Crime, the Hippocampus—and the Lingering Eye

It is also clear that other regions support the hippocampus in its quest to mediate memory formation, regardless of the category chosen. The prefrontal cortex (PFC), which is involved in decision making, planning, and foresight, clearly has a say in what will be encoded, stored, and ultimately retrieved. It communicates with the hippocampus during memory formation.

So what are we going to do? Even given these complexities, clear progress has been made in discerning the relationship between awareness and memory. Though it doesn’t settle the definitional issues, the data presented here show real promise of being practically helpful in a wide variety of settings. There may even be some real practical benefit to the law enforcement community.

The results

The inaugural idea of the project about to be discussed here was to explore whether the hippocampus could actually support indirect expressions of memories when conscious retrieval was failing. There were 2 machines involved in the experimental protocol. The first was a functional MRI, with special focus on images derived from the PFC and the hippocampus. The second was a tracking device that could detect eye movements and could determine where—and for how long—experimental subjects lingered over a given object in their visual fields. The eye-tracking device measured what commentators call REMEs—relational eye movement effects.

With these 2 machines calibrated and ready to work, the following 3-step experiment was deployed:

1. Healthy volunteers were shown pictures of individual faces superimposed on individual scenes (outdoor landscapes). The experiment was reminiscent of how animators draw characters on transparencies (cels), and then place the cels on previously painted backgrounds when recording for film.

2. After a delay, these volunteers were then presented with one of the previously encountered background scenes—sans face—and were asked to recall which face had been previously superimposed on it.

3. Finally, subjects were presented with a choice of 3 faces, and asked to choose which face actually went with a particular background scene in question. There was only 1 correct answer (2 of the 3 faces were distractors).

At each point, the subjects’ eye movements were tracked at the same time their brains were being imaged in the critical 2 seconds after the completion of the conscious retrieval task. The researchers were looking for:

• What the hippocampus did un-der conditions in which subjects matched the correct face with the correct background.

• What the hippocampus did when the subjects didn’t correctly match the items.

• What the REMEs did under both conditions.

What they found was extraordinary (Figure).

Extraordinary findings

Here are the results obtained when subjects were presented with the choice of 3 faces and correctly identified which face went with the background being studied:

• Activity in the hippocampus increased.

• Activity between the hippocampus and the PFC also increased.

• The eyes spent the most time lingering on the correct face (REME scores increased). There was a delay of 500 to 700 milliseconds between hippocampal activation and eye movement.

Here’s what happened when the subjects were presented with the choice of 3 faces and incorrectly identified which face went with the background being studied:

• Activity in the hippocampus increased.

• Activity between the hippocampus and the PFC decreased.

• The eyes spent the most time lingering on the correct face—even though the subject consciously gave a wrong answer.