Welcome!
My understanding, which could be wrong as this isn't my area, is that severe neglect means that you are in a life threatening situation day in and day out... Even if you aren't aware of it? Your brain is. Because it's starving, and periodically at risk of dying from exposure, and having to cope with situations far beyond its normal developmental ability to do so.
So the base levels aren't being met (food, shelter*, human contact**) which is life threatening. Adding in having of cope with situations outside its ability to do so? In addition to asynchronistic development, which is a natural byproduct... Means a whole lot of other life threatening situations (from nearly run over by cars, to assaults or needing to learn to avoid on your own assaults by people/animals, to untreated illness & injury, etc.).
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* Shelter includes adequate clothing for the weather outside/inside, even if one does have a "home". Neglected kids are often dehydrated and suffer from prolonged hypothermia as well as severe sunburn, heat exhaustion, periodic heat stroke.
** Some really horrifying studies (observe & track only, they didn't "do" anything to the kids... They only observed what happened to them as their daily life, and then followed them for decades to see if there were any trends) have been conducted in Russian & Eastern Bloc orphanages, in places where culturally it's not desirable to raise other people's children. A side effect of this is that the staff "handle" the orphanage children as little as possible. Changing diapers, handing bottles and walking away, etc. While these children are often excellently cared for in all other ways (good nutritious food, well dressed, safe place to be/sleep/play, medical needs attended to, educated, etc.... No neglect *except* for the lack of physical human contact) the lack of physical contact had been directly correlated to a large percentage of infants "failing to thrive" (dying), and the vast majority of the rest developing serious psychological conditions (attachment disorders & psychopathy being the 2 most common).
These studies are the foundation why western medicine flipped 180 degrees and now insists on parental involvement in all levels of medical care possible, volunteers are used to "handle" children as much as possible (stroking infants in incubators to carrying children places instead of wheeling them, when possible, etc.) if parents are unable to, & spurred on skin-to-skin & similar studies which have found some amaaaaaazing things medically (too many to list). Similar cross cultural studies have been done on opposite cultures, as well (Like in Bali, where infants are "supposed" to be in arms for their entire first year / never let their feet touch the ground).
