In this commentary, we focus on four points. First, we discuss the assertion that the unitary model explains dissociations that implicate multiple systems. Second, the distinct nature of information utilized in immediate- and delayed-recall supports the distinct memory systems view. Third, the variable nature of capacity limits corroborates this view. Finally, we review event-related fMRI results that suggest support for multiple systems.

Cowan argues that the true short-term memory (STM) capacity limit is about 4 items. Functional neuroimaging data converge with this conclusion, indicating distinct neural activity patterns depending on whether or not memory task-demands exceed this limit. STM for verbal information within that capacity invokes focal prefrontal cortical activation that increases with memory load. STM for verbal information exceeding that capacity invokes widespread prefrontal activation in regions associated with executive and attentional processes that may mediate chunking processes to accommodate STM capacity (...) limits. (shrink)