The Extrastriate Cortex

(1)
University of Sydney, Sydney, Australia
 

Overview (Fig. 15.1)

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Fig. 15.1
The extrastriate cortex
  • The extrastriate cortex is involved in the analysis of specific attributes of visual stimuli
  • (e.g., color, form, movement, and binocular disparity).
  • Visual information is progressively decomposed as it is channeled through processing streams.
  • Nonhuman diurnal primates such as macaque monkeys have approximately 25 cortical visual association areas [1]; humans probably have a similar number [2].
  • In all primates, primary visual cortex (area V1, striate cortex, Brodmann’s area 17) occupies ~12–18% of the neocortex. Although in all primates (including humans) each extrastriate area is substantially smaller than area V1, together, they occupy ~ 25–30% of the neocortices [3]. Thus, V1 and extrastriate cortices together occupy ~ 30–40% of primate neocortices [3].
  • Neurons in each extrastriate area have a degree of functional specificity relating to particular stimulus attributes [4].
  • Each area has a topographic visuotopic map that is more crude than that in V1 [5–7].
  • The main areas are V2, V3, V4, and the middle temporal (MT) area, also known as V5.
  • Visual inputs to extrastriate cortices originate mainly or almost exclusively in area V1.
  • V1 neurons projecting to a given extrastriate area tend to exhibit specific receptive field properties (e.g., direction selectivity) characterizing neurons in the extrastriate area to which they project [8].
  • Some extrastriate cortical areas (e.g., area MT) receive substantial direct input from the retino-recipient dorsal thalamic nuclei such as the lateral geniculate nucleus (LGN) and the retino-recipient part of the pulvinar.
  • Laminae of the LGN (K layers) and regions of pulvinar which project to the extrastriate cortices receive direct input from the superficial, retino-recipient layers of the superior colliculus (SC).
  • Following the damage to the striate cortex, both the colliculo-recipient laminae of the LGN and parts of the pulvinar which provide direct inputs to extrastriate cortices may be responsible for the phenomenon of unconscious vision called “blindsight” [9, 10].

The Ventral and Dorsal Streams (Pathways) (Fig. 15.2 and Table 15.1)

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Fig. 15.2
The dorsal and ventral streams
Table 15.1
The dorsal and ventral streams [12, 13]
Stream
Function
Input
Origin
Passes through
Destination
Dorsal
Spatial location
M and K channels
V1
V2, V3, MT
Parietal cortex
Ventral
Object recognition
P, M, and K channels
V1
V2, V4
Temporal cortex
  • Two broad extrastriate visual-processing streams exist:
    (a)
    The dorsal (“where”) pathway
     
    (b)
    The ventral (“what”) pathway [11].
     

V2 (Table 15.2 and Fig. 15.3)

Table 15.2
Broad overview of the extrastriate cortex [8, 14–27]
Stream
Extrastriate area
Function
 
V2
Thin dark stripes: color processing
Thick dark stripes: orientation selectivity and binocular disparity
Pale stripes: form processing and object recognition
Dorsal
V5/MT
V3
Parietal areas
Direction of movement, binocular disparity
Dorsal V3: direction of movement
Ventral V3: color, orientation
Visuospatial perception and movement planning
Ventral
V4
Inferotemporal areas
Color sensitivity, object recognition
Complex receptive field properties, e.g., face recognition
A347009_1_En_15_Fig3_HTML.gif
Fig. 15.3
V2 inputs and projections (based on Sincich and Horton 2005) [16]
  • V2 receives the bulk of V1 cortico-cortical projections.
  • Receptive fields of area V2 neurons are 2–3 times larger than those of V1 neurons at the corresponding positions in the visual fields [16].
  • V2 organizes visual information for output to subsequent extrastriate processing areas [16].
  • It is arranged into alternating thin and thick dark stripes [17] and pale stripes based on the intensity of staining for the mitochondrial enzyme cytochrome oxidase (CO). Neurons located in the thick and thin stripes contain much more CO than neurons located in pale stripes.
  • Distinct visuotopic maps exist for each of the three stripe types [15].
  • Each stripe type represents a parallel processing pathway for stimulus attributes.
  • This segregation is not absolute; each stripe contains cells sensitive to a variety of stimuli [18, 19, 28].
  • This functional overlap probably represents integration of visual stimuli in visual processing [28, 29].
    1.
    V2 inputs
    • Input to all stripe types is from the same V1 layers and is segregated according to two pathways [30]:
      (a)
      CO blobs to thin stripes
       
      (b)
      Interblob areas to pale and thick stripes
       
     
    2.
    V2 projections
    • The neurons in the thin and pale stripes project to V4 [31].
    • This is a ventral stream area involved in form processing, color, and object recognition [18, 19, 32].
    • The thick stripes project to V5, a dorsal stream area associated with motion processing [17, 18, 22].
    • Some thick stripe neurons project to the SC. Those projections are involved in the control of saccadic eye movement [33].
     

The Dorsal Stream

1.
V5/MT
  • V5 (MT) is concerned with direction selectivity and motion processing [14].
  • V5 is heavily myelinated; in macaques, it is located in the inferior temporal sulcus [34].
  • It receives strong input from direction-selective cells in V1 and from the cells located in thick stripes in V2 [22, 23].
  • Both sources are dominated by M cell pathways [13].
  • V5 contains neurons selective for:
    (a)
    Orientation of elongated contours
     
    (b)
    Direction of movement
     
    (c)
    Binocular disparity (important for depth and motion processing)
     
    (d)
    Wide-field motion contrast [20, 21, 35–37]
     
  • V5 has connections with frontal eye fields important in generating smooth pursuit movements [38].
  • V5 also receives inputs from the pulvinar and koniocellular LGN cells that bypass V1 [39–42].
 
2.
V3
  • V3 is a narrow area of neocortex in front of V2 [43].
  • Its precise location and function are controversial [43, 44].
  • V3 is involved in coding color, orientation, motion, and stereopsis [45, 46].
  • V3 dorsal and ventral halves represent the lower and upper visual quadrants, respectively [43, 45–47].
 
3.
Parietal lobe areas
  • The dorsal stream of visual processing terminates in the parietal lobe [48].
  • These projections are important for:
    (a)
    Constructing a spatial representation of the external world
     
    (b)
    Planning and executing movement [49].
     
  • Parietal lobe area neurons have large receptive fields that send inputs to the:
    (a)
    Frontal cortex (including frontal eye fields) which together with the deep layers of the SC plays an important role in planning eye movements [50–53]
     
    (b)
    Limbic system (cingulate cortex and parahippocampus) which plays an important role in visual memory and visually triggered emotions [27, 54].
     
 

The Ventral Stream

1.
V4
  • V4 is located between ventral V3 and MT [48].
  • The V4 dorsal and ventral areas represent the lower and upper visual quadrants, respectively [48].
    (i)
    V4 inputs
    • V4 receives direct inputs from V1, V2, and V3 [55].
    • V1 inputs are from P, M, and K channels arising from layer 3 CO blobs and interblob regions [56, 57].
    • V2 inputs arise from the thin stripes and pale stripes [19, 31].
     
    (ii)
    V4 projections
    • V4 projects to inferotemporal areas involved in detailed object form analysis [55].
     
    (iii)
    V4 receptive field properties
    Receptive fields of V4 neurons are substantially larger than those of V2 and V3 neurons at corresponding visual field locations.
    • V4 is involved in form processing crucial for object recognition [58].
    • Cells in V4 are principally concerned with color sensitivity; however, cells are also selective for orientation, size, and binocular disparity involved in form and shape perception [24, 59, 60].
    • Visual attention modulates processing in V4 [25].
     
 
2.
Inferotemporal cortex
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Oct 28, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on The Extrastriate Cortex

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