Studies have shown that both serotonin and glutamate receptor systems play a crucial role in the mechanisms underlying drug-induced synesthesia. The specific nature of these mechanisms, however, continues to remain elusive. Here we propose two distinct hypotheses for how synesthesia triggered by hallucinogens in the serotonin-agonist family may occur. One hypothesis is that the drug-induced destabilization of thalamic projections via GABAergic neuronal circuits from sensory areas leads to a disruption of low-level, spontaneous integration of multisensory stimuli. This sort of integration (...) regularly occurs when spatial and temporal attributes match. Destabilization of feedback loops, however, can result in incongruent experiences or binding of random thalamus activation with sensory input in a particular sensory modality. The second hypothesis builds on embodied cognition, cases in which visual images of external stimuli activate task-related neural regions. On this proposal, binding processes that do not normally generate awareness become accessible to consciousness as a result of decreased attentional discrimination among incoming stimuli. (shrink)

Empirical arguments for materialism are highly circumstantial—based, as they are, upon inductions from our knowledge of the physical and upon the fact that mental phenomena have physical correlates, causes and effects. However, the qualitative characteristics of first-person conscious experience are empirically distinct from uncontroversially physical phenomena in being—at least on our present knowledge—thoroughly resistant to the kind of abstract, formal description to which the latter are always, to some degree, readily amenable. The prima facie inference that phenomenal qualities are, most (...) probably, non-physical may be resisted either by denying their existence altogether or by proposing that they are properties of some peculiar sort of mysterious physical complexity, located, for example, within the functioning of the brain. It is argued here, however, that the first, eliminative hypothesis is empirically absurd—while the second is extravagant, vague, ad hoc and (for various additional reasons) profoundly implausible. Taken together, these considerations provide a compelling empirical case against materialism—yet its converse, mentalism, is usually regarded as subject to serious difficulties of its own. I conclude by suggesting empirical and theoretical desiderata, respectively, for the vindication of materialism and alternatively, for the development and defense of a potentially robust and viable mentalist theory of consciousness. (shrink)

(1) The induced colours led to perceptual grouping and pop-out, (2) a grapheme rendered invisible through ‘crowding’ or lateral masking induced synaesthetic colours — a form of blindsight — and (3) peripherally presented graphemes did not induce colours even when they were clearly visible. Taken collectively, these and other experiments prove conclusively that synaesthesia is a genuine percep- tual phenomenon, not an effect based on memory associations from childhood or on vague metaphorical speech. We identify different subtypes of number–colour synaesthesia (...) and propose that they are caused by hyperconnectivity between col- our and number areas at different stages in processing; lower synaesthetes may have cross-wiring (or cross-activation) within the fusiform gyrus, whereas higher synaesthetes may have cross-activation in the angular gyrus. This hyperconnec- tivity might be caused by a genetic mutation that causes defective pruning of con- nections between brain maps. The mutation may further be expressed selectively (due to transcription factors) in the fusiform or angular gyri, and this may explain the existence of different forms of synaesthesia. If expressed very diffusely, there may be extensive cross-wiring between brain regions that represent abstract concepts, which would explain the link between creativity, metaphor and synaesthesia (and the higher incidence of synaesthesia among artists and poets). Also, hyperconnectivity between the sensory cortex and amygdala would explain the heightened aversion synaesthetes experience when seeing numbers printed in the ‘wrong’ colour. Lastly, kindling (induced hyperconnectivity in the temporal lobes of temporal lobe epilepsy [TLE] patients) may explain the purported higher incidence of synaesthesia in these patients. We conclude with a synaesthesia-based theory. (shrink)

Most ?theories of consciousness? are based on vague speculations about the properties of conscious experience. We aim to provide a more solid basis for a science of consciousness. We argue that a theory of consciousness should provide an account of the very processes that allow us to acquire and use information about our own mental states ? the processes underlying introspection. This can be achieved through the construction of information processing models that can account for ?Type-C? processes. Type-C processes can (...) be specified experimentally by identifying paradigms in which awareness of the stimulus is necessary for an intentional action. The Shallice (1988b) framework is put forward as providing an initial account of Type-C processes, which can relate perceptual consciousness to consciously performed actions. Further, we suggest that this framework may be refined through the investigation of the functions of prefrontal cortex. The formulation of our approach requires us to consider fundamental conceptual and methodological issues associated with consciousness. The most significant of these issues concerns the scientific use of introspective evidence. We outline and justify a conservative methodological approach to the use of introspective evidence, with attention to the difficulties historically associated with its use in psychology. (shrink)

There has been some recent optimism that addressing the question of how we distinguish sensory modalities will help us consider whether there are limits on a scientific understanding of perceptual states. For example, Block has suggested that the way we distinguish sensory modalities indicates that perceptual states have qualia which at least resist scientific characterization. At another extreme, Keeley argues that our common-sense way of distinguishing the senses in terms of qualitative properties is misguided, and offers a scientific eliminativism about (...) common-sense modalities which avoids appeal to qualitative properties altogether. I’ll argue contrary to Keeley that qualitative properties are necessary for distinguishing senses, and contrary to Block that our common-sense distinction doesn’t indicate that perceptual states have qualia. A non-qualitative characterization of perceptual states isn’t needed to avoid the potential limit on scientific understanding imposed by qualia. (shrink)

In synaesthesia, ordinary stimuli elicit extraordinary conscious experiences. For example, standard black digits may elicit highly specific colour experiences and specific tastes may elicit unusual tactile sensations. The growing interest in synaesthesia has led to numerous experimental studies of this phenomenon. The purpose of this paper is to review these recent studies and to discuss the relationship between the results of these experimental investigations of synaesthesia and the subjective descriptions reported by synaesthetes. It is argued that when the experimental investigations (...) of synaesthesia are interpreted in the context of synaesthetes' subjective reports, the experimental investigations synergistically advance our understanding of synaesthesia. Ultimately we suggest that a more complete understanding of this fascinating phenomenon will require a clearly articulated combination of well-designed experimental studies and the subjective reports of synaesthetes. (shrink)

Richard Cytowic has argued that synesthetic experimentation by modern artists was based on deliberate contrivances of sensory fusion and not on involuntary experiences of cross-modal association. He has placed artistic experiments with sensory fusion outside the domain of synesthesia research. Artistic experiments, though historically interesting, are considered irrelevant for the study of synesthesia. Contrary to this view I argue that at least Scriabin's and Kandinsky's artistic experiments were based on involuntary experiences of synesthesia. They were investigating perceptual and emotional mechanisms (...) of involuntary synesthetic experiences that meet Cytowic's criteria of synesthesia. Artistic experiments are not only historically interesting, but may also contribute to present synesthesia research. (shrink)

Accounting for phenomenal structure—the forms, aspects, and features of conscious experience—poses a deep challenge for the scientific study of consciousness, but rather than abandon hope I propose a way forward. Connectionism, I argue, offers a bi-directional analogy, with its oft-noted “neural inspiration” on the one hand, and its largely unnoticed capacity to illuminate our phenomenology on the other. Specifically, distributed representations in a recurrent network enable networks to superpose categorical, contextual, and temporal information on a specific input representation, much as (...) our own experience does. Artificial neural networks also suggest analogues of four salient distinctions between sensory and nonsensoty consciousness. The paper concludes with speculative proposals for discharging the connectionist heuristics to leave a robust, detailed empirical theory of consciousness. (shrink)

Although the condition known as synaesthesia is currently undergoing a scientific resurgence, to date the literature has largely focused on the heterogeneous nature of synaesthesia across individuals. In order to provide a better understanding of synaesthesia, however, general characteristics need to be investigated. Synaesthetic experiences are often described as occurring ‘internally’ or in the ‘mind’s eye’, which is remarkably similar to how we would describe our experience of visual mental imagery. We assessed the role of visual imagery in synaesthesia by (...) sampling a large group of synaesthetes and found that they report experiencing more vivid mental images than controls. These findings have important implications for our general understanding of synaesthesia and, in particular, emphasize the need to control for visual imagery in behavioural and neuroimaging paradigms. (shrink)