Figure 3.

Stimuli for visual field mapping and construction of functional field maps (FFMaps). Retinotopic mapping stimuli consist of an expanding, black-and-white, checkered annulus (A) or a rotating wedge (B) counterphase flickered at 6 Hz and presented in a temporal phase mapping paradigm. A single expansion-rotation sequence takes 32 seconds and is repeated 5 times within a 168-second scan run. (The initial 8 seconds of the scan containing the equilibration transient are discarded.) The outer boundary of the stimuli extends to approximately 25° eccentricity. Expanding annuli are not scaled linearly in size and eccentricity but, rather, are scaled nonlinearly so as to activate roughly equal areas of V1, as estimated using the retinocortical mapping function by Balasubramanian et al.⁸⁵ The subject's task during each mapping sequence is to fixate a small dot at the center of the display and press a button when the dot blinks off for 0.125 seconds, varied randomly every 4-8 seconds. This task helps ensure that the patient attends to the mapping stimulus and maintains gaze at the center of the display. Postprocessing of the resulting fMRI data yields an estimate of the visual field coordinates (eccentricity [rho], angle [theta]) at which the mapping stimuli activate each voxel most strongly. Using these coordinates, a circle symbol can be placed at the corresponding location on a diagram of the patient's view of the stimulus display (right). The symbol is colored to show the amplitude of the fMRI response according to the pseudocolor scale below the FFMap. The size of the symbol represents the error in estimating the preferred stimulus location for that voxel (approximately 70% confidence interval). Symbols for all visually responsive voxels are placed on the diagram to yield an FFMap that allows the physician to determine instantly if the patient's visual cortex is responding to all locations within his or her field of view or if there is a zone of impaired brain response, indicated here by cross-hatching in the upper left field. (Color version of figure is available online.)