By the early 1990s, cognitive-behavioral studies had provided evidence that verbal WM contained a few distinct components, including a phonological rehearsal process (see refs. 8 and 9 for reviews). The experiments that follow provide converging evidence for the behavioral work, and, more importantly, tell us something about the neural bases of these components. Unless otherwise noted, these experiments involve visual presentation of the verbal materials.
A useful starting point is a PET experiment reported by Awh and colleagues (10). Subjects were imaged while performing an item-recognition task (11). The task includes a series of discrete trials, and the contents of a trial are presented schematically in the top of Fig. 1. Each trial consisted of four uppercase target letters, presented simultaneously, followed by a 3,000-ms blank delay period, during which subjects had to remember the letters. This delay was followed by a lowercase probe letter, to which subjects responded “yes” or “no” (by pressing one of two buttons), indicating whether or not the probe was identical in name to one of the targets. This task is of interest because of its verbal memory requirement. The task also includes other processes—e.g., perception of the letters, selection of a response, and execution of a response—and thus the corresponding PET image acquired during performance of the task will include activations caused by these processes as well. To eliminate these unwanted activations, subjects also participated in a control task, which presumably contained the irrelevant processes but not the memory process. This task was similar to the memory one except that the probe was presented immediately after the target letters, and the latter remained in view along with the probe. This task was primarily a perceptual one; hence, memory should not have been involved.
When the image acquired during the control task was subtracted from the image acquired during the memory task, there were several activations, most of them in the left hemisphere. (There were also some deactivations, but in what follows we focus on activations.) These activated areas included: left posterior parietal cortex [Broadmann area (BA) 40], Broca’s area (BA 44), left premotor area (BA 6), and left supplementary motor area (BA 6). Given that the latter three areas are known to be involved in the planning and production of speech (12), implicit speech was very likely operative during the memory task. This finding is one piece of evidence for a rehearsal component.
This hypothesis is strengthened by the results of an earlier PET study of Paulesu and colleagues (13). They, too, contrasted an item-recognition task with a control task. When the control image was subtracted from the item-recognition image, there were activations in left posterior parietal cortex, Broca’s area, and bilateral supplementary motor area. These results are in good agreement with those described above. Paulesu et al. (13) included another condition, whose results provide further support for the claim that the frontal speech areas mediate a rehearsal of phonological information. In this additional condition, subjects were presented single letters; for each one, they had to decide whether it rhymed with the name of a target letter. When the image from a suitable control task was subtracted from the image from the rhyming task, there was again activation in Broca’s area, but not in left posterior parietal cortex. Paulesu et al. (13) interpret these results to mean that: (i) Broca’s area mediates phonological processes, including both rhyme judgments and subvocal rehearsal; and (ii) left posterior parietal cortex mediates the pure storage component of verbal WM, and hence was active in the memory, but not the rhyming, condition. Taken at face value, these results show that the rehearsal component can be dissociated from other components of verbal WM.
There is further evidence for such a dissociation in another PET experiment reported by Awh and colleagues (10). This study used a different memory paradigm. It is referred to as a “2-back” task (14, 15) and is presented schematically in the bottom of Fig. 1. Instead of viewing a series of discrete trials, subjects viewed a continuous stream of single letters, each presented for 500 ms, with a 2,500-ms interval between successive letters. For each letter, subjects had to decide whether it was identical in name to the letter two back in the sequence. Two different control conditions were used. One was a search task; subjects saw the same kind of sequence of letters as in the memory condition, but simply had to decide whether each letter matched a single target letter specified at the beginning of the experiment. This control should involve the same perceptual and response processes that are operative in the memory condition, and subtracting it from the memory condition should yield many of the same areas of activation that were obtained in item-recognition tasks. This was the case, as the subtraction image showed activations in left-hemisphere frontal speech regions and posterior parietal cortex. (There were new activations as well, including bilateral posterior and superior parietal cortex, BA 7, and right-hemisphere supplementary motor area; some of these new activations may reflect the greater processing demands of the 2-back task.)
What is of particular interest in this study, though, involves the second control condition. In this “rehearsal” control, subjects silently rehearsed each letter presented until the next one appeared, rehearsed the new one, and so on. Subtracting this control from the 2-back memory condition should have removed much of the rehearsal circuit. Indeed, in this subtraction image, neither Broca’a area nor the premotor area was significantly active. However, the supplementary area continued to be active. That we were unable to subtract out the entire rehearsal circuit may be because in our rehearsal control subjects rehearsed one letter at a time, whereas in our memory