People with amnesia can still remember faces
The hippocampus encodes the stories of your life: the faces you’ve seen, the places you’ve been and the words you’ve read and remembered. Scientists have long thought that damage to the hippocampus impairs all recognition memory. Yet since 2001, a handful of single-case studies tantalizingly suggest that some patients with hippocampal damage still retain the ability to remember faces.
Under what circumstances can this happen? An international team of neuroscientists recruited 10 patients: eight with hippocampal damage and another two with lesions to the medial temporal lobe (MTL), a broader area that encompasses the hippocampus. The patients and matched controls were instructed to memorize a list of faces, buildings or words, and recall was tested at different time intervals.
Patients with hippocampal damaged had no trouble remembering faces compared to controls when tested immediately after learning, but began faltering 2hrs post-learning. After a 3-day delay, they scored roughly 8% lower than the controls. When challenged with a list of inverted faces, however, the patients’ performance dramatically worsened compared to controls. Buildings and words were a whole different matter: even from the get-go, the patients’ recall suffered. Those with MTL damage exhibited even more pervasive memory impairment, performing at chance for all materials tested right after training.
What about faces familiar to the patients? Researchers asked them to study black-and-white photographs of 50 famous people. Immediately after, the patients tried to pick out the faces they'd seen in the study list from a pile of 100 photographs. They also reported whether or not they thought the person was famous. Interestingly, the patients performed worse than the controls at identifying a previously studied face when they said the face belonged to a famous person ("that's Elvis but I'm not sure if he was in the study list..."). But for faces they didn’t identify as a celebrity's, the patients performed on par with controls.
Scientists believe that our brains process faces differently than other objects. Rather than seeing faces as arrangement of features, we take a bird’s eye view, taking in the entire face holistically. Holistic processing, in turn, induces a feeling of general familiarity. The authors suggest that the patients may recognize non-celebrity (unfamiliar) faces through these “gut feelings” independent of hippocampal processing. Pointing out famous/familiar faces in the study list, however, requires the brain to actively link each face to the list, a process that likely depends on the hippocampus. It’s an interesting theory, but the evidence is somewhat tenuous at best.
In contrast, an inverted face is treated as the sum of its components, much like buildings and words. The authors believe that this is why patients with hippocampal damage were no longer capable of recognizing previously studied inverted faces, but managed to recall upright faces, at least for a short amount of time. Since patients with MTL damage are impaired in short-term facial recognition, it’s possible that crucial brain areas in the MTL other than the hippocampus compute faces as single entities and generate a short-lasting recognition memory signal. Unfortunately, the authors didn’t specify how long the patients have recovered from the initial brain lesion: are extra-hippocampal circuits responsible for facial recognition normally, or did plasticity in MTL circuits take over the job from the hippocampus following damage?
Smith CN, Jeneson A, Frascino JC, Kirwan CB, Hopkins RO, & Squire LR (2014). When recognition memory is independent of hippocampal function. Proceedings of the National Academy of Sciences of the United States of America PMID: 24958865
