The behavioral relevance of these two neural populations is controversial, however, with an apparent conundrum between the psychophysical and neurophysiological evidence. Under Livingstone and Hubel's (
1988) original scheme, color vision was considered very poor at form perception and was even called “form blind” (Livingstone & Hubel,
1987), a view that found support from the lowpass, low acuity color contrast sensitivity function (Kelly,
1983; Mullen,
1985). This view held that the boundaries and contours within the visual scene are primarily extracted from luminance contrast, with color vision filling in the surface color between contours and linking areas of common color to define objects, a role which is compatible with a population of nonoriented, color-selective neurons. Psychophysical research over ensuing decades, however, demonstrated that this could not be the only way the brain uses color. Instead, we now know that color vision can extract the key characteristics of edges with very little deficiency. Psychophysical experiments have shown that bandpass spatial frequency tuning, critical for edge detection, underlies the lowpass color contrast sensitivity function (Losada & Mullen,
1994,
1995; Vimal,
1998), orientation discrimination in color vision is only mildly deficient (Beaudot & Mullen,
2005; Reisbeck & Gegenfurtner,
1998; Webster, De Valois, & Switkes,
1990; Wuerger & Morgan,
1999), and chromatic oriented elements can readily be linked into contours and shapes (Mandelli & Kiper,
2005; Mullen, Beaudot, & Ivanov,
2011; Mullen, Beaudot, & McIlhagga,
2000). Hence, there is now little doubt that human color vision is well equipped to extract shape and form, and the large population of spatially selective, orientation-tuned neurons found in primate V1, referred to above, is thought to support this function. This raises the question of the psychophysical role of the small population of nonoriented neurons in V1. The role of these neurons as surface color detectors has largely been surmised, and psychophysical experiments have not yet revealed any direct evidence for nonoriented mechanisms in color vision. Yet, the fact that these neurons are the only population in V1 that exclusively responds to color suggests they have an important role.