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OV2023 visual perception > Extra-striate cortex: functional organisation > Flashcards

Extra-striate cortex: functional organisation Flashcards

1
Q

What is area V1/ striate cortex comprised of ?

A

several thousands of nodules /subunits known as hyper columns that are equipped with neural machinery to encode all all possible stimulus colours, forms (orientations) & directions of motion which are encoded for each point in space via both the left & right eyes

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2
Q

Is the image processing mediated by the cells in V1 cortex sufficient to identify this object as an apple?

A

No.
- The receptive fields of cells in the hypercolumns of this ‘early’ cortical area only analyze each small region of the object.
like individual pieces (pixels) of a jigsaw puzzle
- EXAMPLE hypercolumns in V1 looking at a particular region of image - record the colour of that particular colour in that region of space- E.G the leaf of the apple (which is green) then of the actual apple(red- the cells in these hyper columns reports the colour and contour(orientation) of the apple ))

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3
Q

What do the cells in hyper column do ?

A

only analysing small components of the image e.g telling what colours or orientations are present in the image.

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4
Q

What requires further processing to see the whole image?

A

cells with much larger & more ‘complex’ receptive cells that are able to ‘see the big picture’. which is found in Extrastriate Cortical Areas

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5
Q

Why do we need these bigger more complex cells in the extrastriate corticol areas?

A

this is to perceive the whole of the image .

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6
Q

What is the Traditional View of Human Visual Cortex?

A

100 years ago, the Human Visual Cortex was thought to be restricted to the Occipital Lobe, with the Primary Visual Area (V1, Striate
Cortex, area 17) responsible for mediating all Visual Sensations, since

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7
Q

What was damage to V1 known as ?

A

to result in anopia (blindness

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8
Q

What was Striate Cortex was assumed to do?

A

send information (via direct anatomical
connections) to surrounding ‘Visual Association Areas’ in Occipital
Cortex (i.e., Brodmann areas 18 & 19), where a Unified Percept was
constructed from the form, colour and motion sensations related to
an object & by ‘associating’ these with memories of the same or
similar objects retrieved from past visual experience

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9
Q

What else was the traditional view based on of the human visual cortex ?

A

based on functional localisations - brodman areas

-with each cytoarchitectural (anatomical) Brodmann area has a specific function

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10
Q

What were the different brodman areas of the traditional views based on the human visual cortex?

A

– like area 4 being the Primary Motor cortex - responsible for initiating voluntary movements on the opposite side of the body
– with area 17 being the Primary Visual cortex- where visual sensations percieved
- and areas 18 (surrounds area 17) & 19( surrounds area 18) involved in ‘higher’ visual functions associated with memory-based perceptions.

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11
Q

What is the difference now with he modern view of the human visual cortex ?

A

contains an excess of 30 separate visual areas

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12
Q

What is the modern view of the human visual cortex ?

A

-we now know that V1 is just one – albeit the largest – of several (8-10) other
discrete Visual Areas in the Occipital lobe BELOW
- with another area V2 adjoining V1 on all sides and found in area 18
-& area V3 running parallel to V2, plus others located more dorsally (V3A, V5/MT,
V7) or ventrally (V4, V8, LOC),

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13
Q

What do each of these areas contain ?

A

each of which contains its own retinotopic map of the opposite visual hemi-field.

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14
Q

What is involved in vision in the modern view of human visual cortex?

A
  • the entire Posterior Parietal (PP) Cortex (dorsally: BA 7,5,
    39 & 40) and Inferior Occipito-Temporal (IOT) plus adjoining Inferior Temporal (IT) Cortex (ventrally: BA 37, 20 & 21)

-with these regions each containing at least 10 additional Visual Areas.

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15
Q

What is visual information processing confined to (as the traditional view says) ?

A

the primary area 17 - in occipital lobe
and brodman area 18 and 19
-also extends dorsally up into posterior parietal cortex into brodman areas 5-7 on ,edoal surface of cortex and see it laterally too
-also brodman 39-40 -being dorsal to upper area 17
-also down in the ventral region- inferior temporal cortex-areas 39,21,,20

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16
Q

how much of the Human Cerebral Cortex is specialized for different aspects of visual function ?

A

33%

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17
Q

Why are there so many separate visual areas outside of the striate cortex and extra striate tissues ?

A

the theories are from: Mishkin & Ungerleider/Goodae & Milner)

  • there might be 2 major processing streams that emerge from the primary visual area
  • one of which goes dorsally to PP CORTEX and os mainly based on magnocellular processing
  • and the other which goes down from the primary visual area into the IT (inferior temporal cortex) mainly a continuation of the parvocellular
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18
Q

What is the thinking behind ventral versus Dorsal processing streams ?

A
  • Ventral- V1 sends mainly PARVO information ventrally to areas in Inferior Occipital (V4, V8, LOC) & IT cortex which are Functionally Specialised for Perception = specifically what objects are (based upon their form and colour )
  • Dorsal-whereas V1 also send parallel, mainly MAGNO information dorsally to areas Parietal-Occipital (V5/MT, V3A, V7) & PP Cortex which are Functionally Specialised Where objects are, their direction of Motion & How to interact with them (how to look at the object) for visually-guided action control.
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19
Q

What is the ventral stream?

A

it is involved in what objects are - underlying vp- leading to IT

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20
Q

What is the dorsal stream?

A

leading to PP cortex- concerned on where objects are and how to interact with them.

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21
Q

What is the posterior parietal (PP cortex) based on ?

A

Spatial, Motion & Action Vision - dorsal

22
Q

What is the Inferior temporal (IT cortex based on ?

A

Object Perception, based on form & colour - ventral

23
Q

What are the ventral areas which lead down to IT to tell us what objects are for VP?

A

areas that are in this visual process pathway
-Area V4 (Lingual Gyrus, inferior-medial occipital cortex): functionaly specialised for COLOURS perception
the neurons are selectively activated by coloured stimuli + localized damage results in a selective deficit in colour perception (Achromatopsia)

-Lateral Occipital Cortical (LOC) Area (posterior inferior temporal gyrus): OBJECT FORM
Selectively activated by shapes & objects + localized damage results in a selective deficit in object perception (condition known as Form Agnosia)

-Fusiform Face Area (FFA) Area (posterior fusiform gyrus): FACES
Selectively activated only by faces + localized damage results in a selective deficit in face perception (condition is - Prosopagnosia)

24
Q

Do the px with problems in these areas know they do?

A

px who have localised damage are still conscious of these visual processes

25
Q

What is the major dissociation between ventral versus dorsal damage?

A

px with ventral deficits can still localize objects in space &
interact with them without being able to say what they are.
-this is because there damage is in the ventral stream but their dorsal stream is intact which is telling them where the object is and how to interact with it but ventral stream is loss.

26
Q

Examples of object processing (Evident from fMRI.

A

the are LOC (lateral occipital cortex) is selectively activated by images of real Vs. scrambled objects

  • the activity of the entire cortex is analysed when images are scrambled and the subtraction has been done to show the particular area of the cortex that was activated by the real object than the scrambled object.
  • shows real objects vs to scrambled components of that same image respond to the LOC
27
Q

Why no selective activation of V1 to these real objects?

A

Because cells in the Primary Visual Cortex respond equally well to real objects as to fragmented images containing lots of lines & edges.

28
Q

What is the 2 categories of visual object agnosia?

A

Lissaeuer (1890) called these Seelenblindheit or ‘soul-blindness’

  1. General Form Agnosia (results from damage to LOC)
    where conscious visual sensation of the components of objects is preserved (because area V1 is intact)
    But Px unable to recognize whole objects that they see or understand their meaning
    Although memory of what they look like & their purposes are preserved
  2. Category-Specific Agnosia (damage to FG-fugalgyrus)
    Is restricted to particular classes of object
    Mostly commonly faces (=prosopagnosia), but can be just animals or buildings etc.
29
Q

What is a classic case of form agnosia ?

A

EXAMPLE:
Px : HAS bilateral LOC lesions - due to CO poisoning - L and R eye

what remains Intact is her: Visual Fields, VA good, SA (depth perception good), sees colours, motion.
- Cant identify/understadn what different objects are

  • test - asked to draw a line to match the shape - cannot match the simple shapes or simple objects and doesnt understand what they form ? - e.g dont know what is a toothbrush doesnt understand what it is and what to do with it ?
  • Can’t copy-draw familiar objects – only sees bits of them – although can reproduce them from memory)

BUT: She can also accurately reach out and pick up a real apple: she knows WHERE there are, but doesn’t know WHAT they are!- this is because no damage in the dorsal stream- but damage in the LOC in the ventral stream

30
Q

How does a monkey exhibits perpcepual constancy for a particular face?

A

has inferior temporal cortex cells that are selective repsonsive to different human faces
-this cell - different views of same man- this cells fires regardless of the viewing angle or exprression- can recognise this face regardless of these things.

31
Q

What is Prosopagnosia?

A

damage to Fusiform Face Area

  • Px cannot recognize faces, including their own
  • But can identify familiar people from other visual cues (e.g., hair colour, spectacles) & from their voice
  • Can also interpret emotions in facial expressions, even though unable to identify to whom the face belongs, as this involves the amygdala
32
Q

What does the sulci do?

A

divide the temporal lobe into superior, middle and inferior gyri

33
Q

What are some of the areas in the posterior parietal cortex ?

A
  • central sulcus

- 90 degrees to central sulcus= intra-parietal sulcus

34
Q

What does the intra-parietal sulcus do?

A

separates regions of the parietal cortex below it which is known as the inferior pariteal lobule from superior partial lobule.

35
Q

What does the inferior parietal lobule do?

A

contain separate visual areas associated with initiation of different types of eye movements

36
Q

What does the posterior parietal lobule do?

A

-contains neurons which are responsible for Hand and arm movements

37
Q

What are the dorsal areas which lead down to PP ?

A

-Area V5/MT (junction of inferior occipital & superior temporal sulci): MOTION
Selectively activated by moving stimuli + localized damage results in a selective deficit in motion perception (condition called Akinetopsia)

-Inferior Parietal Lobule (in & below Intra-Parietal Sulcus): initiating EYE movements
Selectively activated during particular eye movements + localized damage disrupts these (e.g., Gaze palsies)

-Superior Parietal Lobule (in & above Intra-Parietal Sulcus): guiding HAND movements
Different regions selectively activated during reaching & grasping actions + localized damage results in a selective hand movement deficit (Optic Ataxia)

but c

38
Q

Are px with Akinetopsia aware of this condition that they may have ?

A

yes

39
Q

Are px with Inferior and superior Parietal Lobule aware of this condition that they may have ?

A

– NO

40
Q

What is the dissociation of px with damage to dorsal areas?

A

they can still perceive what objects are

41
Q

What is the area V5/MT?

A

it is middle temporal

-its required for all motion processiing- all purpose motion processor

42
Q

What are the more specialised complex motion processors ?

A
  • The Medial Superior Temporal (MST) areas

- Rotation + 3D motion-in-depth (moving objects away and to you) + optic flow-

43
Q

What is interesting about V5/MT and MST ?

A

more representation of visual representation than AREA V1

44
Q

What is the reason of over representation of central vision in these motion processing areas isn’t as marked as V1?

A

due to motion in visual periphery being quite important

45
Q

MRI experiment of area V5/MT being selectively activated by moving compared to stationary patterns

A

shown on screen a series of random dots moving in different directions

  • next scan same panel with all random dots but they were stationary
  • they then subtracted patterns of brain activity from the stationary condition from motion condition to reveal parts of the brain that were selectively activated from the moving random dots- corresponding to area V5/MT in l and R hemisphere.
46
Q

Why is area V1 not selectively activated by stimuli?

A

there are loads of neurons in area V1 that are found in mainly layer 4b- but there are loads of neurons in layers above and below layer 4b that like stationary objects too .
So. V1 was equally well activated by the moving compared to the stationary stimuli so when the subtraction was done it was cancelled out and there was no selective activation of the primary visual area V1.

47
Q

What is the classic case of px with area v5/mt damage ?

A

case of Akinetopsia-
-Bilateral area V5/MT & MST lesions (venous sinus occlusion)
-Intact: Visual Fields, VA, colours, objects
+ navigation through the environment & catching slow-moving objects

-But PX reported that visual motion made her feel ‘tired & unwell’ with
Pouring tea; ‘frozen brown glacier’- cant pour tea
Crossing the street; ‘vehicle approach from their sound’- feel cars near her but cant see the cars
Crowded room; ‘people suddenly appearing’- can hear people but cant see them
Can’t see simple or complex objects moving faster than 10-15 deg/s
No smooth pursuit/tracking eye movements to targets >10 deg/s
-can see slow motions are neurones in v1 are still intact

48
Q

Evidence from fMRI: different Posterior Parietal areas are activated during reaching Vs. grasping with the right hand

A

la

49
Q

Patient with Optic Ataxia: V1 intact, but left SPL damaged (opposite right hand

A
  • The Px can correctly describe (PERCEIVE) the target orientation as vertical, but cannot control her moving hand when attempting to insert it into the slot.
  • makes two error-types; (a) incorrect hand orientation & (b) spatial mis-localization
  • her ventral stream is intact - her dorsal is damaged
  • optic- because actions are visually guided
50
Q

What does damage to LOC cause?

A

affect face perecption

  • not selective
  • cant detect any other objects
  • RESULTS IN GENERAL FORM AGNOSIA
51
Q

What does area V5/MT considered to be part of ?

A

magnocellular processing pathway

OV2023 visual perception (20 decks)

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