Forgetting your loved ones or unable to recognise them can be due to a delusional misidentification syndrome.People with this syndrome have a delusional belief that a person/persons close to them as imposters.Mistaken for insanity,this syndrome is called Capgras syndrome.
The Capgras' patient identifies his or her spouse as being an imposter - identical in every possible way, but an identical replica. The patient will accept living with these imposters but will secretly "know" that they are not the people they claim to be.
The official portal of Capgras syndrome is here
The bizarre misperception of body doubles is named after French psychiatrist Jean Marie Joseph Capgras, who in 1923 with colleague Jean Reboul-Lachaux described the case of a Madame M,claimed to have had eighty husbands.
"Invasion of the Body Snatchers " and "I married a monster from outer space " are the two science fiction films based on Capgras syndrome.
The case has been well portrayed by Dr.V.S Ramachandran in his popular book"Phantoms in the Brain"He reported that connections from the face processing areas in the temporal lobes to the limbic system(involved in emotions) have been damaged in these patients. The online format of the 1997 paper by Dr.Ramacandran on neural mechanism behind capgras is available here.
A novel on The Echo maker based on this fascinating syndrome by Richard Powers makes a good summer reading