Extrastriate Cortex - Parallel Processing in Vision Flashcards
1
Q
What are the theories of parallel processing - V1?
A
- V1 “subcontracts” processing to extrastriate areas
o These areas process info relating to from, colour, motion - (Zeki) 4 main pathways working in parallel:
o Colour: parvocellular (detail) – V1 (Blobs) – V2 – V4
o Motion: Magnocellular (when moving not looking at detail) – V1(4B) – V2 – V5
o Form System 1: linked to P1 & colour (Blobs) – V2 – V4
o Form System 2: linked to M1 & motion V1(4B) – V2 – V3
P1 = parvocellular area 1, M1 = magnocellular area 1 - Parvo = ventral stream (what stream)
- Magno = dorsal stream (where stream)
2
Q
What are the theories of parallel processing - V2?
A
- Organisation of V1 is retinotopic
o In V2 there are 3 separate visual maps - Within the thick stripes there is a visual orientation map
- Within the thin stripes there is a colour map
- Within the interstripes there is a disparity map
- Therefore, 3 interleaved maps in V2 each representing different aspect of visual stimulus
o M pathway projects from layer 4B of V1 to thick stripes of V2
o P-B (colour) pathway projects from Blobs of V1 to thin stripes of V2
o P-I (orientation, high acuity perception) pathway projects from interblob regions to the interstripes
-The latter two are both parvocellular (detail)
3
Q
What are the theories of parallel processing - V3?
A
- V3 recieves projection from thick stripes of V2 but main input is directly from magnocellular system in layer 4B of V1
- Since main input to thick stripes V3 is parallel part of magnocellular system
- Third visual complex – region of cortex located immediately in front of V2, includes region named visual area V3
- Dorsal V3 in upper part of cerebral hemisphere (towards parietal lobe)
o Part of dorsal stream
o Receives inputs from V2 & V1 & projecting to posterior parietal cortex
o May be part of larger area: dorsomedial area (DM) – contains representation of entire VF neurons in area DM respond to coherent motion of large patterns covering extensive portions of VF
o Hand-eye coordination - Ventral V3 (ventral posterior area (VP)) in lower part of brain (occipital lobe – goes towards temporal lobe)
o Weaker connections from V1
o Stronger connections w/ inferior temporal cortex
o Extensive area & contains complete visual representation
o Colour-selective neurons are more common in ventral V3 - Dorsal & ventral V3 have distinct connections w/ other parts of brain – contain neurons which respond to different combinations of visual stimulus
4
Q
Describe the V4 complex?
A
- Nearly everything (from V1,2,3) converges onto V4
- Both thin & pale stripes of V2 (parvocellular) & V3 (representing magnocellular)
- Special population of interlaminar cells in LGN project into inferotemporal (IT) cortex near base of brain
5
Q
What are the theories of parallel processing - V4?
A
- V4 – visual area in extrastriate visual cortex of macaque monkey
- Located anterior to V2 & posterior to visual area posterior inferotemporal cortex (PIT)
- Comprises at least 4 regions (L&R V4d, L&R V4v) & contains rostral & caudal subdivisions
- V4 is 3rd cortical area in ventral stream receiving strong feedforward input from V2 & sending strong connections to posterior inferotemporal cortex
o Receives direct inputs from V1 – especially for central space - V4 – 1st area in ventral stream to show strong attentional modulation
o Selective attention (shows changes in spatial profile of its RFs w/ attention) can change firing rate in V4 by about 20% - V4 is tuned for orientation, spatial freq & colour (like V1)
- V4 is tuned for object features of intermediate complexity, like simple geometric shapes (unlike V1)
o V4 is not tuned for complex patterns & objects such as faces - Inferotemporal cortex (IT) cells are targets that V4 cells project to
- V4 directly involved in form recognition as earlier cortical areas – supported 2 streams hypothesis
- Global spatial coherence ‘glass patterns’
6
Q
What are the theories of parallel processing - Inferotemporal (IT)?
A
- Inferotemporal cortex (IT)
- Main target for V4 neurons
- IT has multiple subdivisions
- Essential for pattern perception
- Neurons in inferior-temporal cortex are sensitive to specific, ↑complex forms columnar arrangement (some cells react best to faces)
- RF cover a relatively large area of VF
7
Q
What are the theories of parallel processing - V5/Middle Temporal Area (MT)?
A
- Lies v deep buried within superior temporal sulcus
- Mostly receives magnocellular inputs from V3 & from thick stripes of V2 & as directly from layer 4B of V1
- Some input from thin stripes of V2 & interconnection between V4 & MT
- MT & its subdivisions project to posterior parietal cortex near top of brain
o This area is believed to process info on motion & stereoscopic depth (disparity)
8
Q
What are the theories of parallel processing - V5?
A
- Columnar architecture in MT for stimulus motion
- Neurons w/ similar motion preferences lie nearby one another – w/ an orderly progression from one motion direction to next as move through MT – analogous to orientation columns in V1
9
Q
What are the theories of parallel processing - MT?
A
- MT neurons receive inputs form direction-selective neurons in V1
- MT neurons are velocity selective, each responds best to preferred velocity (speed & direction) within its RF (independent of stimulus pattern)
o Magnocellular so only care about direction & orientation (not pattern) - Direction-selective V1 neuron confounds motion w/ pattern
o A typical V1 neuron responds to a particular orientation (edge or bar) moving in particular direction – response of V1 neuron also ↑ w/ contrast - A typical MT neuron, responds to almost any pattern w/ almost any contrast, as long as it moves w/ right velocity
- Neurons in MT are selective for motion direction
- Neural responses in MT are correlated w/ perception of motion
- Damage to MT or temporary inactivation causes deficits in visual motion perception
- Electrical stimulation in MT causes changes in visual motion perception
10
Q
Describe the receptive fields in MT area & V1?
A
MT area & V1:
* Direction selectivity (90%), velocity tuning broad range of direction-tuning bandwidths. Usually, unidirectional
* Large RFs (good for global analysis – ned to see whole area – magnocellular)
* Two populations:
o Cells like earlier V1 cells – those that do component analysis – 80%
o Pattern-selective cells (20%)
* Carry info about direction which is independent of contour orientation (outline of objects in visual scene)
* Majority of cells also orientation selective
* V strong disparity selectivity w/ binocular interaction
o Due to these broad RF characteristics, MT cells may sometimes be fooled
11
Q
Describe the aperture problem?
A
- Motion of a homogeneous contour is locally ambiguous – motion sensor has finite RF – ‘looks’ at world through something like an aperture
- Within aperture, different physical motions are indistinguishable
- The aperture problem implies motion sensitive neurons in visual cortex will always respond to a contour that crosses their RF – independently of its true length & orientation , as long as its direction is consistent w/ preferred direction of neuron
- Motion of a contour across a small area (e.g. RF) provides ambiguous info about contour’s movement
12
Q
Describe the categories of motion?
A
- 2 or more stimuli that are switched on & off in alteration can produce 2 different motion percepts
- An object perceived as moving when, in fact, a series of stationary images is being presented (apparent or beta motion)
- Phi-phenomenon – example of “pure” motion detection uncontaminated by form cues
- 1st-order motion perception is mediated by relatively simple “motion sensors” in visual system – have evolved to detect a change in luminance at one point on retina & correlate it w/ a change in luminance at a neighbouring point on retina after a short delay
o These sensors detect motion by spatio-temporal correlation & are plausible models for how visual system may detect motion
13
Q
Describe the phi phenomenom & beta movement?
A
- Optical illusion of perceiving continuous motion between separate objects viewed rapidly in succession
- Phi-phenomenon – “pure” motion perception
o Different from optimal apparent movement or beta movement – resembled real movement
14
Q
What is the motion parallax?
A
- When object closer to you tends to move at a speed much faster than object farther away
- Used to determine absolute depth perception & helps display discrepancy in motion of near objects & of objects much further away
15
Q
Describe integrating motion signals and the cells needed?
A
- Component cells:
o Respond to single gratings moving in preferred direction e.g. horizontal to right
o Do not respond to plaids/tartans moving in preferred direction: preferred component is absent
o Are found in visual areas V1 & MT - Pattern cells:
o Respond to overall direction of plaid movement e.g. horizontal to right
o Do not respond well simply to components moving in preferred direction
o Found in visual area MT only - Global motion processing
- Contrast-dependent local motion signal extraction in V1 followed by 2nd stage in MT where global mechanism is limited by signal (not contrast): noise ratio of moving dots (dots going in all directions no response from neurons
o Whole VF moving
16
Q
Describe the medial superior temporal area (MST)?
A
- Cortical area MST right next to MT
- MST neurons have v large RFs, respond selectively to complex optical flow fields: expansion, contraction, rotation
- MST involved in 3D motion perception, inferring 3D motion of objects/observer from optical flow
- Optic flow: motion of all surface elements from visual world
o As move through world, objects & surfaces within visual environment flow around you
o Human visual system can determine current direction of travel from movement of these surfaces/structures
o E.g. pilot requires MST neurons to be activated when landing plane as large field
17
Q
How do neurons in the Superior Temporal Sulcus (STS) respond and to what?
A
Neurons in area STS respond selectively to biological motion
18
Q
Describe cortical interconnectivity?
A
- Feedback pathways carry top-down info
- Processing visual info involves feedforward connections across cortical areas
- Visual cortical pathways:
o Begin in V1 – receives subcortical input from LGN
Feedforward connections extend through a ventral pathway into temporal love & through a dorsal pathway into parietal cortex & prefrontal cortex (PF)
19
Q
Describe the ventral stream?
A
- Ventral (what?) pathway:
- From striate cortex, one route proceeds ventrally into posterior temporal cortex passing through areas V2, V4 and sub-regions of inferior temporal cortex
- As lesions in posterior temporal area result in loss of pattern discrimination & lesions in inferotemporal cortex result in failure to recognise previously presented objects – pathway was assigned task of analysing physical properties of visual object (e.g. size, colour, texture, shape)
- Pathway often known as what? pathway since responses of neurons more closely reflect the attributes of the visual stimulus (rather than its location or motion direction).
20
Q
Describe the ventral pathway?
A
- Ventral pathway - V1, V4, IT progression
- V1 neurons carry info about stimulus orientation and direction
- While some V4 neurons are selective for wavelength and selective for orientation of a bar, many cells are selective for more complex stimuli (glass patterns)
- Neurons encode angles & curves, a higher-level image feature than oriented lines and edges
o IT cortex, last stage in ‘what’ pathway, cells are selective for even more complex stimuli
o There not be a last stage as do you stop processing? - Neurons here may respond to monkey face or cartoon face. The neuron will respond only when outline of face, eyes & mouth are all included. It does not respond to oriented bars, or coloured patches
21
Q
Describe the dorsal stream?
A
- Dorsal (where? how?) pathway:
- The other route proceeds from striate cortex dorsally into posterior parietal cortex via areas MT, MST and regions of posterior parietal cortex.
- Lesions in posterior parietal cortex in monkeys result in failure to be able to select a response location on the basis of a visual landmark, suggesting that this pathway figures in perception of spatial relations among objects, and not in their intrinsic qualities
- Dorsal pathway is also known as where? how? pathway since neural responses carry info about spatial location of stimuli and motion direction
22
Q
Describe the dorsal pathway?
A
- Dorsal pathway V1/MT/MST progression
- V1 neurons respond selectively to direction of motion of an oriented bar. In MT, the middle temporal area, located in superior temporal sulcus, cells respond to motion of a variety of stimuli including random dots (motion coherence stimuli)
- At a later stage along dorsal pathway in area MST (medial superior temporal), cells respond to even more complex motion patterns (optic flow; radial rotational coherence patterns)
23
Q
Describe the receptive fields of cells in the ventral and dorsal pathways?
A
-In both pathways, RF sizes of cells in successive hierarchical stages become larger. In V1, RFs are quite small at centre of fixation, <0.5 deg. They get larger as move towards periphery.
-RF sizes in V4 and MT are roughly 10-16 times the size of V1.
-In MST and IT cortex, receptive fields are even larger, anywhere from 10-40 degrees.
-The second organizing principle is that response properties are increasingly complex functions of the stimulus.
24
Q
Describe the central principles of visual cortical processing?
A
- Topographic (retinotopy)
- Functional specificity (eg orientation columns, blobs etc)
- Modularity (anatomically separate brain areas have distinct functions)
- Parallel processing (simultaneous processing of different attributes of visual information in separate modules)
- Hierarchical organisation (higher areas process increasingly abstract visual aspects eg faces)
- Bottom-up and top-down information flow (not just feed forward but plenty of feedback from higher cortical centres)
25
Q
What are the complexities of perception?
A
- Feature detectors and columnar organisation raise many questions
- Effortless coherent perception = Integration of visual scene
- We have concentrated on ‘what’ an object is
- How we determine ‘where’ an object is
- This is more complex than simply knowing where an image falls on the retina
- Way we interpret our perceptions is influenced by our memories and expectations
26
Q
Describe subcortical pereception?
A
- Subcortical structures involved in non-conscious perception of emotions modulate cortical activity either directly or indirectly
- Amygdala has direct connections to visual areas in ventral stream, to orbitofrontal & anterior cingulate cortices (involved in conscious perception of emotions), & to frontoparietal network – involved in attention.
- Pulvinar (large area in thalamus – integrates lots of info especially visual) has direct connections to visual cortex in dorsal stream, to frontoparietal network and to amygdala.
- Subcortical structures also modulate cortical activity indirectly through downstream connections to sites in the basal forebrain and brainstem that project to many areas of cortex
