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November 08, 2008

Do We Really Forget? Fathoming The Esoteric Realms of Memory

Smells like teen spirit”, but it could be true that we never really loose any memory in our lifetime. Our memories are stored in the synapses (junction, more specifically, gaps between adjoining neurons) as a function of synaptic strength, in the nerve cells like dendrites as proteins, and some other processes which mostly encompasses a chemical interaction. I am excluding memories such as T cell or B cell memories here; memory, here, will refer to neural ones that occur in the CNS.

We know that in dementias such as global multi infarct dementia, Alzheimer’s disease; there are diffuse losses of neurons and losses of cholinergic neurons in particular, respectively. In surgical cases of epilepsy or brain tumor, there are losses of neurons too. In these cases, memory loss may be irrecoverable, though cases are on record which points to shifting of those memories into some other safe havens. But what about the rest of the population? Does an established long term memory vanish completely?

Let’s consider some facts. The numbers of synapses and their strengths are finite, though both can change in response to stimuli. Even the number of neuron themselves can increase, contrary to the belief held earlier. Neuronal stem cell pool has been identified in the brain. Memories stored in the brain are finite too. Memories are inherently dynamic in nature. Even long term memory stored in the neocortex (medial temporal lobe, on the other hand, stores memories as a buffer, like a D RAM chip, a temporary storage) can change location, as much as transferring itself to the other hemisphere (intercortical transfer), when needed; via the optic chiasm and corpus callosum. So, we see that the number of synapses, though finite, can rise to the demand of an enhanced input from sensory cues which are finite too, leading to memories that can jump across their own allocated territories. A finite brain capacity (say C) can certainly contain a finite memory (say M) as long as C is greater than/ equal to M. Certain computer softwares even trespass this limit; a zip file of 2 MB may deliver 3MB of contents on unzipping! Who knows if the brain isn't using this for the past thousand years.

Synapses, simplistically, may be thought of in binary terms: 1, when it is on; 0, when it is off. Both 1 and 0 is a bit in Boolean terms. We leave aside the synaptic strength part here for the sake of simplicity. In addition, memories may shuttle between synapses in such a way so that it is present in the brain, but not represented by any synapse. I will explain. We all have seen those jugglers juggling those colorful balls too many at a time using only their two hands. A similar thing like dipole dynamics may occur in the brain. Added to this is quantum superposition, which allows the situation of BOTH 1 and 0 state at the same time at the synapse. That the brain can be in a quantum state at the core body temperature and the brain can effectively avoid ‘decoherence’ in the background thermal noise has been discussed by Roger Penrose and Stuart Hameroff. We also know that memories aren’t kept as such, but they are fragmented into individual elements, which are mostly matched to existing elements and are associated. This is economic as it saves space, and useful for indexing and contextual retrieval.

a device for administering deep brain stimulationThus it seems that we ought to have immense memory storage. Haven’t we encountered long forgotten memories in our dreams? Electrical stimulations in some parts of the hippocampus (deep brain stimulation or DBS, figure shown) during routine surgical procedures have given rise to ‘deja vu’ phenomena. The patients remembered things considered long forgotten. We may not be aware of the vast database of memories and are liable to infer that we have “killed ‘em all”, but in reality this may not be the case as Norio Ota et al clearly points it out in their paper. It smells like another chapter from your favorite science fiction novel, but it could be true.

Last modified: never; N.B.There is a substantial amount of speculation in this paper. Please exercise your own judgment and enlighten me about any possible error.
Reference: hyper-links, unless specifically mentioned.

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