In 1992, at the age of 70, a US citizen suffered a severe case of viral encephalitis, a swelling of the brain caused by infection. After he recovered two years later, he appeared completely average based on an IQ test (indeed, he scored 103). Yet in other ways, he was completely different. Several decades of his past life were wiped completely from his brain. His only accessible memories came from his 30s, and from the point of his illness to his death, he would never form another memory that he was aware of.

But this severe case of what appears to be total amnesia doesn't mean he had no memory as we commonly understand it. The patient, called E.P., was studied intensely using a battery of tests for more than a decade, with researchers giving him tests during hundreds of sessions. After his death, his brain was given for further study. With the analysis of the brain complete, the people who studied him have taken the opportunity to publish a review of all his complex memory problems.

Aside from memory, there were only a few obvious problems with E.P. Most of his senses were normal except smell, which was wiped out (a condition called anosmia). His vision was perfectly fine, but he had two specific problems interpreting what he saw. One was a limited ability to discriminate between faces, and the other was difficulty in determining whether a line drawing represented an object that's physically impossible (think M. C. Escher drawings).

But the primary problem E.P. experienced came in what we'd probably call conscious memory, or what professionals call declarative memory. This involves, as the names imply, the ability to be aware of something we know, and to state it, whether it's a historic event or the term for an obscure object. For example, E.P. moved to San Diego shortly after his illness, but he was never able to consciously remember the layout of his apartment or where the Pacific Ocean was, even though it was two miles from home. And although he could relate stories about the events of his youth, he'd often get repetitive while doing so—after all, he couldn't remember which parts of the stories he'd already related.

But that doesn't mean he had no memory. We store short-term information (like the digits we're carrying when we're doing math) in a place called working memory—and E.P.'s working memory was just fine. In some tests, he was blindfolded and led along a path up to 15 meters in length. When it was over, he was able to remember his start position successfully. But wait a few minutes, and the entire test faded from his memory. When asked, he'd tell the researchers that he'd been "in conversation" a few minutes earlier.

There's also the matter of unconscious memories. E.P. would claim no knowledge of who the researchers themselves were, but over the course of their hundreds of visits, he came to greet them warmly. He would eventually move to the table where he was tested without being asked to do so.

This ability to form memories without being aware of them extended to some of the tests. E.P. was repeatedly given some short phrases (like "fire warms house") and then asked to fill in the blank when said blank replaced the third word. Over time, his performance slowly improved. But he was never consciously aware of knowing the answer, and the delay involved with producing any answer was the same in that and control words. In fact, he couldn't even recall that (or any other) task. His performance plunged when the rules were changed slightly.

Moreover, whatever this form of memory was, it doesn't plug into the system that allows us to recognize items that are familiar to things we know. So if the phrase was changed to "fire heats," he was unable to fill in the blank. This inability to recognize the familiar extended to a variety of other tasks.

The post-mortem examination of his brain largely expanded on what we had learned using MRI scans. The temporal lobes, which are heavily involved in long-term memory, were eliminated on both the left and right sides of his brain. The damage extended into other nearby regions, including one associated with the sense of smell (which probably explains the anosmia).

The most striking bit of damage, however, was the loss of the amygdala. The amygdala is associated with memory formation, but it has other roles as well, including the establishment of emotional associations (including fear memories). Given the number of things the amygdala is involved with, I'd have expected widespread consequences. Instead, all the researchers saw was a reduced ability to recognize emotions on pictures of faces. It's possible that the reasearchers didn't use more appropriate tests to find additional problems, but E.P. seems remarkably thoroughly characterized otherwise. It doesn't seem likely.

In any case, the study is a nice reminder that a lot of the brain's apparent complexity emerges from the interactions of many systems that perform smaller, more specialized tasks. Rather than being a single, unified system, memory emerges from a combination of multiple features, some of which we're never consciously aware of. And studying those is a challenge, since it's rare to have any of these systems removed with the sort of precision that apparently wiped out a lot of E.P.'s past and continued to erase his present.

PNAS, 2013. DOI: 10.1073/pnas.1306244110 (About DOIs).