Anisotropic object-based shifts of attention arise from unequal influences of visual field meridians on neural attention gradients.

Object-based attention prioritizes the processing of information appearing within a selected object. We previously showed an object-based shift direction anisotropy (SDA) whereby horizontal shifts are more efficient than vertical shifts (Barnas AJ, Greenberg AS. 78: 1985-1997, 2016; Barnas AJ, Greenberg AS. 27: 768-791, 2019), an effect modulated by the visual field meridians (Barnas AJ, Greenberg AS. 77: 2516-2532, 2024). Our present aim was to understand how the functional brain mechanisms of attention give rise to the SDA. We hypothesized that the SDA arises from attentional resources being partitioned differently by horizontal and vertical meridians. To test this, we used fMRI of the visual cortex while subjects shifted attention horizontally and vertically within a single, L-shaped object. An SDA was observed when targets crossed the visual field meridians, and the object vertex was positioned in the upper-left corner. However, no SDA was observed when the object vertex was positioned in the lower-right corner. Cue-related activations revealed that attention spreads throughout the vertical component of each object (even to locations where the target never appeared). In addition, the vertical meridian seemed to impede the spread of attention for the upper-left object's horizontal component. However, for the lower-right object, the horizontal component showed a similar attentional modulation to its vertical component, commensurate with behavioral performance in response to that object. These results suggest the efficiency of horizontal shifts crossing the vertical meridian is enhanced by the support of attentional resources from both hemispheres. As such, we offer a refinement of how the visual field meridians may influence the spread of object-based attention via an anisotropic attention gradient. Our previous behavioral work suggests that inconsistent object-based attention effects (measured as the same-object advantage) arise from differences in shift direction efficiency (a shift direction anisotropy) across the visual field meridians, such that shifts are faster horizontally than vertically. Using fMRI, we present neurobiological evidence that the meridians may cause an unequal allocation of object-based attentional resources. These results necessitate updating theories of object-based attentional selection to account for the effects of visual field meridians.