What happens W h e r e?

The morphology of the aura symptoms and the morphology of the underlying pathophysiological processes are not the same. Loosely speaking, the spatio-temporal activity pattern of the pathophysiological process is translated by one of the many feature maps in the cerebral cortex into a hallucinatory perception pattern.

Which feature map is involved depends on the cortical area the disturbance takes place. In other words, the morphology of the symptoms does not depend solely on the pathophysiology but also on the feature map. Therefore the morphology of the symptoms does not lead to a taxonomy that is exclusively based on etiology. For a usefull therapeutic taxonomy, we need to consider two factors. First, what happened in the cortex and, second, in which cortical area took it place. Both the "W h e r e" and "What" influence aura morphology and the effects are hard to tell apart.

Visualization of the mapping problem: What happens Where? © 2005 Markus Dahlem

While the "What" question refers to the aura etiology, the "Where" question addresses the different feature maps of the cortical areas. Of course, one can try to circumvent both questions. Instead of looking at the aura symptoms, one can record with non-invasive imaging directly the cortical activity during a migraine attack. However, this is only feasible if a patient can trigger the aura. Furthermore, answering these two questions will lead to a better understanding of human cortical feature maps. It is worth to overtaken the obstacles and try to create a taxonomy based on aura symptoms.

A precise drawing of the spatio-temporal development of the visual impairment. By courtesy of Paul VanValkenburgh. © 2005 Paul VanValkenburgh

A road map to a taxonomy based on visual aura symptoms would be as follows (see also Classification of visual disturbances). We need precise drawings of the spatio-temporal development of the visual impairment during the aura, preferably with computer-assisted diagnostic procedures, as described in the next section. Then, we need to know precisely how so-called retinotopic maps are laid out in the various visual areas of the cortex. Last but not least, we need to know which cortical excitation pattern correspond to a neural and which to a vascular disorder.

References

Dahlem MA, Chronicle EP. A computational perspective on migraine aura. Prog Neurobiol 2004; 74: 351-361.
Dahlem MA, Müller SC. Migraine aura dynamics after reverse retinotopic mapping of weak excitation waves in the primary visual cortex. Biol Cybern 2003; 88: 419-424.
Dahlem MA, Engelmann R, Löwel S, Müller SC. Does the migraine aura reflect cortical organization? Eur J Neurosci 2000; 12: 767-770.

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