Grouping of contextual elements that affect vernier thresholds

Abstract

To reveal the mechanisms of spatial interference in the fovea, we examined the capacity of a variety of lateral flanking configurations to interfere with alignment thresholds of a vertical vernier. A single line on each side of the vernier, at the optimal separation of 2-3 arcmin, raises thresholds threefold or more and masks most effectively when its length equals that of the vernier. For an array of equal lines whose length differs from that of the vernier, masking is reduced but not when vernier and flanks have the same length. The reduction for shorter and longer flanks can be reversed by inserting a gap in the row of flanks. By comparing the masking effect of arrays of mixed line lengths, we show that when a pair of flanks, which by itself masks strongly, becomes a component of a coherent contextual configuration, it loses much of its effectiveness to interact. Observers' ranking of the conspicuity of the vernier test pattern among the flanking elements is negatively correlated with the threshold elevation. We conclude that clustering of contextual patterns influences their capacity to mask. Discrimination of a target deteriorates when the target is grouped within an array of surrounding elements.

Combining simultaneous with temporal masking.

“…Increasing the length of the simultaneous mask elements lowered the threshold of the target vernier from approximately 70 arcseconds (dashed line) to 30 arcseconds (dotted line) when no temporal mask is presented (for the two participants that performed both tasks, thresholds went down by 47% and 92%). This decrease in thresholds by increasing the length of the mask's elements replicates earlier findings by Duangudom, Francis, & Herzog (2007, Figure 3, SOA=0) and Malania, Herzog, & Westheimer (2007, Figure 3, 16 flanks). Whereas the threshold changed in the condition with just the simultaneous mask, thresholds as a function of SOA with the temporal mask hardly changed by increasing the length of the simultaneous mask's elements.…”

Spatial grouping determines temporal integration.

“…This suggests that from this spacing on, the anti-verniers are no longer spatially grouped, which allows fusion between the vernier and the central anti-vernier to take place. This result is in good agreement with the results of Westheimer and Hauske (1975), who showed the interference of lateral flanks on vernier acuity is limited to a distance of about 600" (see also Malania, Herzog, & Westheimer, 2007). The model predictions ( Figure 6, filled circles) closely match the experimental findings ( Figure 6, open squares)squares), which is reflected in a correlation between the predicted and observed means of 0.99.…”

How do flankers' relations affect crowding?

“…In the interrupted arrangement, grouping between the target and flankers was as strong as the grouping between the flankers themselves, and therefore we observed a strong effect. Differences in the grouping of flankers and targets has been shown to affect acuity performance in both the fovea and the periphery (Malania, Herzog, & Westheimer, 2007;Saarela, Sayim, Westheimer, & Herzog, 2009). Similarly to our grouping account Saarela and his colleagues suggested that whenever the target stands out from the flankers, crowding is weakened.…”