3.2. Contrastación de hipótesis
3.2.3. Correlaciones Rho Spearman
Perhaps the most well known example of this type of explanation comes from colour vision: the structure of colour quality space is explained by opponent processing theory (Hurvich 1981; Hardin 1988; De Valois and De Valois 1993). According to this theory— which was described above in sections 2.3.1 and 3.3.3—the outputs of the three different kinds of cone cells in the retina are summed and differenced to create three different post-
68 As Clark (2000) puts it, “The structure of similarity and differences among qualitative properties of
sensation must be sufficient to account for the structure of similarities and differences among phenomenal properties.” Of course, the structure of internal properties could be more complex than the structure of a creature’s perceptual discriminations... As Clark puts it, “By no means can there be less structure...since such a finding would render miraculous the creature’s capacities to discriminate. There could be more structure, but it would be unparsimonious to suppose so.” (p.9)
receptor neural channels. The cells in these channels can assume different firing rates, either above or below their ‘base’ firing rate, and this difference can be referred to as the channel taking a ‘positive’ or ‘negative’ value. Two of the three opponent process channels codes chromatic information: one channel corresponds to a red-green opponent relation, whereas the other corresponds to a yellow-blue one. The third channel is an achromatic channel that codes for lightness.
By representing each of the three channels of the opponent processing system as a spatial dimension, every possible state of our opponent processing channels can be understood as corresponding to a point in three-dimensional colour quality space that is discovered by psychophysics. That is, the ‘value’ of each channel (as determined by its level of activation) can be regarded as a Cartesian coordinate along a particular dimension of qualitative variation within the space.69,70
Furthermore, opponent process theory has received some direct neurophysiological support: post-retinal opponent processing cells have been discovered in the LGN that roughly correspond to red-green and yellow-blue dimensions of colour space, although changes in their activity do not directly correspond to changes in the phenomenology of colour experience (De Valois & De Valois, 1993).Similarly, colour-sensitive opponent processing cells have been discovered in various areas of the visual cortex, including V1 and V4 (Zeki, 1993). And while no direct neural correlates of colour experience have been discovered, there is nevertheless good evidence that the brain uses opponent processing mechanisms to encode chromatic information.
Now of course, opponent process theory is just that—a theory. It is an as-of-yet unconfirmed empirical hypothesis, for which there is some promising evidence.
69 Note that this isn’t quite accurate: by plotting each channel as a spatial dimension, we get the so-called
“color cube”. However, our opponent processing states typically cannot occupy every possible position within that space, except under abnormal conditions, and then only briefly. The locations in the cube which our opponent processing state can occupy under normal circumstances and for extended periods of time are within a sub-space of the cube: the colour solid.
70 Similarly, the gustatory (taste) system can also be construed in terms of a quality space built on four
dimensions, corresponding to the four different kinds of taste receptors in the mouth. Any possible taste sensation can be characterized as a point in four-dimensional space, as determined by the relative activation levels of those receptors. (Churchland, 1989). Parallel proposals apply to other sense modalities as well.
Nevertheless, opponent process theory does provide a scientific explanation for the structure of sensory phenomenology, by proposing mechanisms whose activity generates that structure.71 (Or more accurately, by offering theoretical models in which the
psychophysical data can be embedded, by virtue of the model having a relational
structure that is isomorphic to that of the quality space.) In the next section I will examine in more detail how QST fares as scientific explanation by demonstrating how QST fits into the tradition of structuralism in the philosophy of science, and mechanistic
explanations in neuroscience. However, before turning to those issues there are two additional points to note about the importance of providing a neurophysiological interpretation of a quality space.
First, it’s extremely important to note that neuroscience does more than simply provide an explanation for the structural features of the space. For in addition, it also discovers the dimensions along which sensory information is actually encoded in the brain. In other words, although psychophysics can provide us with the number of dimensions of
qualitative variation in a given quality space (e.g., that colour space has three
dimensions), it cannot provide an interpretation for those dimensions. It cannot tell us the coordinate scheme by which the brain encodes qualitative variation. For example, in the appearance property hue circle (a two dimensional cross-section of three-dimensional colour space) one typically uses a polar coordinate scheme, with hue as the angular coordinate and saturation as the radial one. However, that’s not how the brain encodes chromatic information: rather than hue and saturation dimensions, the brain encodes chromatic information using the red-green and yellow-blue dimensions of qualitative variation described above in the discussion of opponent process theory—in essence, a type of Cartesian coordinate system. Without a description of the coordinate scheme by which the brain encodes information in a sensory modality, an explanation of
phenomenal qualities would be necessarily incomplete. We might know that the quality
71 Furthermore—as was mentioned above in section 2.3.1—opponent processing theory not only explains
the structure of colour space, but also a number of different phenomena related to the phenomenology of colour perception, including the unitary-binary distinction, complementary after-images, types of
deficiencies in colour vision, the Bezold-Brucke phenomenon, and many more. (Hardin 1988; Clark 1992; Pautz 2006; etc.)
space for a given modality contained n dimensions of qualitative variation, but we would not know what those dimensions were.72
Second, a neuroscientific explanation of a quality space must provide not only an account of the basic structure of the space, but in addition, it should ideally also provide an
account of the dynamics of state-to-state transformations within the space. Specifically, it should explain how changes to the input of the mechanism (either via external influences on receptoral input or via internal influences from other neural systems) affect transitions from one location in the space to another. Churchland (2005) has provided a detailed account of how opponent process theory explains (and predicts!) such state-to-state transitions in colour experience, and Akins (1996) has provided a meticulous examination of the (nonlinear) response function in thermoreception and its implications for the
dynamics of the phenomenology of temperature perception.73