Chapter 11   Why the Brain has Two Hemispheres

 

This chapter is a good illustration of the Rule of Life, where fish start with a simple non-decisional feeding method and through evolution and an increasingly complex brain lead to human organization, but based on the same simple system and finally showing that a ‘fix’ has to be made in the higher animals without disturbing what has gone before.

 

‘The brain divides into two almost identical halves, so there are actually two of every brain organ (except for the pineal gland), one located in the left hemisphere and one in the right hemisphere. It is not understood why this is so. Perhaps each side serves as a backup for the other, or maybe the two halves are required for some sort of stereo sensory three-dimensional processing’. (Memory and Dreams, G. Christos p 19)

 

These conjectures are essentially correct, but form only part of the picture. The major player is evolution, over a very long time, as well as the particular requirements of fish leading through the land animals to humans. The best that we can do is to look at the animals on our path of evolution as represented by the individuals that are alive today.

 

The following time frames (from Wikipedia) indicate where the animals that have been cited are placed in our representation of evolution. Sharks first appeared in the Silurian, 420 million years ago, but modern sharks date from only 100 million years ago. The octopus first evolved 300 million years ago in the early Pennsylvanian and the dolphin returned to the water in the Miocene about 50 million years ago.

 

On our evolution, the cartilaginous sharks represent the first of the fishes and today they have to keep swimming to pass water over their gills to extract oxygen and to provide lift to prevent themselves sinking to the bottom. ‘Vertebrate animals – mammals, birds, reptiles, and at least some fishes – periodically enter a period of torpor characterized by a profound change in brain waves….. reports from all over the globe …. of sharks resting motionless in caves …. the eyes of these quiescent sharks do follow divers moving about in the caves with them.’ (ReefQuest Centre for Shark Research) It would appear that some level of consciousness remains as a defense measure.

 

These ancient fishes can be compared to the mammals which have ‘recently’ returned to the same environment. ‘Generally, dolphins sleep with only one brain hemisphere in slow-wave sleep at a time, thus maintaining enough consciousness to breathe and to watch for possible predators and other threats’. (Wikipedia, Dolphins, Sleeping) Surprisingly, here is parallel evolution that sees the twin lobes used to the same end! I am not suggesting that one half sleeps whilst one half is awake, but am suggesting a parallel adaptation to the environment. Land animals have privacy in the form of burrows, thick undergrowth, caves etc. in which to retreat and sleep, whilst little privacy is available in the sea and the animals residing there must keep some portion of the nervous system conscious at all times. This shows that all organisms must have the same ‘reality’ and evolve their own reality in order to survive. Animals sleep if they can, otherwise a portion of the brain must be ‘awake’.

 

Starting with the generalized schematic of a bony fish (which is more evolved in that they have (from our point of view) control of buoyancy) shows two olfactory lobes connected to the two-lobed telencephalon (concerned mostly with olfaction) (these are the forebrain) then to the two optic lobes (the midbrain) to the single cerebellum (the hindbrain) and the brain stem. Sharks and catfish that hunt by smell have large olfactory lobes and trout, which hunt by sight have large optic lobes.(Wikipedia, Fish, Central nervous system) Clearly, the use of these two senses define a ‘stereo sensory three-dimensional processing’, because the eyes are on the two sides of the fish. Notice that there is a single cerebellum and brain stem, which suggest that they are of very ancient origin, and the twin nostrils and eyes are decision-making adaptions.

 

However, fish are streamlined for moving through the water with their body presenting a maximum outline to the side and minimum from above and below. Presumably this shape is the most efficient for turning and propulsion, because the tail moves from side to side. Thus the eyes are on the left and right sides of the fish. Over the vast time of evolution and the vast number of animals contributing to survival of the fittest there is great pressure to increase the complexity of biochemical reactions to attain goals, whilst there is great pressure to reduce unnecessary physical parts, if the same result can be attained another way, bearing in mind that it is necessary to build upon something that already exists. This statement I have called the Rule of Life.

 

‘Most fish possess highly developed sense organs. Nearly all daylight fish have color vision that is at least as good as a human’s’. (Wikipedia, Fish, Sense organs) This surprising fact presents a need to investigate some other issues.

 

‘Among these forty-plus independent evolutions, at least nine distinct design principles have been discovered, including pinhole eyes, two kinds of camera-lens eyes, curved-reflector (“satellite dish”) eyes, and several kinds of compound eyes …. camera eyes with lenses, such as are well developed in vertebrates and octopuses’. (River Out of Eden, R. Dawkins p 91) Here is an opportunity to test how a (much less evolved) cephalopod, but which has camera lens vision, fits into the argument presented here.

 

‘Certain areas of the cephalopod brain are particularly interesting with respect to evolutionary convergence because they show a strikingly similar morphological organization to areas of the vertebrate brain that mediate similar functions. For example, the three cortical layers of the cephalopod optic lobe are organized similarly to the deeper layers in the vertebrate retina (Young, 1971). This similarity in the integrational layers is all the more striking because the mechanisms of transduction and physiological responses to light are totally different (e.g., Hardie and Raghu, 2001)’. (Biol. Bul. June 2006 vol. 210 no. 3 308-317) This seems to indicate that the argument is on-track, and perhaps further, that a fundamental principle is involved.

 

Returning to the fact that the eye has hardly changed in form from fish to humans, this shows the versatility of the camera-lens eye, which focuses between infinitely far and close-up (to say 10 cm), using just a tiny variable lens. The patterns of the environment, in which the animal functions, and which are received by the eye are then converted into electrochemical patterns which have then to be recognized by the mind/brain. But, distance, near and far, are linked to the animal’s behavioral patterns. In other words, that which we ‘see’, is different at distance and close up with respect to each animal’s place in the environment. Whilst the eye can handle it, the mind has to resolve two patterns that are fundamentally (logically) different, and needs two parts in each of the brains to make sense of the two different scenarios as the lens changes shape.

 

‘Unique to these intrafusal muscle fibres is an absence of the striations – due to an absence of the overlapping contractile proteins – in the central portion of the cell’s length. This central zone does not actively lengthen and shorten like the rest of the fibre, but is passively stretched and slackened by the contraction and release of the striated, muscularly active parts of the cell. Around this central region is wrapped a long sensory ending, the anulospiral receptor. Because of its spiral arrangement around the cylindrical intrafusal fibre, it can register quite precisely the degree of lengthening and shortening of the central zone.’ (JOB’S BODY, A Handbook for Bodywork, Deane Juhan, p 193) This measuring of distance gives an indication of how far away the object is.

 

The ‘distance pattern’ is logical and obeys the usual laws of nature, but how can this be when we consider space-time? It has been mentioned many times in this book that the Half-truth is the fifth dimension and is necessary for completeness. To continue, this is the space that contains possible predators or food sources. The ‘close up pattern’ contains family members milling around, allows stress-free closeness, grooming, with no food and no enemies to worry about. It is necessary for a single mind to comprehend both patterns, at the same time, and it can reference the required pattern by using the anulospiral receptor. In other words, the electrochemical inputs have to be compared with that stored in the brain and which are being continually updated. Considering that one eye is on each side of the fish, there cannot be ‘true’ spectroscopic vision, and there is a tradeoff that works to the fish’s advantage, else it would not have occurred (Rule of Life).

 

Alternate logical situations are possible, but clearly the die was cast with the simplicity of a multitude of sensors on each side of the animal, such as hearing, sight, olfactory, touch etc. feeding into their own brain, on each side. Of all of the sensors, sight is the most active over a large distance and so it is the principle determinant of the structure of the brain, also the focus distance (that is, how far away something is) of the eye can be computed accurately and simply by using the lens’ muscular contraction sensors, given that there is not ‘true’ stereoscopic vision.

 

The brain of octopus, fish and all the way to humans has two lobes because evolution started with two lobes, and evolution is stuck with it. So, as our eyes moved forward, did our eye architecture change or did the brain change? Surprisingly, the brain did not change, but the left half of both eyes rewired themselves to go to the left lobe and the right half of both eyes rewired themselves to go to the right lobe! (Wikipedia, Human brain, Lateralization) This is sensible because nerves grow easily, but the brain is a component and complex with an organization that has been static for a long time. Another factor is that the brain cannot ‘change’ with respect to the organisms around it because its reality will be compromised if it can’t sense ALL of the other animals and it would quickly become prey, which means it would die out. In other words, it is necessary that the brain remain unchanged if the organism is to survive.

 

In humans, it is well known that the two sides of our brains are ‘different’. The left side is analytical, whilst the right side is the artistic ‘feely’ side. ‘The corpus callosum has been severed in some patients who suffered from epilepsy, to try to control the level of electricity in their brains. These patients with two separate brains are at times observed to act as if they have two different personalities, or two separate minds, although the dominant side of the brain (usually the left), which is the one that houses language, wins out’. (Memory and Dreams, G Christos, p 25) Thus there is no real difference in the lobes, but there is some effect because the thalamus is storing ‘blocks’ of memories in discrete areas.

 

The corpus callosum ‘is a wide, flat bundle of neural fibers beneath the cortex in the eutherian brain at the longitudinal fissure. It connects the left and right cerebral hemispheres and facilitates interhemispheric communication. It is the largest white matter structure in the brain, consisting of 200-250 million contralateral axonal projections’. (Wikipedia, Corpus callosum)

 

‘The corpus callosum is found only in placental mammals (the eutherians), while it is absent in monotremes and marsupials, as well as other vertebrates such as birds, reptiles, amphibians and fish (other groups do have other brain structures that allow for communication between the two hemispheres ….). (Wikipedia, Corpus callosum, Species differences)

 

Over the vast time of evolution and the number of animals contributing to survival of the fittest there is great pressure to increase the complexity of biochemical reactions to attain goals, whilst there is great pressure to reduce unnecessary physical parts, if the same result can be attained another way, bearing in mind that it is necessary to build upon something that already exists. For example, the eye works well, but it does contain ‘engineering’ that could be bettered if we were to design it from basic principles instead of it evolving.

 

This leads to the result that the brain has retained the two lobes even as the corpus callosum has become such a major structure (as above). It seems that the two lobes are ‘coming together’, but evolution has set the pattern and there is no going back.

 

In conclusion, the two lobes of the brain only incidentally serve as a backup for the other, and the lobes are a ‘hang over’ from the bilateral symmetry of the fish, and a reminder of the original ‘simple’ method of decision-making that has been passed to specialized parts of the brain. In other words, the near/far logic is handled as ‘software’ in the brain. This is shown in the quotation, above that ‘other groups do have other brain structures that allow for communication between the two hemispheres’. The brain is unifying as time passes, as shown in the mammals, where the joining apparatus is a major component of the brain.