Chapter

fMRI of Human Visual Pathways

DOI: 10.1007/978-1-4419-0345-7_26

ABSTRACT Functional magnetic resonance imaging (fMRI) of the human brain provides images of changes in local blood flow and oxygenation
that are evoked by sensory, motor, or cognitive events. Functional MRI has been used since 1991 [1] to identify areas of the
brain that respond to visual stimulation and the performance of vision-related tasks. Increasingly, fMRI is accompanied by
diffusion tensor imaging (DTI), which provides images of the speed and direction of diffusion of water molecules in the brain.
Fortuitously, this allows remarkable differentiation of cerebral white mater and the delineation of a variety of major white
matter tracts including vision-related pathways such as the optic radiations. This chapter focuses primarily on fMRI, but
DTI data are also discussed where relevant. Together, the two methods provide a wealth of information about the anatomical
and functional status of key components of the visual system in individual patients even in the presence of pathology. For
example, an imaging-based map of the visual system can be helpful for planning and guiding surgical resection of tumors impacting
critical vision-related brain structures. This is especially true when mass effects or previous surgeries have distorted the
normal anatomy making it difficult to know where key structures are located and if they are still functional. In difficult
cases, identifying the region of “closest approach” of a planned resection to the cortical representation of central vision
or to the optic radiations can help to minimize the risk to eloquent neural tissue and thereby avoid significant treatment-induced
vision loss while still permitting maximum therapeutic effect.

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    ABSTRACT: fMRI is becoming an important clinical tool for planning and guidance of surgery to treat brain tumors, arteriovenous malformations, and epileptic foci. For visual cortex mapping, the most popular paradigm by far is temporal phase mapping, although random multifocal stimulation paradigms have drawn increased attention due to their ability to identify complex response fields and their random properties. In this study we directly compared temporal phase and multifocal vision mapping paradigms with respect to clinically relevant factors including: time efficiency, mapping completeness, and the effects of noise. Randomized, multifocal mapping accurately decomposed the response of single voxels to multiple stimulus locations and made correct retinotopic assignments as noise levels increased despite decreasing sensitivity. Also, multifocal mapping became less efficient as the number of stimulus segments (locations) increased from 13 to 25 to 49 and when duty cycle was increased from 25% to 50%. Phase mapping, on the other hand, activated more extrastriate visual areas, was more time efficient in achieving statistically significant responses, and had better sensitivity as noise increased, though with an increase in systematic retinotopic mis-assignments. Overall, temporal phase mapping is likely to be a better choice for routine clinical applications though random multifocal mapping may offer some unique advantages for selected applications.
    NeuroImage : clinical. 01/2013; 3:143-54.