Perceived magnitude of visual displays: Area, numerosity, and mean size.

Previous studies have shown that the visual system is able to estimate properties such as area, numerosity, and mean size efficiently and accurately. In the current study, we investigated whether our percepts of each of them could be based on ratios of the other two of these three properties. In each trial, observers viewed a display containing various quantities of filled circles and judged whether the magnitude of a property of the display, such as summed area, numerosity, or average size of the circles, was greater or less than a corresponding probe display. We found that mean size judgments were more accurate and precise compared to the other judgments. We then predicted observers' performances for each task using the measured performance for the other judgments. The results showed that the other properties predicted perceived summed area, but not perceived mean size and numerosity. Together, our results suggest that the visual system does not use ratios to compute mean size and numerosity.