I haven’t been really random in awhile, but recently an editor of a science magazine asked me the curious question: how much learning is implicit?

Because I like distant analogies, I’m strongly tempted to claim in response: Implicit learning is the dark matter of memory

To ground the analogy, the idea is that what we normally think of memory (the visible, regular matter) is our conscious experience of memory retrieval.  Implicit learning and memory is this broad set of invisible memory processes that shape our perceptions, actions and thoughts that aren’t directly observable by our subjective experience.

To stretch the analogy, dark matter is thought to constitute 83% of the universe (via Wikipedia).  Is it even possible to quantify the percentage of “memory” that is implicit so that we could compare numbers?

One approach would be based on neurobiology.  Under our “plasticity principle” model of implicit learning, virtually all synapses in the brain have some inherent ability to be modified by experience. Those changes that depend materially on the medial temporal lobe are thought to contribute to explicit, conscious memory while all the other changes are reflected in implicit memory phenomena.  Just based on brain size, we might make the claim that implicit memory should reflect the majority of these plasticity events, perhaps even around 80% of all memory events in the brain.  However, we’d really need some estimate of the rate of change events (or even magnitude) and they might be much more common and significant in the MTL, potentially reducing the percentage by a lot.

Another approach would be based on some quantification of the behavioral consequence of memory and try to carve up changes apportioned based on the relative roles of explicit and implicit memory processes.  After brief consideration of this problem, I’m going to recommend the biological approach.

I’m tempted to invite argument for and against the dark matter hypothesis for students of memory.  Probably the concept is a bit too bizarrely abstract for good debate, though.