As an example of the protophenomenal approach, we can consider the well-known problem of a spectral inversion. In brief, the problem is as follows: Although we agree on the names for various wavelengths, is it possible that you experience red wavelengths the same way I experience blue wavelengths, and vice versa? Before we can solve this problem we need a more accurate phenomenology of color. The plausibility of a spectral inversion derives in part from an oversimplified phenomenology of color, since we have imagined that color can be reduced to a single dimension (wavelength) but a phenomenological analysis shows it to be much more complex (see [3] and the references cited therein).
Setting aside many of the higher-level complexities of color (e.g. its emotional and cultural connotations), yet avoiding the trap of a one-dimensional view, we can observe that it has long been known that we can identify four pure hues, which are termed the ``unique hues,'' an observation that has led to the double-opponent theory of color vision. In this theory the three color receptors (short, medium and long wavelength, henceforth S, M and L) are combined in various ways to yield three orthogonal axes. The light-dark axis is formed by S+M+L and its opposite; the yellow-blue axis is formed by M+L-S and its opposite; the red-green axis is formed by S+L-M and its opposite (here we use a common form of the theory). The two zeroes on each of the two chromic axes (yellow-blue and red-green) define the four unique hues.
The problem of a spectral inversion can be recast in terms of inversions between the poles on one or more of these axes or in terms of exchanges between two or more of the axes. However, we will show by phenomenological analysis that such spectral inversions are impossible, that is, that abnormal neurological connections would lead to abnormalities in conscious experiences that could be detected by the subject. Here the arguments will be summarized briefly.
First, it is fairly obvious that dark and light have phenomenologically distinct characters, and hence are noninterchangeable: in the dark, forms and hues are indistinguishable, but not in the light.
Second, phenomenological analyses of color from ancient times to our own have observed that yellow is intrinsically brighter than blue (the neurophysiological reason being the large overlap between S+M+L and M+L-S). Hence, blue and yellow are phenomenologically similar to dark and light, and hence noninterchangeable. Therefore, in a case of abnormal vision, whatever receptor combination has the largest overlap with S+M+L will be experienced as phenomenal-yellow, and if this does not correspond to spectral-yellow then the anomaly will be detectable.
The case of a red-green inversion is more subtle, but phenomenological analysis again exposes a difference. For example, Goethe observed that green is a phenomenological mixture of yellow and blue, whereas red results from an ``augmentation'' of yellow and blue. Further, the experience of ``unique red'' is nonspectral; that is, it cannot be created by monochromatic light, whereas experience of the other three unique hues (including green) can. (The well-known studies of Berlin and Kay also support the phenomenal differences between red and green.)
Finally, the red-green axis cannot be exchanged with the yellow-blue, because the former is less similar to light-dark than the latter. This phenomenological fact, which has been recognized since ancient times, is consequence of S+L-M (``red'') having a smaller overlap with S+M+L (``light'') than does M+L-S (``yellow'').
As a result of this neurophenomenological analysis, we can begin to understand the topology of color. First we have the three axes, which define three polar oppositions: light-dark, yellow-blue, red-green. Superimposed on this structure are relations of similarity: yellow is most similar to light, and blue is most similar to dark. Green is most similar to yellow and blue and is intermediate in its similarity to light and dark. Red is similar to yellow, but not to blue. These conclusions are objective in that they result from observations made independently by many phenomenologists over the centuries.
Finally, we will consider several more examples of abnormal or nonhuman color perception. For example, if we have S+M-L instead of M+L-S in the yellow-blue channel, then spectral blue-greens will be experienced as yellows, and spectrally orange light will be experienced as green. On the other hand, if we have S+M-L and M+L-S (two asymmetric channels) for the chromic channels, then color phenomenology will have several detectable anomalies: there will be two spectral unique hues (as opposed to three) and one whole phenomenal color quadrant (purple) will be nonspectral (unexperiencable with monochromatic light).
Many other neural anomalies can be hypothesized. However, if a sensory system is too different from our own, we may be neurologically unable to imagine the experience, although we can describe its topology. Since imaginal areas have parallel structures to perceptual areas, we have limited ability to imagine qualia that are distinctly different from what we can perceive.