Ekman, G., Lindman, R. & William-Olsson, W. (1961). A psychophysical study of cartographic symbols. Perceptual and Motor Skills, 13, 355-368.

Cartographic symbols constructed with perspective to give the impression of cubes and spheres and intended to represent population numbers were investigated by psychophysical methods. It was demonstrated that estimates of the "volume" of these symbols merely reflect their perceived area. At least for non-specialized map-readers the symbols are not efficient in creating the desired impression of volume.