CHAPTER 3

This chapter directly addresses the hypothesis from chapter 1: That an enhancement in working memory capacity powered the appearance of the modern mind. By the 1950’s, psychologists had come to a more or less general agreement that people relied on two distinct kinds of memory; short term and long term. By the early 1970’s, experimental psychologists including Alan Baddeley recognized limitations in this two component model of memory. At the time, short term memory research was largely devoted to the study of an acoustic, temporary, limited capacity verbal store, and was typically measured by a simple digit span task where subjects were asked to repeat varying series of numbers. If prevented from rehearsal, short term memories vanished in a matter of seconds.

Competing models attributed this to either fading of the electrical trace of the memory or because later perceptions interfered with them. Both of these models failed to account for significant experimental results. For example; it was unable to explain why some simultaneous tasks interfered with one another (memorizing a list of words while reciting a list of numbers) but others did not (remembering the colors of presented objects while reciting a list of numbers). In 1974, Baddeley and his colleague Graham Hitch proposed a more comprehensive cognitive theory that accounted not only for the standard operations of short term memory but also how memory is enjoined and directed, and how it is related to long term memory.

Baddeley and Hitch’s model of working memory The initial Baddeley and Hitch model included an attentional, panmodal controller or central executive, and two subsystems: the phonological loop and the visuospatial sketchpad but recently Baddeley expanded the central executive’s functions by adding the episodic buffer. This episodic buffer serves as the memory component of the central executive and integrates and temporarily stores information from the other two subsystems. The phonological loop contains two elements: a short term phonological store of speech and other sounds and an articulatory loop that maintains and rehearses information either vocally or subvocally. Baddeley viewed its primary purpose as evolving for language acquisition and comprehension. The visuospatial store was hypothesized to involve the maintenance and integration of visual (“what” information) and spatial (“where” information) elements and a means of refreshing it by rehearsal.

The Central Executive With some modifications, Baddeley and others currently view the central executive either as a unitary system or multiple systems of varying functions including attention, active-inhibition, decision-making, planning, sequencing, temporal tagging, and the updating, maintenance, and integration of information from the two subsystems. In most current models, working memory not only serves to focus attention and make decisions but also serves as the chief liaison to long term memory systems, and to language comprehension and production. Baddeley adopted an attentional control system called the Supervisory Attentional System (SAS), originally proposed by Norman and Shallice (1980) as the basis for his central executive. The SAS takes control when novel tasks are introduced, when pre- existing habits have to be overridden, or when danger threatens and task relevant decisions must be made.

How and why does the central executive make its decisions? Executive attention is the critical component of working memory, whose chief function is the active maintenance of appropriate stimulus representations relevant to goal attainment in the face of interference rich contexts. Miyake and Shah (1999) proposed that the attention and decision making qualities of the central executive may be an emergent property that arises as a function of the dynamic interplay of the multiple and interrelated systems associated with working memory. Barkley (2001) also favored an evolutionary perspective to explain executive functions. He viewed them as a biological adaptation resulting from interpersonal competition in groups. He saw executive functions as a useful social self defense against resource theft and against interpersonal manipulation. He also saw them as advantageous in social exchanges and in imitating and learning from others. He proposed that these executive functions evolved in gradual stages over a period of at least a million years- the ability to attend to relevant stimuli, filter out irrelevant stimuli and to make quick and efficient decisions was favored over static processes.

Neural Structure The executive functions appear to result from the interplay of diverse cortical and subcortical neural systems as well as the dorsolateral prefrontal cortex, the orbitofrontal prefrontal cortex, and the anterior cingulated cortex. There has also been evidence presented for the basal ganglia and the cerebellum. The dorsolateral circuit is generally associated with the classic executive functions-complex problem solving, decision making, verbal fluency, and some of the operations of working memory. Gazzaniga et al (2002) attributed its attentional functions primarily to the anterior cingulated gyrus. Hazy, Frank, and O’Reilly (2006) proposed a complex model, called PBWM (prefrontal cortex, basal ganglia working memory model) which accounts for the mechanistic basis of working memory, the central executive, and its executive functions. They view the prefrontal cortex as critical in maintaining representations of an individual’s perceptions in the broadest sense which are dynamically updated and regulated by reinforcement learning systems based on chemical neurotransmitters (primarily dopamine) activated by the basal ganglia and the amygdala.

Phonological Loop The phonological loop is intimately involved in language use. Basically, it engages in various verbal and acoustic tasks and keeps these going until you are done with them. Baddeley hypothesized that it has two components: a brief sound based storage that fades within a few seconds and an articulatory processor which maintains material in the phonological store by vocal or subvocal rehearsal. Spoken information appears to have automatic and obligatory access to phonological storage and therefore Baddeley hypothesized that it evolved principally for the demands and acquisition of language. Repetition of sounds held in the phonological store, usually by means of the vocal or subvocal articulatory processor will relegate those sounds into long term declarative memory if there is sufficient motivation or emotional salience. A strong motivation to memorize or an elevated emotional meaning will increase the likelihood that the sound will be successfully transferred into long term memory.

Phonological Loop Sounds can be relegated to long term memory even if they have no initial meaning. For example repeating “Won-due, era-due, muru” over and over either vocally or subvocally will eventually transfer them to long term memory. ( phonetic sounds of 1-2-3 in a south Indian language) The phonological loop’s processes also help explain why brain-damaged patients who have lost their ability to repeat sounds vocally can still memorize them. However, those patients who cannot create a sound or speech motor form through the phonological loop cannot memorize new material. Recently, Aboitiz, Garcia, Bosman, and Brunetti (2006) have noted that phonological storage capacity represents a short term memory ensemble that can be phylogenetically tracked to earlier homologues in hominin evolution and to current primate brain systems. Further, they postulated that language has evolved primarily through the expansion of short term memory capacity, “which has allowed the processing of sounds, conveying elaborate meanings, and eventually participating in syntactic processes”. Plays an important role in adult second language learning, the acquisition of native language learning in children, and memorizing stimuli in the visuospatial sketchpad.

Phonological Loop Neural Structure Aboitiz et al. situate the neurological epicenter of the phonological loop in the posterior superior temporal lobe gyrus and the inferior parietal lobes areas. They also agreed with Fuster (1997) who noted that the dorsolateral prefrontal cortex plays an important role with reconciling short term past and short term future and cross temporal contingencies. Thus insuring that one’s present speech is in accord with previous utterances is a function of the complex interactions of the prefrontal cortex, temporal, and parietal areas as well as interconnectivity with other cortical area and subcortical structures. Becker, MacAndrew, and Fiez (1999) have also identified the neural location of the phonological store as the inferior parietal lobe of the speech-dominant hemisphere, particularly the supramarginal and angular gyri. Additionally, Broca’s area has been implicated with phonological tasks that present the items visually.

Visuospatial Sketchpad A temporary store for the maintenance and manipulation of visuospatial information. It also has an important role in spatial orientation and solving visuospatial problems. Studies with brain damaged patients, and with healthy adults, appear to demonstrate that the visual and spatial processes may comprise separate memory systems, although the visuospatial sketchpad is assumed to form an interface between the two systems. Visual information can also be integrated with sensory information such as touch and perhaps smell. Neuroimaging and other neuropsychological studies with a variety of brain damaged patients and other people indicate significant involvement of the primary visual cortex (occipital lobes), the parietal lobes, and the frontal lobes.

Episodic buffer Baddeley initially described the central executive as largely attentional in nature without its own storage capacity, but eventually realized it also must have some way to store information independent of the subsystems (how else could phonological, visuospatial, and long term memory information be integrated?) He thus proposed the episodic buffer as the storage component of the central executive. He endowed the episodic buffer with the ability to bind and integrate the two subsystems, the phonological loop and the visuospatial sketchpad, and also traces from long term memory via a multimodal code. By attending to multiple sources of information simultaneously, the central executive is able to create models of the environment that themselves can be manipulated to solve problems and even plan future behaviors and alternative strategies, so that if a plan fails another may be chosen or generated.

A Memory Primer Cognitive psychologists have long distinguished between declarative memories and procedural memories. Procedural memory is the long-term memory of skills and procedures, or "how to“ knowledge (riding a bike, tying your shoes). Declarative memory is a person's memory of facts or events. It is learned information (phone numbers or the makeup of chemical compounds). Both types appear to use relatively independent neural pathways; selective types of brain damage may affect one type of memory but not the other.

Declarative memory There are varieties of declarative memory. An episodic memory is a coherent, story like reminiscence for an event, often associated with a specific time and place and a feeling signature. It is sometimes labeled personal memory or autobiographical memory. The other type of declarative memory is semantic- the memory for general facts. Reminiscence will include semantic details, but its recall and subjective experience will be psychologically and neurologically different from the recall of the semantic components alone. Episodic memory is advantageous in that people use their episodic memories to simulate future scenarios. This ability has been labeled constructive episodic simulation by Schacter and Addis.

Declarative memory Tulving (2002) has proposed that the ability to simulate and contemplate future scenarios has been the driving force in the evolution of episodic memory. He proposed the term autonoesis to refer to the ability, unique to humans, to form a special kind of consciousness in which individuals become aware of the subjective time in which past events happened. It is also this ability that allows humans to travel mentally in time. Mental time travel, by way of episodic processes, allows awareness of not only the past but of what may happen in the future. “This awareness allows autonoetic creatures to reflect on, worry about, and make plans for their own and their progeny’s future in a way that those without this capability possibly could not. Homosapiens, taking full advantage of its awareness of its continued existence in time, has transformed the natural world into one of culture and civilization that our distant ancestors, let alone members of other species, possibly could not imagine.”

Declarative memory Baddeley proposed that greater working memory capacity would allow for the reflection and comparison of multiple past experiences and that this might allow an individual to actively choose a future action or create an alternative action (based on the success or failure of previous actions) rather than simply choosing the highest path of probably success. Recent neuroimaging studies now link the prefrontal cortex to episodic memory recall. When normal adults are asked to form episodic memories (through experimental manipulation), the left prefrontal cortices are differentially more involved than the right prefrontal cortices, whereas when they are asked to recall them, this pattern is reversed and right prefrontal cortices are more heavily involved.

Working memory capacity Because working memory appears to involve simultaneous attention to task relevant information, as well as its manipulation, processing and storage, experimental psychologists researching individual differences developed the concept of a working memory span and more recently working memory capacity. (synonymous although measurement varies according to the experimental task) Working memory capacity= simply how many items can be recalled during a task. Engle and Kane have importantly noted that there is probably no pure measure of working memory capacity, as a task must be designed in the domain such as verbal, visual, acoustic, spatial etc. Thus it may not be directly measurable. A variety of measures have been developed to asses working memory capacity and these measures do have practical applications.

Working memory capacity Working memory correlates with a number of important practical abilities including reading comprehension, vocabulary learning, language comprehension, reasoning, suppression of a designated event, language acquisition and second-language learning, many neuropsychological measures, fluid intelligence, and general intelligence. The strong relationship between fluid intelligence and working memory capacity is an important one because fluid intelligence is thought to measure ones’ ability to solve novel problems and it appears less influenced by learning and culture and more by some natural or inherent ability to figure out solutions to problems.

Heritability of working memory It is clear that the bulk of human nature and behavior; including predilections, predispositions, fears, personality, psychopathology, and motivations have evolved via natural selection upon genetic mutations over millions of years. In 2001 Morley and Montgomery compiled a list of over 150 possible genes that appear to influence cognition. Thus there is currently good empirical evidence that working memory, its executive functions, and its subsystems have a strong genetic basis. Coolidge et al. (2000) did an analysis of child and adolescent twins as rated by their parents and found that a core of executive functions, consisting of planning, organizing, and goal attainment, was highly heritable (77 percent) and due to an additive genetic influence. Ando, Ono and Wright (2001) also found a strong additive genetic influence (43-49 percept) upon working memory storage and executive functions, in both phonological and visuospatial tasks.

Heritability of working memory Rijsdiik, Vernon and Boomsma (2002) found a 61 percent additive heritability in young adult Dutch twin pairs on the digit span task of the Wechsler Adult Intelligence Scale, which is one measure of phonological storage capacity. Hansell et al (2001) using event related potential slowwave measure of working memory in a visuospatial task, showed solid heritability in a sample of 391 adolescent twin pairs.

Summary Working memory is a highly heritable trait and has received considerable attention. Hard to prove but it appears there may be some pure working memory capacity that varies across individuals and this undoubtedly evolved in primates and in hominin evolution. The nature of this capacity has something to do with attention to task relevant stimuli and the ability to maintain this information in active memory. The neural substrate of working memory, its executive functions and subsystems appears to be the frontal and prefrontal cortex, parietal and temporal cortices, and the basal ganglia among other cortical and subcortical regions. Studies have shown that working memory capacity is limited. When attention is diverted more info gets in and the old info is pushed out because of this limit. Most people average around 7 slots (plus or minus 2) and can hold info in short term store without rehearsal for around 20 seconds.

Phew! PH