You all must be familiar with the ‘memory games’ in which we try to recollect a bunch of words in the correct sequence or try to match identical pieces of a puzzle from a lot kept face down by inverting them one by one to reveal their pattern. We rely on our memory for accomplishing these tasks, hence the name. How are we able to remember and register names, places, incidents, patterns, tastes, odours, sensations and so many different things in our mind? How are memories created in our brain?
The human brain (i. e. the central nervous system) is an arrangement of interconnected cells (i. e. neurons) which relay information from an external sensory stimulus to bring about a desired response. Billions of neurons in the brain are interconnected with one another in a network with adjacent neurons linked by structures termed synapses. A synapse is the major hub of chemical activity in the brain, which conveys information from one neuron to the next. A single neuron is connected via such synapses to several other neurons. Events in our life trigger biochemical interactions and signalling pathways in the brain which in turn strengthen the connections between adjacent neurons.
How is information relayed and stored in synapses as memory? An external stimulus could constitute anything from smelling freshly baked bread to a catastrophic event that triggers the neurons in the brain to fire. The firing of neurons is the conduction of a brief electric impulse known as an action potential through the length of the neuron (through its axon or long tail like process). The more a neuron fires in response to a particular signal, the more activity is elicited at a corresponding synapse and this in turn strengthens the connection with an adjacent neuron. Repeated firing of a neuron or simultaneous activation of synapses contributes to the strengthening process involved in memory formation. Repetition helps in memorising and this is precisely why we most often resort to cramming certain topics of study prior to an examination. Constantly feeding the brain with the same sensory information induces the continuously firing neuron to produce certain proteins which ensure a long term synaptic activity even in the absence of the stimulus. This phenomenon is termed as long term potentiation (LTP).
Alterations in the activity of synapses formed by a neuron in response to an event or incident are the basis of memory formation. If a particular signal is taken away before sensitisation of the synapse — before a connection becomes permanent — then the memory fades away. You forget the name of an acquaintance after a brief encounter; however, the name of your best friend is stored as a long term memory since it is repeatedly accessed.
The accuracy of details stored as memory is not reliable unless the synaptic connection is strengthened over a long period of time via LTP. An analogous comparison for the same would be wet paint that can be smudged and altered, however once it dries with time, it becomes permanent.
Memories can be broadly classified into episodic and procedural memories. As the names suggest, episodic memories are memories formed as a result of episodes or events. Quite often, we associate two events taking place simultaneously and remember one based on other or vice versa. For instance, a particular person is linked to a distinctive smell in our mind just like a characteristic smell is associated with newborns. Procedural memories on the other hand, are memories formed due to routine motor actions, for example the way one signs his or her name.
It has been proposed that children are more receptive in the initial 6 years of age — formative years. This is because nearly all synapses in the developing brain of a child are yet to be strengthened. Children learn by the mechanism of imitation and repetition, hence they tend to form multiple memories by absorbing several stimuli from their environment.
Neurodegenerative disorders like old age related dementia, Alzheimer’s disorder, alcohol induced memory loss and memory loss due to trauma to the brain suggest that memories stored in certain areas of the brain can be lost due to damage to those areas and loss of synaptic connections.
We often choose to remember certain significant details over others and this implies that we emphasise the firing and activating of a few synapses over others. Since a single neuron forms synapses with several other neurons, the process of how a particular synapse is selectively made permanent amongst all those present is a much debated mystery. Phenomena like eidetic memory, commonly referred to as photographic memory, are also a mystery in terms of how all the stimuli are recorded permanently as memories.
There is still a lot to be explored with respect to the human brain especially the procedure of creation and storage of memories.
As Aldous Huxley said, “Every man’s memory is his private literature” for each mind is truly unique.