A device for testing depth perception. It consists of two identically patterned horizontal surfaces, one well below the other; the upper is extended over the lower by means of a sheet of transparent glass. A subject (usually a newborn of a species) placed in the centre of the upper surface and who is unwilling to move onto the transparent glass that projects over the lower surface is assumed to possess depth perception. Many species have been found to possess depth perception at birth, indicating an innate sense, unaffected by learning experience.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann