Neuroimaging analyses of human working memory

Abstract

We review a program of research that uses neuroimaging techniques to determine the functional and neural architecture of human working memory. A first set of studies indicates that verbal working memory includes a storage component, which is implemented neurally by areas in the left-hemisphere posterior parietal cortex, and a subvocal rehearsal component, which is implemented by left-hemisphere speech areas, including Broca's area as well as the premotor and supplementary motor areas. We provide a number of neuroimaging dissociations between the storage and rehearsal areas. A second set of studies focuses on spatial working memory and indicates that it is mediated by a network of predominantly right-hemisphere regions that include areas in posterior parietal, occipital, and frontal cortex. We provide some suggestive evidence that these areas, too, divide into storage and rehearsal regions, with right-hemisphere posterior parietal and premotor regions subserving spatial rehearsal. In a final set of studies, we turn to "executive processes," metaprocesses that regulate the processing of working-memory contents. We focus on the executive process of inhibition as it is used in verbal working memory. We provide evidence that such inhibition is mediated by the left-hemisphere prefrontal region and that it can be dissociated from verbal storage and rehearsal processes.

Figure 1

Schematic representations of trials in two different WM tasks. (Upper) A sample trial for the item-recognition task. It includes the following: (i) a fixation point, (ii) four uppercase letters, (iii) a blank delay interval, and (iv) a lowercase probe letter. The subject’s task is to decide whether the probe names one of the four target letters. (Lower) A sample trial for the 2-back task. Each letter is followed by a blank delay interval. The subject’s task is to decide whether each letter has the same name as the one that occurred two back in the sequence. The durations for all trial events are shown. Adapted from Smith et al. (38).

Figure 2

Percentage of MRI signal change in activation as a function of temporal interval, with working memory load as the parameter. The results for the posterior parietal area are on the top, and the results for Broca’s area are on the bottom. Adapted from Cohen et al. (12)

Figure 3

Schematic model of verbal WM. The model describes some of the cognitive functions involved in the item-recognition task and, where possible, specifies their rough neural implementations. (See text for explanation.)

Figure 4

Images reflecting the activations in three subtractions. Each row presents left lateral, superior, and right lateral views of the brain, with t statistics of activations at or within 15 mm of the surface superimposed in color on a gray MRI of a single subject (not one from this study). The color scale shows the magnitude of the t statistic that corresponds to each color. Shown are all activations that passed a criterion of P < .05 uncorrected for multiple comparisons. The top row depicts the sites of activation subtracting the control from the recent negatives condition; the second row depicts the sites from the subtraction of the control from the standard condition; and the bottom row shows the subtraction of the standard from the recent negatives condition. Adapted from Jonides et al. (39).