Parallel processing in the visual system
Parallel processing means that the visual message does not travel from the eye to the brain precisely compartmentalized in various railway coaches one after the other however it rides there across numerous circus horses, one of which carries color, other contrast, a third luminous intensity, etc.
Jonathan Stone demonstrates this notion on the basis of his own contributions to retinal physiology in a book excelling equally in the style of its text and of its illustrations. It covers the last quarter of a century in which the physiologies of the retina, the lateral geniculate body, and the cortex advanced our understanding, and, as Professor Stone would have us think, our philosophical insight.
If one may allow oneself one specific point in connection with a book that is equally impressive in its treatment of detail and generalities, it associates to the visual streak. This is the name given to the non-radial distribution of specific ganglion cell populations, pronounced in the rabbit, observable in the cat, and manifest also in the macaque. In primates non-radial features of the retina are problematic. The eyes move in utero from lateral to ventral positions, there are differential rates of ocular growth to consider, and the degree to which the retina may slide on, or be dragged on by, the eyeball is as yet unknown. The rabbit's eyes stayput, though I have to confess to complete ignorance of leporid embryology. Bruckner associates the presence of the streak with a need for scanning the horizon, and maybe tree-borne primates do this less than scared rabbits.
Stone's views on the hieratic order of the distribution of ganglion cells in areae centrales (which is the reverse of that given above) seems to indicate that speculation is premature. Ostriches, too, scan the horizon, but their vision is less lateral than that of rabbits. Presumably structure sub-serves function; when it is so, then there is no point in concentrating on mammalian eyes that are at present in any case heavily outnumbered by non-mammalian ones. This aside will have confirmed to you that this book is thought-provoking, stimulating, and worthy of attention by anyone concerned with eyes.