Memory

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A huge topic, which will be only briefly covered here.  A good site to read for more information is Human Memory.

Memory – there are different types of memory and multiple parts of the brain are used.  The mechanism by which memory is stored is still not well understood, but the brain is capable of capturing and storing the synaptic activation’s of each brain wave interval (ie typically 25 ms for 40 Hz EEG conscious and alert functioning).

There are short sensory buffer memory areas in the sensory cortex (typically holiding up to 1 second at “full resolution”); there are short term and working memory areas in the pre-frontal cortex (up to a minute or two), and intermediate (2 – 3 hours) and longer-term memory (lifetime) in the limbic brain.  The long-term memory does seem to be summarised, but is stored away together with “metadata” that contains information about the quality of the experience such as a painful or emotional or joyful memory.

memory_types
Memory types – diagram by Luke Mastin
  • Sensory input has a short memory buffer of less than a 1 second for holding the latest sensory information.  These buffers are located in the relevant sensory processing area, eg Iconic memory in the Occipetal lobe, Haptic memory in the Parietal lobe, Echoic memory in the Temporal lobe.
  • The cerebellum motor areas has an area for recording learned procedures (such as walking or typing).  We learn these procedures early in life, and then store them away and call upon them many times.
  • Short-term memory in the prefrontal cortex holds information for a few seconds (maybe as long as 20 – 30 seconds)
  • There is also a working memory area in the PFC that holds larger amounts of information for a limited duration of time whilst we are “working” on it.  There is a theory of 7 chunks of information (though some people demonstrate the ability to work with larger amounts than this), for example being able remember 7-digit telephone numbers.
  • Between the short-term and working memory and the long-term memory, there is also an intermediate-term memory in the unconscious brain which retains information for 2 – 3 hours.
  • Long-term memory resides in the Limbic brain and it stores information from 30 minutes or so, to the end of a person’s life.  The long-term memory holds episodic memory (specific events), semantic memory (facts), and autobiographical memory (episodic memory about one-self).

There are two types of conscious declarative memory that we frequently utilise:

  • Episodic memory which is a continuous record of our life events and experiences (including autobiographical memory, which includes emotional details of how we felt)
  • Semantic memory which is about facts, eg that the chemical formula for water is H2O.

The Hippocampus appears to be involved in consolidating the memories and submitting them for long-term storage.  It does not appear that everything we experience in our lives is stored long term, just items that we deem to be “important”.

The memories themselves seem to be distributed throughout the brain, and perhaps encoded multiple times.  The precise mechanism is not understood.  A link is maintained as a means of retrieving the memory.  We don’t have a way of erasing memory, but access to memories can be replaced or superseded with a new link to a new memory.  It is know that stressful situations record memories more strongly.  The amygdala can also get involved and block the storage of very stressful situations.  People recovering from Post Traumatic Stress Disorder (PTSD) can struggle with recollection of events, or be continuously troubled with situations that they want to forget.  The memory storage and retrieval operations in the brain happen unconsciously, and thus are very hard for us to control.

Memory in other body organs

It is possible that other body organs can also store memories in some way.  There have been well documented cases of persons receiving an organ transplant suddenly acquiring new skills such as playing music instruments or speaking languages they had never previously learned but were skills of the person they received their donated organ from.  These cases remain controversial and the mechanism by which they could occur is not properly understood.

Memory of past lives

We begin life with almost a blank memory – almost, because it appears that either the amygdala or another Limbic brain area does inherit certain memories or images.  It seems that many people from an early age are able to recognise shapes such as snakes, that could pose a danger to their life.  I vividly remember being afraid of a spider when I was only about 2 years old, too young to have had any previous unpleasant experience with them – but I was immediately afraid of them.  It looks like evolution has provided us with a small pre-programmed selection of potential dangers.

What happens to our life-long memory after death is unclear – whether it is permanently erased as the body dies or perhaps its gets stored forever in the akashic records of the human race.  Some controversial memory experiments back in the 1960’s tried to prove that memory could exist outside the brain and be transmitted from one generation to the next, but this work hasn’t stood up to scientific scrutiny.  There are reports of past-life memories, and whilst they are hard to prove, some reports are very detailed and cannot be easily discounted.  Given our limited understanding of how memories are stored and recalled, and uncanny experiences of some people that have received organ transplants (and seem to have inherited memories from the organ donor), this is an area worthy of further investigation.