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This study explores the cortical cell dynamics of unimodal and cross-modal working memory (WM). Neuronal activity was recorded from parietal areas of monkeys performing delayed match-to-sample tasks with tactile or visual samples. Tactile memoranda (haptic samples) consisted of rods with differing surface features (texture or orientation of ridges) perceived by active touch. Visual memoranda (icons) consisted of striped patterns of differing orientation. In a haptic-haptic task, the animal had to retain through a period of delay the surface feature of the sample rod to select a rod that matched it. In a visual-haptic task, the animal had to retain the icon for the haptic choice of a rod with ridges of the same orientation as the icon's stripes. Units in all areas responded with firing change to one or more task events. Also in all areas, cells responded differently to different sample memoranda. Differential sample coherent firing was present in most areas during the memory period (delay). It is concluded that neurons in somatosensory and association areas of parietal cortex participate in broad networks that represent various task events and stimuli (auditory, motor, proprioceptive, tactile, and visual). Neurons in the same networks take part in retaining in WM the memorandum for each trial, whether it is encoded haptically or visually. The VH association by parietal cells in WM is analogous to the auditory-visual association previously observed in prefrontal cortex. Both illustrate the capacity of cortical neurons to associate sensory information across time and across modalities in accord with the rules of a behavioral task.

A delayed match-to-sample (DMS) task of abstract, visual memory was performed during the uptake period of 18F-fluorodeoxyglucose. The increase in glucose uptake of cortical and subcortical regions ("activation") during the DMS task was compared with that during a control, immediate match-to-sample task using positron emission tomography. Both discriminant analysis and paired t tests supported the observation that the dorsolateral prefrontal area underwent the greatest activation, while a factor analysis revealed the functional correlation matrices of the tasks. Activations in the ventral premotor cortex and supramarginal and angular gyri were highly correlated with the change in the dorsolateral prefrontal cortex. The basal forebrain/ventral pole region showed a smaller but independently significant change. The findings support the role of the dorsal prefrontal region in the nonspatial working memory of humans.

The effects of reversible lesion--by cooling--of dorsolateral prefrontal and posterior parietal cortex were studied in rhesus monkeys performing a cognitive visuomotor integration task. Correct performance required the use of a learned set of cue-response contingencies, some spatial and some nonspatial; in some cases, the task required the short-term retention, through a delay, of the color of the cue or its implicit response direction. Prefrontal cooling impaired performance of the task regardless of its spatial demands, an effect that increased with delay. Parietal cooling, on the other hand, only impaired performance if the task demanded the processing and retention of spatial information (i.e., if spatial active memory was required). Parietal effects were not related to delay. Both prefrontal and, even more, parietal cooling increased response time in all task contingencies. Thus, the results dissociate the respective contributions of the prefrontal and the posterior parietal cortex to the temporal and spatial aspects of information processing in visuomotor performance. They indicate that posterior parietal areas participate in spatial processing and in active memory of spatial information, whereas prefrontal areas subserve a broader role of visuomotor processing and cross-temporal integration of both spatial and nonspatial information. Language: en