the eye
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12
Q

what is the Retina?

A
  • light- sensitive structure
  • where photoreceptors are
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13
Q

what is the fovea?

A
  • most sensitive part of the fovea
  • part of the macula
  • has highest density of photoreceptors, and therefore it is the area of the retina with the highest visual activity
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14
Q

What is the choroid layer?

A
  • controls excessive light, when light enters the eye is not all absorbed by photoreceptors
  • absorbs all protons not used by the photoreceptors
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15
Q

what is the Sclera?

A
  • white of the eye
  • protective covering
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16
Q

what is the optic disc?

A
  • where the retinal blood vessels and fibres of the optic nerve exit the retina
  • blind spot
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17
Q

what is the optic nerve?

A
  • bundle of axons that go from the retina to the brain
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18
Q

what is the macula?

A

contains a high density of photoreceptors and is divided into different regions, and one of these regions is the fovea

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19
Q

what is the blind spot?

A

the optic nerve and retinal artery enter the eye above the retina, creating a physiological blind spot

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20
Q

what are the properties of rods?

A
  • 120 million per eye
  • Periphery
  • Monocrome
  • Low resolution
  • Many to 1 with ganglion cells
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21
Q

What are the properties of cones?

A
  • 6 million per eye
  • Fovea
  • Colour
  • High resolution
  • 1 to 1 with ganglion cells
  • 3 types
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22
Q

what is the duplex retina made up of?

A
  • A cone-driven, photopic (light) system: high acuity, low sensitivity
  • A rod-driven, scotopic (dark) system: low acuity, high sensitivity, colour-blind
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23
Q

what are retinal ganglion cells (RGCs)?

A
  • produce action potentials that project to the CNS via optic nerve
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24
Q

what are single cell recordings?

A
  • identify what type of visual stimulus the RGCs best respond to
  • Place an electrode next to the axon of an RGC to record electrical changes in the axon
  • move the position of a light around until we begin to influence the RGC activity – this area of sensitivity is called the receptive field (RF) of the RGC
  • The RF is the region of the visual scene the cell is responsible for
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25
Q

what is the receptive field?

A
  • Retinal ganglion cells’ firing pattern is influenced by the light in their RF
  • They have a baseline firing rate, so the firing rate can both increase and decrease from its baseline
  • The RGC receptive field has a centre region and a surround region. These regions show a centre-surround antagonism.
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26
Q

what is an ON-centre cell?

A
  • If a light is turned on in the centre region, the RGC firing rate will increase.
  • If a light is turned on in the surround region, the firing rate will decrease
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27
Q

what is an OFF-centre cell?

A

RGCs have the opposite firing pattern to ON RGCs. The firing rate increases when a dark spot cover the centre.

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28
Q

what happens to firing rates in the receptive fields?

A
  • firing pattern increases or decreases depending on where the RF in light hits
  • ON response: increase
  • OFF response: decrease
  • ON and OFF RGCs have opposing response patterns
  • light anywhere outside the RF will not influence the firing activity
29
Q

where are receptive fields smallest?

A

in the fovea (0.01mm)
- have low convergence factor
- high spatial resolution

30
Q

by what factor will the RF size increased at 10mm from the fovea?

A

a factor of 50
- higher convergence factor
- low spatial resolution

31
Q

what purpose does centre-surround organisation serve?

A

helps identify edges in images
- Objects can be distinguished from the background by sudden changes in the reflected light.

32
Q

which are RFs (large or small) respond to which objects?

A
  • a small RF responds best to small objects
  • Larger RFs respond better to larger objects
33
Q

what does centre-surround antagonism allow retinal ganglion cells (RGCs) to do?

A
  • be sensitive to the phase- the relative position of a grating in its cycle
  • An array of RGCs will encode the whole range.
34
Q

what are off systems?

A
  • have an inhibitory centre and an excitatory surround
  • They are maintained as separate systems throughout the visual system, at most stages of visual processing
  • They allow the visual system to detect increment and decrement in light levels.
35
Q

what are the 3 subcategories of RGCs?

A
  • M cells
  • P cells
  • K cells
36
Q

what is a visible consequence of centre-surround antagonism?

A

The Hermann Grid

37
Q

What are M cells (Magnocellular) responsible for?

A

motion

proportion: 10 %
RF size: Large
retinal position: Peripheral
Axon diameter: Thick
conduction velocity: Fast
best objects: Large, low contrast
colour sensitivity: No
temporal response: Transient

38
Q

what are P cells (Parvocellular) responsible for?

A

colour

proportion: 80%
RF size: Small
retinal position: Central
Axon diameter: Thin
conduction velocity: Slow
best objects: small, high contrasts
colour sensitivity: Yes
temporal response: sustained

39
Q

Describe the visual pathway

A
  • Retinal ganglion cells produce APs that project visual information through the optic nerve (2nd cranial nerve).
  • Projections from the contralateral hemifield switch sides at the optic chiasm. Note: the crossing of fibre bundles is called decussation
  • The optic tract projects to the lateral geniculate nucleus (LGN) in the thalamus.
  • optic radiations project to the visual cortex.

retinal ganglion cells → optic nerve→ optic chiasm→ LGN →visual cortex

40
Q

why do nerve fibres from each eye cross?

A

to allow for visual hemifield-specific processing in later visual areas
- The right hemisphere processes visual information from the left visual field, and the left hemisphere from the right visual field

41
Q

What is the lateral geniculate nucleus (LGN)?

A
  • part of the thalamus
  • 80% of axons from RGC project to the LGN
  • rest go by superior colliculus (eye movement) and hypothalamus (circadian rhythm)
42
Q

what are the layers in the LGN?

A
  • 6 layers
  • cells in L1 and L2 are larger than other layers and get input from M cells
  • cells in L3-6 are smaller and receive input from P cells
  • cells with input from K cells are sandwiched between the M and P layers
43
Q

what are Ipsilateral fibres?

A
  • have not crossed
  • input to the LGN in layers 2 (M), 4 (P) and 5 (P)
44
Q

what are Contralateral fibres?

A
  • have crossed
  • input to layers 1(M), 4 (P), and 6 (P)
45
Q

Each layer of the LGN is retinotopically organized: what does Retinotopy mean?

A
  1. Adjacent areas of the retina are represented in adjacent regions of each layer of the LGN.
  2. The spatial relations of the retina are therefore preserved. This is called the retinotopic map.
  3. Each of the six LGN layers has a retinotopic map, and the matching regions are stacked on top of each other. So, the same retinal region is represented in the same location within each layer.
46
Q

colour in LGN in visual function

A
  • Nearly all P cells are colour sensitive. This colour-based centre-surround is the basis of colour opponency
  • M cells respond to all colours and so are not colour sensitive.
46
Q

what are the receptive fields of LGN cells like?

A
  • RFs are also circular in shape, with a centre-surround configuration and two opposing types (ON and OFF). Both are found in magnocellular and parvocellular layers.
  • The inhibitory influence of the surround is stronger than in RGC RFs – this amplifies differences between neighbouring regions of the RGC RF.
47
Q

Acuity in LGN in visual function

A
  • LGN RF sizes vary in each layer, the smallest devoted to the fovea.
  • The largest are in M layers, so best spatial resolution is in P layers.
48
Q

Temporal sensitivity in LGN in visual function

A
  • M layer cells respond well to rapid change in light intensity. - P cells are much slower to respond.
  • The M cells are much more sensitive to motion.
49
Q

what is the structure of the visual cortex?

A

The LGN projects to the primary visual cortex (V1) via optic radiations. V1 aka striate cortex.

50
Q

what are the properties of V1?

A
  • V1 has about 100m cells per hemisphere.
  • V1 is organised in layers.
51
Q

at what layer does LGN input come unto V1?

A

at layer 4:
- Magnocellular (M cells) in upper layer 4.
Parvocellular (P cells) with lower layer 4.

then connect to upper and lower layers

K cells go straight to layers 1-3

52
Q

what are ocular dominant columns?

A
  • cells in layer 4 are driven by the input from one eye only.
  • If a particular block of cells receives input from the right eye, the cells above and below it will also receive input from the right eye.
  • adjacent blocks of cells either side will receive input from the opposite eye
  • This creates a pattern of ocular dominance columns
53
Q

what is the retinotopic map?

A
  • adjacent regions of the retina are mapped onto adjacent regions of the cortex
  • the distribution of cells associated with each retinal region is distorted: 80% of cortical cells are devoted to the central 10deg of the visual field
54
Q

what is cortical magnification?

A
  • disproportionate weighting of cortical power
55
Q

what are the similarities of cortical cells and the RCGs and LGN?

A
  • They maintain the retinotopic map.
  • They aren’t particularly sensitive to the illumination level.
  • They respond best to abrupt changes in luminance (lines, bars).
55
Q

what are the differences of cortical cells and the RCGs and LGN?

A
  • Selectivity to orientation.
  • They are sensitive to size in a different way.
  • They can be binocular.
  • They are more sensitive to colour.
  • They are sensitive to direction of motion
56
Q

what is orientation selectivity?

A
  • most cortical cells have marked preference for particular orientations
  • Cortical RFs are organised and shaped differently so that they obtain a maximum response to a line of a specific orientation
  • staining can tell us about the orientation preference of an array of cortical cells.
57
Q

what are the three types of cortical cell?

A
  • simple cells
  • complex cells
  • Hypercomplex cells
58
Q

what are simple cells?

A

Optimum response to an appropriately oriented stimulus and a certain position within the RF. Phase sensitive

59
Q

what are complex cells?

A

Optimum response to an appropriately oriented stimulus placed anywhere within the RF. Phase insensitive

60
Q

what are hypercomplex cells?

A

Optimum response depends not only on orientation but also on contour length. Maximum response occurs when the bar length matches the width of the receptive field.This is “end stopping” or “length-width inhibition”.

61
Q

what are binocularity cells?

A
  • can be driven by either eye
  • cells in layers other than V1
  • have 2 RFs - LE & RE
  • They are matched in type (e.g. simple) and respond to similar preferred orientations, locations, and directions of motion.
62
Q

how are colour sensitive cells organised?

A
  • in cortical blobs
  • Each blob is centred on an ocular dominance column.
  • Within a blob, cells will either have red/green opponency or blue/yellow opponency – these are not mixed in a single blob.
  • Blobs receive their input from lower layer 4, which gets its input from the parvocellular LGN layers
  • These cells show no preference for a particular orientation, unlike most other V1 cells.
62
Q

which is direction sensitivity?

A
  • when cortical cells display preferences for stimuli moving in a particular direction.
  • Motion-sensitive cells usually respond only to one direction, i.e., they do not respond to motion in the opposite direction. This is direction selectivity.
  • Simple cells respond to slow motion.
  • Complex cells respond to faster motion.
63
Q

what are columns of cells?

A

Each column consists of cells with the same orientation preference and the same ocular dominance preference.

64
Q

what is a hypercolumn?

A
  • A set of 18-20 columns (~1mm) traverse a complete range of orientations and ocular dominance. This collection of adjacent columns is referred to as a hypercolumn (HC).
  • Each HC contains the neural machinery required to simultaneously analyse multiple attributes of an image (orientation, size, colour, direction of motion) falling on a localized region of the retina
  • A foveal HC covers ~0.05deg of the visual field;At 10deg eccentricity, each one covers ~0.7deg.
    Neighbouring HCs look at neighbouring regions of the retina
65
Q

what is the ice cube model?

A

the structural arrangement of columns

Cognitive Psychology 2 (22 decks)

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