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)

In grapheme-color synesthesia, a number or letter can evoke two different and possibly conflicting color sensations at the same time. In this study, we investigate the relationship between synesthesia and executive control functions. First, no general skill differences were obtained between synesthetes and non-synesthetes in classic executive control paradigms. Furthermore, classic executive control effects did not interact with synesthetic behavioral effects. Third, we found support for our hypothesis that inhibition of a synesthetic color takes effort and time. Finally, individual differences (...) analyses showed no relationship between the two skills; performance on a ‘normal’ Stroop task does not predict performance on a synesthetic Stroop task. Across four studies, the current results consistently show no clear relationship between executive control functions and synesthetic behavioral effects. This raises the question of which mechanisms are at play in synesthetic ‘management’ during the presence of two conflicting sensations. (shrink)

Recently, swimming-style colour synaesthesia was introduced as a new form of synaesthesia. A synaesthetic Stroop test was used to establish its genuineness. Since Stroop interference can occur for any type of overlearned association, in the present study we used a modified Stroop test and psychophysiological synaesthetic conditioning to further establish the genuineness of this form of synaesthesia. We compared the performance of a swimming-style colour synaesthete and a control who was trained on swimming-style colour associations. Our results showed that behavioural (...) aspects of swimming-style colour synaesthesia can be mimicked in a trained control. Importantly, however, our results showed a psychophysiological conditioning effect for the synaesthete only. We discuss the theoretical relevance of swimming-style colour synaesthesia according to different models of synaesthesia. We conclude that swimming-style colour synaesthesia is a genuine form of synaesthesia, can be mimicked behaviourally in non-synaesthetes, and is best explained by a re-entrant feedback model. (shrink)

In Nyāya philosophy, a special kind of extraordinary sensory connection is admitted named jñānalakṣaṇā pratyāsatti or jñānalakṣaṇa sannikarṣa. It is held that sometimes our sense-organ can be connected to such an object which is not amenable to the operating sense-organ. In such cases, cognition (jñāna) plays the role of sensory connection and connects the content of itself to the operating sense-organ. The paradigmatic example of jñānalakṣaṇa perception is to ‘see’ fragrant sandal through visual sense from non-smellable distance. This hypothesis of (...) jñānalakṣaṇa has been criticized by the opponents being considered as counterintuitive, mysterious and theoretically overloaded. This paper tries to demystify the notion. It shows that although it seems to be metaphysically mysterious phenomenon at first sight, it is not so at all. The paper explores the psychological process involved in this sensory connection. The hypothesis is shown to have sufficient explanatory power, because the Naiyāyikas have used this hypothesis to explain five different epistemic situations. Hence, this paper argues that it is not a theoretical overload. The opponents counter-argue that all those five cognitive situations can be explained without admitting jñānalakṣaṇa. Moreover, if we admit jñānalakṣaṇa, then a particular kind of inference will become redundant. The paper answers all those objections and defends the hypothesis. The second part of the paper presents an empirical evidence in support of the hypothesis. The arguments leveled against the hypothesis of jñānalakṣaṇa can be contested on the ground that they try to disprove something which is supported on experimental ground. Experiments represent universally acceptable objective facts supported by experience—denying which amounts to anubhavavirodha, which philosophers would want to avoid. Hence, supporting jñānalakṣaṇa on the ground of scientific experiments can be considered as a philosophical stand. Now, there is a clinically recognized and neurophysiologically proved condition, called synaesthesia, where stimulation of a particular sensory modality automatically and involuntarily activates a different sensory modality simultaneously without a direct stimulation of the second modality. As for example, when a sound → colour synaesthete listens to a particular tone such as C-sharp, she visualizes particular colour, such as blue, in her mind’s eye; for a grapheme → colour synaesthete a particular number or alphabet is always tinged with a particular colour. This paper shows that the cognitive process involved in synaesthesia lends support to the hypothesis of jñānalakṣaṇa pratyāsatti. It has been proved through several experiments that it is a genuine perceptual phenomenon and is not a confabulation of memory. There are several alternative theories which explain the phenomenon neurophysiologically. The paper discusses the most popular one: the cross-activation hypothesis. There are two major objections against the project of comparing jñānalakṣaṇa with synaesthesia. First, synaesthesia is a neurological condition present in a few numbers of people whereas jñānalakṣaṇa is claimed to be universal phenomenon. Second, syneasthesia is a sensory experience whereas jñānalakṣaṇa involves application of concepts. The paper answers these questions. Firstly, multimodal processing in the brain is a universal phenomenon; secondly, there is a form of synaesthesia where top-down processing is involved. In those cases, concepts play important role for having synaesthetic experience. (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)

Synesthetic experiences of color have been traditionally conceptualized as a perceptual phenomenon. However, recent evidence suggests a role of higher order cognition in the formation of synesthetic experiences. Here, we discuss how synesthetic experiences of color differ from and influence veridical color processing, and how non-perceptual processes such as imagery and color memory might play a role in eliciting synesthetic color experience.

Is synaesthesia cognitively useful? Individuals with time–space synaesthesia experience time units as idiosyncratic spatial forms, and report that these forms aid them in mentally organising their time. In the present study, we hypothesised that time–space synaesthesia would facilitate performance on a time-related cognitive task. Synaesthetes were not specifically recruited for participation; instead, likelihood of time–space synaesthesia was assessed on a continuous scale based on participants’ responses during a semi-structured interview. Participants performed a month-manipulation task, which involved naming every second month (...) or every third month in reverse-chronological order, beginning and ending with a target month. Using hierarchical multiple regression, we found that time–space synaesthesia corresponded with faster performance on both versions of the task. We propose that time–space synaesthesia may expedite the cognitive manipulation of time-based information. Our results also indicate that synaesthesia is far less unusual than widely believed. (shrink)

We examine a condition in which units of time, such as months of the year, are associated with specific locations in space. For individuals with this time-space synaesthesia, contiguous time units such as months are spatially linked forming idiosyncratically shaped patterns such as ovals, oblongs or circles. For some individuals, each time unit appears in a highly specific colour. For instance, one of the synaesthetes we studied experienced December as a red area located at arms length to the left of (...) their body. For the same individual May was a blue area located roughly at arms length to the right of their body. We studied four synaesthetes who report spatial associations for the months of the year. We found that the time-space associations experienced by these individuals were consistent across test–retest. In addition, month names directed visual attention to particular locations in space. For some synaesthetes, this directing of spatial attention was quite rapid-in accord with their reports that month names involuntarily bring to mind spatial locations. (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)