Neurophilosophy

A study by Pasko Rakic, of Yale Medical School in New Haven, Connecticut, published in the Proceedings of the National Academy of Sciences, shows that ultrasound affects axonal growth in mouse foetuses.

Ultrasound scanning (or sonography) uses high frequency sound waves, whose echoes are reflected and recorded, to obtain real-time images of growing foetuses. The procedure is used routinely on pregnant women, to determine babies’ growth rates and to check for developmental abnormalities.  

During development, neurons are generated by cell division in the subventricular zone before migrating outwards to form the layers of the cerebral cortex. The inner layers of the cerebral cortex are formed first, with subsequent waves of neurons migrating over them afterwards to form the outer layers. 

Rakic and his colleagues had noticed this in earlier studies, the results of which they discussed at the 2004 Society for Neuroscience meeting. The inadvertent findings of these preliminary studies led them to investigate further.   

In this study, they used ultrasound to inject dyes into the migrating cortical neurons of 335 embryonic mice. During their experiments, they noticed that a “small but significant number” of these cells remained in the inner layers of the cortex, or in the adjacent white matter, instead of completing their migration to the outer cortical layers; the number of affected cells increased with exposure time. This was also observed in mouse embryos which had not been exposed to ultrasound, and it is suggested by the authors that the stress of handling may have contributed to disrupting the process of neuronal migration. 

Rakic emphasises that there is no evidence that ultrasound scans affects neural development in humans:

We don’t want to push the panic button. It would be very wrong if women stopped having medically indicated ultrasounds because of this. Our study in mice does not mean that use of ultrasound on human fetuses for appropriate diagnostic and medical purposes should be abandoned.

Mice in the studies carried out by him and his team were exposed to ultrasound continuously for up to 7 hours; furthermore, because the foetal mouse brain is so small, and is positioned very closely to the source of the sound waves, the radiation to which the mice foetuses were exposed is far more intense than that to which human foetuses would be exposed to.   

Dyslexia, autism and schizophrenia are all believed to occur, at least partly, as a result of aberrant neuronal migration during neural development, and Rakic cautions that women should avoid unnecessary ultrasound scans until further research has been done.