You enter an unfamiliar building for a job interview. The receptionist tells you to make a left turn at the end of the corridor to get to your interviewer’s office. Easy instructions, but your brain has to remember them nonetheless. For the past decade, theoretical neuroscientists have proposed that, to do this job, neurons in the parietal cortex act as a kind of memory container: once you have learned that you need to make a left, dedicated ‘left-turn’ neurons are persistently active until you have reached the end of the hall, have turned, and can forget about it again. In addition to having lots of supporting evidence and enjoying intuitive appeal, the memory-container model has the advantage that, once the appropriate neurons are activated, they can potentially hold on to the ‘left-turn’ memory indefinitely (for instance, allowing you to get a drink of water before heading to the office).
However, a recent paper in the journal Nature has added to a growing list of evidence contradicting this model. In the paper, Princeton researchers Christopher Harvey, Philip Coen, and David Tank describe how ‘left-turn’ neurons in the parietal cortex of mice fire in a stereotypical cascade as the animals navigate along a virtual-reality corridor. The sequence begins with a small number of ‘left-turn’ neurons activating the next group and then falling silent again (see image), while the new group in turn activates yet another subset, and so forth until the end of the cascade is reached at the end of the corridor. In contrast to the memory-container model, this kind of dynamic activation sequence could be more similar to your car’s sat nav, constantly keeping you up to date on when you will have to turn left. Like a sat nav, dynamic memories could become more prominent when you are navigating through a complicated environment and have to make a left turn at the right time or in the right place (say, for instance, that there are lots of possible left turns, and you must remember to turn behind the drink fountain). Perhaps previous researchers may have failed to pick up on such dynamics because their memory tasks did not involve this aspect (in a typical experiment, a participant will receive instructions to make an eye movement to a certain location, remember the location for a few seconds, and then execute the movement).
The notion of dynamical memories is particularly interesting to our research because it relates to the idea that memories are an anticipation to act in a certain way (turn left) at a specified place (the end of the corridor) and a specified time (in about 10 seconds) – something we have been exploring in recent papers as well (i.e., research briefing from May 7th).
Harvey CD, Coen P and Tank DW (2012) Choice-specific sequences in parietal cortex during a virtual-navigation decision task. Nature; 484(7392):62-8