The two defining characteristics of hyperthymesia are
“1) the person spends an abnormally large amount of time thinking about his or her personal past, and 2) the person has an extraordinary capacity to recall specific events from his or her personal past”.
A hyperthymestic person can be asked a date, and describe the events that occurred that day, what the weather was like, and many seemingly trivial details that most people would not be able to recall. They often can recall what day of the week the date fell on, but are not necessarily calendrical calculators as people with autism or savant syndrome sometimes are; the recall is limited to days on a personal “mental calendar”. The mental calendar association occurs automatically and obsessively. Unlike some other individuals with superior memory, hyperthymestic individuals do not rely on practiced mnemonic strategies.

Two cases of Hyperthymesia relating specifically to a Eidetic memory have been reported. Although the first recorded case was in the 1800s and little is known about the subject, there have been plentiful studies carried out on the second, A young English girl who has a perfect Photographic memory spanning almost 18 years. This is thought to be in direct relation to her advanced IQ, which has been recorded by MENSA as 191…. WOW!

Although memory for personal events is extraordinarily detailed,it isn’t particularly skilled at rote memorization. Some studies indicate that the prefrontal cortex, an area of the brain associated with executive function and decision making, is active when subjects are trying to suppress information. Not surprisingly, some performed poorly on tests of these functions. A New Scientist article reviewing the Neurocase study speculates that “they may be better at storing memories than most while also being worse at blocking their retrieval.”
The blog Yes, ICantSeeYou reports on a University of California Irvine lecture in which Larry Cahill held a human brain before an audience, pointed to the hippocampus and said, “The memory must go through there in order to stick.” However, hyperthymesia case studies failed to find anything special in the hippocampus of those subjects. Cahill explains, “My hope is that a sign may well point somewhere else entirely.”
According to a January 2009 article in USA Today, MRI scans of Price’s (woman with hyperthymesia) brain showed “two abnormally large areas”: the caudate nuclei and a portion of the temporal lobe. Cahill explains the caudate nuclei processes “automatic habits,” while the temporal lobe collects and retains facts. This breakthrough may signal that when these brain areas work in concert, they create a type of event recall that’s as automatic as remembering how to brush your teeth. According to Cahill, “[W]hat we’re looking at is a new chapter in the book on memories and the brain.”