The main connections in the left hemisphere.
The Magazine New York Times recently published an editorial about mapping the connectome, all the connections that connect all the neurons in a person’s brain. Many of these connections are formed and strengthened as a result of our experiences, and their sum sums up everything about our personalities: the memories we’ve formed, the skills we’ve learned, the passions that drive us.
There is even data to suggest that some neurological disorders are in fact “connectopathies,” characterized by aberrant connections or an unusual degree of connectivity between neurons. Some studies have shown that autism spectrum disorder (ASD) is associated with reduced functional connectivity in the brain, but other experiments have found increased connectivity in autistic brain. A new study may have reconciled these conflicting findings. Researchers at the Weizmann Institute of Science in Israel found that brain regions with high levels of connectivity in controls have reduced connectivity in ASD, and regions with lower connectivity in controls have increased connectivity in people with ASD.
The scientists analyzed fMRI scans of high-functioning autistic adults and controls, obtained from five different datasets. When the scans of the controls were superimposed, a typical, canonical template of connectivity was apparent. Certain regions had high interhemispheric (between the right and left sides) connectivity: primary sensorimotor regions such as the sensorimotor cortex and occipital cortex. Others showed low interhemispheric connectivity: regions such as the frontal cortex and temporal cortex, which are involved in higher-order association. Overall, the control brain scans looked pretty much the same.
Autistic brains, on the other hand, were all different. Each had regions of high and low connectivity, and there were regions where connectivity increased relative to controls and decreased. But no standard template emerged; when these scans were superimposed, no clear areas of high and low connectivity were discernible.
This comparison of grouped brain scans, rather than just the individual ones, revealed that the unusual pattern in ASD brain connectivity is due to the topology of their connectivity patterns and not necessarily the strength or weakness of any particular connection. It also revealed that people with ASD have more individualized, idiosyncratic connectivity patterns than controls. Each autistic brain differed from the norm, but each did so in its own way. The researchers couldn’t even find subgroups of ASD brains that resembled each other, though they noted that larger datasets could reveal some of these.
Autism is not diagnosed by anatomical criteria, but by deficits in social and language skills and excessively repetitive behaviors. When the researchers checked whether the degree of disruption in the connectivity pattern was related to an individual’s behavioral symptoms, they found that was exactly the case.
This effect, like the degree of distortion of the connectivity pattern, was most pronounced for homotopic interhemispheric connections – connections that went from a specific location on one side of the brain to the corresponding location in the other hemisphere. But it was seen to a lesser extent for connections between different brain regions, on the same or on different sides. In contrast, this distortion of the connectivity pattern did not correlate with IQ scores.
Since the study didn’t involve children, the authors weren’t sure when or how these effects might develop — whether they have a developmental origin or are some kind of compensatory mechanism that occurs later in life. They did note that the idiosyncrasies did not seem to arise from anatomical differences in autistic brains, as they did not have significantly atypical shapes or significantly asymmetrical hemispheres.
An intriguing possibility is that because neural connections are formed by the routine behaviors and experiences we have every day, people with ASD each display peculiar connectivity patterns in their brains because they each experience the world and interact with their environment in a peculiar way. Their peculiar behavior prevents them from participating in the typical interactions that typical brain scans create.
Natural Neuroscience2014. DOI: 10.1038/nn.3919 (About DOIs).