severe early neglect is irreversible sadly. Children can "recover" but not in the sense of healing the brain damage, more in the sense of learning to live with it.
For example, as a foster carer I learnt that f a child has no sense of right nd wrong by 18 months, they NEVER will. That is not to say they cannot CHOOSE to follow social conventions and use moral guidelines they are taught, but it will be an academic teaching, not a gut feeling.
Children who are not touched, hugged, tickled, etc do not learn wherre the outline of their own body is, and this is hard to overcome too.
Children who don't get "babytalk", ie parents repeating things in exaggerated, loud, clearly pitched voices miss out on crucial stages of language development.
eg "WHERE is baby's din- dins??? HERE is baby's din- dins"
the reptition of syllables is extremely important, and most loving parents do it intuitively.
Children who are not cooed over by grandparents, friends and neighbours etc do not even learn that they are an individual entity which people notice and respond to.
This damage is VERY profound, and children do not recover.