Precise Functional Localization of the Foveolar Representation in Anesthetized Macaque Visual Cortex

The foveola-the central ~1{degrees} of the visual field-supports the highest-acuity spatial vision, yet localizing its cortical representation in the primate brain is technically demanding, particularly under anesthesia where gaze cannot be behaviorally controlled. Here we describe a hierarchical, meridian-based 7T BOLD fMRI strategy for sub-degree functional localization of the foveolar representation in anesthetized macaques. Using drifting grating bars presented at three successively finer spatial scales (4.5{degrees}, 1.5{degrees}, and 0.5{degrees}), we independently mapped the vertical and horizontal meridian representations in early visual cortex through a coarse-to-fine refinement scheme. The vertical meridian was identified by the progressive transition from unilateral to bilaterally symmetric activation converging at the V1/V2 border, whereas the horizontal meridian was defined by simultaneous activation of dorsal and ventral V2/V3 borders at the fundus of the calcarine sulcus. The functional intersection of the two meridia defined candidate foveolar coordinates, which were then refined using localized 0.2{degrees} spot stimuli. Brain surface-based rendering revealed multiple discrete foveolar loci, one at the lateral end of each of the V1/V2, V2/V3, V3/V4, and V4/TEO borders. These distributed loci define the "foveolar core", a newly discovered visual area with distinct functional properties. This meridian-based hierarchical approach provides a reproducible framework for resolving central visual representations in anesthetized primates, providing a new avenue for studying foveolar cortex and circuitry.