Intro arrow 0. Left & Right Brain
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0. Left & Right Brain
1. Masking Alpha Channel
2. Rods & Cones
3. LGN: Magno & Parvo
4. SC: Superior Colliculus
5. Primary Visual Cortex
6. Dorsal - Ventral Stream
7. Eye Movements
8. Oculomotor System
9. Balance System
10. Ectopia & Microgyrus
11. Genetic Etiology
12. Reading
13. Animals
14. Conclusion / Solution
15. Different Theories
16. Peace of Mind
0.1 Left and right brain: function & interaction

One side of the brain thinks and sees in wide-angle (global red area)

while the other zooms in on the detail (highlited white areas).

There is a well-known myth about the hemispheres that grew from "split-brain" research in the 1960s. In a drastic treatment for epilepsy, surgeons operated a number of patients by cutting the corpus callosum, the thick bundle of nerve fibres that forms the main connection between the cerebral hemispheres.


The surgery revealed as described "two spheres of consciousness" locked in the one head, the left-hand side having speech and a rational, intellectual style, while the right was inarticulate, but blessed with special spatial abilities.

Many theories have grown up around these hemispheres such as two personalities in one head, with:

  • Left Brain: Coldly logical, verbal, dominant, etc.
  • Right Brain: Imaginative side, emotional, spatially aware, suppressed, etc.

To most neuroscientists, these notions are seen as simplistic, so there was general satisfaction when, a couple of years ago. A simple brain scanner test appeared to reveal the true story about one of neurology's greatest puzzles.

Exactly what is the difference between the two sides of the human brain?

Clinical neurologists Gereon Fink of the University of Düsseldorf in Germany and John Marshall from the Radcliffe Infirmary in Oxford, pursued the idea that the difference between the two hemispheres lay in their style of working:


  • Focuses on detail.
  • Mental skills that need us to act in a series of discrete steps or fix on a particular fragment of what we perceive, skills such as recognising a friend's face in a crowd or "lining up" words to make a sentence.


  • Concentrates on the broad, background picture.
  • Panoramic focus that made it good at seeing general connections; this hemisphere was best able to represent the relative position of objects in space and to handle the emotional and metaphorical aspects of speech.

 There is a similar balance structure in auditory processing between 'Global' and 'Detailed'. The main difference between Left Hemisphere (LH) and Right Hemisphere (RH) can be found at the micro-anatomical level. There are numerous asymmetries that influence how neurons spread information. At the cellular level, pyramidal cell dendrites branch further from the soma and ultimately into more branches with more dendritic spines, in the RH than in the LH.


Such circuitry favors more input from relatively distant sources in the RH, and from close sources in the LH. Thus, cortical mini-columns, macro-columns, and functional areas are more highly overlapping and more densely interconnected in the RH than in the LH.


Because functional and structural levels of brain organization are interdependent, some effects at each level get passed through to higher levels. (See topic: 0.5 Left and right brain: Language processing)


This is a clip of how neurons, branches and networks work.


One side of the brain thought and saw in wide-angle while the other zoomed in on the detail. To test this idea, the imaging laboratory at London's Institute of Neurology, scanned the brains of people who were looking at a series of images called "letter navons" (image left) and asked their subjects what they saw: F: Local element - S: Global image


  • Concentrating on the small letters (F) fired areas on the left side.
  • Mentally stepping back, to take in the overall shape (S) fired areas on the right side.

Fork or hat?

In a test split-brain patients matched household objects, when seeing "a cake on a plate".


Left connected to a picture of a fork and spoon -> function.

Right connected to a picture of a broad-brimmed hat -> appearance.

This evidence supports the idea of a highly modular brain in which thinking in logical categories is a strictly left hemisphere function while mental imagery and spatial awareness are handled on the right.


But, says Joseph Hellige, a psychologist at the University of Southern California, this picture changed dramatically as soon as brain-scanning experiments began to show that both sides of the brain played an active role in such processes.

Processing styles seem to distinguished the two halves. Under the scanner, language and space turned out to be represented on both sides of the brain:



Grammar and word production.Left
Objects at particular locations.

Intonation and emphasis.Right
General sense of space.

With all this evidence, researchers have come to see the distinction between the two hemispheres as a subtle one of processing style, with every mental faculty shared across the brain, and each side contributing in a complementary, not exclusive, fashion. A smart brain became one that simultaneously grasped both the foreground and the background of the moment.

Next they had the problem to work out exactly how the brain manages to produce these two contrasting styles. According to Hellige, he and many other researchers originally looked for the explanation in a simple wiring difference within the brain.


The theory held that, there are different neuron connections:

  • Left: make sparser, short-range connections with their neighbours.
  • Right: more richly and widely connected.


The result would be that the representation of sensations, memories and even motor plans would be confined to smallish, discrete areas in the left hemisphere, while exactly the same input to a corresponding area of the right side would form a sprawling, even impressionistic, pattern of activity.


Supporters of this idea argued that these structural differences would explain why:

  • left-brain language areas are so good at precise representation of words and word sequences.
  • Right brain seems to supply a wider sense of context and meaning.


A striking finding from some people who suffer right-brain strokes is that they can understand the literal meaning of sentences, their left brain can still decode the words, but they can no longer get jokes or allusions. Asked to explain even a common proverb, such as "a stitch in time saves nine", they can only say it must have something to do with sewing. An intact right brain is needed to make the more playful connections.


Even though this theory has no anatomical backing (just try counting neural connections under a microscope), computer simulations made it seem a decent enough hypothesis. For example, researchers including Robert Jacobs at the University of Rochester, New York, showed that varying the richness and distance of interconnections between neurons in an artificial neural network changes the network's performance. It can be made good at recognising either specific shapes or at grouping shapes generally.

But wiring differences are not the only contender to account for the origin of the brain's hemispheric bias.

One of the main reasons why Fink and Marshall's Nature paper attracted so much attention is that it was seen to support a quite different theory: that the bias is orchestrated by "higher" cortex areas.

Visual perception seems to emerge in the brain through a hierarchical process in which "low" areas of the brain send out signals when they detect simple aspects of the image falling on the retina, such as vertical or horizontal lines, or movement in different directions. These signals are then turned into meaningful scenes by "higher" areas. But this is not a passive process. High-level attentional areas can tell low-level sensory areas what they should be concentrating on.

Fink and Marshall's experiment appeared to show exactly this. Fink says that areas around high-level regions known to be crucial for directing the brain's attention, the inferior parietal cortex and its junction with the temporal cortex, fired every time attention switched between local and global features.

But very little about the brain is ever straightforward.

 The team replicated the test using an "object navon" (image left), an image in which a large shape such as an anchor is made up of smaller shapes such as cups. The pattern of activity was utterly reversed, the scans showed left-brain activation for processing the global picture and right-brain activation for the local elements.


Why should using an object navon reverse the side of the brain that is spurred into activity?

The team have yet to find an answer. Fink has a strong feeling that the wayward result is something to do with the fact that in the object navon, the local elements are very small, much smaller than the letters making up the letter navons. It could be that the difficulty of discerning such small shapes changes the nature of the task. Instead of the brain increasing the sensitivity of the local pathway, it may be busy inhibiting awareness of the global shape, so apparently creating a metabolic hot spot in the "wrong" hemisphere. This, of course, is speculation and the team plans to run more tests when they find how to match the ease of switching attention between the local and global views of their object and letter navons. This may mean altering the relative sizes of the elements and perhaps using more geometrical shapes.

Overall, the bulk of the evidence still suggests that the left brain is orchestrated to a state of local bias, while the right-side processing is tilted towards the global. But just how these attention effects express themselves in terms of the activity of individual brain areas such as V2 and V3 depends rather on the nature of the task.


'Right Brain' or 'Left Brain' - Myth Or Reality? By John McCrone " The New Scientist" (download pdf )

"Hemispheric specialization for global and local processing: the effect of stimulus category " The Royal Society/Biological Sicences (download pdf )

The well known Hermann-grid illusion, gives a good reflection of this shifting, balance seeking activity. When we look at the image above, our focus shifts from the whole image that is black and look for a point/detail at a crossing point of lines, this activety goes back and forth, also trying to get 'visualisation', understanding of the grid itself, who is white in contrast to the black square. This non grip having situation starts to pop-up the blurry points and confusion in our brain.


[ Next : 0.2. Optical Illusions ]



The 'Golden Ratio' and 'it's Fibonacci spiral' is a reflection of the ideal visual movement between whole and detail.



The purpose of this site is to present questions and new ideas about the above subjects.

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