Proper understanding of processes underlying visual perception requires information on the activation order of distinct brain areas. We measured dynamics of cortical signals with magnetoencephalography while human subjects viewed stimuli at four visual quadrants. The signals were analyzed with minimum current estimates at the individual and group level. Activation emerged 55-70 ms after stimulus onset both in the primary posterior visual areas and in the anteromedial part of the cuneus. Other cortical areas were active after this initial dual activation. Comparison of data between species suggests that the anteromedial cuneus either comprises a homologue of the monkey area V6 or is an area unique to humans. Our results show that visual stimuli activate two cortical areas right from the beginning of the cortical response. The anteromedial cuneus has the temporal position needed to interact with the primary visual cortex V1 and thereby to modify information transferred via V1 to extrastriate cortices.A natomy of connections between primate visual cortices suggests a hierarchical organization of signal processing (1). However, the order of processes at different functional areas cannot be directly deduced from the anatomical hierarchy without relevant timing information. In monkeys, several areas become active immediately after the primary visual cortex (V1), while another set of areas is active at clearly longer latencies (2, 3). The areas showing early responses, such as V5͞middle temporal area (MT), medial superior temporal area, and frontal eye field, are specialized in analyzing dynamical visual information and in visuomotor transformation (for reviews, see refs. 4 and 5). The areas with longer latencies, such as V4, are sensitive to object form and color and participate in object recognition (6, 7). The dissimilarities in activation latencies and functional properties of these areas suggest diversity in the type and amount of necessary input and preprocessing before activation. Given the differences between monkey and human visual cortices (for a review, see ref. 8), we aimed to explore the dynamics and distribution of early cortical activation in humans. Neuromagnetic signals were recorded while the subjects viewed pattern reversal or luminance stimuli at the four visual quadrants. The signals were analyzed with minimum current estimates (MCEs), which require minimal human intervention for determining the location and orientation of the cortical currents (9). The individual three-dimensional estimates were aligned with a nonlinear transformation according to individual brain shapes (10, 11), and both the individual and group average MCEs were compared with the existing maps of human visual cortices (12-16). Materials and MethodsSubjects and Stimuli. We studied five female and five male subjects (mean age 27 years, range 20-42 years). The stimuli were generated with a Macintosh computer and presented with a dataprojector (VistaPro, Electrohome Ltd., Ontario, Canada) on a back projection screen, with viewing d...

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Although both MEG and functional MR imaging can provide useful information for neurosurgical planning, in the present study, MEG proved to be superior for locating the central sulcus. Activation of multiple nonprimary cerebral areas may confound the interpretation of functional MR imaging results.

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