An Event-related fMRI Study of Exogenous Orienting: Supporting Evidence for the Cortical Basis of Inhibition of Return?

This event-related fMRI experiment examined the neural substrates of exogenous visuospatial attention. Exogenous attention produces a biphasic response pattern denoted by facilitation at short cue–target intervals and inhibition of return (IOR) at longer intervals. Whereas the volitional orienting of attention has been well described in the literature, the neural systems that support exogenous facilitation and IOR in humans are relatively unknown. In direct comparisons to valid facilitation trials, valid IOR trials produced unique foci of activation in the right posterior parietal, superior temporal, middle temporal, middle occipital, anterior cingulate, and dorsal medial thalamic areas. Valid IOR trials also resulted in activation of motor exploratory and frontal areas previously associated with inhibition and oculomotor control. In contrast, invalid IOR compared to facilitation trials only activated anterior cortical structures. These results provide support for both attentional and oculomotor theories of IOR and suggest that IOR may be mediated by two networks. One network may mediate the inhibitory bias following an exogenous cue, whereas a separate network may be activated when a response must be made to stimuli that appear in inhibited locations of space.

Enlargement and Rupture of Distal Basilar Artery Aneurysm after Iatrogenic Carotid Occlusion

A case of angiographic enlargement and fatal rupture of a previously asymptomatic distal basilar aneurysm in a 12-year-old girl is reported. She had been treated by carotid sacrifice for a giant intracavernous carotid aneurysm. After superficial temporal-middle cerebral artery bypass, this patient underwent a trapping procedure and decompression of her symptomatic giant aneurysm. Despite postoperative patency of her bypass graft, the involved middle cerebral circulation was irrigated substantially by retrograde flow through her posterior communicating artery. An incidental distal basilar aneurysm involving the origin of her superior cerebellar arteries, posterior cerebral arteries, and multiple perforators was treated by a wrapping procedure. Eleven days after carotid ligation, she suffered a fatal subarachnoid hemorrhage from her basilar aneurysm. This catastrophe was undoubtedly produced by our failure to consider the additional hemodynamic stress placed upon the distal basilar artery by carotid sacrifice and may have been preventable by a more aggressive attack on this previously asymptomatic lesion.

Basal Arterial Occlusive Disease

Seventeen patients with basal occlusive disease have been seen over the past 4 years. Nine of these had a classical moyamoya appearance on angiography. Of these nine, seven were adults and two were children. Eight had ischemic episodes, and one had a hemorrhage. One had neurofibromatosis with a chiasmal glioma and had received radiotherapy. Eight patients underwent superficial temporal-middle cerebral artery (STA-MCA) bypass, and one refused operation. Six patients improved, one had a stroke on the opposite side, and one died. Eight patients had unilateral basal arterial occlusive disease. Of these, three had ischemic episodes, four had hemorrhages, and one had only headaches. Four were adults, and four were children. Five underwent STA-MCA bypass with improvement, one with headaches had an aneurysm treated, and two were not operated upon. Considerations regarding the cause, therapeutic options, and disease course in these patients are discussed.