Third Kind of Eye Cell Discovered

Until very recently, the textbook description of the eye contained two kinds of photoreceptive cells, rods and cones:

The retina contains two types of photoreceptors, rods and cones. The rods are more numerous, some 120 million, and are more sensitive than the cones. However, they are not sensitive to color. The 6 to 7 million cones provide the eye's color sensitivity and they are much more concentrated in the central yellow spot known as the macula. In the center of that region is the " fovea centralis ", a 0.3 mm diameter rod-free area with very thin, densely packed cones. . . . [C]ones can be divided into "red" cones (64%), "green" cones (32%), and "blue" cones (2%) based on measured response curves. . . . The green and red cones are concentrated in the fovea centralis. The "blue" cones have the highest sensitivity and are mostly found outside the fovea, leading to some distinctions in the eye's blue perception.

New research indicates that there is a third kind of photoreceptive cell, an upsilon cell. These cells sense motion and there are roughly one for every 10,000 rods and cones. According to Science News:

Upsilon cells and detectors of other image features, such as changes in brightness, constitute the forward layer of the retina's three-tiered structure. Various types of relay cells make up the middle layer, and the back layer contains the rods and cones.

Upsilon cells had previously been observed in cats, but not in primates.

The source article abstract is here.