Is Finding Mutations the Path to Discovering the Genesis of ASD? 35in a human body DNA replication continues for an average of 70 years whereas
in a mouse such replication is only possible for about 3 years. Therefore, the
degree of somatic cell mutations in humans may be hundreds of times higher
than in mice [102].
Structurally speaking, the human brain, as well as those of nonhuman pri
mates generally, are similar to those of other mammals. Also, the former are
typically larger and proportional to their larger body mass. As far as making
comparisons of brain size for various species, the generally accepted approach
is to use EQ, the encephalization quotient. EQ adjusts for the nonlinear nature
of the body to brain relationship. The EQ for humans averages 7–8; other pri
mates usually have EQs that range from 2 to 3. As summarized in Table 1.1, the
EQ for dolphins exceeds that of other primates, with the obvious exception of
humans, whose EQ of 7.4–7.8 significantly surpasses that of dolphins and the
other mammals listed.
Primate brain enlargement is primarily the result of extensive grown in
the cerebral cortex, and particularly in the prefrontal cortex and in those
parts of the cortex that control vision. Visual processing in primates
involves complex networks and interconnectedness that includes 30 or
more identifiable areas of the brain. Estimates suggest that brain areas
related to visual processing comprise over half of the entire surface area in
the neocortex of primates. The activities of the prefrontal cortex are crucial
in key functional capacities, including attention, working memory, execu
tive control, motivation, and planning. This occupies a far larger space in
the primate brain than in the brain of other species, and the percentage of
space occupied by the prefrontal cortex in the human brain is particularly
high (https://en.wikipedia.org/wiki/Brain).
Table 1.1 Encephalization quotients for a range of species.
Species Encephalization quotient (EQ)Human 7.4 – 7.8
Chimpanzee 2.4 – 2.5
Rhesus monkey 2.1
Bottlenose dolphin 4.14
Elephant 1.13 – 2.36
Dog 1.2
Horse 0.9
Rat 0.4