Scientific Understanding of Consciousness
Neural Substrates of Visual Working Memory
Cortex, (2003) 39, 927-946
Seeking the Neural Substrates of Visual Working Memory Storage
Bradley R. Postle1, T. Jason Druzgal2, Mark D’Esposito3
1Department of Psychology, University of Wisconsin, Madison, USA;
2Program in Neuroscience, University of Pennsylvania Medical School, Philadelphia, USA;
3Wills Neuroscience Institute and Department of Psychology, University of California,
It is widely assumed that the prefrontal cortex (PFC) is a critical site of working memory storage in monkeys and humans. Recent reviews of the human lesion literature and recent neuroimaging results, however, challenge this view. To test these alternatives, we used event-related fMRI to trace the retention of working memory representation of target faces across three delay periods that were interposed between the presentation of each of four stimuli. Across subjects, only posterior fusiform gyrus demonstrated reliable retention of target-specific activity across all delay periods. Our results suggest that no part of frontal cortex, including PFC, stores mnemonic representation of faces reliably across distracted delay periods. Rather, working memory storage of faces is mediated by a domain specific network in posterior cortex.
Baddeley and Hitch, in 1974, proposed a multiple-component model of “working memory” that has been vastly influential within cognitive psychology (Baddeley and Hitch, 1974). This model comprised, in simplified outline, two independent buffers for the storage of verbal and of visuospatial information and a Central Executive to control attention and the management of information in the buffers (Baddeley, 1986). Human working memory is widely viewed as a fundamental cognitive capacity that contributes critically to such high level cognitive functions as learning, reasoning, and language comprehension. Since the introduction of this multiple-component model of human working memory, many cognitive psychologists have proposed alternative models that employ a wide variety of mechanisms to produce working memory behavior.
In the neuroscience tradition of working memory research, there are two dominant views about the neuroanatomical basis of working memory storage. According to one class of models, PFC supports working memory storage during delay periods. A second class of models posits that stimulus representations may be stored across delay periods in posterior cortical regions, whereas PFC’s working memory-related functions support extramnemonic executive control operations, such as maintaining task set, manipulating or transforming mnemonic representations, and using and the information held in working memory to organize behavior.
The results of this study suggest that FC does not store mnemonic representations of faces across distracted delay periods. Rather, they suggest that this function is performed by FG. The model of working memory storage functions that emerges from the results of this and other human neuroimaging experiments is that they are mediated in a domain-specific way by discrete, segregated networks in posterior cortex. In contrast to faces, for example, working memory storage of verbal material is supported by left posterior perisylvian regions associated with language comprehension functions, and visual working memory storage of spatial and object features of stimuli is supported by posterior regions of the dorsal and ventral visual processing streams, respectively. At least three candidate models might explain this accumulating pattern of data: Working memory storage may be accomplished by 1) sustained activity in the same networks that process perceptual information about the to-be-remembered stimulus; 2) the operation of domain-specific short-term memory buffers located proximally to these sensory networks; or 3) temporary activation (e.g., by attention) of the long-term memory representations that correspond to the memoranda.
Our finding that FC (including PFC) does not govern working memory storage is consistent with an emerging view that this region’s contribution to working memory function is to control task-related behavior via functions operating at a level that is abstracted from the processing of individual stimuli. Examples of these functions, none of which are stimulus specific, include control of attention, transformation of mnemonic representations from their encoded state, abstraction across trials of patterns and regularities with which to guide behavioral set, response selection, and mediation of the effects of interference in working memory.
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