The effect of number of flankers observed in the present study partly replicates the results obtained in the unilateral presentation conditions in our prior work (Chanceaux & Grainger,
2013). As in our prior work, the number-of-flankers effect was obtained using a 2AFC task where the alternative response was never present as a flanker. This is thought to limit the influence of positional uncertainty in driving any effect of flanker interference, with results mainly reflecting participants' difficulty in identifying targets rather than localizing them. Chanceaux and Grainger (
2013) calculated the extent of the crowding zone for the conditions tested in their study (identical to the present study) using the parameters of Nandy and Tjan's (
2012) model of inward-outward asymmetries in the crowding zone. This model was able to account for continuing interference in the four-flanker condition (two to each side of the target) compared with two flankers, but predicted no further decrement in performance in the six-flanker condition. This was in line with the results obtained using homogeneous X flankers (e.g., XXXTXXX) in Chanceaux and Grainger's study, but is not in line with the results of the present study. In the present work we found a significant decrease in performance as the number of flankers was increased from four to six (see
Figure 3). This discrepancy with respect to our prior work is likely due to the lesser interference generated by homogeneous flankers compared with different flanking stimuli, as already demonstrated by Chanceaux and Grainger (
2013). This is likely due to grouping of homogenous flankers (the X flankers in Chanceaux & Grainger's study), and therefore constitutes further evidence for reduced crowding by flankers being grouped separately from targets (e.g., Manassi et al.,
2012; Zhang, Zhang, Xue, Liu, & Yu,
2009). Could grouping have had an influence in the present study? According to grouping accounts of crowding, more visually distinct targets should stand out from flankers and would be subject to less crowding (Malania et al.,
2007; Manassi et al.,
2012). This is one possible basis of the effects of target-flanker similarity seen in the present study. In
Table 1 it can be seen that more similar flankers generated greater crowding, in line with prior research on effects of target-flanker similarity (e.g., Andriessen & Bouma,
1976; Bernard & Chung,
2011; Freeman et al.,
2012; Kooi et al.,
1994; Nazir,
1992). However, regression analyses revealed that most of the influence of target-flanker similarity was driven by flanker complexity in the present study.