Abstract
Neural activity measured in the absence of any external stimuli or overt behaviors exhibits systematic and bilateral correlations across the brain. Such correlations are often taken to imply functional connectivity between different brain regions, but the underlying causes of these correlations are unclear. We used functional magnetic resonance imaging to study spontaneous activity in the human visual cortex (V1-V3) at a fine scale, capitalizing on the detailed retinotopic maps in these visual areas. We found that the strongest correlations in spontaneous activity are between points on the cortex with functional receptive fields at the same distance from the point of gaze in the visual space, irrespective of whether the receptive fields are in the same quadrant or hemi-field. This long-distance iso-eccentric organization of the spontaneous activity is robust. Moreover, when we visually stimulated at one eccentricity in one quadrant of the visual field, we detected a signal from cortical locations corresponding to the same eccentricity in the non-stimulated quadrants. While these findings may suggest iso-eccentric connectivity in the measured visual areas, there is no anatomical evidence for such connections. We show that the seemingly strong evidence for neural connectivity can be quantitatively accounted for by temporally varying and diffuse feedbacks from two or more higher cortical areas that exhibit different degrees of eccentricity biases. Our findings provide a method for assessing the integrity of such feedbacks.