Sensory & Motor Systems 4/13/2012
Chapter 10- The Central Visual System
Anatomy of the striate Cortex
The Primary visual field (also known as striate cortex or V1):
- Is Brodmann’s area 17
- Is located in the occipital lobe of the primate brain
- Much of area 17 lies on the medial surface of the hemisphere, surrounding the calcarine fissure
Retinotopy: is an organization whereby neighboring cells in the retina feed information to neighboring
places in their target structures- in this case, the LGN and striate cortex.
- In this way the two-dimensional surface of the retina is mapped onto the two-dimensional surface
of the subsequent structures.
- The mapping of the visual field onto a retinotopically organized structure is often distorted,
because visual space is not sampled uniformly by the cells in the retina.
*** Recall: there are many more ganglion cells with receptive fields in or near the fovea than in the
periphery. Thus, the representation of the visual field is distorted in striate cortex: the central few degrees
of the visual field are overrepresented or magnified, in the retinotopic map.
- A discrete point of light can activate many cells in the retina, and often many more cells in the
target structure, due to the overlap of receptive fields. The image of a point of light on the retina
actually activates on a large population of cortical neurons; every neuron that contains that point
in its receptive fields is potentially activated.
o Thus, when the retina is stimulated by a point of light, the activity in striate cortex is a
broad distribution with a peak at he corresponding retinotopic location.
Lamination of the Striate Cortex
The cells of different Layers:
- Spiny Stellate cells: are small neurons with spine-covered dendrites that radiate out from the cell
o They are seen primarily in the two tiers of layer IVC
o Axons make local connections only within the cortex
- Pyramidal cells: are also covered with spines and are characterized by a single thick apical
dendrite that branches as it ascends toward the pia mater and by multiple basal dendrites that
o These are found in the outer layer IVC
o Only pyramidal cells send axons out of the striate cortex to form connections with other
parts of the brain.
- Inhibitory neurons: lack spines and are sprinkled in all cortical layers
o These form only local connections.
Inputs and Outputs of the Striate CotexA A Layer
Patterns of intracortical connections.
(a) Rada corrections (b) Honzontal
Binocular input Monoculatinput FIGURE 10.
The mixing of information from the
two eyes. Axons project inom layer MC
more supericial layers Most layer ll neurons
receire binocular input from both left and
Layo fight ees
LeReye Right eye
Other Pons and
Patterns of outputs from the stri
A A Layer LayorIVC Layer Layer ll Layer IVC Layer AGUREI0. Patterns of intracortical connections. (a) Rada corrections (b) Honzontal corrections. Binocular input Monoculatinput FIGURE 10. The mixing of information from the two eyes. Axons project inom layer MC more supericial layers Most layer ll neurons A receire binocular input from both left and Layo fight ees IVA IVB Layer MC Layer VI LeReye Right eye input input White ma10r FIGURE IOI8 Other Pons and LGN Patterns of outputs from the stri cortical superior cortex, areas colliculusSensory & Motor Systems 4/13/2012
Chapter 10- The Central Visual System
In the LGN, every layer receives retinal afferents and sends effecters to the visual cortex. In the visual
cortex, only a subset of the layers receives input from the LGN or sends output to a different cortical or
- Axons from the LGN terminate in several different cortical layers, with the largest number going
to layer IVC.
- Magnocellular LGN neurons project to layer IVCx and parvocellular LGN neurons project to
o Therefore the layer IVC contains two overlapping retinotopic maps, one from the
magnocellular LGN and the other from the parvocellular LGN
- Koniocellular LGN axons follow a different path, bypassing layer IV to make synapse in layers II
Innnervation of other Cortical Layers from Layer IVC:
Most intracortical connections extend perpendicular to the cortical surface
along radial lines that run across the layers from white matter to layer I.
- This pattern of radial connections maintains the retinotopic organization
established in layer IV.
o Therefore, a cell in layer VI, for example, receives information
from the same part of the retina as does a cell above it in layer
- However, the axons of some layer III pyramidal cells extend collateral
branches that make horizontal connections within layer III.
- Layer IVC Stellate cells project axons radially up mainly to layers IVB
and III where, for the first time, information from the left eye and right
eye begins to mix.
Whereas all layer IVC neurons receive only monocular input, most neurons in
layers II and III receive binocular input coming from both eyes.
- Layer IVCx which receives magnocellular LGN input, project mainly to
- Layer IVCb which receives parvocellular LGN input projects mainly to layer III.
- In layers III and IVB, an
axon may form synapses
with the dendrites of
pyramidal cells of all layers.
Striate Cortex Outputs
- Layer II, III and IVB
pyramidal cells send their
axons to other cortical areas
- Layer V pyramidal cells
send axons all the way
down to the superior
colliculus and pons
- Layer VI pyramidal cells
give rise to the massive
axonal projection back to the LGNScreen
in striate cortex
receptive field The visual stimulus is a
bars of various orientations (eft) elicit
(right) The optimal
Screen Light stimulus Border of receptive field Microelectrode in striate cortex recording action potentials receptive field The visual stimulus is a bars of various orientations (eft) elicit (right) The optimalSensory & Motor Systems 4/13/2012
Chapter 10- The Central Visual System
Physiology of the Striate Cortex:
- Receptive fields of neurons in layer IVC are similar to magnocellular and parvocellular LGN
neurons providing their input.
o This mean they are generally small monocular center-surround receptive fields
- In layer IVCx the neurons are insensitive to the wavelength of light, whereas in layer IVCb the
neurons exhibit center-surround color opponency.
- Each neuron in layers IVCx and IVCb receives afferents from a layer of the LGN representing
either eye. Monocular neurons from either eye are also clumped together in V1 rather than
- The axons leaving layer IVC diverge and innervate more superficial cortical layers.
o As a consequence of divergence, there is a mixing of inputs from the two eyes. We say
that such neurons have binocular receptive fields, meaning that they actually have two
receptive fields, one in the ipsilateral and one in the contralateral eye.
o Retinotopy is preserved because the two receptive fields of a binocular neuron are
precisely placed on the retinas such that they are looking at the same point in space.
- Most of the receptive fields in the retina, LGN,
and layer IVC are circular and give their
greatest response to a spot of light matched in
size to the receptive field center.
- Outside layer IVC, we encounter cells that no
longer follow this pattern.
Hubel & Wiesel found that many neurons in V1
respond best to an elongated bar of light moving across
their receptive fields. But the orientation of the bar is
- The greatest response is given to a bar with a
particular orientation; perpendicular bars
generally elicit much weaker responses (This is
known as orientation selectivity.)
- The orientation selectivity of nearby neurons relates to orientation selectivity of a specific neuron
(this is known as orientation column).
o As an electrode passes tangentially (parallel to the surface) through the cortex in a single
layer, the preferred orientation progressively shifts.
o If an elec