Unique hues have been recognized since Hering to represent the basic color categories of red, yellow, green, and blue (for review see
Abramov & Gordon, 1994). They are pure instances of hue in that they do not appear perceptually mixed with other hues. All color normal subjects seem to accept these hues as “unique” within certain constraints and research has shown that some can consistently identify unique hues of different contrast within a range of a few nanometers of dominant wavelength (
Webster, Miyahara, Malkoc, Raker, 2000). Whereas previous studies noted small, but significant differences in setting both categorical hues for spectral colors (
Jordan & Kulikowski, 1995; Jordan & Kulikowski,
1997) and unique hues (
Mollon & Jordan, 1997;
Wuerger, Atkinson, & Cropper, 2005), according to
Webster at al. (2000) there are big interobserver variations in the perception of unique hues and even bigger interexperimental variability of unique hues where maximal ranges of unique blue, green and yellow overlap (for review see
Kuehni, 2004). Usually, perception of unique hue is tested with a small stimulus extending a few degrees within the central visual field with short durations up to two seconds.
Nagy (1979) shows that invariance or lack of invariance of unique hues with increasing intensity is dependent on stimulus duration (brief flashes of 17ms and one second were tested) with a tendency to greater hue stability for longer duration. Unique hue invariance after 5 minutes adaptation to unique hues has been tested by
Cicerone et al. (1975). They used a smaller overall field size than described here and found slightly different results. Our results do not quite correspond with
Cicerone et al. (1975), but our conditions are different in that we had full visual field adaptation, while they had a 2.6° adapting stimulus presented in a dark field and unique hue perception was governed by chromatic versus dark field contrast in both dark and chromatic adaptation situations. Unique hue invariance in the central versus peripheral visual field have been tested by
Parry et al. (2006) who found that unique hues are invariant with retinal eccentricity (up to 24°) whereas nonunique hues undergo a shift when viewed peripherally. Unique hue changes under long-term adaptation conditions have been shown by
Neitz et al. (2002). They reported that repeated adaptation to red and green illuminants (4 to 12 hours a day up to 24 days) produce changes in unique yellow perception that can last for weeks. This effect was attributed to a plastic neural mechanism that is adjustable in adults.
Here we tested stability of unique hues after complete adaptation to unique hue illuminants, with mean adapting times ranging from 8 to 22 minute and maximum time for some observers extending over an hour as seen in
Figure 3a. The results show that, notwithstanding the statistical effects, overall perception of unique hues remains quite stable after complete adaptation to a particular unique hue color (
Figures 2 and
Figure A1). This confirms our previous observations on color constancy (
Murray et al, 2006;
Stanikunas, Vaitkevicius, Kulikowski, Daugirdiene, & Murray, 2005) that two separate processes are responsible for color evaluation: one is responsible for background color evaluation while the other is responding to chromatic contrast. In our case, after complete adaptation to the illuminant, the observer acquires a new neutral (gray) reference point, but unique hue perception is maintained, most likely computed from the chromatic contrast between the unique hue stimulus (Munsell chip used for the setting) and the full field background, which is physically colorful, but is perceived as a neutral field.