Monthly Archives: September 2003

Mysteries of the Brain Part – 9 – HUMAN MEMORY

This post was written on Thu, 25 Sep 2003.

Human Memory :
The cells responsible for brain processes and memory are called neurons. Neurons have specialized projections called dendrites and axons. Dendrites bring information to the cell and the axons take information away from the cell. So Neurons are connected to each other through these projections. A neuron can have several dendrites but it usually has only one axon. But fear not! At the end of the axon there are several terminals. These terminals can be connected to several other neurons. So our nervous system is made of neurons connected to each other through their projections. Each of these connections is called a synapse. There are a HUGE number of synapses in the brain. In a new born child each neuton in the cortex has about 2500 synapses!! And the human brain has about 100 billion neurons…… Imagine that!!


See the above image. THis shows a neuron with its dendrites, axon and terminals at the end of the axon. Now see the image below.This shows the synapses between the dendrites of one neuron and the axon of another.


And the number of connections is ot fixed. The human brain is very “plastic”. It can grow/reduce based on your needs. A new born child has about 2500 synapses per neuron. By the time the child is 2-3 years old this grows to 15000 synapses per neuron!! The child is curious.
He wants to learn. And tragically, the number of synapses per neuron in an adult brain is about 1/2 that of a child. He is no longer curious….
He wants to read only about his profession. He is too busy impressing people to be able to think any orignal thoughts. What he does
not understand he attributes to God :-). But what is happening here????!!! Do the number of synapses decrease? Does this mean that the brain is
deleting synapses? Yes! This is called synaptic pruning. Like a gardener, the brain  removes the synapses that are not needed….But how is that
decided? If a synapse has not been used for a long time and does not have an emotional significance then it is removed. Snip! snip! and there
goes your knowledge. When was the second battle of Panipat fought? Who defeated whom?? Forgot? You got snipped by your own brain sweetie.



So how do we learn something new? When we experience something a set of neurons activates(“fires”) together. They form a connection
between each other. If the experience is very exciting then the neurons fire more intensely and the connections between them become stronger.
The next time a similar event occurs there is a strong possibility that the same neurons will fire again and you will remember! So the whole event is coded in these inter neural connections. If you experience only one aspect of an old memory you might experience the whole thing again because the neurons associated with that intense memory will fire together again. You might have noticed that a certain song, a certain taste  will take you back to a time when you experienced it first. But this happens only if the orignal experience was intense enough to produce that kind of firing of neurons. I know a set of songs that take me back to those old times….and a certain taste of coffee that will take back to those exam days and I will remember not only the coffee but allso the exams and the books… the whole experience.


That is why to remember something that we have forgotten you can try and think about a related thing. So when you think about something related the neuron for the memory that you want to remember may fire with it and you may remember.


How do neurons communicate with each other? This is an electro chemical process. An electric signal travels down an axon and when it reaches the terminal a set of chemicals get released which travel across the synapse to the dendrites of the connected neuron. This causes the electric signal to travel from the dendrites of the new neuron to its cell centre etc.


A temporary memory is a simpler connection. A loop: a neuron X is connected to Y and Y is connected to X. After some time the connection is not there anymore. Only if your brain feels the need will this be connected to the larger permanent memory.


A few other facts about memory:

Some of our most emotional memories are triggered by smell!!

  •     Excitement enhanhances memory formation and depression inhibits it.

The human brain has a special area to store faces. That is why sometimes you will recognize a face but not be able to recall his name.


You have more memories than you can remember. The neurons may be there but you are able to reach them because of a lack              of connections  to them. An isolated incident/experience may spark it again.


” Lulled in the countless chambers of the brain,
Our thoughts are linked by many a hidden chain
Awake but one, and lo, what myriads rise” (Alexander Pope)

Enjoy this video by VS Ramachandran :





Go, wondrous creature! mount where Science guides:
Go, measure earth, weigh air, and state the tides:
Instruct the planets in what orbs to run,
Correct old time and regulate the Sun;

Mysteries of the Brain Part – 8


This post was written on Thu, 11 Sep 2003.

This today’s VERITAS we study the human memory. We will see how storage and retreival of memory happens and where all this memory is
stored. In this part we will concentrate on the methodology of memory storage and the kinds of memory. There will be a VERITAS next week
in which we will study the brain cells responsible for memory and how they connect to each other.


Lets divide memory into three parts: sensory memory, short term and long term. As we go through life our senses constantly perceive the
world around us. This information gathered by our senses goes into sensory memory. But we do not want to learn everything. If we did then every
sound, every sight every smell of every moment would be stored! and that would be too much. Most of the information received by our senses
is immediately lost. Only the events that interest us go into short term and long term memories. The rest are immediately forgotten. You can
go through a whole book without remembering anything. The key here is attention. If you pay attention to something only then does a sensation
have a chance of becoming a longer lasting memory.

The sensations that are not immediately forgotten go into short term memory. This is the sketch pad of our brain. This is a temporary or working memory. This memory lets you store numbers while you are doing calculations. It lets you remember the beginning of a sentence when you are nearing its end. This memory isnt too much. It can handle “seven plus minus two” pieces of information at a time according to George Miller. There is not one short term memory “sketchpad” but two : the phonological loop which stores words and numbers as sounds and the visual-spatial loop which stores images briefly. That is why you can drive and listen to music at the same time! Imagine what would happen
if there were only one short term memory sketch pad?!!! This memory decays rapidly unless you “want” to store it in the long term memory.


Information that is needed for a longer time than just a few seconds is tranferred from short term memory into the long term memory.
There are two types of long term memories : the declarative and the procedural. The procedural memory is a memory of skills acquired.
We learn swimming and a few years later we still know how to swim even if we do not practice. This kind of memory is the longest lasting!
The declarative memory is about facts and is further divided into two parts : The episodic(storage of events in a serial form) and declarative( the meaning of a word, a multiplication table etc).


The procedural memory is very impressive. A person may forget his name but a person will NEVER forget a skill like walking or riding a cycle. It is that good. People with amnesia( memory loss) are sometimes surprised when they realize that they know how to do something without remembering it. Imagine asking a person if he knows how to drive a car and he says “No” and then you make him sit in a Ferrari and he gives Schumi a run for his money! This is all procedural memory. And we have seen in an earlier VERITAS that dreaming is a very important component of storing such long term memories. So when you want to learn a new skill do not do it in one day. Space the learning over a set of days. For each day you will dream and the memory of that skill will be etched deeper!


The key thing in memory storage is attention. You make the decision to store or not store something depending on how interested you are in that. Skills are stored longest in procedural memory because our survival depends on remembering these!


Our brains have developed a lot in the last few thousand years. But we retain the brains of our animal ancestors. The reptilian brain is still there. It is called the limbic system. The mammal brain is there. The cortex is a recent development and is uniquely human. The Amygda,is a part of the limbic system and stores emotions. So emotions are stored in the deepest reptilian parts of our brain. The cortex is responsible for storing numbers, facts etc, the human thingies.


Now an interesting aspect of the memory that we have all experienced: Deja Vu: sometimes you get a stong feeling that you have been
through a situation before. You get a very strong feeling that this has happened earlier. It can be a pretty mysterious feeling. People associate Deja Vu with all sorts of explanations: you dreamt about something and that has some true! Or You have seen this thing in a past life! Or the Matrix program is being changed! crazy people! They dont research anything and just come up with conclusions. Modern science has a better explanation: The limbic system is responsible for the emotional tag on a memory. A memory may have some emotional component and the limbic system gives it the correct “tag”. Familarity is an emotion and the limbic system gives a feeling of familarity to certain events. The cortex is responsible for getting the facts of a event together. It KNOWS if an event has happend before or not. It knows when it has happened. Sometimes the limbic system makes a mistake.It gives a wrong emotional tag to an event. It makes something that is happening now and HAS NEVER HAPPENED BEFORE a emotional tag of familarity. When this wrong tagged information is combined with the cortex fact that it is a new thingie you get a feeling of Deja Vu. You think that this event is familiar but another part of the brain, the part we trust for facts says that NO! This
is new!!!!!


But Deja Vu is a very interesting thing.It is a relationship between our reptilian brain and our human developments. If I were less interested in Physics I would love to do research on Deja Vu. I get a feeling that I have said this before!!! 🙂


Remember Me. I will be back.



Go, wondrous creature! mount where Science guides:
Go, measure earth, weigh air, and state the tides:
Instruct the planets in what orbs to run,
Correct old time and regulate the Sun;