Brownian thought space

Cognitive science, mostly, but more a sometimes structured random walk about things.

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Chronically curious モ..

Friday, February 08, 2008

Memory as a developmental change

What's memory? (Here's the Wikipedia take). Last December, I had the chance to hear and interact with Eva Jablonka, famous for her work on epigenetics*. And from that, and reading about other stuff, it occurred that memory, in a sense, might be pretty much the same thing as a developmental change. Why? Because both are changes in cellular properties in response to the environment, and that code the environment in a meaningful way. For example, the training of immune T cells is encoded in the expression of surface markers (like the CDn family). This is a developmental change, and is involved in (immune) memory. One way to think about this is that developmental programs might occur over different timescales, and with respect to different developmental stages. So, some programs might turn on and be relevant at the blastula stage, and others at the adult stage. At each stage, the 'environment', in the inclusive sense (both biological and non), interacts with the various genes and gene products and all the related chemicals to get to the next stage. Why is this relevant? Well, while talking to Eva J, I wondered if longterm memory in the brain was actually a page out of an older and extremely important 'memory' system- the germline. Why 'memory'? You can think of an organism as reflecting in some way its natural history. The 'memory' of this natural history is simply the product of the NeoDarwinistic selection process: the current organism. One might think of the gamete Mother Cells as being the memory stores, which keep both long-term (genetic) and short-term (epigenetic?) memory marks. So here's the idea: perhaps, the epigenetic mechanisms for storing 'memory', which had been perfected in the germline cells, were recruited for storing memory of a different kind- that between sensory inputs and their optimal outputs (i.e., optimal given the environment & the rest of the geno/phenotype of the organism). Actually recent evidence from a unicellular slime mold suggests something slighty different. Japanese scientists showed that Physarum polyephalum amoebae show learning. The learning consists of altered movement patterns in response to external stimuli- these patterns are retained through a period when the stimulus is absent, and re-deployed when the stimulus comes back on. So, the single cell has both the capacity to deply a motor pattern in response to an external stimulus and to keep the memory trace for that. This suggests that germline cells (for the single-celled amoeba, the one cell is both the germline and the soma ;) and memory cells might just be two separate cellular programs generated from a single cell with both capacities. In either case, this implies that, perhaps, during development, (a) germline and neural tissue share some common origins and/or (b) germline and neural tissue share some interesting patterns of gene/protein network states. In fact, there are a bunch of proteins of the testis-brain protein family that seem to be present in both the germline and neural tissue. These proteins seem to be involved in chromatin remodeling and in RNA localizations. Still, why developmental? Possibly, neural tissue and germline tissue, relies heavily on epigenetic markers for retaining environmental memory from recent times. So, during development, precursors to these are put aside early so that the remaining cells can do all the chromatin remodeling they please. Neural and germline cells are put through only very few division cycles to maintain their status as faithfully as possible. Now, wouldn't it be cool if neural changes were able to be transmitted directly down to the germline in adults? :) *A recent article explores the concept in greater depth

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