A small proportion of the population experiences synaesthesia, in which a stimulus (the inducer) causes a percept (the concurrent) in its own sensory domain, and in another domain, or another sub-domain of the same sense. This thesis is concerned with synaesthesiae in which numbers and letters take on spatial locations or colours. In Paper 1, alphabet-form synaesthesia is investigated. The majority of alphabet forms belonging to native English speakers are straight, horizontal lines. Any breaks, gaps or direction changes tend to fall in line with the parsing of the Alphabet Song. Synaesthetes show greater inducer-concurrent consistency than controls; their spatial attention can also be cued by letters. In Paper 2, synaesthetes with alphabet forms and number forms took part in case or parity judgement tasks. Synaesthetes behave similarly to controls on the parity judgement tasks (i.e. both groups categorise small numbers more quickly with the left hand than the right hand). In the case judgement task neither group shows an equivalent effect for letters of the alphabet. Controls alone show a QWERTY effect, in which letters on the left of the keyboard are categorised more quickly with the left hand than the right hand. A large-scale study of letter-colour and number-colour synaesthesia in Paper 3 shows that correlations between letter frequency and saturation, alphabetical position and saturation, magnitude and luminance, magnitude and saturation are seen when luminance and saturation are considered as across-hue and within-hue variables. Papers 4 and 5 are concerned with synaesthetic bidirectionality, wherein concurrents can elicit implicit mental representations of their inducers. While no experiment in these papers shows evidence for bidirectionality, this may be due to the presentation of concurrent colours as graphemes instead of colour blocks. However, priming effects appear during a synaesthetic Stroop task when numbers are presented as digits, suggesting a stronger role for digits than other notations in number-colour synaesthesia.

Humans and other animals learn to associate flavours with aspects of consuming foods, which can result in acquired liking or aversion for that flavour. Two main processes of learning have been proposed to be critical: flavour-flavour (FFL) and flavour-nutrient (FNL) learning. This thesis addresses two main research questions primarily in the context of FNL; firstly, does liking for a flavour acquired through FNL persist once energy has been removed? It has been suggested that acquired flavour liking is resistant to extinction, but there are conflicting results within the human literature, which has concentrated on FFL. Studies One and Two explored this but failed to demonstrate acquired liking, although they tentatively suggested that extinction did not occur for acquired liking as pleasantness ratings remained stable after removal of energy. The second research question investigated whether liking acquired through FNL was modulated by expectations. Study Three manipulated viscosity of a yoghurt drink to determine if this impacted upon FNL, as thicker products have been shown to signal higher energy content. Expectations were influenced by viscosity, but with little impact upon pleasantness ratings and little evidence that FNL was enhanced. Studies Four and Five used labelling to influence expectations regarding a yoghurt-based breakfast. Study Four found that when no information was provided, liking changed as predicted from FNL. Contrary to prediction, when congruent information about energy content was provided, this acquired liking was not demonstrated, and ratings remained stable across sessions. Study Five did not replicate this finding, with pleasantness ratings in line with FNL literature. Addition of a hedonic label actually resulted in decreased pleasantness of the breakfast over time, suggesting a contrast effect with the flavour not delivering what was expected. Methodological limitations are recognised, with measurement of liking and contingency awareness discussed as potential explanations for weaker findings.

Previous research from the Sussex Ingestive Behaviour Group suggests that satiety beliefs generated by product information and satiety-relevant sensory characteristics (thick consistency and creamy flavours) can enhance the satiety response to covertly added energy in beverages. However these characteristics in low-energy beverages can generate rebound hunger effects. This thesis explored whether this enhanced-satiety concept can be translated to real consumers. Study 1 examined the extent of energy reduction that could be tolerated without rebound hunger effects. The original enhanced satiety concept was not replicated, although there was tentative evidence that energy compensation was more accurate for small energy additions. Study 2 explored whether enhanced satiety would prevail following repeated exposures in consumers' own homes. Enhanced satiety was found before and after exposure. Additionally focus groups suggested that diet-concerned consumers may be particularly interested in such products. Therefore in Study 3 this population, represented in the literature by those reporting high dietary restraint, was studied suggesting that those high in restraint and disinhibition compensated more accurately for energy in unenhanced beverages. A final complication for consumers is that believed healthy foods are often overconsumed. Two final studies demonstrated that health labels generated beliefs about the sensory experience and expected satiation and satiety of beverages. Tasting overrode the effects of these beliefs, although expectation-experience congruency led to assimilation of healthy beliefs, and indulgent-based fullness. Portion size selection was unaffected. Together the findings from these studies suggest that the enhanced satiety concept may have some utility in the real world, although it remains unclear as to how little caloric content can be tolerated whilst still enhancing satiety, and whether diet concerned consumers would benefit. Finally whilst health information may have a role in appetite expectations the interaction with sensory experience is important for generating overall product evaluations, and sensory experience is likely to override label information in dictating portion size selection.

Sensory substitution devices (SSDs) systematically turn visual dimensions into patterns of tactile or auditory stimulation. After training, a user of these devices learns to translate these audio or tactile sensations back into a mental visual picture. Most previous SSDs translate greyscale images using intuitive cross-sensory mappings to help users learn the devices. However more recent SSDs have started to incorporate additional colour dimensions such as saturation and hue. Chapter two examines how previous SSDs have translated the complexities of colour into hearing or touch. The chapter explores if colour is useful for SSD users, how SSD and veridical colour perception differ and how optimal cross-sensory mappings might be considered. After long-term training, some blind users of SSDs report visual sensations from tactile or auditory stimulation. A related phenomena is that of synaesthesia, a condition where stimulation of one modality (i.e. touch) produces an automatic, consistent and vivid sensation in another modality (i.e. vision). Tactile-visual synaesthesia is an extremely rare variant that can shed light on how the tactile-visual system is altered when touch can elicit visual sensations. Chapter three reports a series of investigations on the tactile discrimination abilities and phenomenology of tactile-vision synaesthetes, alongside questionnaire data from synaesthetes unavailable for testing. Chapter four introduces a new SSD to test if the presentation of colour information in sensory substitution affects object and colour discrimination. Chapter five presents experiments on intuitive auditory-colour mappings across a wide variety of sounds. These findings are used to predict the reported colour hallucinations resulting from LSD use while listening to these sounds. Chapter six uses a new sensory substitution device designed to test the utility of these intuitive sound-colour links for visual processing. These findings are discussed with reference to how cross-sensory links, LSD and synaesthesia can inform optimal SSD design for visual processing.

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To most people, a question like “What colour is the letter A?” may seem nonsensical, but to a grapheme-colour synaesthete, each letter and word has an automatically evoked colour sensation associated with it. This thesis asks whether the synaesthetic colours for letters and words are shaped by the same influences that inform the typical use of language – that is, if grapheme-colour synaesthesia is fundamentally psycholinguistic in nature. If this is the case, the colour experiences of synaesthetes for letters and words can also be used to investigate long-standing questions about how language acquisition and processing work for everyone. This thesis addresses two aspects of the psycholinguistic roots of synaesthesia: structure/morphology and meaning/semantics. The first two studies on word structure collected colour responses from synaesthetes for compound words (e.g. rainbow), the constituent morphemes of those words separately (e.g. rain and bow), and the letters that in turn form those words (e.g. R, A, B, etc.). These studies showed that synaesthetic word colouring does indeed encode linguistic properties such as word frequency and morphological structure. Furthermore, both linguistic and colour elements of words were important in determining their synaesthetic colour. The second two studies turned to the semantic aspect of language, asking how the meanings associated with words (e.g. red, fire) and even individual letters (e.g. A, Q) can influence the colours that a synaesthete experiences for them. The first of these studies indicated that the synaesthetic colour for a word like red or fire was measurably influenced by the colour that word typically evokes (e.g. the red of red and the orange of fire). The second showed that trends in letter-colour associations in large-scale studies (e.g. A is typically red) may be rooted in connections to particular words (e.g. A is red because A is for apple and apples are red). Overall, this thesis shows that both word structure and meaning have a systematic, measureable effect on synaesthetic colour, which allows these colours to then be used as a new tool to investigate psycholinguistic questions.

Sensory-specific satiety (SSS) is a significantly greater pleasantness decline for a consumed (Eaten) food, than foods that are tasted but not consumed (Uneaten). SSS occurs during consumption, reaches optimal magnitude immediately afterwards, and returns to baseline within two to three hours. The phenomenon is dependent on the sensory properties, rather than the energy or macronutrient content of the food. To the extent that an Uneaten food shares similar sensory properties with the Eaten food, the Uneaten food may be subject to pleasantness decline: a transfer effect. Repeated exposure to a food stimulus may alter liking in the long-term, through mere exposure, monotony, and dietary learning paradigms resulting in an association between the novel target food and either a known food stimulus, or a consequence of consumption. Novel foods are more susceptible to these effects than familiar foods, for which learned associations may have already formed. Repeated consumption alone does not modulate SSS, but to date such studies have not tested novel foods. Through six experiments this research explores the influences of long-term pleasantness changes of novel foods and the number and type of Uneaten foods present during SSS testing, on the magnitude of SSS for snack foods. While no evidence of mere exposure or dietary learning was found, and in some instances experiments failed to induce SSS, these negative results are likely due to methodological, and sometimes procedural issues in the design and conduct of experimental testing. Findings revealed SSS to be vulnerable to a number of procedural and methodological factors, such as: portion size; baseline novelty and pleasantness ratings; hunger; perceived ambiguity of measurement scales; and expectations raised by the type and number of Uneaten foods present during testing.

For people with synaesthesia, a percept or concept (inducer) triggers another experience (concurrent) which is usually in a different modality. The concurrent is automatic, and in the case of certain types of synaesthesia also consistent, however the relationship between the inducer and concurrent is not fully understood and shall be investigated in this thesis from different perspectives. The first is using hypnosis to suggest synaesthesia-like phenomenological experiences to participants, and measuring behavioural responses to see whether they behave in a similar manner to developmental synaesthetes. Results from hypnotic; 1) grapheme-colour (GC) synaesthesia; 2) motion-sound synaesthesia; suggest that phenomenological experiences similar to developmental synaesthesia can be experienced by highly susceptible participants, but is not associated with the same behaviour as developmental synaesthetes. Developmental GC synaesthetes were tested to determine whether a grapheme presented preconsciously binds with the concurrent colour to the extent that it influences behaviour or evokes the phenomenology of colour. Two techniques were used, gaze-contingent substitution (GCS) and continuous flash suppression (CFS). Using GCS, it was shown that although digits can be primed preconsciously, they don't bind with their concurrent colour to influence behaviour. Nevertheless, many synaesthetes still experienced colours though they didn't necessarily match the primed digit. CFS experiments showed that the colour of a grapheme's concurrent, or whether the grapheme is presented in the correct or incorrect colour for that synaesthete, doesn't influence the time for conscious perception of a grapheme, even though colour words presented in the correct colour are perceived faster than those in the wrong colour. Phenomenological differences were compared to the behavioural measures using questionnaires modified using factor analysis (the R-RSPA and R-ISEQ). Overall, inducers must be seen consciously for them to bind with the concurrent, and experiencing the phenomenology of synaesthesia is not sufficient to behave like a synaesthete.