Wednesday, December 21, 2011

The way memories are recorded and reused is analogous to the way tracks of water form on a window pane

New droplets fall into the paths of old droplets
On a rainy bus ride in 2004 I was staring out of the window, hoping to push myself into thinking up a novel conceptualization of the neurological basis of memory. Looking up from an article on axonal migration, my attention turned to the tracks of water that had formed on the window. You know how they look. Water droplets leave trails of water behind them as gravity pulls them down the pane. Early drops form trails that subsequent drops fall into and travel within. Of course these are trails of water on glass, whereas memories are recorded in tracts made of neurons and their projections. But the analogy felt substantial to me, and I think that it can be carried at least a short way before it fails.
The qualities of the surface of the glass and the placement of the water droplets on the glass determine how the drops will travel. This is similar to how the current physical state of the brain and the locations of the activations therein determine how neural pathways are selected. During every perception, cognition and experience old neural pathways are being retraced (memories, familiar perceptions) and new pathways are being created.

Some water droplet's trails extend long distances and subsequent drops fall directly within their borders. Depending on dirt and smudges on the glass other trails dry up before they extend very far and when a subsequent drop reaches this point, it is free to delimit its own path. Similarly our memories and experiences involve the activation of neural trails some of which are only small snippets of the original encodings.
Altering synaptic weights among groups of neurons creates patterns in the nervous system that are stable and that serve as preexisting pathways that constrain the future propagation of nervous energy. In this way, previous experiences imprint stable conduits that act as barriers for subsequent stimulation, thus the activation patterns elicited by new stimuli will not be random, they will be constrained by past activity. This accounts for how humans respond to disparate situations in the same inflexible ways, it accounts for how stubborn and resilient our personalities are, and accounts for how most of our actions are simply pastiches of previously used techniques. Consciousness and higher-order thought really are composed of multiple stable states organized in different configurations. In this way, consciousness is akin to the entire window and its current streaming activity.
This perspective feels limiting and deterministic, but it is important to point out that our window is huge and that we should stimulate and push ourselves to increase the number and complexity of stable states in our repertoire ensuring that our pane of glass has many possible tracks. Also, the pane of glass on the bus sits passively as water streams down its side and creates the pathways. Humans, on the other hand do not have to have sit passively and be written upon. With the right environmental provocations we can intentionally seek out activities that will influence the creation of new pathways.
I had always wondered how memories, thoughts and behaviors were made possible by something physical. I wondered how alterations in the connections between brain cells allowed mental representations that could remain stable over days or years. Like the water droplets, neural energy finds the path of least resistance, and information in the form of action potentials is pushed down those avenues that are best facilitated by the synaptic weights in the network. The analogy here is very simple. Today it seems so obvious as to be trivial, but I remember it being powerful and instructive for me at first consideration.