Computation and measurement of slant specified by linear perspective.

One of the striking features of vision is that we can experience depth in two-dimensional images. Since the Renaissance, artists have used linear perspective to create sensations of depth and slant. What is not known is how the brain measures linear perspective information from the retinal image. Here, an experimental technique and geometric computations were used to isolate slant related to linear perspective from slant induced by other cues. Grid stimuli, designed to induce strong impressions of slant, were sufficiently simple to allow accurate predictions on the basis of numeric computations. Measurement of slant about the vertical axis as functions of slant depicted on the screen and slant of the screen relative to the observer showed that linear perspective explained 95% of the slant judgments. Precision and accuracy of the judgments suggest a neural substrate that is able to make highly accurate comparisons between orientations of lines imaged at different retinal locations. The neural basis of slant from the linear perspective has not yet been clarified. Long-range connections in V1, however, and cells in V2, V4, lateral occipital cortex, and caudal intraparietal sulcus have features that suggest an involvement in slant perception.