Extrastriate Cortex - Parallel Processing in Vision

Question 1

What are the theories of parallel processing - V1?

Answer 1

• 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)

Question 2

What are the theories of parallel processing - V2?

Answer 2

• 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)

Question 3

What are the theories of parallel processing - V3?

Answer 3

• 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

Question 4

Describe the V4 complex?

Answer 4

• 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

Question 5

What are the theories of parallel processing - V4?

Answer 5

• 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’

Question 6

What are the theories of parallel processing - Inferotemporal (IT)?

Answer 6

• 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

Question 7

What are the theories of parallel processing - V5/Middle Temporal Area (MT)?

Answer 7

• 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)

Question 8

What are the theories of parallel processing - V5?

Answer 8

• 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

Question 9

What are the theories of parallel processing - MT?

Answer 9

• 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

Question 10

Describe the receptive fields in MT area & V1?

Answer 10

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

Question 11

Describe the aperture problem?

Answer 11

• 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

Question 12

Describe the categories of motion?

Answer 12

• 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

Question 13

Describe the phi phenomenom & beta movement?

Answer 13

• 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

Question 14

What is the motion parallax?

Answer 14

• 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

Question 15

Describe integrating motion signals and the cells needed?

Answer 15

• 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

Question 16

Describe the medial superior temporal area (MST)?

Answer 16

• 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

Question 17

How do neurons in the Superior Temporal Sulcus (STS) respond and to what?

Answer 17

Neurons in area STS respond selectively to biological motion

Question 18

Describe cortical interconnectivity?

Answer 18

• 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)

Question 19

Describe the ventral stream?

Answer 19

• 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).

Question 20

Describe the ventral pathway?

Answer 20

• 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

Question 21

Describe the dorsal stream?

Answer 21

• 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

Question 22

Describe the dorsal pathway?

Answer 22

• 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)

Question 23

Describe the receptive fields of cells in the ventral and dorsal pathways?

Answer 23

-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.

Question 24

Describe the central principles of visual cortical processing?

Answer 24

• 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)

Question 25

What are the complexities of perception?

Answer 25

* 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

Question 26

Describe subcortical pereception?

Answer 26

* 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