Visual System III: Visual Cortical Areas
Primary visual cortex (V1 or
Brodmann’s area 17)
The primary visual cortex (V1) lies
along the calcarine fissure of the occipital lobe and receives its major input
from the lateral geniculate nucleus (LGN).
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These
connections are organized retinotopically so that adjacent areas of the
retina project up the visual pathway via neighbouring axons. However, this
retinal projection is not a simple map, as the critical factor is the
relationship of the photoreceptors to the projecting ganglion cell of the
retina. This means that the centre of vision (especially the fovea) dominates
the retinal projection to V1 because of the near one-to-one relationship of
photoreceptor to ganglion cell at the fovea in contrast to the peripheral
retina (see Chapter 24).
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The LGN
projection to V1 is mainly to layer IV and is different for the M and P
channels, while the projection from the intralaminar part of the LGN is to
layers II and III of V1 (see below).
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The LGN
input to layer IV of V1 is so large that this cortical layer is further
subdivided into IVa, IVb, IVcα and IVcβ, with each subdivision having slightly
different connections. In general, however, the cortical neurones in layer IVc
of V1 have centre surround or circular symmetrical receptive field
organization (see Chapter 24). These layer IVc neurones then project to
other adjacent neurones within the cortex, in such a way that several neurones
of this type converge onto a single neurone, whose receptive field is now more
complex in terms of the optimal activating stimulus.
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These
cells respond most effectively to a line or bar of illumination of a given
orientation and are termed simple cells. These cells in turn project in
a convergent fashion onto other neurones (complex cells), which are
predominantly found in layers II and III, and which are maximally activated by
stimuli of a given orientation moving in a particular direction. This direction
is often orthogonal to the line orientation.
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The
complex cells project to the hypercomplex or end-stopped cells,
which respond to a line of a given orientation and length. This series of cells
originally described by Hubel and Wiesel is thus organized in a hierarchical
fashion, with each cell deriving its receptive field from the cells immediately
beneath it in the hierarchy.
The Hubel and Wiesel model
Hubel and Wiesel further discovered
that these neurones were organized into columns of cells with similar
properties; the two properties that they originally studied being the eye that
provides the dominant input to that neurone (giving ocular dominance columns)
and the orientation of the line needed to activate neurones maximally (giving orientation
selective columns).
They represented these two sets of
columns as running orthogonally to each other, with the area of cortex
containing an ocular dominance column from each eye with a complete set of
orientation selective columns being termed the hypercolumn.
This hypercolumn, which is 1 mm2
in size, is capable of analysing a given section of the visual field that is
defined by the corresponding retinal inputs from both eyes. In the case of the
fovea, where there is near unity of photoreceptors to ganglion cells, this
visual field is very small, while the converse is true for more peripheral
retinal inputs. Therefore a shift of 1 mm in the cortex from one hypercolumn to
another leads to a shift in the location of the visual field being analysed,
with most of these being concerned with foveal vision (see below).
However, there are two main
complicating factors with this model:
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The
accommodation of the M and P channels;
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The
discovery of cytochrome oxidase (a marker of metabolic activity) – rich areas
in layers II, III and ivb (and, to a lesser extent, layers V and VI), which
show no orientation selectivity but colour and high spatial frequency
sensitivity.
These cytochrome oxidase-rich areas
in layers II and III are grouped together to form blobs, at least one of
which is associated with each ocular dominance column, with the areas between them
being termed interblobs. Both the blobs and interblobs, together with
the cytochrome-rich layer IVb, have distinct projections to V2 and other
extrastriate areas – projections that correlate well with the M and P channels.
This arrangement of channels and connections suggests that visual information
is processed not so much in a hierarchical fashion, but by a series of parallel
pathways (see Chapter 10).
Functions of V1
The major function of V1, apart
from being the first site of binocular interactions, is to deconstruct the
visual field into small line segments of various orientation as well as
segregating and integrating components of the visual image, which can then be
relayed to more specialised visual areas. These areas perform more complex
visual analysis but rely on their interaction with V1 for the con- scious
perception of the whole visual image. This occasionally presents itself
clinically in patients with bilateral damage to V1, in which they deny being
able to see any visual stimulus even though on formal testing they are capable
of localizing visual targets accurately (a phenomenon known as blindsight).
Visual association or
extrastriate areas
The extrastriate areas are
those cortical areas outside V1 that are primarily involved in visual
processing. The number of such areas varies from species to species, with the
greatest number being found in humans. These areas are found within Brodmann’s
areas 18 and 19 and the inferotemporal cortex. They are involved in more
complex visual processing than V1, with one aspect of the visual scene tending
to be dominant in terms of the analysis under- taken by that cortical area
(e.g. colour or motion detection). In general, damage to these areas tends to
produce complex visual deficits, such as the ability to recognize objects
visually (visual agnosia) or selective attributes of the image such as colour
(central achromatopsia) or motion. In addition, a number of other parts of the
central nervous system are associated with the visual system including the posterior
parietal cortex (see Chapter 34); the frontal cortex and frontal eye
fields (see Chapters 34 and 56); and the subcortical structures of the hypothalamus
(see Chapter 23) and upper brainstem (see Chapter 25).
Often these projections are grouped
together into a ventral stream which passes through the temporal lobe
and is important in object recognition and a dorsal stream passing
through the parietal lobe that is
more concerned with object location.
