J Psychiatry Neurosci. 2011 Nov;36(6):412-21. doi: 10.1503/jpn.100138.
Can Asperger syndrome be distinguished from autism? An anatomic likelihood meta-analysis of MRI studies.
Whereas grey matter differences in people with Asperger syndrome compared with controls are sparser than those reported in studies of people with autism, the distribution and direction of differences in each category are distinctive.
In development, timing is of the utmost importance, and the timing of developmental processes often changes as organisms evolve. In human evolution, developmental retardation, or neoteny, has been proposed as a possible mechanism that contributed to the rise of many human-specific features, including an increase in brain size and the emergence of human-specific cognitive traits. We analyzed mRNA expression in the prefrontal cortex of humans, chimpanzees, and rhesus macaques to determine whether human-specific neotenic changes are present at the gene expression level. We show that the brain transcriptome (transcriptome includes all mRNA transcripts in the cell; it reflects the genes that are being actively expressed at any given time, with the exception of mRNA degradation phenomena such as transcriptional attenuation.) is dramatically remodeled during postnatal development and that developmental changes in the human brain are indeed delayed relative to other primates. This delay is not uniform across the human transcriptome but affects a specific subset of genes that play a potential role in neural development.
By comparing the gene expression profiles in human, chimpanzee, and rhesus macaque prefrontal cortices throughout postnatal development, we have found that there is no uniform shift in the developmental timing between humans and other primates. We find instead a significant excess of genes showing neotenic expression in humans. This result is in line with the neoteny hypothesis of human evolution (6) and provides insight into the possible functional role of neoteny in human brain development. Specifically, we show that at least in one of the 2 cortical regions studied, the neotenic shift is most pronounced at the time when humans approach sexual maturity, (body matures; brain does not) a process known to be delayed in humans relative to chimpanzees or other primates (6, 24). Furthermore, the neotenic shift particularly affects a group of genes preferentially expressed in gray matter. Intriguingly, the timing of the shift also corresponds to a period of substantial cortical reorganization characterized by a decrease in gray-matter volume, which is thought to be related to synaptic elimination (21, 25, 26). The developmental pace of changes in gray-matter volume has been associated with the development of cognitive skills among humans (e.g., linguistic skills) (27) as well as with the development of disorders (e.g., attention-deficit/hyperactivity disorder) (28).
Although the precise causes and consequences of the human neotenic shift remain unknown, together these observations suggest that ontogenetic timing differences between the human and the chimpanzee prefrontal cortex transcriptomes may reflect differences in sexual and cognitive maturation between the 2 species. According to this logic, delayed gray-matter maturation in the human prefrontal cortex may extend the period of neuronal plasticity associated with active learning, thus providing humans with additional time to acquire knowledge and skills.
Things to think about:
Maturation of gray matter in the human prefrontal cortex is delayed by neotenic shifts. Developmental delay is neotenic. There is a “fuzzy boundary” (?) between too much / too little grey matter volume for the brain to function well; there also seem to be 2 types of brain organization: social navigation vs. factual and problem-solving. (See today’s Temple Grandin post.) These are probably not ‘separate’ paths, but are developmental stages. (Social is juvenile; factual is adult.)
The gray matter volume in any specific human brain may vary between mature and neotenic states. Humans vary in degrees of neoteny. It might be more accurate to dump “Autism Spectrum” for an inclusive classification “The Neoteny Spectrum” which includes all contemporary homo sapiens.
How do we know which human brains (volume of grey matter) are mature and which are neotenic? What volume of grey matter is the reference for maturity vs. neoteny? We can begin with behavior.
Some changes in grey matter volume occur during puberty: therefore it would be useful to compare the pre-puberty and post-puberty states of grey matter in individuals, especially those diagnosed with a “brain disorder” in early childhood. Mixing data from before and after stages of brain reorganization may be completely misleading do to variations in timing.
Grey matter differences in individuals reflects maturity vs. neoteny (a spectrum of rates of development) and not fundamental developmental disability; if one is ONLY interested in “The Social Brain,” and designates this juvenile stage of development as the ONLY legitimate human brain, then the “problem” of Asperger’s is the product of inattentional blindness and ignorance. Bad science!
CASE IN POINT: It is said that Asperger’s have “less than normal” volumes of gray matter, but if this conclusion has been derived from pre-puberty testing, then it is entirely possible that Asperger children, in terms of specific brain development, (intellect, language, concrete – visual thinking, facts and problem-solving) simply MATURE FASTER – and gray matter volume is reduced to a more effective and efficient volume well before their peers.
Social brains are neotenic; the neurotypical neotenic brain never fully matures. Which means that the social orientation (obsession) of modern social humans is the product of extreme neoteny. Neoteny is evident in a lack of logic, rationality, analytical thinking and effective problem-solving, which are absent in everyday life and most seriously, in our political leaders.
“Juvenile” neotenic behavior is evident in the inability to recognize that facts and physical reality exist. Instead, emotions are paramount; self absorption is rampant, magical thinking prevails, action is missing, narcissitic orientation is “normal”, and “worship of” childlike celebrities is a substitute for adult models and personal development. Adult children never leave home but remain dependent on parental support. Violence is characteristic of juvenile males; violent behavior usually decreases as males age, but today “frivolous” violence is the perpetual activity of neotenic males and is encouraged by popular culture.