George Sperling¹ (), Lingyu Gan¹; ¹University of California, Irvine

Household measuring cups, weight scales, and measuring tapes, do not know what they are measuring, they simply make a real-number, substance-invariant measurement. Because there is a virtual infinity of visual substances, the brain also must make substance-invariant measurements but it uses a different mechanism: salience processing. The brain makes substance-invariant judgments by ignoring the nature of the substance to be measured, whether it is particular set of shapes, colors, textures, or whatever, and representing the just locations being occupied, and then measuring the locations versus the substance itself. The set of possible locations is called a salience map, occupied locations are designated by a positive number, salience. Originally, this salience architecture was used to describe priority in visual processing and visual search (Koch and Ullman, Human neurobiology, 1985). However, the same architecure can be used for other purposes (e.g., direction of feature motion, Lu and Sperling, Nature, 1995). Because what is judged is not the substance itself, but merely the locations occupied by the substance, the judgments are indifferent to the nature of the substance. Here we demonstrate three judgments that are substance invariant: Distance between two items in frontal plane, the centroid or numerosity of a group of items. Judgment accuracy is independent of whether the items are similar or different or whether they are isoluminant with the background, provided the items are distinctly visible. These three parametrically studied judgments are of novel stimuli. We also demonstrate partial substance invariance for text: Many different colors of large letters on an isoluminant gray background are easily readable but very small letters require high contrast black or white. Conclusion: The brain solves the problem of measuring many different substances by representing just the locations of the substances in a salience map and measuring the salient locations.