Biomedical Engineering Reference
In-Depth Information
cause otherwise the posterior cortex perceptual modules could implement the short-
term memory function by their own recurrent collateral connections. This approach
also emphasizes that there are many at least partially independent modules for short-
term memory functions in the prefrontal cortex (e.g., several modules for delayed
saccades; one or more for delayed spatial (body) responses in the dorsolateral pre-
frontal cortex; one or more for remembering visual stimuli in the more ventral pre-
frontal cortex; and at least one in the left prefrontal cortex used for remembering the
words produced in a verbal fluency task - see Section 10.3 of [92]).
This computational approach thus provides a clear understanding of why a sepa-
rate (prefrontal) mechanism is needed for working memory functions, as elaborated
in Section 16.3.1. It may also be commented that if a prefrontal cortex module is
to control behaviour in a working memory task, then it must be capable of assum-
ing some type of executive control. There may be no need to have a single central
executive additional to the control that must be capable of being exerted by every
short-term memory module. This is in contrast to what has traditionally been as-
sumed for the prefrontal cortex [98].
16.3.4
Role of prefrontal cortex short-term memory systems in visual
search and attention
The same model shown in Figure 16.9 can also be used to help understand the im-
plementation of visual searchtasks in the brain [70]. In such a visual search task,
the target stimulus is made known beforehand, and inferior temporal cortex neurons
then respond more when the search target (as compared to a different stimulus) ap-
pears in the receptive field of the IT neuron [15, 16]. The model shows that this
could be implemented by the same system of weakly coupled attractor networks in
PF and IT shown in Figure 16.9 as follows. When the target stimulus is shown, it
is loaded into the PF module from the IT module as described for the delayed match
to sample task. Later, when the display appears with two or more stimuli present,
there is an enhanced response to the target stimulus in the receptive field, because
of the backprojected activity from PF to IT which adds to the firing being produced
by the target stimulus itself [67, 70] (see Figure 16.11). The interacting spatial and
object networks described by [82]) in Chapters 9-11, take this analysis one stage
further, and show that once the PF-IT interaction has set up a greater response to the
search target in IT, this enhanced response can in turn by backprojections to topo-
logically mapped earlier cortical visual areas move the “attentional spotlight” to the
place where the search target is located.
16.3.5
Synaptic modification is needed to set up but not to reuse short-
term memory systems
To set up a new short-term memory attractor, synaptic modification is needed to
form the new stable attractor. Once the attractor is set up, it may be used repeatedly
when triggered by an appropriate cue to hold the short-term memory state active by
continued neuronal firing even without any further synaptic modification (see [37]
Search WWH ::
Custom Search