**5. Conclusion**

therapeutic proposals resulting from these studies yet yielded tangible benefits. Studies of the impact of the disease on dynamical system elements, on the other hand, have yet to be included in the exploratory spectrum, a reflection of the field's emphasis on theoretical issues concerning the relation of its theoretical models to the physical structure that is the object of the disease. Increasingly, however, not only are dynamical elements considered to be fundamental features of brain operation but their role in global operation are specifically targeted by the disease. This is also to say that their exploration is likely to prove fruitful to the understanding of the cognitive dimension that is specifically impacted by the disease and that constitutes its hallmark. This chapter builds on these observations to propose that the global dynamical operation targeted by the disease is constituted by a construct subsumed within the percept of the self.

The evidence for the specific effect on the dynamical systems that underlie the self is three fold and is framed against the backdrop of the studies just mentioned that demonstrate the basic need to dynamically structure a coherent and stable self image that can make and execute motor plans for interacting with events in the world beyond the self: first, Alzheimer's specifically affects a major brain network closely implicated with the self, the Default Mode Network (DMN); second, there is a specific loss in the ability to maintain functional integration over distributed regional zones that are likely to be subordinated to self performance; and third, parametric indices of dynamical metastability, an index relating global dynamics

We begin with the evidence linking the DMN to the self percept [46]. The DMN was first identified by nuclear imaging studies that showed that this region displayed consistently higher levels of activity during passive task conditions. This led to the hypothesis of its role in monitoring the external environment, body, and even emotion [47]. Additionally, the DMN activity is generally and persistently elevated relative to a number of other areas of the brain, a feature needed for guiding global activity. Task related increases in activity in other zones, moreover, coincide with a relative decrease in DMN activity suggesting a reciprocal relation between the two that is related to the performative context of the task [17]. It is likely therefore that the dynamical ground state of the DMN situates the self image in the context of effecting motor plans and tasks in order to contextualize them as ones own. Consistent with this interpretation its high metastability index reflects a sustained exploratory state where the self image can be accessed for task execution. In other words, the DMN appears to function as the center of a global and distributed network system [16] that assimilates the body image as a coherent and stable self reference that is persistently ready to engage the environment [27].

Secondly, and strikingly, fMRI observed activity patterns in the DMN substantially change during the progression of Alzheimer's. Posterior cingulate and right inferior temporal cortical activities, for example, decline whereas the activity of the bilateral inferior parietal cortex increases [48]. What is significant about these zones is that they form central connectivity hubs within the DMN that exhibit causally influential connections. Thus changes in connectivity strength between these zones appear to reflect a weakening of causally influential, functionally significant, relations between the principal nuclei of the DMN, with a corresponding

to coordinative oversight, is significantly altered in AD patients.

118 Neuroplasticity - Insights of Neural Reorganization

and decreasing ability to sustain the self percept.

Though later years of life are often portrayed as a time of self realization, it is also the period of life when health related disabilities are especially protracted. Among the most debilitating is the loss of self awareness and self control experienced in advanced Alzheimer's dementia. This realization has impelled numerous studies seeking to identify its etiological roots; its physiological basis, nonetheless, remains uncertain and an overarching framework has yet to be determined. Increasingly, the realization that brain activity is dynamically sustained offers a prospectus within which to frame Alzheimer's related behavioral manifestations. Dynamical systems constitute an operational ground not only as processional motifs but as functional units at progressively higher levels where their coherent assembly is used to guide global operation. This chapter proposes that the peculiarly debilitating and widely regarded hallmark aspects of AD are likely to relate to its ravaging of a global dynamical system that serves as the neural substrate for our sense of self.
