The Primary Visual Cortex




(1)
University of Sydney, Sydney, Australia

 




Overview






  • The primary visual cortex (V1) receives visual information from segregated magnocellular, parvocellular, and koniocellular channels of the lateral geniculate nucleus (LGN) [1, 2].


  • Separation of these channels is largely preserved in V1 processing [3].


  • V1 also receives modulatory input from several non-LGN cortical and subcortical areas.


  • V1 codes image features including size, orientation, motion, and depth (Table 14.1) [3–6].


    Table 14.1
    Visual information coded by magnocellular, parvocellular, and koniocellular channels [3–6, 8, 9]

























    Visual component
    Cell channel

    Form (edge and corners)
    Parvocellular

    Motion and dynamic form
    Magnocellular

    Color
    Parvocellular, koniocellular

    Depth
    Magnocellular

    Texture
    Magnocellular


  • V1 sends these basic image descriptors to extrastriate visual association areas 2, 3, 4, and 5 (V2, V3, V4, and V5) for higher visual analysis of specific stimulus attributes (see Chap. 15, The Extrastriate Cortex) [7].


Structure of V1




1.

Anatomy (Fig. 14.1)

A347009_1_En_14_Fig1_HTML.gif


Fig. 14.1
(a) Location of V1 within the occipital lobe, with (b) visuotopic projection of the hemifield




  • V1 is located within the occipital lobe of the cerebrum [10].


  • It extends along the medial wall from the posterior pole on either side of the calcarine sulcus.


  • Like all cerebral cortex, it contains six principal layers.


  • V1 is called the striate cortex due to a heavily myelinated stripe in layer 4, the stria of Gennari [11].


  • Layer 4 is heavy myelinated due to high density of LGN projections.

 

2.

Visuotopic organization



  • V1 in each hemisphere represents the contralateral visual hemifield [12].


  • Each side receives uncrossed ipsilateral and crossed contralateral fibers from the LGN.


  • In V1 the peripheral field is represented anteriorly and the central field posteriorly.


  • Like the LGN, the central field is magnified compared to the periphery (Fig. 14.1) [13].

 

3.

Layers of V1 (Fig. 14.2)

A347009_1_En_14_Fig2_HTML.gif


Fig. 14.2
V1 layers and connections




  • V1 has six main layers (layer 1 superficial, layer 6 deep) [14–17].


  • These contain two main neuronal cell types: pyramidal and stellate [18, 19].


  • An overview of the connections of the layers is outlined in Table 14.2 and Fig. 14.2.


    Table 14.2
    V1 layers, input and output [6, 14–17, 20–22]
































    Layer
    Input
    Output

    1
    Modulatory subcortical input

    Koniocellular LGN input
    Other V1 layers

    2 & 3a

    Projections from layer 4
    Extrastriate areas

    4
    Most LGN inputs

    Layers 3, 5, and 6

    5
    Projections from layer 4
    Subcortical areas (thalamus, pons, and midbrain)

    6
    Projections from layer 4
    Subcortical areas (thalamus, pons, and midbrain)


    aLayers 2 and 3 are functionally similar and often grouped together

 

4.

Cytochrome oxidase blobs



  • Layers 2 and 3 contain areas that stain for the mitochondrial enzyme cytochrome oxidase (CO), known as the “CO blobs.”


  • CO blob neurons have color-opponent receptive fields and lack orientation selectivity [23].


  • CO blobs have been postulated to be important in color processing [4]; however, no evidence suggests that blob neurons are more color sensitive than other V1 neurons (see Chap. 24, Color Vision).

 


Connections of V1 (Fig. 14.2)




1

Inputs to V1

(i)

Lateral geniculate nucleus inputs



  • LGN inputs, predominantly into sublayer 4C, determine the activation of V1 neurons.


  • The inputs are segregated according to:

    (a)

    Eye (left or right)

     

    (b)

    LGN cell type (magnocellular (M), parvocellular (P), or koniocellular (K)) [24–26]

     


  • This segregation is maintained at the first synapse in V1.


  • M cells terminate in layer 4Cα which projects to sublayer 4B; P cells terminate in layer 4Cβ which projects to sublayer 4A [1, 2, 5, 25, 27].


  • Sublayer 4A receives collateral input from P and K cells and projections from cells in 4B.


  • Sublayer 4A may have a role in synthesizing information from separate M, P, and K streams [28].


  • K cells predominantly terminate in the CO blobs in layer 3 and in layer 1 [6].


  • Neurons in layer 4 send off connecting axons principally to layer 3 and also to layers 5 and 6.

 

(ii)

Non-lateral geniculate nucleus inputs



  • V1 receives modulatory inputs from cortical and subcortical areas.


  • These regulate the signals sent from V1 to higher-order areas for further processing.


  • Subcortical inputs include those from the thalamic intralaminar and pulvinar nuclei (which synapse in V1 layer 1), the amygdala, and the basal forebrain nuclei [29–32].


  • Cortical inputs are received from the claustrum and V2–V5 [24, 33, 34].

 

 

2

Output pathways for V1

(i)

Output from layer 3



  • Layer 3 provides output to a number of extrastriate visual cortical areas (V2, V3, V4, and V5) [21].


  • The major output is to V2 [ 35] (see Chap. 15, The Extrastriate Cortex).


  • Cells within the CO blobs in layer 3 send axons to CO thin stripes in V2, while cells outside the CO blobs send axons to the thick and pale stripes of V2 [6, 35].

 

(ii)

Output from layer 5



  • Layer 5 provides a major input to the thalamic pulvinar nucleus [29].


  • This provides input to V1 and extrastriate areas regarding visual attention.


  • It also projects to the superior colliculus, pretectal area, and pontine nuclei that control eye movement [22, 36].

 

(iii)

Output from layer 6



  • Layer 6 provides direct feedback to the LGN and thalamic reticular nucleus [37, 38].


  • This allows V1 to regulate LGN input (see Chap. 13, The Lateral Geniculate Nucleus).

 

 


Binocularity and Ocular Dominance Columns




1.

Ocular dominance columns



  • Input arriving from the LGN into layer 4C remains segregated according to the right or left eye [39].


  • Subsequent connections to binocular cells in layers 3, 5, and 6 combine input from both eyes [40].


  • Binocular neurons tend to display a preference for one eye and are organized into ocular dominance columns (ODCs) (Fig. 14.3) [39, 41].

    A347009_1_En_14_Fig3_HTML.gif


    Fig. 14.3
    Ocular dominance columns


  • The ODC extends from layers 1 to 6 reflecting the laterality of layer 4C cells within that column [42].


  • ODCs are organized into adjacent, alternating bands of V1.


  • In V1 each point in visual space is represented by two ODCs, one for each eye.

 

2.

Development of ocular dominance columns



  • Cortical visual development occurs after birth in response to visual stimuli.


  • Equal binocular input is required for normal development of the ODCs [43].

 

3.

Visual deprivation



  • Development of ODCs can be profoundly affected by visual deprivation [43].


  • Monocular visual deprivation causes functional connections from the normal eye to be retained and nonfunctional connections from the deprived eye to decay [44].


  • This can result in a reduction of binocularly driven cells (Fig. 14.4).

    A347009_1_En_14_Fig4_HTML.gif


    Fig. 14.4
    (a–c) Relative binocularity of V1 neurons found in ocular dominance columns (Based on data from Chino et al. [45], Sakai et al. [46], Wensveen et al. [47], and Smith et al. [48])

 


Receptive Field Properties of V1 Cells






  • V1 cells transform the visual signal from LGN cells.


  • They code image features including orientation, motion, and depth [24].


1.

Orientation sensitivity

(i)

Simple cells



  • Simple cells have elongated center-surround receptive fields aligned at a particular orientation [49].


  • These receptive fields are formed by the summation of overlapping LGN cell fields (Fig. 14.5).

    A347009_1_En_14_Fig5_HTML.gif


    Fig. 14.5
    Simple cell receptive fields. (a) Summation of LGN receptive fields; (b) Stimulus responses


  • Simple cells respond to a line of light at a specific orientation.


  • Their receptive fields vary according to orientation and length [50].

 

(ii)

Complex cells (Fig. 14.6a)

A347009_1_En_14_Fig6_HTML.gif


Fig. 14.6
(a) Complex cell receptive field; (b) End-stopped cell receptive field




  • Complex cells receive input from several simple cells with the same orientation [51].


  • They respond to line stimuli of specific orientation anywhere within a larger receptive field [52–54].

 

(iii)

End-stopped cells (Fig. 14.6b)



  • These respond only if a correctly oriented stimulus is of appropriate length.


  • If the stimulus extends beyond the receptive field, the response is diminished, but not if the extending part of the stimulus has a different orientation to the receptive field.


  • Through this mechanism, end-stopped cells can detect curvature, direction, and contrast [55–57].

 

 

2.

Motion sensitivity

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Oct 28, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on The Primary Visual Cortex

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