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)

This review addresses the role of early sensory areas in the awareness of tactile information in humans. The results of recent studies dealing with this important topic are critically discussed: In particular, we report on evidence from neuropsychology, neurophysiology, neuroimaging, and behavioral experiments that have highlighted the crucial role played by the primary somatosensory cortex in mediating our awareness of tactile information. Phenomena, such as tactile hallucinations, tactile illusions, the perception of supernumerary limbs, and synaesthesia are also discussed. The research (...) reviewed here clearly shows that the activation of SI is necessary, but not sufficient, for the awareness of touch. On the basis of the evidence outlined here, we propose a neurocognitive model that provides a conceptual framework in which to interpret the results of the literature regarding tactile consciousness. (shrink)

Synesthetes are people who report having perceptual experiences that are very unusual, such as ?seeing? sounds as colors or ?smelling? colors as odors. It is commonly assumed these days that such synesthetic experiences must be instances of misperceptions. Against this widespread assumption, I will highlight that there is reason to think that at least some synesthetic experiences can be viewed as truly veridical perceptions, and not as illusions or hallucinations. On this view, which I will back up by conceptual arguments (...) and empirical data, synesthesia does sometimes enable the individual to truly pick up on worldly features. In failing to take this possibility on board, the participants in this debate have thus unduly restricted the scope of dialectical options. Finally, the reassessment of synesthetic experiences that I defend in this paper will turn out to have important ramifications not just for synesthesia research, but also for perception theories more generally. (shrink)

In cases where one sense-modality is stimulated by another, we speak of synesthesia, i.e., of a subjective experience of multiple distinct sensations as being quite literally conjoined. The term “synesthesia” is derived indirectly from the Greek words “syn,” meaning “together,” and “aisthesis,” meaning “sensation.” This article focuses on the question of whether synesthesia is in fact a mental disorder or a creative gift. Both the commonsense views that have emerged in recent times, and neurological research, demonstrate that our knowledge of (...) this relatively uncommon phenomenon is slowly but constantly expanding. Proper experimental research conducted with the right sorts of synesthete, and philosophically and scientifically nuanced conceptual studies of synesthesia, can all be helpful when seeking answers to the question posed above, while also confirming general claims about the extent to which our perceptions are really subjective. Most synesthetes see themselves as gifted, and claim that this “conjoining of the senses” enriches the quality of their lives. (shrink)

Despite some principal similarities, there is no systematic comparison between the different types of synesthesia . This comprehensive review compares the three principal types of synesthesia and focuses on their phenomenological features and their relation to different etiological models. Implications of this comparison for the validity of the different etiological models are discussed.Comparison of the three forms of synesthesia show many more differences than similarities. This is in contrast to their representation in the literature, where they are discussed in many (...) respects as being virtually similar. Noteworthy is the much broader spectrum and intensity with the typical drug-induced synesthesias compared to genuine and acquired synesthesias. A major implication of the phenomenological comparison in regard to the etiological models is that genuine and acquired synesthesias point to morphological substrates, while drug-induced synesthesia appears to be based on functional changes of brain activity. (shrink)

Researchers have long suspected that grapheme-color synaesthesia is useful, but research on its utility has so far focused primarily on episodic memory and perceptual discrimination. Here we ask whether it can be harnessed during rule-based Category learning. Participants learned through trial and error to classify grapheme pairs that were organized into categories on the basis of their associated synaesthetic colors. The performance of synaesthetes was similar to non-synaesthetes viewing graphemes that were physically colored in the same way. Specifically, synaesthetes learned (...) to categorize stimuli effectively, they were able to transfer this learning to novel stimuli, and they falsely recognized grapheme-pair foils, all like non-synaesthetes viewing colored graphemes. These findings demonstrate that synaesthesia can be exploited when learning the kind of material taught in many classroom settings. (shrink)