Let’s say you’re in a crowded bar when somebody suddenly shoots at a patron. You clearly see a man carrying a firearm, but all hell breaks loose as you and everybody else scramble for the exits. In the terrifying seconds following the crime, you lose track of who discharged the firearm: it could have been 1 of 3 suspects. Afterward, the police interview you, but it is hopeless. Even bringing in the suspects for a lineup isn’t going to help you recall. There will be no “Perry Mason” moments, when the perpetrator breaks down under the weight of guilt and confesses to the crime. How can the authorities make an arrest?

They will be in for a tough time. Law enforcement officials know better than anybody about the long and storied literature concerning false memories, and the extraordinary unreliability of eyewitness accounts. People make things up all the time in high-pressure situations—seeing things that weren’t there, omitting things that were. The brain obviously records the information as it occurs in real time. But accuracy becomes only one version when the brain makes a conscious effort to retrieve the events.

What if there were a technological way to distinguish between these 2 versions of the same experience . . . the event that actually happened, and the version you perceived to have happened? The topic of this month’s column is an amazing result that promises to make this distinction possible. Using a combination of eye-tracking devices and noninvasive imaging, a group of researchers have uncovered an interesting way to get at a more accurate version of an event, even when conscious retrieval breaks down.

To talk about this extraordinary discovery, we will first need to review a few processes regarding human memory and some of the neural substrates that undergird it. Feel free to migrate to the “The results” section if words like “explicit memory” and “relational memory” processing are working parts of your vocabulary.

Thinking about memory

Present-day researchers use many ways to categorize human memory, some of which are clearly contradictory. That’s because there are many types of memory systems and subsystems, and not everybody agrees on just what they are. Is learning to ride a bike different from learning a foreign language? If a memory involves an emotionally competent stimulus, is that memory qualitatively different from the memory of a boring list of dead monarchs in a history class?

To make matters more complicated, many of these memory systems work in a semi-independent fashion. From a research perspective, the only clear thing you can say is that memory is not yet a unitary phenomenon.

One popular way to classify human memory abilities is based on whether the memory requires conscious awareness for retrieval. This idea was eventually transformed into a classification system involving so-called declarative memories and non-declarative memories. Declarative memories involve information you can physically declare—for example, that Thomas Jefferson was the third president of the United States. Non-declarative memories involve information you can’t declare—such as the learned ability to ski. Declarative memories are often called “explicit” memories; non-declarative memories are often called “implicit” memories.

The reason this classification system is so convenient is that there are distinct neural mechanisms and subsystems that underlie their function. Declarative memories are considered to be the province of the crown jewel of the medial temporal lobe—the hippocampus. The organ is ground-zero for helping to convert short-term declarative memory traces into long-term information. Damage to the hippocampus can lead to a selective inability to retrieve conscious events and facts while leaving non-declarative memory traces virtually intact.

Of course not everyone agrees with this rubric—the view is necessarily simplistic and has had recent empirical challenges. Certain patients with hippocampal damage possess deficits that are also associated with implicit memory formation. Some researchers believe the hippocampus is involved in yet another hypothesized memory category, so-called relational memory. This is the memory for the associations between discrete elements in an individual experience. Research findings in some labs implicate the hippocampus in establishing this type of memory too, whether a subject is aware of the experience or not.