Response Type & Reduction

An important question for the Chalmers/Thompson version of Fregean

representationalism is how to understand the response type r in the above definition. According to Thompson, we can understand the response type for colour experiences to be “one in which the vehicle of content is an instance of the relevant response type” (p.103). Similarly, in Chalmers’ representationalist theory, the response type in the MoP for properties is characterized in entirely phenomenal terms: i.e., “the property that causes phenomenally green experiences...” His characterization is thus intended to be inherently non-reductive, as it makes essential reference to phenomenal notions in characterizing the content. Indeed, Chalmers explicitly claims that the response type r cannot be specified in non-phenomenal terms.52

52 _{Presumably, the same would apply to sensory representations of objects and locations to: i.e., “the object}

that...”; “the visual location that...”; etc. The terms would be specified in a phenomenal type way (i.e., not in a way that is directly reducible to neuroscience or psychophysics, etc.) However, I will argue that those MoPs can be so reduced.

However, I will argue that neuroscience and psychology can provide an empirical framework in which one can reductively explain the phenomenal content (i.e., MoPs) of sensory representations. For example, the phenomenal MoP for a sensory representation of a propertycan be characterized in terms of a psychophysically-defined “quality space” (described below), which can then be given a neurophysiological interpretation.

Likewise, this account can provide a reductive explanation of the MoPs associated with locations and objects as well – in terms of neural mechanisms feature-maps and visual indexes, respectively (described in section 3.6 below and in chapter 6).

The most obvious way to specify the response type r in entirely non-phenomenal (i.e., reductive) terms is simply in terms of a neural response type: roughly, the idea would be that the conditions on extension for a phenomenal MoP for a property would be

something like “the property that causes neural response type X in me under normal conditions”.53 _{(Indeed, this is precisely the strategy of tracking/externalist}

representationalism to individuate the tracked properties, as described above.)

However, the problem with this proposal is making the necessary connection between neural response types and types of phenomenal experiences. It has become widely believed that this is impossible. (Hence the motivation for non-reductive

representationalism!) The argument for this claim rests on the possibility of, e.g.,

philosophical zombies and undetectable spectrum inversion—in short, the idea that there could be a duplicate that is physically or functionally indistinguishable from me, yet whose experiences are radically different than mine (or even absent altogether). This view is held not only by those who defend a non-reductive view of phenomenal qualities, but also by reductive externalist representationalists. Indeed, externalism is committed to the claim that identical internal brain states could give rise to phenomenally different

53 _{Similar proposals would apply to objects and locations (e.g., “the object that is causing neural response x}

in me”). There are distinct types of neural responses that underlie our ability to track objects and external locations in our environment, which I will describe in more detail below.

experiences by being related to different external properties (in much the same way that identical internal states of a voltage meter could ‘mean’ either 6dB or 6lbs).54

What is required then is a reductive way of describing the phenomenology of sensory experience that is not vulnerable to the objection that the correlation between response type and type of phenomenal experience is entirely contingent. In other words, we need what Nagel (1974) calls an “objective phenomenology”; that is, a way to “describe…the subjective character of experiences in a form comprehensible to beings incapable of having those experiences.” (p.449) Such a theory would allow us to characterize the response type r in non-phenomenal terms. Happily, such a theory is in fact available, and is known as quality space theory. At essence, quality space theory (“QST”) is primarily intended to be a radically empirical theory of sensory phenomenology. It is an inherently third-person approach that provides a way in which the phenomenal qualities present in sensory experience (colours, tastes, sounds, smells, etc.) can be both

investigated/described by the empirical sciences and explained in objective terms. In what follows, I will briefly describe the quality space approach to sensory

phenomenology.

3.3.3.1 Quality Space Theory

In any sensory modality (or sub-modality), phenomenal qualities stand in certain structural relations such as relative similarity and difference (e.g., red is more similar to orange than it is to blue). A quality space is a spatial ordering of the global structure of similarity and dissimilarity relations that hold between phenomenal qualities in a given sensory modality in which relative similarities between qualities are represented by their relative distance. The similarity relations in question typically vary across multiple dimensions within a single sensory modality: for example, colour qualities vary along three dimensions: hue, saturation, and lightness. By representing each of these

dimensions spatially such that the relative similarity of colour qualities in a given

54 _{Indeed, this is why externalists must hold a Russellian view of the nature of phenomenal content if they}

wish to defend a reductive account of sensory phenomenology: i.e., they can’t provide a reductive account of phenomenal qualities in terms of brain states, so they provide a reductive account in terms of external properties.

dimension corresponds to their relative distance across that dimension, each colour quality can be characterized as a location in a three-dimensional space, the totality of which constitutes the so-called “colour solid”.

The structure of a quality space for any given sensory modality (or sub-modality) is initially determined by psychophysical experiment; e.g., systematically altering the properties of test stimuli along some physical dimensions and observing the resulting effect on a subject's discriminative capacities. Specifically, by using psychophysical measures such as discriminability (matching), relative similarity, and just-noticeable- differences, and applying statistical techniques such as multidimensional scaling (described below) to this kind of psychophysical data, one can construct a unique

multidimensional ordering of the phenomenal qualities in a given sensory modality. Next, this ordering (“quality space”) can be given a neurophysiological interpretation which explains the structure that is discovered via psychophysics.55 (Indeed, while

psychophysics can discover the structure of the quality space for a given sensory modality, it is merely an expanadum that requires an explanans. For the latter, we must turn to neuroscience.)

For example, the structure of colour appearance can be explained by the opponent

process theory of colour vision, which was briefly described in section 2.3.1. The outputs of the three different kinds of cone cells in the retina are summed and differenced to create three different post-receptor neural channels. Two of the three opponent process channels code chromatic information (one channel corresponds to a red-green opponent relation, whereas the other corresponds to a yellow-blue one), and the third channel is achromatic and 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

55 _{Essentially, this amounts to finding the realizers of the representational vehicles of phenomenal qualities;}

those information-carryingstates that have both the right structural relations and causal role to underlie phenomenal qualities.

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. It is important to understand exactly how the quality space approach to sensory phenomenology applies to the current representationalist proposal. The wrong way to understand this version of representationalism is to claim that the relevant response type r for a phenomenal MoP for an experience of colour should be characterized in terms of brain states--namely, opponent processing states. So, for example, according to this (incorrect) characterization, a particular surface reflectance property is in the extension of the phenomenal MoP for redness if it causes one’s opponent processing channels to go into the neural state which codes for redness under (statistically) ‘normal’ conditions. However, this version of representationalism would face exactly the same sort of

problems connecting neural states and phenomenal states that were described above. The right way to understand this version of representationalism—which I will refer to as “quality space representationalism”—is to recognize that what is relevant to the response type r is not that r is a particular type of neural state, but rather that r has a particular relational structure.In essence, quality space theory (hereafter, “QST”) offers a purely structural description of the phenomenology of sensory experience. This abstract structure can then be given a neurophysiological interpretation that explains both the topology of the structure and the discriminative capacities of the subject, but it is the structural details that are relevant to individuating the response type, not the

neurophysiological ones.

As described thus far, the quality space approach to sensory phenomenology plausibly allows for a reductive account of Fregean representationalism by providing an empirical, reductive way of characterizing the response type r in Thompson’s “response-dependent” MoPs. However, I now want to propose a type of Fregean representationalist theory based on QST that is very different from the standard Chalmers/Thompson account. Specifically, recall that (as was described in the chapter 1) there are two parts to a representationalist theory: (i) a theory of the nature of phenomenal content, and (ii) a theory of sensory intentionality that explains how and why we are able to represent that

content. According to the Chalmers/Thompson version of Fregean representationalism, both of these issues are solved by appeal to the above-described conditions on extension. That is, according to the Chalmers/Thompson account, (i) Fregean content of a sensory representation is exhausted by its CoE such that the conditions themselves are identical with its phenomenal MoPs, and (ii) these conditions determine the reference/extension of a sensory state, thereby explaining the intentional directedness of sensory states.

However, in what follows, I will argue that—contra Chalmers and Thompson—the CoE imposed by MoPs for propertiesare not identical with the phenomenal content of a sensory state. Rather, I will argue that the MoP of a sensory representation should be identified with the response type r itself, whose representational content can analyzed in terms of a psychosemantic theory known as functional role semantics.