Mixed with the two classes of 
double-opponent cells (red-
green and yellow-blue) were 
ordinary broad-band, center-
surround cells. Again, these 
broadband cells differed from 
cells in the upper geniculate 
layers and from cells in 4Cß in 
having several times larger 
center sizes. Blobs also contain 
cells that are indistinguishable 
from geniculate type 2 cells, 
resembling double-opponent 
cells but lacking the receptive-
field surround.

    Margaret Livingstone and I 
have proposed that the blobs 
represent a branch of the visual 
pathway that is devoted to 
"color", using the word broadly 
to include blacks, whites, and 
grays. This system seems to 
separate off from the rest of the 
visual path either in the lateral 
geniculate body or in layer 4 of 
the striate cortex. (The 
geniculate probably projects 
directly but weakly to the blobs. 
It seems likely that layer 4Cß 
also projects to them, and it 
may well form their main 
input. Whether 4Ca projects to 
them is not clear.) Most blob 
cells seem to require border 
contrast in order to give 
responses at all: either 
luminous-intensity borders, in 
the case of the broad-band, 
center-surround cells or color-
contrast borders, in the case of 
the double-opponent cells 
would respond. As I argued 
earlier, this amounts to saying 
that these cells play a part in 
color constancy.

    If blob cells are involved in 
color constancy, they cannot 
be carrying out the computation 
exactly as Land first envisioned 
it, by making a separate 
comparison between a region 
and its surround for each of the 
cone wavebands. Instead they 
would seem to be doing a 
Hering-like comparison: of red-
greenness in one region with 
red-greenness in the surround, 
and the same for yellow-
blueness and for intensity. But 
the two ways of handling color-
-r, g, and b on the one hand and 
b-w, r-g, and y-b on the other--
are really equivalent.