MODELS OF HUMAN MEMORY 17 The auditory counterpart to a visual sensory memory (or iconic memory) is the

so-called echoic memory. It is comparable to iconic memory in that a brief mental echo seems to exist for a few seconds after an auditory stimulus has been heard (for detailed information about sensory memory, cf. Ashcraft, 1995).

In the history of memory research, many names were used for what can be seen as the core component of the modal model (Atkinson and Shiffrin, 1968). Pri-mary memory, immediate memory, short term-store or SHORT-TERM MEMORY. The nameWORKING MEMORYemphasizes both tasks of this memory component – storage and processing. In cognitive science, there is a broad agreement that working memory is indispensable for all kinds of cognitive tasks. Baddeley illus-trates the need for a working memory with a simple arithmetic problem: “Suppose I ask you to multiply 23 by 7 in your head” (Baddeley, 1999:15). Baddeley shows that even for this simple task a list of subtasks has to be accomplished. Storing, processing or recalling complex numbers or single figures sums up to a complex cognitive task that asks for a lot of storage and processing over a short period of time. As Baddeley points out, when the problem is solved, there is no need to store all intermediate steps anymore. All processes will be banished from the ac-tive working memory the moment the task is fulfilled. Mental arithmetics is just one of numerous cognitive tasks. Storage and manipulation of information is just as well needed in all other mental tasks as, for example, language processing. In section 2.2.3, features of the working memory system will be discussed in more detail on the influential model of Baddeley and Hitch (1974).

The most permanent part of human memory is the LONG-TERM MEMORY

(LTM). This is probably the memory component that comes closest to scholarly considerations of memory (cf. the wax tablet or the storehouse metaphor). Con-trary to the components introduced above, long-term memory is supposed to be equipped with an unlimited capacity. Its main task is to store information over periods ranging from more than a few seconds up to an entire lifespan. This discriminates LTM from the two other components of human memory in which storage of information rather serves as an associated feature for superior tasks.

Being the ‘storehouse of memories’, long-term memory covers the entire knowl-edge in ones life, or, as Cicero puts it: “Memory is the treasury and guardian of all things”. Intensive investigations have led to some systematic order in the otherwise vague concept of human memory. Tulving, refering to an early con-cept of SEMANTIC MEMORY states that a “[. . . ] useful concept in science fre-quently is one whose definition not only makes very clear what it includes, but also what it excludes” (Tulving, 1972:384). He established a two-part classifica-tion of long-term memory and discriminated betweenSEMANTIC MEMORY and

EPISODIC MEMORY. Semantic memory refers to knowledge about the meaning of words, the properties of objects and general concepts about the world. Thus, Semantic knowledge is the knowledge of facts that is shared with other people.

Episodic memory “is a more or less faithful record of a person’s experiences.

Thus, every ’item’ in episodic memory represents information stored about the experienced occurrence of an episode or event.” (Tulving, 1972:387). Contrary to semantic memory, information allocated to episodic memory does refer to au-tobiographical facts and personally experienced events. Tulving’s categorization covers memories about facts of all kinds. Memory is more than just a storehouse filled with facts both autobiographical and generally shared ones. Human mem-ory also contains knowledge about riding a bike or swimming, about reading and writing. This memory for skills is also part of long-term memory. An elaborate account about the stored knowledge of skills in particular or the complex architec-ture of the entire long-term memory in general would by far exceed the limits of this chapter. For detailed information about the nature of long-term memory and current developments in research see introducing literature, for example Ashcraft (1995) or Haberlandt (1999). For a extensive anthology of influential papers on the topic of human memory see also Andrade (2008).

2.2.3 The Multi Component Model

In the early multi-store models of human memory (Waugh and Norman, 1965;

Atkinson and Shiffrin, 1968), short-term memory was thought to be a unitary component. Intensified research in the late 60s and early 70s suggested that short-term memory had to consist of various specialized subparts. Baddeley and Hitch (1974) introduced their influential multi-component working memory model that has been refined over the years (cf. Baddeley, 2000; for an overview cf. Badde-ley, 2003, 2007). The working memory model – serving as a platform for both storage and processing of information – consists of three subcomponents. (i)THE CENTRAL EXECUTIVE is supposed to be the core component which controls all activities within the system. The central executive is aided by two specialized sub-components: (ii) THE PHONOLOGICAL LOOP and the (iii)THE VISUO-SPATIAL SKETCH PAD.

. Figure 2.3: The Multicomponent Model (Baddeley & Hitch, 1974).

The Phonological Loop. The first of the two slave-systems of the model bases on assumptions that working memory relies largely on some form of acoustic or

2.2. MODELS OF HUMAN MEMORY 19 speech code (cf. Waugh and Norman, 1965; Atkinson and Shiffrin, 1968). Bad-deley and Hitch (1974) report a range of effects that support the assumption of phonological coding, including the PHONOLOGICAL SIMILARITY EFFECT, the

WORD LENGTH EFFECT and theIRRELEVANT SPEECH EFFECT. The phonologi-cal loop is considered to consist of two independent subsystems: aPHONOLOG

-ICAL STORE and aREHEARSAL BUFFER. The store is supposed to hold acoustic information, while the buffer serves to refresh items in the store that otherwise are subject to decay.

Early noteworthy findings are reported from studies of immediate recall of unrelated items in visual presentation in the mid-60s. Conrad (1964) run experi-ments testing visually presented lists of arbitrarily grouped and unrelated letters.

It turned out that immediate recall of phonological similar letters (‘BCPTV’ or

‘FMNSX’; cf. Conrad, 1964:77) was more error-prone than for phonological dis-similar lists of consonants. Subsequent work revealed that not only processing lists of unrelated consonants is affected by phonological similarity of the items.

(Baddeley, 1966) showed that words that sounded alike (‘map can cap man map’) were also more likely to be confused than words in control lists (‘pen day cow bar rig’) (cf. Baddeley, 1966; quoted in Baddeley, 1986:50). Baddeley (1986) states that the assumption of a subordinate system within the working memory model that encodes verbal material in a phonological code is backed by other phenomena than the phonological similarity effect, too.

A second phenomenon supporting the hypothesis of phonological encoding was first reported in Baddeley et al. (1975). They examined immediate recall on lists of different-sized, unrelated words. The findings suggest a causal link be-tween length of words and memory span size. Monosyllabic words (e.g ‘some, harm, hate’) resulted in increased memory spans compared to polysyllabic words (e.g. ‘association, considerable, individual’; examples from Baddeley, 1986:51).

A follow-up study with short and long names of countries (e.g. ‘Malta, Burma, Chile’ versus ‘Czechoslovakia, Switzerland, Afghanistan’; cf. Baddeley, 1986:50) was able to repeat this word length effect. Under the assumption that memory traces are kept active in the working memory system via subvocal repetition, it is a logical consequence that longer words decrease memory span as it takes longer to say them under the breath. Both phenomena are supported by experimental find-ings in studies applying so-called ARTICULATORY SUPPRESSION (AS). In AS, participants are required to repeat irrelevant words (e.g ‘the’) while being pre-sented visual stimuli. Repetitive speech is supposed to prevent subvocal rehearsal of the experimental material. Therefore, the transformation of visually presented information into the phonological code is blocked. As a consequence of this, both the ‘phonological similarity effect’ and the ‘word length effect’, do not occur.

This also holds for a third phenomenon which is reported in Salamé and Bad-deley (1982). They run a series of five experiments and showed that immediate recall on visually presented unrelated list items decreased when irrelevant spoken material was presented simultaneously. They suggest that the effect is indepen-dent of any semantic attribute of the auditory presented irrelevant speech. Their first experiment showed that meaningless syllables affected immediate memory as strong as meaningful words. Their findings of anIRRELEVANT SPEECH EFFECT

supported previous findings about disruptive effects on immediate memory of vi-sually presented stimuli. For example, Colle and Welsh (1976) showed that pro-cessing lists of visually presented items was adversely affected by simultaneously presented irrelevant speech in a language not familiar to the participants (Colle and Welsh, 1976; cited in Salamé and Baddeley, 1982). Their experiments also yielded the result that articulatory suppression ended the disruptive effect of irrel-evant speech effect. As for the previous phenomena, the ‘phonological similarity effect’ and the ‘word length effect’, this again argues in favor of the hypothesis of an encoding of verbal material into a phonological code.

The Visuo-Spatial Sketch Pad. The second slave system of the model is sup-posed to fulfill the same tasks for visual and spatial information as the phonolog-ical loop does for acoustic information. Just as hypotheses suggest an acoustic code underlying verbal information, there are assumptions that a visual code un-derlies storage of visual and spatial information. One alternative is to assume that such information is stored and generated from a more abstract code. Early findings support the view of a direct storage of visual and spatial information. Baddeley (1999) reports an experiment that asked participants to fold a three-dimensional cube out of two-dimensional ‘unfolded’ shapes consisting of small squares. Par-ticipants had to do so in order to judge if two arrowheads on different sides meet at the ridge of the cube. Results show that the amount of folds necessary to physi-cally create the cube were systematiphysi-cally related to reaction times in the (mentally accomplished) folding experiment (for details about the experiment and visual-ization of the shapes, see: Baddeley, 1999:59ff.). Baddeley (2007) reports clinical studies with two groups of unilaterally brain-damaged patients. All patients ac-complished experiments which required immediate reproduction of items. Stimuli were either presented auditory (words or digits) or visually (objects in a ‘Corsi-Block-Tapping-Test’). The tests were accomplished to assess either verbal or vi-sual memory spans. Results reveal that patients could be grouped with regard to their performances. One group of subjects shows decreased performance on the verbal memory tests, while spatial working seems to be unaffected in comparison to normal subjects. In the second group the reverse pattern can be found: verbal working memory seems to be unaffected while performance on the visuo-spatial

2.2. MODELS OF HUMAN MEMORY 21