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OV2023 visual perception > essential concepts > Flashcards

essential concepts Flashcards

1
Q

what are the key tasks for the visual system?

A

. to gather information about the spatial and temporal distribution of light reflected from objects and surrounding scene

. reconstruct information to form meaningful representations of the visual world

. the visual system consists of an extremely complex network of many, many millions of neurons

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2
Q

what is the organisation of neurons in the retina?

A

. neurons with different structures and functions occupy different layers of the given tissue

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3
Q

what is the function of photoreceptors ?

A

. transduction of light stimulus into a change in membrane potential and send information into bipolar cells

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4
Q

what is the function of bipolar cells?

A

bipolar cells relay information into the ganglion cells

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5
Q

what is the function of ganglion cells?

A

process information and send that information to brain

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6
Q

what is one method of studying the visual system?

A

. anatomy microscopy: look at the arrangement of neurons and their patterns of synaptic connectivity

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7
Q

what is another method for studying the visual system?

A

. pathway tracing: inject a substance into a local region of the visual pathway and examine where the neurons there transport it down their axons , so revealing their long-range connection with other parts of the system

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8
Q

what is the concept of pathway tracing?

A

visual pathways/processing occurs in sequence

e.g. from photoreceptors to bipolar cells to ganglion cells to LGN to visual cortex

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9
Q

what are other methods for studying the visual system?

A

. functional imaging
- examine brain activity in neurologically-intact (i.e healthy) people while they perform specific visual tasks ( e.g. MRI)

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10
Q

what is the concept of functional imaging?

A

. visual neurons excited by similar features of the image cluster together in specialised brain areas

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11
Q

what is clinical neurology ?

A

. another method for studying the visual system
. identify the specific visual deficits that brain-damaged subjects experience and relate these the location of the lesion ( damage) site

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12
Q

what is the concept of clinical neurology?

A

. ‘loss-of-function ‘ is due to loss of the brain region specialised to carry out that function

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13
Q

what is the visual system organization ?

A

. neurons with different structures have different functions
. neurons with similar functions cluster together (e.g. in the same cell layer)

. MAPS of the visual field ‘ over-represent’ the high acuity central ( 15 deg ) of vision

e.g. 66-75% of area VI processes information from the fovea and macula

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14
Q

what is important to note when processing the visual image?

A

. every visual neuron possesses a ‘receptive field’

. serial ( step-by-step sequential ) pathway = associated with reconstructing the image

. parallel(side-by-side) pathways (e.g. for colours vs. motion)= associated with functional specialisation/divisions of labour

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15
Q

what is the primary visual pathway?

A 
. retinal ganglion cells
. optic nerve chiasm tract
. lateral geniculate nucleus 
. optic radiation ( white matter pathway) 
. primary visual ( V1) cortex
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16
Q

what happens at the optic chiasm?

A

. axons that come from the nasal retina cross the midline at the optic chiasm and go to opposite LGN

. axons from the temporal retina remain uncrossed at the chiasm

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17
Q

what happens to nasal retina of RE?

A

. nasal retina of the RE is looking at the left half of visual field
. information from nasal axons goes to chaism , crosses the midline and ends up in the right LGN

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18
Q

what happens to temporal retina of RE?

A

. temporal retina of RE is looking at the left half of the visual field

. information from the temporal retinal ganglion cells from the right eye travel to the chiasm , but thoses axons remain uncrossed and ends up in the right LGN

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19
Q

what is left hemianopia ?

A

. damage to the primary visual cortex in the right hemisphere causes a visual field loss of hemifield on the opposite side

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20
Q

where is the highest density on retinal neurons/retinal ganglion cells ?

A

. in the fovea and macula

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21
Q

how is the high cell densities in fovea and macula represented ?

A

. over-representation of these retinal regions is replicated at all subsequent levels of the primary visual pathway

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22
Q

what is receptive fields?

A

. all neurons have a receptive field (RF)

. this means that they are

  1. receptive ( respond) only to specific features of the visual image in
  2. a particular, spatially restricted, region of the visual field
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23
Q

how is the receptive fields of photoreceptors determined?

A

photoreceptors: their RFs are determined by
1. the luminance (brightness) and wavelength-sensitivity of the particular rhodopsin (visual pigment ) they contain in their outer segment

  1. their position in the retina :i.e. where they are looking in space
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24
Q

how are all other visual neurons receptive field determined?

A

all other visual neurons: their RFs are determined by

  1. their excitatory and inhibitory synaptic inputs they receive
  2. their position in the visual field map in tissue in which they reside
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25
Q

explain receptive fields?

A

. e.g. the RF of a ‘red’ cone
. if you were to fixate at the red dot with the fovea
. the red light that is reflected from the red dot passes through optical apparatus of the eye and then cross the retinal layers
. the red light will be absorbed by visual pigment
. the red dot stimulus is what the red cone interested in
. if the red dot became blue and blue light reflected by the retina - the red cone will not respond to blue light

26
Q

why would the red cone not responds to a blue dot?

A

because the stimulus is not the one that red cone likes- even if location of blue light is the right one

27
Q

what does a red cone respond to?

A

the red cone only responds to a red dot that is in its receptive field

28
Q

what happens when red cones supply excitatory input to a bipolar cell directly below them in the retina?

A

. the bipolar and ganglion cells will respond to red in the same region of space

29
Q

what is the region in space the bipolar cell is interested in determined by?

A

the region in space the bipolar cell is interested in , is determined by where in space the outer segments of the three red cones supplying input to bipolar cell are looking

. receptive field for bipolar cell will be for red stimuli and in the same place , the three cones are looking at

30
Q

what is single cell electrophysiology?

A

. another method for studying the visual system
. record the electrical activity of single neurons at different levels of the system in animals with similar visual functions to our own and determine the specific type of stimulus that the neuron is excited by

31
Q

what is the concept behind single cell electrophysiology?

A

. visual neurons are only excited by specific features of the visual image in their RF

32
Q

what is do receptive field increase in size with?

A

. receptive fields increase in size with eccentricity

  • smaller in central vision than more peripherally
  • related to visual resolution and acuity
33
Q

what else do receptive field increase in size with?

A

. increase in size at increasingly high levels of the visual system

. e.g. cones have tiny RFs, but those of bipolar cells are larger, and those of ganglion cells are larger still

. with single cells in the highest visual cortical areas having RFs that cover more than 20deg of visual space

. due to convergent inputs and spatial summation at each successive (serial) step in the visual pathway

34
Q

what else do receptive fields increase in size with?

A

. receptive field increase in complexity at increasingly high levels of the visual system

. e.g. cone responses depend on the intensity of the light in their RF, but bipolar and ganglion cell responses depend on luminance-contrast in different parts of their RF

. with most single cells in the primary V1 cortex , having RFs that respond to lines/edges ( e.g. contours) and those in the highest visual cortical areas having RFs that respond only to real object

35
Q

what is the problem with photoreceptors?

A

. 120 million rods and 6 million cones per retina

  • whose outer segments form an array / mosaic over the retinal surface
  • as a consequence each of the photoreceptor outer segment samples a tiny part of the visual image - this breaks down the visual image into 72 mega pixels
36
Q

what is the problem for the rest of the visual system?

A

is to re-build the visual image to create meaningful representations for perceiving real-world objects

37
Q

how does the visual system re-build visual images to create meaningful representation for perceiving real world objects?

A

. RFs increase in size at each visual processing stage, so that cells analyze progressively larger regions of the image and RFs increase in complexity so that what they analyse gradually resembles real object perception

38
Q

what are receptive field of retinal ganglion cells interested in?

A

. the receptive fields of retinal ganglion cells have centre-surround organisations which are interested in either light in centre and dark in surround and vice versa

39
Q

what are neurons in the primary cortex interested in?

A

. in the primary cortex , neurons are interested in lines and edges

40
Q

what is interested in geometric and abstract features?

A

. extrastriate areas beyond V1 , contain neurons that interested in simple geometric and abstract features

41
Q

what is interested in real-world object?

A

. in the highest level in extrastriate cortical areas furthest from V1, there are neurons interested in real-world object categories

42
Q

what is the concept behind parallel image processing?

A

. different aspects of the visual image are processed by different anatomically and functionally specialized neurons at all levels of the visual system

43
Q

what is important about rods?

A

. outer segment of rod cells contains photosensitive chemicals and high concentration of rhodopsin which can capture and respond to 1 photon

. rods, high light sensitivity = scotopic vision ( night )

44
Q

what is important about cones?

A

. contain lower concentration of rhodopsin in their outer segment and therefore have lower light sensitivity
. responsible for photopic vision ( day + colour vision)

45
Q

what are the three classes of ganglion cells in retina?

A

. midget ganglion cells
. parasol ganglion cells
. small bi-stratified cells
which are responsible for processing different aspects of the retinal image

46
Q

what are the three types of ganglion cell in the retina?

A

. midget ganglion cells
. parasol ganglion cells
. small bi-stratified cells

.which are involved in processing different aspects of the retinal image and have axons that connect to different neurons with LGN
. this parallel processing is continued here and also involves different cortical cells

47
Q

what is midget ganglion cells?

A

. small cells
. concerned with high acuity and red/green colour vision
. they make connections with parvo cellular layer of LGN

48
Q

what is parasol ganglion cells?

A

. dendritic cell
. concerned with low acuity vision and motion vision
. have axons that travel down the optic pathways and end up in two different layers of the LGN which are called mango cellular layers

49
Q

what is small bi-stratified cells?

A

. concerned with blue/yellow colour vision

. they connect to cells in konio cellular layer of the LGN

50
Q

where does parallel image processing begin?

A

. parallel image processing begins in the three ganglion cells of the retina

51
Q

what happens to parallel processing in higher areas of the visual cortex?

A

. parallel processing continues to higher areas of the visual cortex
. primary ( V1, striate ) cortex subcontracts the labour of processing specific stimulus attributes to higher functionally specialized ‘extra-striate’ cortical areas, via parallel pathways emanating from specialized cell populations in area V1

52
Q

what is LO?

A

. neurons in V1 interested in lines and edges send information to extra striate area known as LO
. within LO object perception first appears

53
Q

what is area V4?

A

. neurons in the primary visual cortex that get input from red/green colour processing cells of the LGN and yellow/blue they send information to area V4
. area V4 is concerned with colour perception
. area V4 is also in the occipital lobe

54
Q

what is area V5/MT?

A

. neurons that get magno cellular input which is associated with motion
. theses neurons send information to area known as V5/MT which involved with motor perception

55
Q

what is area V3/7?

A

. neurons that are interested in retinal disparity send information to area V3/7 which process information related to 3 dimensions
. specialised for depth perception

56
Q

what does damage to V1 cause ?

A

. NB: damage to V1 causes ‘ anopsia’ = loss of all vision

57
Q

what does isolated damage to functionally specialized higher cortical areas beyond V1 cause ?

A

causes selective vision loss

  • colour perception: area V4, lingual Gyrus
    . local damage cause a -chromat-opsia
  • form/object perception: lateral occipital (LO) area
    . local damage causes from agnosia
  • faces: fusiform gyrus
    . local damage causes prosop-agnosia
58
Q

where do photoreceptors send information?

A

send information to bipolar cells of the retina

59
Q

what is visual sensitivity?

A

sensitivity to light

rods have high sensitivity

60
Q

what is visual acuity?

A

spatial resolution

cones highly sensitive to spatial resolution

OV2023 visual perception (20 decks)

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