The Neuron- Up Close and Personal
There are a number of ways to examine the relationships resulting from brain sizes and structures:
1) Absolute Magnitude
This refers to the total size of a brain of a certain species, with no reference to body size or neuronal activity. If the organization of the brain as a whole is irrelevant, then total brain size seems an obvious candidate for use as an estimate of total information-processing capacity. If we used this as a comparison method, we would find that the largest brain sizes and highest brain weights are found in porpoises, elephants, and whales, followed by man (Figure 396- Kuhlenbeck). Whale brains weigh generally between 4000-7000 g, elephant brains around 4000-5000 g, dolphin brains around 1700 g, and human brains weigh between 1300-1700 g. Of course, remember that variations in brain weight within a species are quite considerable.
From this information, can we speculate that whales, with the largest total brain size, are therefore the most intelligent species? Let us look at a study reported by Kuhlenbeck (1973) with regards to intelligence and brain size. Studies of brains of "outstanding" or "genius" (link- what is genius? What is outstanding?) human individuals have been interpreted to show some statistical correlation between high brain weight and intellectual capacity. However, in individual cases, a person with a low brain weight around 1017g was highly gifted while another with a brain of 1800g was extremely mentally handicapped. In addition, one of the highest recorded human brain weights, mentions Kuhlenbeck, is said to have reached 2850g and this person was reported to be "an epileptic affected with idiocy" (Kuhlenbeck, 1973, 732). How does this affect our use of absolute magnitude of brain size to correlate with intelligence?
For another example, the average weight of a "typical" (link-what is typical?) adult male is +- 1400 while an adult female brain weight averages at +- 1300. Does this mean that males are more intelligent, more advanced than female humans? If we correlated large brain size with increased intelligence, then we would have to assume this comparison, yet on a whole it has not been documented that males are any more intelligent than females.
Thus, it seems that statistical correlation of brain weight and "superior intellectual ability" remains rather inconclusive. Therefore, perhaps we should find another method for comparing brain sizes and structures of various species.
2) Brain Weight vs. Body Size
A first most obvious manner of comparison would be the ratio of body weight to brain weight. Using Cuvier's fraction E/S (link- define E: brain weight; S: body weight), we find the following ratios:
Cat: 1/100; Dog: 1/120-1/300; Lion: 1/550; Horse: 1/600; Hippopotamus: 1/2789; Human: 1/40; Mouse: 1/40; Elephant: 1/560; small birds: 1/12; Frog: 1/172; Shark: 1/2496.
Notice that the human and mouse ratios are roughly identical and the horse and elephant ratios are also roughly identical. Note also that these ratios are according to relative brain weights from adult individuals.
However, a complexity in this method is that brain weight in vertebrates does not in general appear to increase linearly with body weight, so that heavy vertebrates have proportionally smaller brains than light vertebrates, and many small mammals have, in terms of these simple ratios, relatively larger brains than that of humans. (link- Fig. 2.3, Macphail) . This may lead us to question-- if the increase in brain weight that accompanies increases in body weight does not necessarily increase intelligence, what then is the function of the 'extra' brain matter?
In order to remedy inconsistencies in the simple ratio method, let us try "allometry" (link- glossary: the science of relating quantitatively the size of one part of the body to another). The following equation was developed in the late 19th century by Snell: E=CS^r, where E is the weight of the brain, S the body weight, C is a constant "cephalization factor", and r an epirically determined exponential constant. Kuhlenbeck suggests this value to be around 0.56 for mammals. Macphail asserts that this exponent would be approximately 0.66 for most mammals.
Once an acceptable value of r is determined, then we see that brain weight is determined by two other factors, S, the body weight and C, the cephalization factor. This equation, then, gives us a way of establishing the relative capacity of brains of different species with different body weights. When we enter values for the weights of brains and bodies of two species, then a value of C can be determined for each species. We can then find the encephalization quotient (EQ) which is the ratio of C over the average mammalian value. For example, if a certain species has an EQ of 2.0, this means that the species has a value of C twice as high as that expected in a mammal of comparable weight with average encephalization. Or if a species has an EQ of 0.5, then this species has a level of encephalization half that of an "average" mammal" (link- once again,what is normal?). Let us look at the following table of encephalization quotients (using Macphail's 0.66 as the constant r value):
Does this information agree with your intuitions regarding relative intelligence of mammals? We see that man is at the top, with dolphins a close second, and on down. Dolphins have a high reputation for intelligence, but do we also assume that dogs are more intelligent than cats? How do we determine if a mouse is more intelligent than a rat? This data seems to indicate that higher primates are generally more "encephalized" than lower primates relative to mammals as a whole and that smaller mammals and rodents are below average.
None of this data necessarily has a definitive link with intelligence. Only behavioral data could show the significance of levels of encephalization of a species. Let us look at other methods for comparing species' brains before we make any final conclusions as to the relevance of this information.
3) Volume of Gray Matter vs. Volume of Nerve Cells
As with the above section, there are two methods for thinking about neuron (link- glossary: neuron) density within the brain. The first, most basic method is to take the ratio of volume of gray matter and volume of body of nerve cells. According to Kuhlenbeck, the average value of this ratio for the human cerebral cortex is 27, meaning that or each volume of nerve cells there are 27 volumes of other substance. The latter may include glial elements (link- glossary: glial cells), intercellular space, vascular components, etcetera.
A second method as food for thought is looking at nerve activity. Macphail suggests that in mammalian cortex in general, the number of neurons per unit volume (i.e. neuron density) declines with increases in brain size. However, as neuron density declines with increasing brain size, so too does neural connectivity (link- glossary: neural connectivity= estimated from length of dendritic trees of cortical neurons) increase. In other words, the activity of a neuron may be proportional to the length of its dendritic tree, and thus activity per unit volume of brain is independent of a species' brain size. Take a look at the figure (link- Figure 2.1 Macphail, 26) and see what you think.
How do we make sense of all this? There seem to be two main limitations to this discussion. First, the independence of neuronal activity and brain size is assumed from the linear relationship between dendritic tree length and activity. However, I could not find asolute evidence to support or refute this assumption. Second, the data for both density and connectivity were obtained from mammalian cortex, so we don't know if it would hold true for other types of vertebrates.
4) Brain Structure vs. Brain Size
What if we look at specific brain organization and structure? It is important to note that there are regions in mammals for which there is no corresponding area in non-mammals and vice versa. This may or may not have implications for intelligence. The signifiance of anatomical data depends on the relation of structure to function. Thus, we'll focus on mammalian species predominantly.
What if we made a comparison of the size of brains (brain weight) and brain parts, based on indexes that directly reflect size differences (ratios) in species of equal body weight? Well, since I myself could not do the study, we will look at someone who hasŠ
S. Heinz et al. isolated the following brain structures from 3 different Families of primates- Prosimians (ex: lemurs), Simians (ex: monkeys), and Tenrecinae ( ex: other kind of monkey???) medulla oblongata, mesencephalon, cerebellum, diencephalon, telencephalon, bulbus olfactorius, paleocortex, septum, hippocampus, schizocortex, striatum, neocortex, area striata gray, and the corpus geniculatum laterale. They also found the total brain size of these animals. They found that the relative size of some brain parts shows strong differences between individual primate species. To be specific, "a classification of the various brain components based on the trends and intensities of size modifications from low Insectivora through prosimians, monkeys, and apes to man may be suggested as follows: an extremely strong progressive structure is the neocortex; strongly progressive structures are striatum, cerebellum, and diencephalon; fairly progressive structures are shizocortex, septum, hipocampus, and mesencephalon; conservative structures are medulla oblongata and paleocortex and amygdala; and a regressive structure is the olfactory bulb (Heinz, 15).
Does this give us relevant information about intelligence? What about evolutionary trends? What can we hypothesize about the fact that lower primates have smaller amounts of neocortex than higher primates and man? Perhaps this suggests that human primates are more advanced than lower primates due to their relative brain structure sizes? What about accounting for ecological or behavioral adaptations? Differences in relative size of brains and brain parts may reflect differences in these adpatations. However, simple interrelationships are hard to identify as they can be masked by a wide spectrum of factors.
What else can we say about comparing individual structures of the brain? Let's keep goingŠ
5) Cortical Folding
One distinction between the brains of mammals is that the brain surface is highly convoluted in some species and in others there is minimal degree of convolution (this is called "lissencephalic"- link, glossary). What is the significance of the amount of folding in the cortex? The thickness of the neocortex (link-glossary: forms the greater part, if not all, of the surface in mammals) varies very little within or between mammals. Cortical folding seems to be the consequence of maintaining or increasing the ratio of neocortex volume to brain volume with increasing brain size (Macphail, 247).
Indexes of cortical folding have been made by various scientists. Von Bonin (1941) devised an index of cortical folding using the ratio of the total cortical surface (as if it was laid out flat and unfolded) to the total exposed cortical surface. According to these indexes, the ratio increases fairly regularly with overall brain volume. For example, man has an average ratio of 2.86; dolphin and whale ratios span from 4.0 to 8.55. Thus it would seem that man has less folding than these cetaceous species. In addition, it is interesting to note that although the brains of most of these animals were larger than the human brains, some dolphins have a brain volume considerably below that of the human average. What does this mean, in layman's terms? It would seem that cortical folding is, thus, mostly a matter of brain volume. Some mammals may show more or less folding than man, but there is no evidence that these convolutions indicate more or less behavioral complexity.
This idea of cortical folding leads us to consider the significance of the neocortex in general. Macphail asks a particularly relevant question: "Given that folding could be the inevitable consequence of maintaining a constant ratio of cortical volume to whole brain volume, does this ratio in fact remain constant (as might be suggested by the reasonably regular relationship between brain size and index of folding), or is there an increase in relative neocortical volume in some mammalian groups?" (247)
First of all, what is neocortex??? Neocortex is the structure in the brain that differentiates mammals from other vertebrates and it is assumed that the neocortex is responsible for the evolution of intelligence [.ŠŠŠŠŠŠŠ.]
Is there a relationship between the amount of neocortex and brain size? Relevant data have been provided by a number of authors (Macphail, Stephen, Harman) that there is within different mammalian orders a good correlation between brain size and both neocortical volume and neocortical surface (see Figure 7.3 and 7.4, Macphail, p. 248). There are some slight differences, however, in that rodens have less relative neocortex than primates, carnivores, and ungulates, and that some types of opossums have even less than rodents.
What is the significance of these statements? First, it seems that man possesses no more neocortex than would be expected according to brain size. Second, although primates generally have more neocortex than some other mammals, they clearly are not unique in their neocorticalization.
One may expect, knowing the "role" (see above, or see glossary) of neocortex, that man and primates show so little superiority in terms of amounts of neocortex. Yet, what happens when data about neocortex is collected or analyzed slightly differently? Let's look at another method of research and see if there is an effect on outcomes and conclusions about neocortical significanceŠ
Researchers Stephan and Andy wanted to compare the size of a structure in a given mammal with the size that would be expected in a basal insectivore of comparable body weight. They found that man does indeed "possess much more neocortex than would be expected in a basal insectivore of comparable body weight, yet man does not possess much more neocortex than would be expected in a basal insectivore of comparable brain weight" (Macphail, 251). Basal insectivores (for example, hedgehogs) are assumed to be relatively un-specialized in terms of their behaviors and adaptations. A progression index was calculated (footnote: the expected size of a structure in a basal insectivore for a given body weight is calculated for each structure by applying the simple allometry formula to data gathered from basal insectivores of various weights, and extrapolating in the usual way to other body weights; the ratio between the actual size of a structure and the expected size in a basal insectivore is the "progression index"-- see Fig. 7.6 for progression indices for a number of brain structures in primates, Macphail 250). According to the figure, we see that the neocortex shows a dramatic progression in size as we go from the insectivores to the 'lower' primates and on to the 'higher' primates. The importance of this figure is that while neocortex maintains a relatively constant percentage of brain volume as it expands (Fig. 7.5), other forebrain structures gradually decrease in percentage of brain volume as the volume increases.
Thus, we can say with relative accuracy that size of neocortex is related to size of the brain. Man has just the amount of neocortex expected in an average primate of comparable brain size (but not body size). In other words, if man has more neocortex than, say, another primate, it would most likely be due to the possession of more brain for a given body weight.
Thus, both studies we have seen conclude that the main statistic that makes substantial distinctions between mammals is relative brain size rather than relative amounts of neocortex.