74. Proposition of an MEG phantom as the evaluation standard

Visual-evoked magnetic fields (VEFs) can detect early components for pattern-reversal stimuli which are generated from the primary visual cortex (V1). The present study applied VEFs for the localization of the V1 and evaluation of visual function in 16 patients (10 males) with occipital lesions (7 with vascular malformation, 6 with neoplasm and 3 with cerebral hemorrhage). Monocular pattern-reversal stimuli were independently presented to the left and right half visual fields of the left and right eyes. VEFs were measured by a helmet shaped MEG system (VectoreView, Elekta-Neuromag, Helsinki). The source of the P100m was estimated by a single dipole model and was superimposed on the three-dimensional magnetic resonance image. VEFs were not detected for the corresponding visual stimuli or the latency of P100m response was delayed in the 5 patients with homologous hemianopsia. In contrast, the P100m source was estimated on the calcarine fissure in the 11 patients with no visual deficit, even in patients with lesions adjacent to the V1. VEFs provide a noninvasive method to objectively evaluate visual-field deficit in patients with occipital lesions. Localization of the P100m source is also useful for presurgical mapping.