More thoughts on the neurobiological account for working memory

Central executive and attention control
The memory system is generally thought to be composed of two elements, short-term memory and long-term memory. Baddeley and Hitch (1974) developed "working memory" model from the short-term memory system, refering to short-term memory system that is involved in a temporary storage of information and processing (Gastherole & Baddeley, p. 2). The working memory system sublines human cognitive activities, such as "reasoning, language comprehension, long-term learning and mental arithmetic"(p.2). The WM system has three main componants that Baddeley use for explaning relationship between language and memory-with 'central executive' as center componant'; phonological loop', and 'sketchpad'serving as "slave systems" to central executive. The central executive involves in regulating information flow, and retrieval of information from long-term memory. Baddeley also quoted other people's studies in the book that the executive might be involved in planning, task coordination, and consious awareness, and selection and control of action. The concept comes from Shallice's (1988) model of the "attentional control of action." Shallice (quote) reported a paradoxical phenomanon of frontal lobe damage patients. Due to the impairment to the central executive (SAS, "Supervisory Attentional System), the patient can't selectively intervene the activated schemas for apporpriate actions. That is, the patient either have difficulty in inhibiting other distracting schemas, or show perseverance of an already activated schema.

Working memory cells
Coming from neurophysiological point of view, Fuster (2003) looks at memories as cortical networks. The formation of memory is through "the modulation of transmission of information acorss synapses, the neural elements anatomically associating cells with one another" (p. 113). Therefore, memorys are associative with one another. Working memory under such sense is what Fuster called "attention focused on the internal representation of a recent event for a pending action" (p.155). The main propoerties of working memory is not to form a new memory, but to hold active at the need of processing a new action. Thus it's components can be both new items such as sensory inputs, associations; and old, past experience (long-term memory).

The most notable neuronal evidence for this sustained attention is found in monkeys performing a "delayed-response task" (todo:elaboration). Cells in monkeys' prefrontal cortex were found to maintain discharged at a higher level in delayed period (i.e. when the stimulus is obsent from the environment) than in periods between trials. These cells are later called "memory cells". The characteristics of the finding are, first, the delayed-period activation was dependent on the impending need to perform an act "that's contingent on the signal in memory". Second, it was not dependent on the expectation of reward. Third, the accuracy is correlated with the monkey's ability to remember the signal. Forth, the performance could be attenuated when the animal was distracted. (p. 156). In short, these cells discharge selectively to the retention of a stimulus, and they can be distracted by other stimulus concommitently.

Physiologically, single neuron recordings (Goldman-Rakic et al.,1996) reveal neurons in prefrontal cortex firing specifically to the same one or a few locations in a two-dimentional visual field when the animal was performing visual delayed response tasks. According to Fuster, the significance of Goldman-Rakic and collegues' work is that they dissociate two subareas of cortex that's specialized in working memory function: neurons engaged in the memory of the object predominate in the ventral aspect of lateral prefrontal cortex, whereas the neurons for memory of special information predominate in the dorsal apspect. This examplifies that the nature of working memory is not a single unit or localization, but more a distributed network(p.158).

Other studies involving tasks during a state of attention, so called "working memory" tasks also found "memory cells" not limited to prefrontal areas. For example, visual memory were found in the inferior temporal cortex (quote) (Fuster & Jervey, 1981; Miller et al., 1993), spatial memory cells in posterior parietal cortex (quote) (Andersen et al., 1990), and tactile memory cells in somatosensory cortex (quote) (Zhou & Fuster, 1996). In human visual cortex, increased activity was found when the stimuli was obsent but the subject was attending a specific visual field (Kastner et al., 1999).

The inferences can be drown from these findings, according to Fuster (2003) are that, working memory consists of a large cognitive network (perceptual and executive memory), and the selective attention during a task of working memory consists of the sustained and selective activation of this network.



Dopamine projection

Dopamine in the prefrontal cortex:
In the study of Goldman-Rakic et al.(1996), they pointed out that pyramidal neurons and non-pyramidal neurons in prefrontal cortex is critical for the formation of a "memory field" found in the area. Dopamine has inhibitory and excitatory effect on the prefronal cells. Prefrontal cortex is also known as the richest in dopamine than any other cortical areas. Tus the role of dopamine projection to prefrontal area and its effect on working memory tasks worths further discussion.

More evidence has been made between the dopamine dysregulation in prefrontal cortex in dschizophrenia(Goldman-Rakic et al. 2004). Working memory deficits have been postulated to be "underlying the myriad impairment of disorders" in schizophrenia (p. 4). In Diamond (1989) young infants still can't perform without errors in working memory plus inhibition tasks due to late developmental maturity of human dorsal lateral prefronal cortex.