the role of the hippocampus -- 9/22/21
Today's selection -- from Wayfinding by M. R. O'Connor. The brain’s hippocampus navigates both physical space and social space:
"Scientists have created a multitude of categories to try and [characterize navigation in humans and other mammals]. Vector navigation involves moving along a constant bearing relative to a cue that could be magnetic, celestial, or environmental. Piloting is defined as navigating relative to familiar landmarks. True navigation generally means wayfinding toward a distant, unseen goal. Dead reckoning, also called path integration, is keeping track of every stage of a journey in order to compute one's location.
"As it turns out, both rats and humans are the worst at path integration, which is precisely the kind of navigation that cognitive-map theorists propose the hippocampus does. In Eichenbaum's opinion, this is extremely problematic. 'One of my complaints about the path integration theory is how bad we are at it,' he said. Dead reckoning is applicable in short distances at local scales, but it is a strategy that isn't actually advisable in real-world navigation because it is so prone to accumulated error (except, it would seem, for those who have mastered complex environments like the Arctic tundra or Australian desert). Can the navigational capacities of humans be fully explained by the cognitive map theory of the hippocampus, or is there more going on?
|Basic circuit of the hippocampus|
"Eichenbaum most readily describes navigation as what it is not. 'I think navigation is not about Cartesian maps,' he offered. 'It's a story or memory problem.' The hippocampus is not so much about spatial memory, he said, as it is about 'memory space.' Parsing this distinction is important. True navigation, in his opinion, is what happens when we travel to an unseen place. It requires planning a future (envisioning the place we want to go), calculating or remembering the route to get there (a sequence or narrative), and then orienting to ensure we are on the right track, often by comparing our memory (or perhaps a description we've been told) to our real-time perception of movement through space. 'There are huge memory demands to solving the problem of navigation,' he said. 'Memory steps in at every moment.'
"Space and its role in hippocampal function has been oversold, says Eichenbaum, for whom space is just one of many 'fabrics' that we hold memory in. He believes that the hippocampal cells called place cells are much more flexible and capable of adapting to different dimensions. One of those dimensions is temporal, and for this reason, Eichenbaum doesn't call them place cells, he calls them time cells. 'Time is a philosophically interesting question. Do we make it up?' he mused. 'As you navigate, you are moving in space and time together, and the hippocampus is mapping both.' His research has led him to believe that the organization of our episodic memories is supported by these time cells, and that mapping sequences of memories in time is just as critical to navigation as mapping geographic space. The trick is trying to design experiments that can demonstrate the difference because 'you can't usually parse space and time.'
"He motioned me to his desk and opened a video file on the computer. I was looking down at the faint outline of a healthy, plump white rat with black markings. Its head was obscured by wires attached to the electrodes inserted in its brain. Eichenbaum had conducted this particular experiment in the laboratory across the hall a few years earlier. It appeared to be like so many others. A rat is released in a figure-eight maze with a reward at the end. But this one had a treadmill at the stem of the maze. Before the rat could find its way to the reward it had to step on the treadmill, which was programmed to randomly speed up and slow down. As the rat started running in place at these different speeds, the electrodes in its brain recorded the firing of three different hippocampal cells, represented by a colored pixel on the screen. 'Watch closely,' said Eichenbaum. 'First you'll see a blue dot, then a green, and last a pink dot.'
"As the rat started to run, I saw each cell fire in the order Eichenbaum described. I could tell that watching the video still thrilled him four years later. But what did these colored pixelneurons prove? By holding behavior (running) and location (in place) constant, and randomizing the treadmill speed, Eichenbaum had decoupled the distance the rat traveled from the time it spent running and could track which neurons were mapping each variable. The results show that the hippocampus was encoding both time and distance simultaneously. Then, when the treadmill stopped and the rat continued through the maze, the very same neurons began to fire because they were encoding space. Experiments like these, in which hippocampal cells 'map' multiple dimensions, are why Eichenbaum believes the hippocampus is capable not only of organizing physical space but of creating 'temporally structured experiences into representations of moments in time.'
"After years of studying rats in mazes, Eichenbaum has come to understand the hippocampus as the 'grand organizer' of the brain. 'It's organizing and integrating all these bits and pieces of information in a contextual framework,' he said. 'It does create a map. I'm all for the cognitive map in the original sense that it's a map where you put stuff to remember where they are in relationship to each other. That is a specific, limited, concrete sense of moving in geographic space and how did I get from here to there. The other sense is this abstract term, how did I navigate graduate school? What's the path to the presidency? In human language, these are both legitimate. But which one is the hippocampus? Is it the specific one or the generic one? I think the hippocampus could mean to map things in time. And there are other spaces in addition to geometric space. It doesn't have to be Euclidean or linear. That's just a really good example of what the hippocampus does, but it has other functions.'"
"In the last five years there has been more interest in designing tests to explore what those other spaces could be. A few years ago, a team of researchers in New York and Israel wondered whether the hippocampus could map social space: the relationship and interactions among individuals with different roles and levels of power. They asked individuals to participate in a roleplaying game in which they moved to a new town and had to find a place to live and work, and they found that the hippocampus was activated during the tasks, indicating it's a circuit that is important for 'navigating' social relationships. Another study, authored by Sundeep Teki and others in 2012 and called 'Navigating the Auditory Scene,' found that professional piano tuners had something in common with London's taxi drivers: more hippocampal gray matter. The more years a person spent tuning pianos, the larger this part of their brain was. Sound, in their case, was the space that the hippocampus mapped. Different pitches and beat rates were landmarks, and routes were created from one previously tuned note to another. A study in the same journal two years earlier reported that musical training actually induces plasticity in the hippocampus. After just two semesters of training, the researchers saw evidence in fMRis that music academy students' hippocampi had enhanced responses to hearing sounds. Had their hippocampal cells become music cells?
"Eichenbaum thinks that results like these are perhaps more faithful to the original idea of the cognitive map described by Tolman back in 1948. A close reading of that now-historic paper reveals that he thought the cognitive map might be multidimensional, a tool capable of mapping multitudes of life experiences. And these new studies are also relevant to how we answer the question Eichenbaum first posed to me: What is navigation really? Insights into time cells, social space, and music highlight how complex human navigation in the brain is: not just a calculation based on reading a Cartesian map but an unfolding memory or a narrative sequence, human relationships, sensory experiences, personal history, or paths into the future. 'The hippocampal system,' Eichenbaum once wrote, is 'encoding events as a relational mapping of objects and actions within spatial contexts, representing routes as episodes defined by sequences of places traversed..."