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Nervous System


Psychologists had been arguing about the definition of intelligence since there are different types. Finally in 1993, they came up with the "Three Stratum Theory" as illustrated in Figure 31c. Sitting at the top of this one-plus-many relationship is the General Intelligence Factor (the g factor)
Intelligence IQ under which are the broad abilities and further down in the ladder are specialized skills related to the broad abilities. Fluid intelligence is about learning, reasoning and problem solving; while crystallized intelligence is the recollection of previous experiences, such as vocabulary, cultural knowledge, ... The other abilities in the middle of Figure 31c are self-explanatory. By examining wildly separated twins and non-kin adopted into the same home, it is found that heritability (of intelligence) is less than 30% before starting school, rising to 80% among western adults. Thus, nature seems to win over nurture at the end as revealed by IQ tests. IQ test was invented in 1904 to identify children who would fail elementary school without special help.

Figure 31c Intelligence
[view large image]

Figure 31d Brain Anatomy and IQ [view large image]

Today their use can be considered contentious, partly because they do not find equal amounts of intelligence everywhere. It is culturally bias. Figure 31d shows the purported correlations between brain anatomy and IQ.

Barin Mass vs Body Weight Limits of  Intelligence The informal definition of intelligence in dictionary is the capacity to acquire and apply knowledge and the faculty of thought and reason. In general it is perceived in terms of memory, reasoning, responsiveness, ... In neuroscience intelligence is vaguely related to brain mass, neural connections, and transmission time, these properties are loosely associated respectively with memory, reasoning, responsiveness in the layman's terms. Recent studies in the 2010's suggest that human intelligence has reached its limits beyond which any attempt for improvement would bring only diminishing returns.

Figure 31e Brain Mass vs Body Weight

Figure 31f Limits of Human Intelligence [view large image]

A more detailed explanation is provided in the followings :
  1. Brain Mass -- It was thought that heavier brain means higher intelligence because it would have more neurons (gray matter) and axons (white matter). However, "more" doesn't mean "better" as shown by the Kleiber's Law in Figure 31e, in which human is at the apex of intelligence. Further increase in brain mass entails more increase in body weight such that the add-on brain cells are allocated for neural housekeeping chores (such as controlling more muscle fibres) unrelated to intelligence.

  2. Neural Connections -- The human brain contains about 120 billion neuronal cells, with an approximately equal number of non-neuronal cells (glial cells). These cells pass signals to each other via as many as 1000 trillion synaptic connections. The communication networks (at about 2% of our body weight) consume about 20% of the energy that we expend at rest. In newborns, it is an astounding 65% making the infants to rely on parental care for its survival. Further increase in neural connections will demand more resource at the expense of the other biological processes in our body.

  3. Transmission Time -- The transmission time between neurons depends on the traveling distance. That's why the motion of elephants appears to be rather slow. The transmission time is also determined by the speed of the signal. It is found that thicker axons carry signals faster. This advantage is negated by greater consumption of energy and occupies more space (the same trade-off as for more neural connections).

  4. Neuron Density -- It is found recently that unlike other mammals, cortical neurons in primates enlarge very little as the brain increases in size. This kind of packing strategy allows for greater number of cortical cell as brains enlarge; and it also permits faster communication, because the cells pack more closely. However, thermal motion triggers random firing of the action potential if the axons or neurons become too small.
Restricted by the above-mentioned constraints, it is doubtful that a major evolutionary leap could lead to a smarter brain. The evolution of brains started about 600 million years ago. Since then all kinds of animals have now evolved to a point where the brain circuits have arrived at a similar design for running tasks such as vision, smell, navigation, ... Such evolutionary convergence usually suggests that a certain anatomical or physiological solution has reached maturity so that there may be little room left for improvement. Figure 31f shows the various options for improvement in brain structure and the trade-offs in each.

Cephalopods The class of Cephalopods includes octopuses, squids, cuttlefish, and nautiluses. It is within the phylum mollusks which encompasses such animals like clams, snails, and octopuses. Cephalopods has the distinction of the most intelligent invertebrate on a par with the cleverer vertebrates such as chimps, dolphins, and crows. Although there are differences in synaptic chemicals (they have fewer) and their axons are not covered with myelin, there are lot of similarities between the two kinds as shown in Table 03. If consciousness is defined as the notion of "self" and making decision based on previous experiences, then the cephalopods have at least the rudimentary form as experiments show repeatedly that they can navigate maze, use tools, mimic other species, learn from each other, and solve complex problems (such as eating clams with closed shell and trying to devour a hermit crab with stinging anemone on its back). The earliest ancestor of cephalopods is probably a snail-like creature existed about 500 million years ago. The present day nautilus is a living fossil not much different from those cephalopods in the ancient time. While their brains are nowhere near as complex as their younger relatives, they have many of the same abilities in

Figure 31g Cephalopods

learning, memory and spatial awareness. It seems that they have beaten the mammals to develop intelligence by at least two hundred million years (see Figure 31g).

Structure Similarity
Location In the head (unlike the ganglia running along the length of the body in other mollusks)
Lateralization Brain split into 2 halves connected by a bundle of nerve fibres
Lobes Brain divided into specialized lobes
Sulcus The lobes are folded to increase surface area
Neuron Distance Inter-neuron distances are shorter (than the other mollusks) to allow faster communication
Other Parts There are corresponding parts to the hippocampus and cerebellum in higher vertebrates

Table 08 Similarities between the Brains of Cephalopods and Clever Vertebrates

Since these two kinds of creatures developed their brain along different pathways, the specializations listed in Table 08 indicate that they are the minimum requirements for intelligence.

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