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General Observations
Whereas the older term ‘short term memory’ suggested a passive process restricted to
retention, the currently more favored term ‘working memory’ also entails the active
maintenance and use of information for cognition and behavior. Visual working memory
(VWM) entails both feature and feature memory. This process is closely related to
attention, because attention is required to select and maintain items in VWM. This is
particularly important because the capacity of VWM appears to be 3-4 items, although
this is influenced by the nature and interactions between these items, and may actually
reflect limited resources rather than fixed memory ‘slots’ (this remains controversial, i.e.
Luck & Vogel vs. Bays & Husain). Together, attention and working memory provide a
moment-to-moment internal representation of key task-related items in the external
world, closely related to consciousness. Working memory contents, under some
circumstances, may be converted into long-term memory stores. One early but still
influential model of working memory (Baddeley and Hitch) entails a central executive
(for top-down attention, manipulation etc.) over several slave storage systems (including
a ‘visuospatial sketchpad’). Most current computational / connectionist models assume
that maintenance of information involves recurrent connections between neurons.
Neural Mechanisms
Many neurons in visuomotor regions of the brain (but not all) show sustained activity
between a visual stimulus and a motor action. Working memory has long been
associated with prefrontal cortex, especially lateral prefrontal cortex. Dorso-lateral
prefrontal cortex (DLPFC), as opposed to ventral PFC, has been associated more with
spatial memory, in particular in the right hemisphere. However, some current
investigators associated DLPFC more with executive control. DLPFC forms part of a
VWM network that includes a number of areas closely related to attention and eye
movement, including FEF, Cingulate, PEF, and some subcortical structures. Some have
argued that these neurons are more concerned with coding past sensory events, and
others that they are concerned with planning future action, but our lab has found that
the same neurons in one area (at least superior colliculus and FEF) can encode both at
different times, tending to transition from target to planning codes. Recently, Anna
Heurer, working in my lab, found that –as in perceptual attention— there are different
neural mechanisms for the retrograde selection of content from VWM, with inferior
parietal cortex (SMG) playing a role in spatial selection and LOC playing a role in
feature selection. Finally, although distinctions have been made between spatial and
feature memory, as in perception these two must be bound together somehow to
meaningfully encode the location and identify of an object, perhaps through some sort
of trans-cortical synchrony of rhythmic activity.