The ACTION Network
We consider first the nature of the frontal lobes themselves. They have both cortical and sub-cortical components; the former we will not discuss, since their parcellation is similar to that of 6-layered posterior sites, with a similar hierarchy (Mesulam, 1998). However there are no comparable subcortical structures accompanying the posterior cortex to those for frontal regions. The basic feature of the sub-cortical components of these frontal areas are the basal ganglia. They are two-layer systems activated unidirectionally from the cortex, the upper layer being the striatum (from its appearance), being decomposable into caudate and putamen. Beneath these, and close to the relevant part of thalamus (the mediodorsal nucleus) is the globus pallidus, comprising two components, termed internal and external. The unidirectional flow of information is : cortex -> striatum (STR) -> globus pallidus (internal) (GPi) -> thalamus (TH) -> cortex (Passingham, 1993) so there is a feedback of activity round the above long loop. There may also be similar feedback more rapidly round the short cortex TH loop, and although there is known topography in these connections the presence of any reverberation of such feedback activity is unknown. There is also the presence of the indirect loop, involving the globus pallidus external (GPe) as well as the sub-thalamic nucleus (STN), as shown in figure 1, a schematised version of the frontal lobe architecture we have termed elsewhere the ACTION network, since it is so important for the generation of actions (Taylor, 1995; Taylor & Alavi, 1996).
