Categories of human memory

Human memory systems have been categorized in many different ways. Various conceptual dichotomies such as long-term/short-term memory (Miller, 1956; Brown, 1958; Peterson & Peterson, 1959; Broadbent, 1958; Atkinson and Shiffrin, 1968;

Cowan, 1988, 2000), primary/secondary memory (Waugh and Norman, 1965), semantic/episodic memory (Tulving, 1972, 1983, 1993, 1995, 2001), implicit/explicit memory (Graf and Schacter, 1985; Schacter, 1987; Graf, 1994; Duffy, 1997), and declarative/nondeclarative memory (Squire, 1987; Squire and Zola, 1996;

Eichenbaum, 1997), etc. have been used to classify human memory. Tulving (1995) has combined these dichotomies and proposed a more general scheme of human memory, which contains at least five major categories of memory system, whereby each system may contain several subsystems (see Table 1).

Procedural memory (or nondeclarative memory) involves knowledge about how to perform an action. “The operations of procedural memory are expressed in the form of skilled behavioral and cognitive procedures independently of any cognition” (Tulving, 1995: 840). The skillful performance of many motor or non-noetic tasks such as driving a car, riding a bike, or getting dressed and so on relies mainly on procedural memory. Priming is a kind of perceptual learning, which

is expressed in enhanced re-identification of objects that one has already encountered before. “A perceptual encounter with an object on one occasion primes or facilitates

System Other terms Subsystems Retrieval

Procedural Nondeclarative Motor skills

Cognitive skills Simple conditioning Simple associative learning

Implicit

Perceptual representation system

Priming Structural description

Visual word form Auditory word form

Implicit

Semantic Generic Factual

knowledge

Spatial Relational

Implicit

Primary Working Short-term

Visual Auditory

Explicit

Episodic Personal Autobiographical Event memory

Explicit

Table 1: Major categories of human learning and memory (Tulving, 1995: 841) the perception of the same or a similar object on a subsequent occasion, in the sense that the identification of the object requires less stimulus information or occurs more quickly than it does in the absence of priming” (Tulving, 1995: 841). Priming and procedural memory are essential to human beings in order to survive in their environment. According to Tulving, these two memories are probably phylogenetically very early forms which also develop early in human infants.

Primary memory is commonly referred to as short-term memory or working memory. It is responsible for the registration, organization, and storage of incoming information for a short period of time. Semantic memory refers to the factual information or the general knowledge of the world. It represents our knowledge system and enables “cognitive operations on the aspects of the world beyond the reach of immediate perception” (Tulving, 1995: 841). Semantic memory and episodic memory are sometimes termed declarative memory or propositional

memory. Episodic memory contains recollections of personally experienced events in the past. According to Tulving (1995: 841), “Episodic memory enables individuals to remember experienced events as embedded in a matrix of other personal happenings in subjective time. It depends on but also transcends the range of the capabilities of semantic memory.” Conscious awareness, which is also referred to as auto-noetic consciousness, plays an important role in describing the memory of past happenings.

The differentiation between implicit and explicit memory, according to Markowitsch (1999), does not refer to different memory systems but to different forms of memory or the way in which these two kinds of memory are retrieved.

Implicit memory refers to the expression of what a person has learned without necessarily recollecting when, how, or where learning had occurred. In contrast, explicit memory specifies the expression of what the person is consciously aware of as a personal experience (Douglas and Brian, 1992; Tulving, 1995). The retrieval of procedural, priming, and semantic memories are based on implicit operations while that of primary and episodic memories is carried out by means of explicit operations.

To account for the relationship between memory systems and memory processes (encoding, storage, and retrieval), Tulving has proposed the SPI model.

SPI stands for serial, parallel, and independent; encoding is serial, storage is parallel, and retrieval can be independent. This model assumes that the relations among different systems are process specific. “Different systems are dependent on one another in the operations of interpreting, encoding, and initial storing of information.

Once encoding has been completed, different kinds of information about the same initial event are held in various systems in parallel… Access to different kinds of information about a given event is possible in any system independently of what happens in other systems” (Tulving, 1995: 844).

Aside from the memory categories proposed by Tulving, there are some other important categories of human memory, which are based on different criteria such as the duration of retaining information or the way in which information is received.

Regarding memory as a function of duration, we can subdivide memory into a short-term memory system and a long-term memory system. The former can retain information only for a short period of time (a matter of seconds or, at most, a few minutes) and the latter can store information permanently. In considering the function of memory as a system for storing and retrieving information received through our senses, we can classify the memory systems according to different senses (visual or auditory memory, for example).

The storage of visual and auditory information in human memory has been well investigated in a number of laboratory experiments, while the storage of

olfactory, tactile, or gustatory information has not been well explored. However, it may be assumed that, for each kind of sensory information, there is a corresponding memory system. When people receive different sensory input from their environment simultaneously, different sensory registers work in parallel and hold information in the same form in which it is received for a few milliseconds. Neisser (1967) termed the memory systems responsible for storing visual and auditory input over a matter of milliseconds as iconic and echoic memory accordingly. Following Baddeley (1997), however, these two kinds of short-lived memories should be regarded as a part of the processes involved in perception. Both of the memories seem to be able to prolong the initial stimulus to enable later processing to be carried out by the corresponding short-term visual and auditory memory systems. Further processing involves the manipulation and the integration of the sensory-based information with information from other sources and the information stored in long-term memory.

This should be performed by means of the working memory system (see Section 3.3). It should be noted that even with long-term storage, the representation of sensory-based information in memory may still retain sensory characteristics. Such memory involves the recollection of faces, scenes, voices and tunes.

The question of how human cognitive systems process and store information over shorter or longer periods of time is a core issue in any discussion on learning.

Short-term memory, which is also termed working memory by some theorists, is particularly relevant for the acquisition of new information. A slight difference between short-term and working memory, however, is that short-term memory focuses on the duration of information storage while working memory focuses more on the processing of new information. In the following subsections, I shall describe these two memory systems in more detail.