Binding Memories and Decision Context in the Temporal Lobe

Invasive detection of single human neurons allows analysis of their role in decision making.

Is AI as good as human brains? No and no- AI is not as energy efficient by a long shot, not as flexible, and particularly not equipped with sufficient world models (aka general common sense and context) to be dependable in decision making outside its narrow areas of expertise, such as coding and hacking through computer security barriers. But both are black boxes to some degree, with the brain being particularly hard to study, even for adepts.

A recent paper provided an example of advanced research in human brains. Sadly, it relies on people with epilepsy, whose cases are so severe that they are getting brain surgery to remove the epilepsy-activating locus. As they are already being given a highly invasive treatment, with electrodes probing likely areas of their opened brains in order to find the smallest possible excision site, there is a long history of researchers asking for access to these patients for unrelated studies in general neurobiology. This paper uses 16 such patients to read electrodes inserted into their medial temporal lobes, including the entorhinal cortex and hippocampus, areas well known to function in memory and its integration into navigation and other kinds of decisions. Fortunately or unfortunately, the entorhinal cortex is the most common activating nucleus location for temporal lobe epilepsy, the most common kind of focal epilepsy.

The testing procedure was relatively bare-bones. The subjects were first shown a large number of images to find responsive cells from among their implanted electrodes. Then the experimenters set up a task system. They would ask a question first, and the subject would have to remember that question as two pictures were then presented successively. These questions required some kind of comparison, such as "Which person shown is older", or "Which item is more expensive?". The idea was to track how the picture-responding cells (and other cells) activated while the items were being presented and the decisions were being made. But the researchers were particularly interested in other cells- those that responded to the question being asked, or perhaps responded to some combination of the question plus the image. In finding such cells, they could investigate how memory and subject binding between context and object is handled in the brain. 

The figure above shows the firing of individual cells. On the left is a cell that responds only to the image of a cookie. Other pictures (below) do not activate it, and nor do any of the context-leading questions, to any special degree. On the right is a cell that is quite special- it responds to a combination of one image (the train) and one question, but not any of the other questions, and not any of the other pictures. 

The figure above shows another kind of neuron firing- one that responds only to one question/context: is the person in picture one older than the one in picture one? But it does not respond to any picture in particular way, nor to the other questions. The dotty portions of the figure show the detailed firing of these cells, which is counted/binned/summarized in the graphs at the bottom.

This Venn diagram shows the counts of neurons with particular characteristics, from among the total of 3,109 neurons that were probed in these 16 patients. Due to the pre-experiment screening procedure, cells that respond to images (the "stimulus") were predominant. But cells that respond to the questions asked (the "contextual stimulus") also came up surprisingly frequently. In green are cells that respond to one or more questions only. Cells that responded to some combination of both picture and question are represented by the smallest circle, giving various possible computations of the decision being asked for. 

These graphs show a summary of the experiment through time, from where the question was asked, to the presentation of each picture, and to the decision being made about four seconds down the line. In the bottom, picture-specific neurons fire separately as they are stimulated by each picture in succession. What is interesting is the top graph, showing activation of context-stimulated neurons, not just as the question is being asked, but then re-activated as the subjects need to recall the question in the context of each picture. The control graph (red) is where the experimenters shuffled the data (pictures and questions) randomly, getting a net null level of activity. Note that there is a delay of about 40 milliseconds between the picture-driven activity peak and the question/context activity peak, indicating some time span for evaluation that is taking place, all followed by about a second until (on average) the decision finally arrives from the subject (at the end of the graph). In addition, note how the context neuron firing becomes (on average) even more intense as the trial progresses, from its original activation as the question is asked, through the presentation of each picture. This suggests that these neurons are not so much for storage of memories as for their activation and use in decision-making.

The main finding is that neurons corresponding to what is needed for a decision in this instance are clearly available if one looks carefully in the brain. There are neurons that "see" the picture, regardless of other distractions, neurons that "understand" the question, regardless of other pictures or questions (and remember that question later on), and there are neurons that get the binding of both conditions, between the picture and the question. Additionally, the researchers find that the stimulus neurons were primarily located in the entorhinal cortex, while the context neurons arose mostly in the hippocampus. This tracks with the memory functions (heavily concentrated in the hippocampus) helping to keep the question in mind as the trial went forth. On the whole, we are seeing "binding"- that mythical beast in the philosophy of consciousness, and of mental processes more generally.

• Universities should have nothing to do with business schools.
• What is our political system about, after all?
• Report number 9.
• Triple win from the Norwegian wealth tax.