…How we put the pieces together

Earlier this week, I began wondering down the rabbit hole that meanders towards the fundamental answers about how we process time.  I presented you with a theory pursued by David Eagleman, a neuroscientist at Baylor University that hypothesizes that our experience of time from moment to moment is based on the firing rate of our neurons, supposedly those most closely associated with our sensory input mechanisms and our memory.  The theory posits that as the firing rate of said neurons increases, our experience of time increases in length and vice versa.  It’s essentially the same idea that’s behind HDTV, the more information per second, the richer the experience.  The perception of time is a function of rate exclusively, and is a perception in the moment.

A competing theory, one that I think makes more sense, is that our perception of time is not at all in the moment.  It is only in assembling a narrative, a chronological categorization of events and stimuli that we begin to gain an idea of the passing of time.  Let’s think about this anecdotally for a moment.  It seems highly unlikely that in each moment we are aware of the passage of time, after all, most people’s perception of time is anything but precise, anything but a simple sum of the passing seconds as measured by clockwork machinery.  Now, consider how much of your existence, your idea about who you are, where you’ve been, what you know, involves placing experiences into a chronological narrative, holding disparate patterns of actions and events together and placing them in the correct order.  Even simple action patterns, like getting ready for work, or later, being able to recall what you actually had for breakfast that morning, requires this ability.  That this ability, this proclivity for “Episodic” or “Narrative” memory, is so central to our perception of the world and events in it leads one to think that perhaps the process of holding these events in the order that they occurred, encoding them with this underlying pattern, and subsequently recalling this ordered group of memories, is at the heart of our mental perception of time.  If this were true, one would hypothesize that similar sets of neurons are performing both the task of providing a “temporal code”, a means of keeping order of patterns in time, and also linking up the disparate pieces that make up an episodic memory.  For those familiar with episodic memory, the most likely candidate for this task would be the hippocampus.

Lending credence to this theory is this recent paper published in the Journal Neuron by members of the Cognitive Neurobiology Lab at Boston University headed by noted memory researcher Howard Eichenbaum.  The hippocampus, already known to be heavily involved in the linking of separate “parts” of episodic memories, also posseses a neural map, a collection of “place cells” that help us to keep track of where we are in a space, and where events happen to us in this space.  These place cells, well studied and categorized elsewhere, seem to be able to map any area we enter into discreet spatial relationships, firing more at specific predictable locations; they are also capable of remapping their “spatial code” when entering a new location.  The new paper from the Eichenbaum lab presents for the first time evidence for what they are calling “time cells”, cells that map a temporal sequence as it plays out over time.  These “time cells” “encode successive moments during an empty temporal gap between key events” according to Eichenbaum et al.  What this group has found is a set of cells whose variability in firing rate may be correlated with place, but is also correlated with location in time.  The key I think is that these cells are most highly correlated with both a specific place in a specific time, providing a means of encoding when we are in a specific place at a certain time.  This is at the heart of episodic memory, where and when.  When looking at the ensemble of time cells, the group was able to see sequences of firing that seemed to suggest that these cells were replaying the timing characteristics of the original action.  By changing the delay window in which they measured the activity of these time cells, the group was able to find several different subsets of these neurons that behave in qualitatively different manners.  Absolute time cells showed similar delay activity across the varied time trials, these cells seem to be firing in a pattern that signals the absolute passage of time.  There also seemed to be cells that rescaled their temporal pattern to stretch across the now elongated temporal window.  The majority of neurons found, however, seem to go through another process, which the Eichenbaum lab has termed “remapping”.  These neurons changed their behavior in much more significant ways than either of the first two groups.  Some that were originally inactive became active in the longer window or vice versa.  Some of them even categorically changed the firing pattern that they exhibited in the small temporal window.  You can begin to get a picture of how this system may be working, one set of neurons keeping absolute time, one set replaying the sequence unrelated to absolute time, and a third group possibly showing the integrated output of the combination of these populations.  I don’t suggest that this is actually what is going on, but it is certainly a possibility.

All of this provides evidence for the idea of temporal consciousness as a function of our processes of episodic memory and recall, but it doesn’t totally explain mechanisms for this ability.  We still don’t have a concrete answer for how the absolute time cells keep absolute time, are they running on genetic circuits that keep time in an absolute and repeatable way like tiny biological gears in a clock?  We have just started to explore this area so as of now there is now good answer.  For the mean time, I think we can accept that your brain doesn’t speed up in “near death” experiences.

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