The first system called sensory memory (SM) processes mainly visual and auditory information from our environment (Myers, 2010). It is made up of several components associated with each sense and filters information experienced by the senses – iconic SM which receives visual information has a very short retention time of merely a half of a second while echoic SM which receives auditory stimuli has a slightly longer retention time of three to four seconds (Sperling, 1960; Myers, 2010). If attention is brought to sensory information, it is transferred to the next system called short term memory (STM) (Atkinson & Shiffrin, 1968).
This system processes visual and auditory information received from SM for approximately one minute and also temporarily uses retrieved information from the long-term memory for problem-solving (Myers, 2010). Miller’s (1956) study has shown that STM is not only limited in retention duration but also in capacity having on average a storage capacity of seven bits of information whether verbal or numerical (Myers, 2010). The last system is long term memory (LTM), often referred to as the ‘permanent storage’ (Atkinson & Shiffrin, 1968).
As its name suggests, it has unlimited capacity to store a vast amount of information such as motor skills, language, autobiographical and factual information (Baddeley, Eysenck, & Anderson, 2009). The three basic memory stores are distinct from each other in terms of encoded information, capacity and duration of information retention (McLeod, 2007). One of the strongest evidence to support this distinction lies in Murdock’s (1962) experiment – when presented with a list of words, the tendency was that the participants would more likely recall the first words (primacy effect) and the last words (recency effect) than the words in the iddle of the list (Myers, 2010). This is known as the serial position effect whereby the first words are recalled since they have been transferred to LTM and the last words were still accessible in STM (Myers, 2010). However, the middle words were present for too long to be in the STM but not long enough to be encoded in LTM, giving evidence that STM and LTM are two different and separate stores (McLeod, 2008). Research on patients suffering from amnesia also support the multi-store model (Groome, 2006).
One classic case is reported by Corkin (1968) about a patient HM who suffered from anterograde amnesia – He was unable to form new memories and lost part of his existing memories (Groome, 2006). However, despite his inability to create new memories, he was still able to have a conversation thus indicating that his STM processes were intact with normal capacity and duration (Wickelgren, 1968 as cited in Groome, 2006). He lost the capacity of only one store hence supporting the idea that STM and LTM are separated (Groome, 2006).
Another case is a patient known as KF who suffered damaged to his STM – he was still able to temporarily recall visual information but could not process auditory information which made conversation difficult (Myers, 2010). On the other hand, his LTM processes were retained, again providing evidence that STM and LTM are separated systems (Myers, 2010). Although KF’s case study explicitly provides strong evidence for distinction of STM and LTM, it also questions the idea of having only one STM (Groome, 2006).
The damage in KF’s STM seemed to have only affected the echoic and verbal aspects leaving the visual aspects intact (Groome, 2006). This suggests that STM is more complex and has separate stores for visual and auditory processing (Groome, 2006). Based on this finding, Baddeley and Hitch (1974) argued that STM illustrated by the multi-store model is too simplistic (McLeod, 2008). They developed another model of STM known as the Working Memory (WM) which comprised of a two subsystems controlled by the central executive (Groome, 2006).
The central executive takes on cognitive processes such as problem-solving while controlling allocation of data to the two subsystems namely Phonological loop (PL) which deals with auditory and speech-based information and visuo-spatial sketchpad (VSS) which, as its name suggests, holds information about visual and spatial information (McLeod, 2008; Groome, 2006). By analyzing in more depth KF’s case study, it is reported that KF had suffered impairment of his PL but still had his VSS abilities, hence providing support that STM is not unitary but in fact divided into subsystems (Warrington & Shallice, 1972 as cited in Groome, 2006).
Similarly, HM who had an impaired LTM was unable to form new memories but surprisingly could still learn new motor skills although he was not aware that he actually learnt the skills (Groome, 2006). This finding questions the unified nature of LTM and suggests that there are two memory systems (Squire, 2004). McDougall (1924) investigated the term implicit memory which involves unconscious learning and explicit memory which involves learning with consciousness (Graf & Schacter, 1985; Schacter, 1987; Brooks, 2012).
Cohen and Squire (1980) suggested that amnesiacs such as HM had an impaired explicit or declarative memory which hindered their abilities to recall previous or make new memories of facts and events (Groome, 2006). They referred implicit memory as procedural memory involved in knowledge of skills which would explain why HM could still learn new motor skills (Groome, 2006). According to Tulving, Schacter and Stark (1982), declarative or explicit memory can be further separated into two conscious systems: episodic and semantic memory (Squire, 2004).
Episodic memory contains autobiographical contents, is a record of a person’s personal history such as birth dates and deals with past experiences (Tulving 1972; 1993; 2002). In contrast, semantic memory is a storage system of facts, meanings of words and general knowledge of the world such as the capital of cities (Tulving, 1972). Research on memory is arduous and the problems questioned are not easily solved (Tulving, 1985). At the beginning, the answer to the title question would be ‘three’ but when looking into further research based on studies of amnesiacs, the number increased to about approximately ‘seven’.
However, being aware of the difficult nature of memory and the limitations of its study, the more reasonable and sensible answer would be ‘at least three and probably many more’ (Tulving, 1985). References Atkinson, R. C. , & Shiffrin, R. M. (1968). Human memory: A proposed system and its control processes. In K. W. Spence & J. T. Spense (Eds. ), The psychology of learning and motivation (pp. 13-195). New York, NY: Academic Press. Baddeley, A. D. , Eysenck, M. W. , & Anderson, M. C. (2009). Memory. New York: Psychology Press. Baddeley, A.
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