Page 157 - Migraine, the heart and the brain

The post-ictal impact of a migraine attack on cognition
by, migraine. This leaves the global-local task, investigating organization of local and global visual stimuli, as the test differing between migraineurs and controls, a nding which is consistent with the previous observation that migraineurs have higher perceptual thresholds for the recognition of global shape (20).
The global-local task was developed by Navon (14) and taps into the organization
of visual information into coherent objects or Gestalts and assesses this performance
by presenting participants with hierarchically organized visual stimuli, such as a
global letter that is made of a number of local letters. Healthy humans have a strong
preference to perceive the global letter rst and faster, suggesting that they organize
the local information into a coherent global whole the so-called global precedence
effect (14). The rst evidence for speci c dysfunction in migraineurs comes from two
recent studies (20,21) showing that migraineurs did not differ from healthy controls
in a visual task tapping into very early sensory processes (presumably performed by
brain area V1) but have higher thresholds than controls for the perception of global
shape (presumably performed by the extrastriate area V4). This suggests a locus in
the cortical information-processing stream later than visual stages (V1) but earlier
than at encoding into working memory, which implies higher visual areas that are
either directly involved in feature integration or that provide integration processes
with global information. As demonstrated by Badcock and colleagues (23), the global
precedence effect can be eliminated by ltering out low-spatial frequency information
in Navon-type stimuli. It is thus possible that in migraineurs the processing through
low-spatial frequency tuned visual channels is impaired. This would t with the
observation that channels that prefer higher flicker rates (commonly tuned to lower
spatial frequency ranges) can have lower sensitivity in migraineurs during their inter-
ictal period (24). Hence, patients may not be able to process or to integrate more 8 global features into perceptual representations of objects and events.
The organization of perceptual information is followed by attentional selection processes, in which no difference was found in our study between migraineurs and controls by ANT test. Human attention is thought to fall into a least three different abilities, which are handled by three neurally dissociable networks (15): alerting (activating the system in response to the presence of a relevant stimulus), orienting (selecting a particular stimulus or location for further processing), and executive control (biasing response selection towards appropriate responses). Not one of these abilities seems to be impaired by migraine.
The outcome of attentional selection processes are thought to be transferred to working memory, which organizes further processing or storage of the selected
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