Working Memory: Capacity Limitations - Drew & Vogel - Article

What is working memory?

Working memory (WM) is a cognitive system that keeps information activated for a limited amount of time. This way, this piece of information can be quickly accessed and manipulated. Working memory helps us to connect information and shape it into complex concepts. Working memory can only store a limited amount of information. The limited capacity of the working memory represents the limited processing capacity of human cognition. This limit is due to the limitation of the focus of attention. Long-term representations (memories) can be active simultaneously, but only a few can be attended.

There is no clear evidence whether WM capacity limits are modality-specific or a single central limitation. In the article, the limitations of WM are considered separately for all modalities.

What is working memory?

Working memory (WM) is a cognitive system that keeps information activated for a limited amount of time. This way, this piece of information can be quickly accessed and manipulated. Working memory helps us to connect information and shape it into complex concepts. Working memory can only store a limited amount of information. The limited capacity of the working memory represents the limited processing capacity of human cognition. This limit is due to the limitation of the focus of attention. Long-term representations (memories) can be active simultaneously, but only a few can be attended.

There is no clear evidence whether WM capacity limits are modality-specific or a single central limitation. In the article, the limitations of WM are considered separately for all modalities.

What research has been done of working memory and which theories exist?

The most common paradigm to investigate WM capacity uses digit span tasks. Participants are given a random list of numbers that they have to repeat. Most people can memorise about seven items, but there is a lot of individual variation. This not only holds for digits, but also for words and other stimuli. Basically, this rule of “seven, plus or minus two” (initiated by Millder in 1956) holds for every kind of “chunk” of information. Chunking can also be done voluntarily, by snipping meaningless information into meaningful chucks. This makes recalling the chucks easier, because chunking uses pre-existing information in the long-term memory.

Baddeley’s model of working memory states that the so-called phonological loop stores language- or sound-based information for a short period of time. An individual can keep this information active by repeating it. The phonological and semantic meaning has influence on the way words are stored in this loop. For example, longer words are harder to store, while words that are similar in meaning are not. Apparently, the phonological properties influence the storage limit. Another rule is that the verbal WM is limited by how much an individual can say in about two seconds. Here it’s not about items, but about time. If the rehearsal of items in the loop is prevented, the amount of remembered items drops dramatically.

Early investigation of the capacity of the visual working memory has been done by Sperling. He flashed letters on a screen and asked participants how many they could remember. The results suggested that individuals have a very limited memory for items. However, Sperling’s estimation of visual WM capacity has two possible limitations. Subjects could have suffered from output interference, because they were asked to name or write the letters they remembered. This may have interfered with the retrieval process. Additionally, Sperling used alphanumeric characters. This makes it unclear whether the verbal and/or visual WM had been used for this task. Subjects had to transform the visual image to verbal labels. Therefore, this report is not clear about the exact capacity of the working memory. Another way to investigate the limitation of visual WM is by using Philips’ sequential comparison paradigm, or the change detection paradigm (Figure 1 in the article). Subjects have to report whether items (objects) stay the same or have changed during the task. Typically, subjects start to fail the detection task when the amount of items in the array exceeds about four items at once. The amount of features (colour, orientation, etc.) these objects have doesn’t matter. However, when the complexity of objects increases, the memory capacity decreases. There seems to be a relation between the required resolution of items in memory and the number of items that can be maintained in the visual working memory. Another factor influencing visual WM is the ability to discriminate between representations. Maybe the relationship between object complexity and change detection has more to do with the resolution for making discriminations than with the number of representations that can be maintained in working memory.

Many experiments use the blood-oxygen level-dependent (BOLD) activation to investigate the neurobiological response during retrieval. BOLD activity is mainly found in the prefrontal, posterior parietal, and inferotemporal cortices. It is hard, however, to determine if a raise in activation reflects WM or a more general response due to increased task load. There is a hint that the phonological loop is situated in Broca’s area. This study also suggests that when the WM capacity was exceeded, additional executive processes in the prefrontal areas are activated to assist in performing the task. Activity in the posterior parietal cortex might be sensitive to working memory capacity limits. Activity in the interparietal sulcus increases when the retention period increases, but this phenomenon reaches its maximum when the subject has to process more than four items (Figure 3 in the article). This result suggests that the activity in the IPS reflects the number of item representations that can be maintained in the visual WM. After all, four items is the maximum amount of items that can be held in the visual WM. Later research found that the lateral occipital complex also gets activated during visual WM activation. Event-related potentials (ERPs) can be used to filter specific activation related to working memory. Contralateral delay activity (CDA, measured about 250 ms after stimulus presentation) increases when the number of items in a visual WM task increases. Again, the maximum of this phenomenon lays around four items.

How does individual variation complicate research of working memory?

Of course, there’s much individual variation in WM capacity. Individual working memory capacity is associated with many cognitive and aptitude measurements. WM capacity seems to be a core mental construct, underlying overall cognitive ability. Researchers recognise that WM capacity and intelligence are positively correlated. Almost every intelligence test has some sort of memory span component or WM processing component. A popular method is the operation span (OSPAN). Subjects are asked to read a mathematic equation, judge the correctness of the equation, and have to read a random word aloud. After some trials, the subject has to recall as many words as possible. The OSPAN is correlated to language comprehension, intelligence, and verbal and visual working memory. Individual performance on the change detection paradigm is also related to intelligence and school performance. These findings are in line with the idea that there is a single central working memory capacity: measures of verbal and visual WM capacity are both predictive of cognitive ability.

How do attentional control and working memory relate?

There is another construct related to working memory capacity: attentional control. The WM system can be used to control the stream of information and to diminish distracting information. For example, people with low-WM-capacity are more likely to hear their name in an auditory stream task. Maybe low-capacity individuals can process the same amount information, but are also processing irrelevant information. Researchers found that high-memory-capacity individuals are more efficient in deflecting irrelevant information than low-memory capacity individuals. Low-capacity individuals actually hold more information in their WM, but these individuals store irrelevant information instead. Individual differences in WM capacity may be the consequence of the attentional control processes. It determines what information is stored in working memory or long-term memory.