The density effect in centroid estimation is blind to contrast polarity

Human vision is highly efficient in estimating the centroids of spatially scattered items. However, the processes underlying this remarkable skill remain poorly understood. A salient fact is that in estimating the centroids of dot-clouds, observers underweight densely packed dots relative to isolated dots; thus, when an observer estimates the centroid of a dot cloud, the weight exerted on the subject's response by a given dot tends to be suppressed by other dots near it. The current experiment sought to determine whether dots of contrast polarity equal vs. opposite to a given dot differ in how they alter the weight it exerts. Six observers were tested in a task that used brief (180 ms), Gaussian clouds that mixed 9 white and 9 black dots on a gray background. On each trial, the observer strove to mouse-click the centroid of the stimulus cloud weighting all dots equally. The model used to describe the results allows the weight exerted on the subject's response by a given dot to depend on its peripherality in the stimulus cloud as well as on the density of same- and opposite-polarity dots surrounding it. For four observers, peripheral dots exerted lower influence than central dots on responses; the other two showed little effect of peripherality. For all observers, dots in high-density regions exerted less weight on responses than dots in low-density regions. Concerning the primary research question: dots of opposite vs. the same polarity as a given dot suppressed the weight it exerted with equal effectiveness. This suggests that the site of the interaction producing the density effect is a neural population that registers positive and negative contrast polarities in the same way.