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

Abstract

The foveola-the central ~1{degrees} of the visual field-supports the highest-acuity spatial vision, yet localizing its cortical representation 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 localization of the foveolar representation in anesthetized macaques. Drifting grating bars presented at three successively finer scales (4.5{degrees}, 1.5{degrees}, 0.5{degrees}) independently refined the vertical and horizontal meridian representations, each identified by a characteristic activation signature: bilateral convergence at the V1/V2 border for the vertical meridian, and simultaneous dorsal and ventral V2/V3 activation at the fundus of the calcarine sulcus for the horizontal. Their functional intersection defined candidate foveolar coordinates, which were refined using 0.2{degrees} spot stimuli. Surface-based rendering resolved discrete foveolar loci at the lateral end of the V1/V2, V2/V3, V3/V4 and V4/TEO borders, together delimiting the previously described 'foveolar core'. Pooling these with awake data, we place the loci in a common stereotaxic frame, yielding a first anatomical atlas of the core. Intrinsic-signal optical imaging further shows that this territory lacks the ocular-dominance and orientation columns of surrounding cortex. The approach provides a reproducible framework for locating and targeting foveolar cortex in anesthetized primates.