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).
• 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).
• 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).
• 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.
• 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.
• 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:
• The accommodation of the M and P channels;
• 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.