Form – The Where System

Visual information is processed along parallel tracks: different kinds of neurons in the retina are wired up to the photoreceptors in different ways, to extract different kinds of information. Some cells are sensitive to movement, others to cone opponency, and others to spatial contrast. Within each track, information is communicated along a series of hierarchical stages beginning with these retinal neurons, moving via the optic nerve to the thalamus and on to primary visual cortex (V1) and then higher cortical regions (V2 and up) where it is ultimately integrated to produce our experience of motion, color and form. One useful model divides the neocortex of the visual brain into two broad subdivisions, a dorsal stream involving brain areas on the top (dorsal surface) of the brain, and ventral stream involving brain areas on the bottom of the brain. These two streams, referred to as the Where pathway and the What pathway, are concerned with extracting where an object is located in visual space (and where the viewer is located in reference to the external world), and what the object is (what color, shape, texture). The Where system is thought to be more primitive (evolutionarily older) than the What system, and is responsible for the perception of motion, space, position, three-dimensionality, figure/ground segregation, and the overall organization of the scene. This system is colorblind, has high contrast sensitivity, operates quickly, and has a slightly lower acuity than the “What” system. The “What” system provides us with the ability to identify objects, faces and color.

Luminance and Equiluminance
Luminance is a measure of the amount of light emitted from a surface that falls within a defined area. It is measured in candelas per square meter; and is what artists call “value”. Quantifying luminance for a given surface indicates the amount of luminous power the eye will detect when looking upon that surface from a given angle. The envelope that encompasses photoreceptor sensitivity is called the Vλ (V lambda) function; luminance is calculated by taking the dot product of the spectral distribution of the light reflected or emitted from a surface and the Vλ function. Because the Vλ function is considered a constant from person to person, luminance is therefore an accurate measure of the physical stimulus. From luminance, the brain computes brightness, which is a subjective experience. Two objects may have exactly the same luminance but appear different brightness, as shown in the demonstrations below.

Brightness illusion by Ted Adelson. “A” and “B” have the same luminance, but not the same brightness.	Brightness illusion by Bart Anderson. The chess pieces in both top and bottom are physically identical, yet have different brightness.

Brightness illusion by Ted Adelson. “A” and “B” have the same luminance, but not the same brightness. Brightness illusion by Bart Anderson. The chess pieces in both top and bottom are physically identical, yet have different brightness.

Surfaces that have the same luminance but differ in color are said to be equiluminant. Equiluminance is difficult to demonstrate on a computer monitor: the monitor needs to be carefully calibrated; if it is not, than there will be residual luminance contrast and our Where system is very sensitive to luminance contrast. Stimuli at equiluminance are blind to the Where system, and have a bizarre “jumpy” visual quality that some have said is reminiscent of the way dyslexics see the world. For example: