Tuesday, 28 August 2007

I'd Forgotten to Post about this one

A Brain Post. Summary: Long-term memory is DRAM, and the refresh mechanism allows errors.

From ScienceMag.Org:
Rapid Erasure of Long-Term Memory Associations in the Cortex by an Inhibitor of PKM
Little is known about the neuronal mechanisms that subserve long-term memory persistence in the brain. The components of the remodeled synaptic machinery, and how they sustain the new synaptic or cellwide configuration over time, are yet to be elucidated. In the rat cortex, long-term associative memories vanished rapidly after local application of an inhibitor of the protein kinase C isoform, protein kinase M zeta (PKM{zeta}). The effect was observed for at least several weeks after encoding and may be irreversible. In the neocortex, which is assumed to be the repository of multiple types of long-term memory, persistence of memory is thus dependent on ongoing activity of a protein kinase long after that memory is considered to have consolidated into a long-term stable form.
Not only is Long-term memory DRAM, but the refresh mechanism can be chemically interrupted, causing what appears to be permanent degradation of long-term memory.

DRAM - dynamic random-access memory - is the "memory" on your PC or laptop. It's cheaper to make than SRAM, Static RAM, which maintains its contents even when switched off as long as there's power applied. The memory in your mobile phone is SRAM Flash RAM, the addresses are still there even when you change the battery. The Hard Disk on your PC is also SRAM Flash Ram, essentially. When your system "boots up", the saved contents in it are loaded into the blank DRAM of the computer's memory. It's blank, because if DRAM isn't refreshed every 64 milliseconds, it loses its contents.

This work implies that the long-term memory in rats - and thus presumably all mammals, and probably all vertebrates - is DRAM. It has to get refreshed every few days (at most) by some form of inate mechanism. Interrupt the mechanism, and memory will fail.

(My thanks to Hildy for the correction. Even Jove nods)

1 comment:

Hildy said...

Hi, just a note from a fellow geek: SRAM is based on the noble flipflop, which doesn't hold its state when powered down. For persistent storage you're probably thinking of Flash, the various EPROM flavours or maybe the new fancy PMRAM (which uses a magnetic substrate to store bits).