Monitoring and VIsual Working Memory

Prefrontal cortex is known to be important for working memory processes, but there is debate surrounding whether it is important for the online maintenance of information, the online monitoring of information, or both. Some theories hold that prefrontal cortex (and dorsolateral PFC in particular) are important for the "executive" aspects of working memory, such as monitoring, evaluation, or retrieval/maintenance strategies, as opposed to maintenance itself.

A 2000 J Neurosci paper speaks to this point. Michael Petrides gave monkeys a visual working memory task in which either the delay between display and test or the number of items was varied. These monkeys either had lesioned mid-dorsolateral prefrontal (DLPFC) or anterior inferotemporal (aIT) regions, the latter being a part of the brain that is known to be involved in object processing and could potentially be the neural locus of the active maintenance of visual objects.

The author found a double dissociation between the effects of DLPFC and aIT damage, such that aIT damage affected the robustness of memory accuracy to increased delay, whereas medial DLPFC damage affected the robustness of memory accuracy to increasing set sizes. According to Petrides, this evidence suggests that aIT is more critical for maintenance, whereas medial DLPFC is more critical for the monitoring of multiple stimuli and responses. (Although Petrides makes claims that DLPFC representations are symbolic, the evidence presented here really can't speak to issues of representational format.)

One idea that may tie all of this evidence together is that representations become more abstract in more anterior regions, and that maintenance of a representation in an anterior region is used to select among representations in more posterior regions. According to this perspective, executive functions are accomplished by maintenance within more anterior regions, and the types of executive functions required in this task may involve deciding which of the items in each set size to attend to, or they may involve selectively biasing each of these items' representations so that they are maximally different from one another. Consistent with much previous work, this kind of activity would occur in dlPFC. In contrast, a more posterior region (such as aIT) might contain the object or item representations biased by more anterior regions; activity here should be particularly important for robustness across delays, since this is where the actual item information per se is being maintained.

Several previous posts have discussed the anatomy that is relevant to visual short term memory. For example, Xu and Chun determined that inferior intraparietal sulcus activity is minimally sensitive to set size (pretty much regardless of object complexity), while superior intraparietal sulcus activitiy is sensitive to set size only for simple objects. Likewise, Vogel & Machizawa found parietal and lateral occipital areas were indicative of visual WM span (though these were ERP waves, and could certainly have their source in other regions of the brain).

On the other hand, at least one study has shown that differences in span may be accounted for by differences in "selection efficiency." This function certains suggests a strong role for dlPFC, which is known to be involved in processes like selection, and overcoming interference. And studies mentioned in this post, using TMS, found that disruption of neural activity in right & left PFC in humans resulted in lower visual recognition memory.

Related Posts:
Multiple Capacity Limitations for Visual Working Memory