There is, however, a potential alternative explanation to the present results. Specifically, it is possible that the configurations used in the present experiment led to a linear increase in the number of visual hemifields or quadrants over which sample items were presented. Previous studies have demonstrated that information in VWM may be represented by distinct resources in the left and right hemispheres (Buschman, Siegel, Roy, & Miller,
2011; Delvenne,
2005; Umemoto, Drew, Ester, & Awh,
2010). Moreover, information presented in the same hemifield or quadrant is subject to more competition than information presented in distinct hemispheres (Alvarez & Cavanagh,
2005; Scalf & Beck,
2010) or quadrants (Carlson, Alvarez, & Cavanagh,
2007). Monte Carlo simulation (see Methods section) indicated that although the average number of hemifields over which the items were presented increased from Separation 1 to Separation 3 (1.3, 1.7, and 2.0, respectively), items could only be presented in two hemifields in both the Separation 3 and Separation 4 conditions. Thus, although items were always distributed across resources in both hemispheres in these conditions, the proportion of binding errors continued to decrease, suggesting that the proportion of non-target responses is independent of the allocation of hemispheric resources. In contrast, the average number of quadrants in which sample items were presented increased roughly linearly with separation condition, with items presented in 1.8, 2.3, 2.9, and 3.7 quadrants, respectively. Thus, while in the Separation 1 condition a quadrant with one sample item was very likely to contain at least one other item, in the Separation 4 condition items were often presented within independent quadrants. Consequently, it is possible that the proportion of binding errors is affected not only by the absolute distance between each of the items but by the competition for representation by independent resources within each visual quadrant. This interpretation, however, is still consistent with the conclusion that competition for representation during encoding increases the number of binding errors. Ultimately, whether items compete for representation within a visual quadrant (Carlson et al.,
2007) or as a function of the distance between items (Bahcall & Kowler,
1999; Franconeri et al.,
2007; Kastner et al.,
2001; McCarley & Mounts,
2007,
2008; McCarley et al.,
2004; Mounts & Gavett,
2004; Mounts,
2000; Mounts et al.,
2007), the functional mechanism underlying these effects remains the same: Items compete for neural representation within spatially defined receptive fields. When items are presented close together in space, the amount of competition will increase, resulting in an increase in binding errors. Moreover, these effects are also likely to occur at early stages of sensory processing rather than during maintenance stages, as previous studies have demonstrated that competition for representation within a hemifield occurs during early sensory encoding (Buschman et al.,
2011; see
General discussion section).