Is regression a sudden loss of previously acquired skills or is it the gradual decline in a particular area of function?  Using the right tools, both parents and clinicians can document the gaining and loss of skills in different areas, and they agree on what they see.  However, rather than being a single event, regression is slow, starting at around 12 months and showing continual declines through diagnosis.  Thanks to an NIMH day long symposium on the biological causes of regression, scientists got together to discuss this loss of skills on a biological level.  They think that the decline or loss of skills is due to biological events that disrupt the formation of specific brain circuits at critical times in development.  This can be because neurons stop developing, or maybe the brain goes overboard in shaping and pruning back connections.  If you want to see the amazing full day of presentations that aired in 2016, click here:  https://videocast.nih.gov/Summary.asp?file=19500&bhcp=1.  A link to the Ozonoff study described in the podcast is here:  https://www.ncbi.nlm.nih.gov/pubmed/29524310