Sensory memory, short-term memory, and long-term memory comprise the recognized major systems of human memory. The division of memory between short and long-term began with metacognition by John Stuart Mill and James Mill in the 19th Century as they explored how their own memories functioned (Baars, Banks, & Newman, 2003). The concept of sensory memory originated in the 1960's when Sperling's research showed that the senses records a full picture of a series of letters before the brain can consciously read them (Lefton and Brannon, 2006). What is truly fascinating about the types of memory is that the physiology of the brain is tied to the processing of memory making the human evolutionary superior to every other animal.
The American Psychological Association (2009) defines sensory memory as, “the initial memory processes involved in the momentary preservation of fleeting impressions of sensory stimuli”. This rapidly fading sensory memory is only 250 milliseconds long and almost like a photograph capturing visual, auditory, tactile, or chemical stimulus for a brief moment (Lefton and Brannon, 2006). Short-term memory (STM) is defined as “preservation of recent experiences and with retrieval of information from long-term memory; short-term memory is of limited capacity and stores information for only a short length of time without rehearsal” (APA, 2009). Short-term memory lasts somewhere between 20-30 seconds at most. Short-term memory is associated with what the brain is currently considering at the moment which is five groups or “chunks” of information. The APA (2009) defines long-term memory (LTM) as, “memory processes associated with the preservation of information for retrieval at any later time.” The brain can store long-term memories for a lifetime, and it is accepted that barring disease or injury there is no limit to the amount of information that the brain can store (Nelson, 2006).
The primary reason that the brain has developed in this way over time is that natural selection has caused the brain to physiologically develop into what it is today. Neuroscientists currently call the human brain the "triune brain", composed of three separate but interconnected levels of the human brain: a) the brainstem and cerebellum), b) the limbic system, and c) the cerebral cortex. Grey matter in cerebral cortex becomes progressively larger as animals have evolved their way up the evolutionary ladder (How the Brain Works, 2009). Sherwood, Subiaul, and Zawidzki (2008) describe in detail how the human brain has evolved to produce the unique characteristics, of which greatly increased memory is one substantive advantage. Brain size is one important factor, the more developed prefrontal cortex is another which allows humans the ability to better focus of attention, which in turn enhances learning and memory. The human brain has also developed many other unique aspects of neurobiology making man better suited than primates. Studies have shown that the brains of fetuses display the capacity to recall acoustical vibrations as early as 30 weeks-old and retain that information for up to four weeks. Other similar tests show that 30 week-old fetuses have short-term memory for up to 10 minutes (Dirix, et. al, 2009). Even before birth the brain has adapted physiologically to learn and remember which gives the human the top spot in the evolutionary pyramid.
Practically, the three major systems of memory each serve unique purpose and fiction as the brain takes in information. It is not practical or necessary to remember every detail. The sensory memory is a base survival mechanism. If the body senses danger or damage it is important to retain that information briefly, assess it, and react. For example, if someone places their hand on a hot stove the brain signals damage is occurring through pain. In less than a millisecond the brain retains the information than the pain they are feeling is due to the stove and the hand moves away. It is key for a survival situation for the brain to have a sensory memory to react in a advantageous way without further conscious effort. Short-term memory is a simple tool for problem solving where the brain can take in, use, and dispose of information. An example of this is that when driving, the brain is keenly aware of the environment and surrounding risks. As short-term memory of what is around them us used by the consciousness it is discarded. Perhaps the bad driver that was next to them has exited off the free way. Now it is no longer important that the car was yellow and speeding so the memory of the car memory can be replaced with new, more important information for problem solving. Long-term memory comes into play when a critical or emotional event happens. If the yellow speeding car causes an accident it is now important that the information is retained long enough to report to the police minutes later or a court months later (Lefton and Brannon, 2006).
The human brain is more than an evolutionary supercomputer processing and storing information. The emotions that a person experiences also help the brain categorize and store important information. The brain also is not passive, if information is important it can be revisited and studied to ensure it remains available (Lefton and Brannon, 2006). The question of how the brain remembers is tied to the larger questions of consciousness, life, and the soul that are without solid scientific answers but what is known is critical to the understanding of psychology.
American Psychological Association. (2009). Psychology Matters Glossary. Retrieved October 10, 2009 from http://www.psychologymatters.org/glossary.html
Baars, B.J. , Banks, W.P., & Newman J.B. (2003). Essential Sources in the Scientific Study of Consciousness. Retrieved October 10, 2009 from http://books.google.com/
Dirix, C E, Nijhuis, J G, Jongsma, H W, & Hornstra, G. (July-August 2009). Aspects of Fetal Learning and Memory.(Report). Child Development, 80, 4. p.1251(8). Retrieved October10, 2009, from Health Reference Center Academic via Gale: http://find.galegroup.com.lumen.cgsccarl.com/itx/start.do?prodId=HRCA
How the brain works.(Disease/Disorder overview). (2009). A Guide to Alzheimer's Disease (Harvard Special Health Report), (p.8). Harvard Health Publications Group. Retrieved October 10, 2009, from Health Reference Center Academic via Gale: http://find.galegroup.com.lumen.cgsccarl.com/itx/start.do?prodId=HRCA
Lefton, L.A. & Brannon, L. (2006), Psychology (9th Edition), In Psychology: The Pearson Custom Library, ORG 5001, Survey of Psychology I. (2008). New York: Pearson, 209-258.
Nelson, A. (March 2006). What is memory?. Improving Memory (Harvard Special Health Report), (p.5(3)). Harvard Health Publications Group. Retrieved October 10, 2009, from Health Reference Center Academic via Gale: http://find.galegroup.com.lumen.cgsccarl.com/itx/start.do?prodId=HRCA
Sherwood, C C, Subiaul, F., & Zawidzki, T W (April 2008). A Natural History of the Human Mind: Tracing Evolutionary Changes in Brain and Cognition.(Report). Journal of Anatomy, 212, 4. p.426(29). Retrieved October 10, 2009, from Health Reference Center Academic via Gale: http://find.galegroup.com.lumen.cgsccarl.com/itx/start.do?prodId=HRCA