Future Investigations

Future investigations of human area V2 will utilize new techniques to address several anatomical issues related to two- and three-dimensional mapping and several physiological issues related to compartmental organization. Current methods of two-dimensional and three-dimensional mapping techniques can be expected to be used to provide more complete reconstructions of the distributions of several anatomical markers. For example, although cytochrome oxidase stripes have been visualized in limited portions of V2, it has not yet been possible to map the complete distribution of human V2 cytochrome oxidase stripes. Similarly, the distribution of immunoreactivity for CAT-301, characteristic of the magnocellular compartment of V2, has been observed in limited regions of flattened V2, and it will be possible to reconstruct its complete distribution in three-dimensional reconstruc tions of histological sections and on unfolded cortical maps. These same approaches can be applied to the localization of a variety of other anatomical markers, such as the distribution of callosal afferents and the distribution of immunoreactivity for SMI-32 or various neurotransmitters.

The previously described anatomical studies are primarily concerned with the distribution of anatomical markers in individual brains that have been first imaged by MRI, then sectioned and stained, and then reconstructed back into three-dimensional models and two-dimensional maps. Additional investigations will expand these single-subject reconstructions to build a standardized brain and standard map. These investigations will build upon existing methods of brain warping and map warping to allow for a probabilistic mapping of these anatomical variables.

Future investigations will explore further the connections that area V2 makes with cortical and subcortical targets. One approach might use highly diffusible markers, with greater efficacy than DiI, to trace axonal pathways. Another approach might use physiological methods of microstimulation combined with fMRI to detect cortical targets activated following local magnetic or electrical stimulation. Finally, the more recently developed method of diffusion tensor magnetic resonance imaging might be employed to study the organization and targets of axonal fascicules that leave V2 for other cortical areas.

Future functional studies will provide better insight into the role that human V2 plays in various aspects of visual perception. These studies are likely to proceed along three different fronts. The first of such studies will utilize refined psychophysical methods, in conjunction with fMRI, to better identify the role of V2 in color, form, motion, texture, and illusory contour processing. The overall goal of such experiments will be to distinguish the perceptual contribution ofV2 that is distinct from those of area V1 and higher cortical areas. This work will proceed in parallel with similar investigations in macaque monkeys using microelec-trode and fMRI techniques.

The second type of future physiological studies will employ fMRI techniques to study the temporal organization of processing in the visual cortex. These studies will explore the temporal sequence of color, form, motion, and texture processing as these signals radiate out of V1. Although it is clear that the visual cortex is organized in an anatomical hierarchy, little is known about the timing of information flow within this hierarchy. Thus, whereas different stimulus attributes are processed by different compartments in V2

and by different cortical areas, it remains unclear, for example, whether fast motion processing via area MT interacts with slower form- and color-processing elements via feedback to area V2.

The third type of future physiological investigation will employ state of the art fMRI techniques to explore the functional properties of the modular compartments of human V2. Previous fMRI studies of human V2 have summed signals across large expanses of cortex to yield a picture of the role V2 plays in perception. Future studies will be directed at the modular segregation of such signals. Current fMRI methods have begun to address the compartmental organization of visual cortex. The visualization of presumed ocular dominance columns in human V1, which are approximately 1 mm in width, increases the likelihood that the larger V2 modules can be similarly visualized.

Finally, better functional localization of visual cortical areas such as V1, V2, V3, VP, V4, and MT in individual brains will allow more highly detailed solid models necessary for the interpretation of multisource electrical or magnetic evoked potentials. The resolution of these multisource models will allow a wide variety of studies that can examine the dynamics of visual perception.

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