Online research in philosophy

The NMDA receptor (NMDAR) channel has been proposed to function as a coincidence-detection mechanism for afferent and reentrant signals, supporting conscious perception, learning, and memory formation. In this paper we discuss the genesis of distorted perceptual states induced by subanesthetic doses of ketamine, a well-known NMDA antagonist. NMDAR blockage has been suggested to perturb perceptual processing in sensory cortex, and also to decrease GABAergic inhibition in limbic areas (leading to an increase in dopamine excitability). We propose that perceptual distortions and hallucinations induced by ketamine blocking of NMDARs are generated by alternative signaling pathways, which include increase of excitability in frontal areas, and glutamate binding to AMPA in sensory cortex prompting Ca++ entry through voltage-dependent calcium channels (VDCCs). This mechanism supports the thesis that glutamate binding to AMPA and NMDARs at sensory cortex mediates most normal perception, while binding to AMPA and activating VDCCs mediates some types of altered perceptual states. We suggest that Ca++ metabolic activity in neurons at associative and sensory cortices is an important factor in the generation of both kinds of perceptual consciousness.

In this paper I offer an explanation of the ineffability (linguistic inexpressibility) of sensory experiences. My explanation is put in terms of computational functionalism and standard externalist theories of representational content. As I will argue, many or most sensory experiences are representational states without constituent structure. This property determines both the representational function these states can serve and the information that can be extracted from them when they are processed. Sensory experiences can indicate the presence of certain external states of affairs but they cannot convey any more information about them than that. So, format- or code-conversion mechanisms that link different systems of representation (linguistic and perceptual) to each other will fail to extract any relevant information from sensory experiences that could be coded in language. They only way to establish specific roles for sensory experiences in communication and the organization of behavior is to attach to them, by associative links, words, or other behavioral responses. If a sensory experience has no linguistic label associated to it in a particular subject, then no linguistic description can token, or activate, that state in the subject. In other words, no linguistic description can cause a subject to undergo an unlabeled perceptual state. On the contrary, complex, or syntactically structured perceptual states can be built up, on the basis of descriptions, by mechanisms of constructive imagination (conceived here as one sort of format conversion). It is this difference between complex and unstructured representational states that gives us an understanding of the phenomenon we call the ineffability of qualia.

The consensus in contemporary philosophy of mind is that how a perceptual experience represents the world to be is built into its sensory phenomenology. I defend an opposing view which I call ‘moderate separatism’, that an experience's sensory phenomenology does not determine how it represents the world to be. I argue for moderate separatism by pointing to two ordinary experiences which instantiate the same sensory phenomenology but differ with regard to their intentional content. Two experiences of an object reflected in a mirror can possess the same spatial phenomenology while representing that object to occupy different spatial locations. So, contrary to the current consensus, the representation of spatial location is not fixed by an experience's sensory phenomenology.

How could neural processes be associated with phenomenal consciousness? We present a way to answer this question by taking the counterintuitive stance that the sensory feel of an experience is not a thing that happens to us, but a thing we do: a skill we exercise. By additionally noting that sensory systems possess two important, objectively measurable properties, corporality and alerting capacity, we are able to explain why sensory experience possesses a sensory feel, but thinking and other mental processes do not. We are additionally able to explain why different sensory feels differ in the way they do.

CHAPTER On sensory consciousness Consciousness, like love, is something so intimate and vital to our sense of ourselves as human, that explanation, ...

One of the central assumptions made in much of contemporary philosophy of mind is that there is no appearance-reality distinction when it comes to sensory states. On this assumption, sensory states simply are as they seem: consciousness is an intrinsic property of sensory states—that is, all sensory states are conscious—and the consciousness of one’s own sensory states is never inaccurate. For a sensation to be felt as pain, for example, is for it to be pain. This assumption, which I call the Cartesian assumption, can be seen everywhere from the standard arguments against physicalism—such as those advanced by Kripke, Nagel, and Levine—to current theorizing about consciousness. I here argue that this assumption is false and that it goes wrong in two ways. I further argue that the appeal of the Cartesian assumption is due to a commitment many still have to a poorly motivated and misguided Cartesian model of consciousness and its relation to mental states. As an alternative to this Cartesian concept of mind, I argue for a theory of consciousness which claims that the “phenomenal character” of a sensation or perception—the “what it’s like” to have that sensation—is determined by the content of a higher-order thought one has of that sensory state.

Hegel's discussion of the concept of “habit” appears at a crucial point in his Encyclopedia system, namely, in the transition from the topic of “nature” to the topic of “spirit” (Geist): it is through habit that the subject both distinguishes itself from its various sensory states as an absolute unity (the I) and, at the same time, preserves those sensory states as the content of sensory consciousness. By calling habit a “second nature,” Hegel highlights the fact that incipient spirit retains a “moment” of the natural that marks a limitation compared to “pure thought” but that also makes perceptual consciousness possible. This makes Hegel's account analogous in important respects to John McDowell's “naturalism of second nature.” But Hegel's account of habit can be seen as a version of a Kantian synthesis of the productive imagination—and hence presupposes a given material that can become one's own by means of habit. This does not mean that Hegel falls into the Myth of the Given, but it does suggest that an appropriate account of second nature might be committed to something McDowell wants to deny: that nonconceptual states of consciousness play a role (even if not a justificatory role) in perception.

Steven Lehar allows too much to his direct realist opponent in using the word “subjective” of the sensory field per se. The latter retains its nonconceptual, nonmental nature even when explored by perceptual judgement. He also needs to stress the evolutionary value of perceptual differences between person and person, a move that enables one to undermine the direct realist's superstitious certainty about the singular object.

I argue that John McDowell’s attempt to refute Wilfrid Sellars’s two-component analysis of perceptual experience and substitute for it a conception according to which perceptual experience is the “conceptual shaping of sensory consciousness” fails. McDowell does not recognize the subtle dialectic in Sellars’s thought between transcendental and empirical considerations in favor of a substantive conception of sense impressions, and McDowell’s own proposal seems to empty the notion of sensory consciousness of any real significance.

"Consciousness" is a multiply ambiguous word, and if our goal is to explain perceptual consciousness we had better be clear about which of the many senses of the word we are endorsing when we sign on to the project. I describe some of the relatively standard distinctions made in the philosophical literature about different meanings of the word "conscious". Then I consider some of the arguments of David Chalmers and of Ned Block that states of "phenomenal consciousness" pose special and intractable problems for the scientific understanding of perception. I argue that many of these problems are introduced by obscurities in the term itself, and propose a distinction between epistemic and non-epistemic senses of the term "phenomenal consciousness". That distinction helps explain why phenomenal consciousness seems so mysterious to so many people. States of "phenomenal consciousness" are not states of one, elemental (and inexplicable) kind; they are a ragtag lot, of differing levels of complexity, corralled under one heading by a regrettable ambiguity in our terminology.