September 30, 2008

The Binding Of Memory And Hebbian Learning

Imagine this. Your physics professor is taking his regular class, and a boy sneezes. The professor is startled and the chalk falls from his hand. The whole class breaks into laughter. The memory of this particular incident gets stored in your head.

The whole process is somewhat like this. The visual scenes (your teacher’s attire, the chalk and the blackboard etc) are analyzed and processed in the visual association areas; the sounds (sneeze, laughter) get processed in the sound processed in their respective association areas and so on. Thus memory seems to be broken down to its individual elements; and these elements confine themselves in the areas in the neocortex where they were first processed.

human brain showing hippocampus
Hippocampus (means sea horse in Greek) is a small banana shaped structure inside the brain. It gets its input from these association areas via another structure called the parahippocampal cortex. Hippocampus, as if, queries those areas: ‘what happened’, ‘when did it happen’ and ‘where did it happen’? Then the hippocampus binds all those information in the form of an event. For this particular episodic memory (classroom drama); the hippocampus wires together the respective areas so that the whole event is now bound together into an ‘engram’, the proposed neuro-anatomical representative of a particular memory.

While all these are happening in the medial temporal lobe (MTL), more specifically the hippocampus, the actual memory elements are still in the neocortex. With each recapitulation, voluntary (by thinking about the incident) or involuntary (someone else’s sneezing reminds you of that event), the association gets stronger. Thinking of sneezing (memory stored in neuron A) reminds you of chalk falling (neuron B). This way as A becomes active (fires), B is associated too, and this leads to wiring them. This is known as 'Hebb’s rule', after Donald Hebb, a Canadian scientist. Simply put, it says, ‘cells that fire together, wire together’.

The strengthening of synapses as a basis of learning were later found to have been mediated in part to LTP or long term potentiation, a chemical process. While the memories are being strengthened this way, the neocortical areas become more and more inter-connected. This releases the MTL connection, as the memory finally gets settled in the neocortex. This 'plasticity' is important. The neocortex stores memories effectively, has a large storage space, but it learns slowly. The MTL learns quickly, but has little storage space. One can think of neocortex as the ‘hard disk’ and MTL as the ‘RAM’ of a computer. Freeing the MTL would enable it to acquire memory more efficiently.

No wonder my teacher said, ‘Read once, write twice, think thrice’. Its time we consolidated our memories by recap.  

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