Abstract
In a series of recent experiments (cf. Barnas & Greenberg, VSS 2015–18) we demonstrated that reallocating object-based attention across the visual field meridians is faster horizontally than vertically, referred to as a ‘shift direction anisotropy’ (SDA). We revealed that the SDA is (1) observed whether attention shifts occur within or between objects, (2) not present when targets/objects are sequestered into quadrants, and (3) driven by target, rather than object, location. Here we present a theory, grounded in neurobiology, (along with new data) that accounts for these findings. We theorize that the source of the SDA depends upon anatomical segregations of the visual system that determine the manner in which pools of attentional resources resolve competition between targets. It is well-established that the left/right cerebral hemispheres are organized contralaterally, imposing an interhemispheric boundary along the vertical meridian (anatomically, the longitudinal fissure). Some suggest that this explains independent attentional capabilities between the hemispheres during attentional tracking (Alvarez & Cavanagh, 2005) and visual search (Clevenger & Beck, 2014). Additionally, lower/upper visual field representations are segregated anatomically, forming an intrahemispheric boundary along the horizontal meridian (anatomically, the dorsal and ventral aspects of retinotopic cortex). We extend the independent attentional resources explanation to object-based attention and propose that the SDA occurs due to impaired attentional reallocation across the intra-hemispheric boundary. Vertical shifts experience a processing cost because stimuli compete within a single pool of attentional resources, requiring additional processing time to resolve the competition. Conversely, horizontal shifts experience a processing benefit because competition is resolved quickly due to two independent pools of attentional resources. Confirming this hypothesis, our new data show that a perceptually enhanced horizontal meridian is sufficient to create an artificial boundary that further subdivides attentional resources into four pools. As a result, competition across the intrahemispheric boundary is reduced, equating RTs and ameliorating the SDA.
Acknowledgement: US-Israel Binational Science Foundation, University of Wisconsin-Milwaukee Research Growth Initiative, and Greater Milwaukee Foundation