The Operation of Memory Systems in the Brain - Computational Neuroscience: A Comprehensive Approach - page 506

within milliseconds (given the point that some other neurons will be close to thresh-
old) through the modified recurrent collateral synapses that store the information.
In this way, the neurons in networks with continuous dynamics can influence each
other within a fraction of the synaptic time constant, and retrieval can be very rapid
[82, 92].

16.3 Short-term memory systems

16.3.1 Prefrontal cortex short-term memory networks, and their rela-

tion to temporal and parietal perceptual networks

A common way that the brain uses to implement a short-term memory is to maintain
the firing of neurons during a short memory period after the end of a stimulus (see
[24] and [92]). In the inferior temporal cortex this firing may be maintained for a few
hundred ms even when the monkey is not performing a memory task [18, 89, 90, 91].
In more ventral temporal cortical areas such as the entorhinal cortex the firing may
be maintained for longer periods in delayed match to sample tasks [108], and in the
prefrontal cortex for even tens of seconds [23, 24]. In the dorsolateral and inferior
convexity prefrontal cortex the firing of the neurons may be related to the memory
of spatial responses or objects [30, 118] or both [63], and in the principal sulcus /
arcuate sulcus region to the memory of places for eye movements [22] (see [82]).
The firing may be maintained by the operation of associatively modified recurrent
collateral connections between nearby pyramidal cells producing attractor states in
autoassociative networks (see [82]).
For the short-term memory to be maintained during periods in which new stimuli
are to be perceived, there must be separate networks for the perceptual and short-
term memory functions, and indeed two coupled networks, one in the inferior tem-
poral visual cortex for perceptual functions, and another in the prefrontal cortex for
maintaining the short-term memory during intervening stimuli, provide a precise
model of the interaction of perceptual and short-term memory systems [67, 70] (see
Figure 16.9) . In particular, this model shows how a prefrontal cortex attractor (au-
toassociation) network could be triggered by a sample visual stimulus represented
in the inferior temporal visual cortex in a delayed match to sample task, and could
keep this attractor active during a memory interval in which intervening stimuli are
shown. Then when the sample stimulus reappears in the task as a match stimulus,
the inferior temporal cortex module showed a large response to the match stimu-
lus, because it is activated both by the visual incoming match stimulus, and by the
consistent backprojected memory of the sample stimulus still being represented in
the prefrontal cortex memory module (see Figure 16.9). This computational model
makes it clear that in order for ongoing perception to occur unhindered implemented
by posterior cortex (parietal and temporal lobe) networks, there must be a separate
set of modules that is capable of maintaining a representation over intervening stim-