Short Term Storage

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The short-term storage of information in working memory appears to be accomplished via two mechanisms: one that retains information and another that "rehearses" that information in order to keep the memory traces active during a retention interval. This is perhaps best illustrated for verbal information. A task that has been used frequently to study the mechanisms of verbal working memory in neuroima-ging experiments is the item-recognition task. In this task, participants are presented with a small number of target items, typically randomly selected letters, to store for a retention interval of several seconds. Following this interval, a single probe item is presented and participants must decide whether this item was a member of the memorized set. When participants engage in this task in PET and fMRI settings, a number of easily replicable sites of activation exist compared to a control condition that does not require memory at all or in which the memory requirement is minimal. One frequent site of activation is in the posterior parietal cortex, typically more prominently in the left hemisphere than the right. In addition, a set of activations appears in frontal areas, including the inferior frontal gyrus on the left, premotor cortex (more prominently on the left than on the right), and supplementary motor cortex. These brain regions, and all other major regions discussed throughout this article, are shown in Fig. 2.

The frontal cortical areas that are activated in this task are quite similar to those activated in a task that requires one to make judgments of rhyming, a task that

Figure 2 Brain diagrams highlighting the major structures discussed in the text. The upper figure shows a lateral view of a left hemisphere of the human brain, and the lower figure shows a view of the right hemisphere as seen from the midline of the brain. Major structures of relevance to memory are labeled.

Figure 2 Brain diagrams highlighting the major structures discussed in the text. The upper figure shows a lateral view of a left hemisphere of the human brain, and the lower figure shows a view of the right hemisphere as seen from the midline of the brain. Major structures of relevance to memory are labeled.

presumably requires production of a speechlike representation. So, it is likely that these frontal areas are the ones involved in rehearsal, which involves internally generating and regenerating a speechlike code for the stored verbal material. The posterior parietal sites have been suggested as sites for the storage of verbal information as well as for switching attention between one item and another.

The purported dissociation between the frontal and parietal sites is nicely supported by a study that used a different task involving verbal working memory, the two-back task. In this task, participants see a series of letters presented at a pace of one every 2.5 sec, and they must judge whether each matches in identity the one that appeared two letters back in the series. This task clearly requires storage and rehearsal of each letter, as well as other processes that we discuss next. Compared to a task in which participants must simply judge whether each letter in the series matches a single target (say, the letter "P"), the two-back task produces activations in regions similar to those in the item-recognition task. This is as it should be if both tasks involve storage and rehearsal. Beyond this, though, the two-back task has also been compared to another condition, one in which participants had to silently rehearse letters to themselves with little storage requirement (e.g., say the letter "P" for 3 sec, followed by silently saying the letter "M," and so on). Subtraction of the activation in this rehearsal condition from that in the two-back condition revealed much lower activation in the frontal areas. The rehearsal condition is presumed to involve the explicit production of silent speech. Subtraction of the activations in this condition from those in the two-back condition reduces frontal but not parietal activation; therefore, one can conclude that the frontal activations in the two-back and other verbal working memory tasks must reflect an inner rehearsal process as part of those tasks. These same frontal regions are also activated in tasks that require a recall response, so they are not unique to the peculiarities of the item-recognition task or the two-back matching task.

Just as we can identify the frontal sites used in verbal rehearsal, we can also identify the parietal sites used in verbal storage. Evidence that the parietal sites are used in part for storage comes from a study in which subjects memorized a set of nonsense letter strings (e.g., "MAVER") and then kept these items in memory during a retention interval of some 50 sec, during which they underwent PET scanning. After the scan, they had to retrieve the items to be sure that they had been stored accurately. Scanning during just the retention interval allows one to isolate storage processes or at least to concentrate scanning on storage. One study using this procedure found posterior parietal activations, leading to the conclusion that these activations reflected storage processes and not encoding or retrieval processes.

Storage and rehearsal should not be restricted to verbal information, of course, if they are general properties of working memory as Baddeley has supposed. Indeed, many studies have investigated the storage and rehearsal circuitry used for spatial information as well. The clearest result of these studies is that the circuitry activated by spatial information in a working memory task is quite different from that activated by verbal information, even when the tasks are quite similar and only the material differs. For example, in an analog to the item-recognition task, subjects are presented with a set of dots on a screen and asked to store their locations in memory. Following a retention interval of several seconds, they are presented with a single probe dot, and their task is to decide whether it appears at the same location as one of the locations they have stored. This task has the same formal structure as the item-recognition task for letters, yet it yields activations that are quite different. In common are activations in the posterior parietal and premotor cortex, although with a tendency for greater activation in the right than the left hemisphere. However, quite different are activations in the occipital cortex, superior frontal cortex, and inferior frontal cortex, most prominently in the right hemisphere.

The common activations in parietal and premotor cortex between verbal and spatial versions of the task suggest that there are some processes in common between the tasks, possibly having to do in part with allocating attention to several items in memory. However, the differences in activations suggest that the mechanisms by which information is stored and rehearsed may be different. Indeed, there is evidence of a similarity in circuitry between processes mediating spatial working memory and those mediating shifts of attention to various locations in the visual field when stimuli are being perceived. This leads to the conclusion that spatial rehearsal may amount to a successive allocation of attention to internal representations of spatial locations, a process possibly mediated by premotor mechanisms near the frontal eye fields. This region, together with parietal cortex, may also play a role in maintaining the representations of the spatial locations as well, a conclusion that is consistent with lesion studies and electrophysiological studies of monkeys in spatial working memory tasks. So, we can see that, although storage and rehearsal are common features of spatial and verbal working memory, they appear to be implemented in the brain in different ways.

Of course, visual information that is stored need not be spatial in nature. Features such as the shape of an object or its color are not spatial, even though they are visual. As described earlier, the brain honors this distinction in simple visual processing, and indeed neuroimaging research suggests that spatial memory and memory for other visual information are processed differently in the brain as well. One experiment that demonstrates this used pictures of three faces presented sequentially in three different spatial locations. After a retention interval, a probe picture was presented in one of the locations. When subjects were tested on their working memory for objects, they had to decide whether the probe face was the same as any of the previous three; when they were tested on spatial working memory, they had to decide whether the probe was in the same location as one of the original faces. The elegance of this design is that it involves the very same stimuli, and only the nature of the memory task changes. The results show that this change in task produces an important difference in brain activation: The object task activated regions of dorsolateral prefrontal cortex, whereas the spatial task activated a region posterior to this in the premotor cortex. Beyond this, a meta-analysis of several spatial and object working memory tasks suggests that there is also a dorsal-ventral difference in activation in the posterior cortex. Spatial working memory tasks activate more dorsal structures in the posterior cortex, whereas object working memory tasks activate more ventral structures.

B. Executive Processes

In addition to storage components, the model of working memory proposed by Baddeley includes a component due to executive processes. Although there is not yet a clear taxonomy of executive processes in hand, descriptions of them typically include the following: (a) focusing attention on relevant information and inhibiting attention from irrelevant information; (b) scheduling processes in tasks that require multiple processes; (c) planning and prioritizing a sequence of steps to meet some goal; (d) updating and checking the contents of working memory; and (e) coding internal representations for time or place of occurrence. All of these processes involve the manipulation of information that is temporarily stored in working memory. Research on executive processes using neuroimaging techniques has revealed a heavy contribution of frontal mechanisms regardless of the executive process in question.

As an example, recall the verbal item-recognition task. In that task, subjects are presented with a set of letters that they have to retain for several seconds, after which they have to decide whether a probe letter matches one of the letters in memory. Several studies have introduced an inhibitory component in this task in the following way. Trials were included in which the distractor probes (probes that did not match an item in the current memory set) were letters that did match a letter in the memory set from the previous trial. Thus, these probes were relatively familiar because they had been memorized recently. This design creates a situation in which participants have a sense of familiarity about the probe item, but they must remember that it does not match the memory set on the current trial. On such trials, subjects take longer to give a "no match'' response. Both PET and fMRI studies show that there is a site in the left lateral prefrontal cortex that is activated on these trials, and the activation occurs most prominently at the time the probe is presented. Furthermore, older subjects, who show a greater interference effect on these trials, also show less activation at this left lateral site, and patients with damage to this area show a dramatically increased interference effect compared to patients with damage elsewhere in the frontal cortex. Taken together, this evidence suggests that the left lateral site is involved in resolving the conflict between familiarity and source memory that arises on these trials.

Another example of a task in which executive processes interact with storage processes is the two-back task described earlier. Recall that, in this task, single letters are presented in succession and subjects must judge whether each letter matches the one two earlier in the sequence. To succeed at this task, one not only has to store the recent stream of letters but also has to update this stored set as new letters are presented, dropping older letters and adding newer ones. This task is similar to the item-interference task in that it includes an inhibitory component, as described earlier. In addition to this executive process, the letters that are stored in memory also have to be tagged by their order of appearance so that the subject can keep in mind which one is two back, which is one back, which is three back, and so on. Thus, the two-back task must recruit an executive process responsible for temporally tagging information, a sort of short-term episodic memory requirement. Indeed, the two-back task shows evidence of activations in the dorsolateral prefrontal cortex in addition to other sites that may well be responsible for temporal tagging. The dorsolateral prefrontal activation that arises in this task seems to be a common broad site of activation in many tasks that require manipulation of the information stored in working memory, and so this leads to the general conclusion that prefrontal mechanisms may be responsible for a wide array of executive processes.

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