As many humans there are, as many realities. In fact just like the finger prints, DNA prints (exception of identical twins), each of us have a unique Brain print with no exceptions as no two humans can be present at one place simultaneously and experience the same reality.
Our Brain along with the spinal cord (CNS -Central Nervous System) and nerves (PNS -Peripheral Nervous System which includes the ANS ie., Autonomic Nervous System and SNS ie., Somatic Nervous System) with the help of various sensory organs (special ones - Eyes for 3D color sight/vision, Nose for Olfaction/smell, Ears - Audition/Hearing and Equilibrium/Balance and acceleration, Tongue - Gustation/Taste and General ones - like the Touch (skin and viscera), Pressure, Vibration, Proprioception, Pain, and Temperature (hot and cold) senses the presence of energy and matter(within the human capability/limitations) and based on prior and real time knowledge base generates a sense of self and the surroundings - which we call our reality.
As the design is almost the same for all of us the our knowledge and experiences are the main confounding factors in the creation of our realities. That being said knowledge and experience are very powerful tools that can change our reality.
Our Brain does a marvelous job at Capturing and Deleting information/data almost simultaneously to keep us aware of our surroundings and self, and that is what we perceive to be "Real" and gives us a sense of "Present" time or "Real" time.
Our Brains possess a "very powerful filtering mechanism", meaning that humann brain is designed to "Forget" or "Delete" information/data and NOT to "Remember" it as most of you might think. Had it not been so we would be unable to move on and all information would pile up very quickly and confuse us and we would not be able to sense ourselves or our surroundings in real time as the past would be superimposed on it. There would be no "Reality" and no distinction between past, present and future and thus no sense of "Time".
That is not to say that Our Brains do not have the capabilities to remember concepts, ideas, information, and things from the past. It surely does in the various forms of "Memory":
Firstly, The Real Time Memory - lasting a few milliseconds or less (when we are moving fast) providing us enough time to create and comprehend reality and reflexly respond to its changes. It is then quickly erased or deleted as the brain is constantly bombarded with new information/data. This type of memory does not make any physical changes in our brain, it's purely electrochemical in nature. The Brain possesses the power to "scoop out" certain data - "filtering mechanism" - and push it back into short term/ long term memory (awake/sleeping memory). What it filters is "very unique" to each one of us and to each circumstance and is dependent upon an intricate balance between our knowledge base and prior experiences in those circumstances and our human body's basic anatomic and physiologic needs to stay safe and reproduce (survival instincts). It is also in part determined by our rewarding experiences (interests) and previous punishments (fears) which we can label as our "Motivating" factors and how an interplay of those relates to our present. What is very-very interesting about this type of memory is that our brain can speed up tremendously to "Slow Down Time" in life threatening situations when our bodies pump up our brain with "Adrenaline". This is much like the slow motion effect on a "film" when the projector rolls faster. This happens to provide us more time to better assess our surroundings, and to link it to our past knowledge and experiences to come up with a move to save ourselves (our survival instinct!).
Another important aspect of our survival instinct, which the "Magicians" have exploited for thousands of years to create "Illusions" is that we are easily distractable especially when it comes to our sense of Sight/Vision. In order to keep us safe, we involuntarily focus our attention on "Moving objects" as they could be dangerous to us. Magicians have been using these movements and sound effects to perform their tricks as we easily and involuntarily focus our attention on those and not on what they don't want us to see and they very easily get away with it.
Secondly, we have the Short-term memory which lasts from a few seconds to a few minutes and helps us in assessment of the present circumstances including our reflex responses and modulate them in the future as we learn and somewhat plan the very near future which changes into our present as the very near past gets deleted. This also does not make much physical changes in our brain. Some of this information, both reflexly and voluntarily can be pushed back further into long term memory based on its "repetitive utilization", "active recall" and on our levels of "Motivation".
Thirdly, we possess a Long-term memory (often life long) which as a result of repetitious use for example our Name, Spouse's name, name of our close family members, relatives and friends, own phone number, Social Security Number or some form of ID number, name of our Street, Apartment or House number etc. It is also mostly the one related to our parenting, schooling, interests, occupation, religion, voluntary active recall and our "Motivation" - these are things we cannot afford to forget or simply love too much! This type of memory has been shown to make physical changes like hypertrophy (increase in size) of certain brain areas based of excessive utilization and has been consistently shown to exist in various occupations.
Recent studies on sleep physiology have hinted that during sleep, we also possess these three different types of memories but are stored in separate regions of the brain and do interact and affect our reality, learning, behavior, and dreams.
Thus, our Memories are there to assist us with our future by analysis of the past and present to create a "Knowledge Base" which constantly changes our "Reality".
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Dr. Harish Malik