Investigations of synesthesia

This thesis explores various aspects of the phenomenon of synesthesia where stimulation of one sense modality (such as in audition or vision) evokes sensations in another modality. Chapter 1 presents an overview of the range of synesthetic phenomena and outlines the questions explored in this thesis. Chapter 2 examines the diagnosticity and utility of four tests of genuineness (TOG) based on the new method implemented in the Synesthesia Battery (Eagleman, Kagan, Nelson, Sagaram, & Sarma, 2007). The TOG were found to adequately differentiate between groups of synesthetes and controls. New cut off scores for the tests were derived using signal detection theory (SDT) and the derivation of subjective signal strength (d'). Diagnosticity of the tests at the individual level was found to vary depending on the test and was not a function of the new method but rather the phenomenology of the type of synesthesia being examined. The new method produces results that are consistent with the traditional testing method and therefore both the new method and the Synesthesia Battery which implements it, are useful new tools to aid future researchers. Chapter 3 focuses on a previous claim by Day (2005) that colours are matched between graphemes based on their shape similarity. Using the data from the TOG for graphemes, we found that synesthetic colour associations tend to be shared between graphemes that are different category (letter number) pairs rather than same category (letter letter) pairs. This suggests a categorical transfer of colour between graphemes rather than colour transfer based simply on shared early visual features. Chapter 4 investigates the nature of the synesthetic Stroop effect. We administered both the synesthetic Stroop screen colour naming experiment and the synesthetic Stroop associated colour naming experiment to synesthetes and non-synesthetes. In both experiments we administered unique stimulus ensembles to each participant. Ensembles comprised graphemes for which the participants had strong colour match consistency in the TOG for graphemes. This differed from previous designs which administered synesthetes' stimulus ensembles to controls. Synesthetic Stroop effects were elicited in non-synesthetic individuals in both experiments. Utilising the unique stimulus ensembles for individual participants enabled the first comparison between synesthetes and non-synesthetes in the associated colour naming paradigm. No significant difference in effects were found between the groups. In the screen colour naming experiment, Stroop interference effects were found to correlate with the colour match consistency score in the grapheme TOG. These results suggest that the synesthetic stroop effect is not unique to synesthesia, and that Stroop effects likely reflect the strength of paired stimulus dimensions. This interpretation was supported by our examination of a single synesthete, for whom projected synesthetic colour is elicited by both graphemes and geometric shapes, and of which, the graphemes synesthetic colour is reported to be stronger. Consistent with the self report, we found that synesthetic Stroop effects were elicited by geometric shape inducers, and these effects were significantly weaker than those elicited by grapheme inducers. This held for both types of Stroop test. Chapter 5 describes two cases of poly-modal and multi-concurrent synesthesia. They challenge current theoretical accounts of the condition, particularly the localised crosswired hypothesis. Chapter 6 reports on the conduct of a preliminary inquiry into the relationship between absorbed attention and synesthesia. Using the modified Tellegen Absorption Scale (MODTAS) (Jamieson, 2005) we found that synesthetes report significantly higher levels of absorbed attention in everyday activities. We make some speculative suggestions as to why this might be the case and propose future inquiries. Chapter 7 investigates the multisensory nature of synesthesia. We administered the sound induced flash and fusion illusions (Shams, Kamitani, & Shimojo, 2000) to synesthetes and controls. Synesthetes did not report the illusion any more often than control subjects. We discuss the assumptions of the research question and general limitations of the study. Chapter 8 discusses the implications of the research presented in this thesis as well as its limitations. Recommendations for future research that include longitudinal and developmental investigations are also made.