Other Multiplicity

Parallel processing streams in the cortex

The human brain may be divided into two major portions – the left hemisphere and the right hemisphere. The external surface of the hemispheres, the cortex (or grey matter), is about 2 mm thick and consists of six thin layers. It is within the cortex that much of the higher processing of the brain takes place and different parts of the cortex are connected to each other via the underlying white matter. The two cortical hemispheres are only connected to each other at a few places – the largest of these connections is the corpus callosum.

The surface of each cortical hemisphere is divided into four lobes which are defined by major fissures which appear in the folded cortical surface – the occipital lobe, the temporal lobe, the parietal lobe, and the frontal lobe. If you apply different stains to the cortex, we can see that it is not a uniform structure. In 1909 Brodman published maps, developed by using staining techniques, in which the human cortex was found to be divided into many different areas (Figure 1).

Figure 1 Brodman's (1909) maps of the human cortex

Figure 1 Brodman’s (1909) maps of the human cortex. The top map shows the left hemisphere viewed from the side. The front of the brain is on the left and the back of the brain at the right. Area 17 at the back of the brain (also known as visual area 1, or V1) is the first cortical area which processes visual information. The bottom map shows the right cortical hemisphere as viewed from the left (with the left hemisphere removed). The small black shaded part in the middle of the figure represents the corpus callosum which provides the major connection between the two hemispheres. You can see by the size of the corpus callosum that the two hemispheres of the brain are quite separate structures.

We don’t know much about the brain mechanisms underlying personality and the self, which presumably involve processes within the frontal lobe, but over the past century we have learned much about how the brain processes visual information in the occipital, temporal, and parietal lobes. By the 1980s researchers were starting to publish ‘wiring diagrams’ showing how different cortical areas in primates were connected to each other (Figure 2).

Figure 2 Wiring Diagram

Figure 2  ‘Wiring diagram’ showing connections between different cortical areas processing visual information. [1] Processing of this information begins in visual area 1 (or V1) which was labelled by Brodman as his area 17.

Data from studies in which microelectrodes were inserted into living neurons to measure their activity, from studies of people who had suffered brain damage, and from studies of normal observers in visual testing laboratories, all served to support a common idea. That idea was that although we subjectively experience our view of the world as a unitary experience, underlying this, our brain splits up visual information into separate parallel streams.

Images received by our eyes, are processed by at least two [2], or three [3], major streams within our cortex.

One of these streams processes the motion of objects. This pathway doesn’t see fine detail too well, but it is very good at seeing very faint images (such as a faint shadow), and it is both colourblind and blind to stationary objects.

The second pathway analyses the form of objects [4]. This pathway is also colourblind, but it sees fine detail very well, although it is not as sensitive as the motion stream to faint images.

The third steam analyses the colour of objects [5]. This stream doesn’t see fine detail too well and it doesn’t see faint images too well either – but it does see colour.

The existence of these streams is revealed by rare conditions resulting from brain trauma.

Individuals, for example, have been found who have had damage to the brain which selectively hits the motion stream, but leaves the form and colour streams intact. Someone with this sort of specific damage is said to suffer from akinetopsia and they experience the world like a series of stationary snapshots [6]. Other people, similarly, have specific brain damage which destroys their ability to discriminate colours, but leaves their form and motion processing intact [7].

The existence of separate parallel processing streams in occipital, temporal, and parietal cortex is now well established in visual neuroscience. In the normal individual these streams are integrated so that they appear to act as a functional whole. But under unusual circumstances, or as the result of trauma, the true nature of the system is revealed. That is, the human cortical visual system is a system comprising several parallel visual systems, which, in the normal individual appears to be a single unitary system.
  

  1. Figure redrawn from Maunsell & Newsome, 1987
  2. Van Essen & Maunsell, 1983
  3. Livingstone & Hubel, 1988
  4. Livingstone & Hubel, 1988
  5. Livingstone & Hubel, 1988
  6. Zeki, 1991
  7. Livingstone & Hubel, 1988

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References and further reading

Brodman, K. (1909). Verglcichende Lokalisationslehre der GroBrhirnrinde. Leipzig, Germany: J. A. Barth.

Hubel, D.H., & Wiesel, T.N. (1959). Receptive fields of single neurons in the cat’s striate cortex. Journal of Physiology (London), 148, 574-591.

Livingstone, M.S., & Hubel, D.H. (1988). Segregation of form, color, movement and depth: Anatomy, physiology and perception. Science, 240, 740-749.

Maunsell, J.H.R., & Newsome, W.T. (1987). Visual processing in monkey extrastriate cortex, Annual Review of Neuroscience, 10, 363-401.

Sincich, L.C., & Horton, J.C. (2005). The Circuitry of V1 and V2: Integration of Color, Form, and Motion. Annual Review of Neuroscience, 28, 303-326.

Van Essen, D.C., & Maunsell, J.H.R. (1983). Hierarchical organization and functional streams in the visual cortex. Trends in Neuroscience, 6, 370-375.

Zeki, S.M. (1991). Cerebral akinetopsia (visual motion blindness): A review. Brain, 114, :811–824.


 


 

 

Cortical multiplicity

Parallel processing streams

Integration

Split-brain studies

 

 

     
         
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