Spatial stimulation

The first variable is how changes in spatial stimulation affect synaesthetic photisms, with strokes along the vertical (1) and horizontal (2) axes producing photisms that are rotated 90 degrees with respect to each other for all participants. The depth axis (12) is also experienced for MLS with descriptions of their photisms going "towards" them, however CS shows no obvious influence of this, and ES' photisms are more in line with the previous horizontal stimulation. The distance between touched regions (3 & 4) show a literal representation of spatial distance, with wider tactile gaps producing wider visual gaps while narrower gaps reduce this distance (MLS & CS) or eliminating it entirely (ES). Other variations in spatial pressure also seem to have a closely knit relationship to visuospatial representations, with circular strokes (5, 6) producing circular photisms and pressure on three points on their hand produce 'dots' that appear to represent the allocation of pressure (16) although not always accurately (CS). Finally the representation of objects was tackled in two ways, with the passive receiving of a circular object into their palm (17) or holding the object in their fingertips (18). Synaesthetes uniformly had circular photisms to the passive receiving of the object, but are split on how holding the object is visually represented. For holding an object in their fingertips, MLS experienced photisms with multiple points, possibly representing the contact points between their fingers and the object, quite differently CS and ES both have circular representations closer to the passive representation of the object.

3.5.3.4.2. Perspective

Varying the relationship between the individuals' perspective and the spatial location of tactile stimulation has been found to modulate photisms in acquired synaesthesia (Armel & Ramachandran, 1999), this section will explore if this is also true of developmental synaesthesia. Situations in which the proximal stimulation stayed the same (i.e. identical stroking patterns) but changes to the positioning of the hand in relation to the face are examined first. When the synaesthetes had a diagonal line (little finger to thumb) stroked into their palm while it was facing them (7), MLS & ES reported photisms that moved from top left to bottom right, however when the same stimulation is given to the palm facing away (8), the photism 'flips' for these synaesthetes so that it remains consistent with their visual perspective. When the hand is moved in relation to the face, MLS and CS have their photisms remain central while ES reports that their photisms move horizontally (13) or vertically (15) with their hand, with ES also reporting that they occur outside of her visual space. Changing the body's position while keeping the head position constant does not appear to influence photisms for any synaesthetes (14).

3.5.3.4.3. Motion

Synaesthetes were asked to report the direction of any visual motion that occurred from the touch stimulation. CS did not explicitly report any visual motion from the stimulation for any stimulus. Clockwise (5) and anti-clockwise (6) tactile stimulation produced clockwise and anti- clockwise motion in MLS and ES’ synaesthetic photisms respectively. Likewise, horizontal (2), vertical (1) and towards (12) stroking motions produced congruent motions in MLS and ES as well.

3.5.3.4.4. Texture

ES reported that exploring textures in particular had a strong influence on their photisms, so stimuli from the consistency test were brought back for her to explore and report her photisms to. The results can be seen in table 3.4.

Table 3.4. Photisms to active exploration of different textures by ES

The rougher grainier textures appear to have more isolated 'specks' in the photisms of ES, similar to the previously delivered three points of pressure (16). As the textures become smoother the photisms become more integrated and uniform in their visual texture, and colour saturation appears to increase with smoothness, similar to correspondences in non-synaesthetic children and adolescents (Ludwig & Simner, 2013) but opposite to a previously investigated synaesthete, for whom roughness related to increased saturation (Simner & Ludwig, 2012).

There are many interesting commonalities in how synaesthetic photisms change in response to variations of touch. The first is that the spatial organization of touch relative to their visual perspective appears to be reflected in the spatial distribution of colour in their photisms. Despite this, varying the location of the hand had mixed effects with the photisms following the hand position for ES, or remaining central for MLS and CS. The orientation, spatial distribution and motion of strokes appear to be accurately mapped in most of the synaesthetes' photisms (however CS did not report motion). Orientation and motion processing involve shared sensory areas between tactile and visual processing (Blake et al., 2004; Hagen, et al., 2002; Zangaladze et al., 1999; Zhang et al., 2005). The spatial distribution of tactile stimulation appears to have a close relationship with the distances between colours in the synaesthete's photisms. The ability to accurately interpret distances between points on the skin appears to be enhanced in tactile-vision synaesthesia from earlier experiments using the JVP texture task. This brings two previous bodies of work into consideration. Firstly it could mean the use of the RAG, an area used in interpreting distances between skin locations (Sathian et al., 1997; Spitoni et al., 2013; Zhang et al., 2005). Secondly, it could be the result of early visual area recruitment which has been incorporated for interpreting tactile distances in work on visually deprived individuals (Merabet et al., 2004, 2007, 2008; Sathian & Stilla, 2010). Other potential avenues for future investigation include research into shared external spatial representations for both the tactile and visual modalities (Spence, Pavani & Driver, 2000). For instance, the processes that interpret external tactile spatial cues affect early visual processing (Eimer & Driver, 2000) or how multi-sensory representations of external stimuli both involve the intraparietal sulcus (Macaluso & Driver, 2001). Together these may provide clues as to how such a tight spatial mapping is achieved between the inducing touch and concurrent photisms in synaesthesia.

3.6.

Discussion

The present experiment represents the largest questionnaire and behavioural study to date on how both touch and visual photisms are influenced through the presence of tactile-vision

synaesthesia. In a sample of 21 synaesthetes, respondents noted that the most common inducers of synaesthesia were emotional human touch, itchiness and sexual experiences. Variations in textural quality appeared to vary the content of visual photisms the most, which typically consisted of colour, motion and texture. Some synaesthetes do not report consistency or that inanimate objects create photisms which may make some unsuitable for consistency testing. In our consistency testing, one synaesthete reached significance, while others reporting consistent photisms had a tendency in that direction. In our behavioural testing, synaesthetes have a significantly superior ability to discriminate distances on the skin, but did not show superior orientation or tactile-visual integration abilities. These findings indicate either superior RAG involvement or the recruitment of additional visual areas to aid in discrimination. Finally, the photisms themselves appear to contain a close tactile to visual spatial mapping and depend on their visual perspective rather than sorely on the skin location of tactile stimulation.

3.6.1. Consistency testing

Of the three synaesthetes that underwent consistency testing, only one reached significantly higher levels of consistency than controls, while the final two tended in that direction. As a group, these three tactile-vision synaesthetes are more consistent than controls. This data suggests that consistency may indeed be a suitable measure of genuineness for some tactile-vision synaesthetes. During consistency testing, controls use many tactile-visual correspondences which aid their consistency, making the test less sensitive for identifying synaesthetes (Ludwig & Simner, 2013; Ward et al., 2008). Another factor potentially reducing sensitivity in the present experiment is the time period of the test-retesting being an hour unlike Simner and Ludwig's (2012) case study, which tested a synaesthete over four months and sixteen days for controls. While shortened periods of retesting may be suitable for other forms of synaesthesia (Ward, Huckstep & Tsakanikos, 2006) this may provide too much of an aid to controls in maintaining their touch-colour pairings. Further sensitivity could be added in several ways; firstly objects that show high consistency for controls could be omitted from further testing as they would not be sensitive to identifying synaesthetes. In addition future studies could either focus on tactile dimensions for which only synaesthetes have consistent-mappings or by introducing distracters for controls that do not affect synaesthetes' photisms. For example, since texture is the strongest reported modulator of colour for synaesthetes applying the same texture to objects varying in dimensions that do not influence synaesthetic photisms, might make texture-colour consistency a stronger measure of genuineness. However this still leaves the problem of testing synaesthetes that do not have photisms to inanimate objects. A

remaining commonly reported inducer is that of human touch which may also be suitable for consistency testing, variations in orientation and pressure seemed to influence the colour of the photisms reported in the photism-illustration task, in addition, human touch could have variations in texture through additional materials at the point of contact. This also has the added benefit of being passively received, since variations in exploration strategies by users may add to inconsistent tactile sensations and potentially more inconsistent colour selections as a result.

3.6.2. Weight-luminance

Previous investigations into touch-colour links for controls and synaesthetes found multiple associations between softness, roundness and smoothness with luminance and saturation (Ludwig & Simner, 2013; Simner & Ludwig, 2012). This study adds further evidence of a negative weight- luminance correspondence found in non-synaesthetes (Walker et al., 2010), and furthermore that this has an impact on synaesthetic photisms. Whether weight is the primary cause of the luminance association is unclear however, as previous negative pressure-luminance associations have been observed in controls and synaesthetes, and could also explain the present findings (Ward et al., 2008). One way these explanations could be disentangled is through changing the surface area that an object makes contact with the skin, this way the same weight can be felt as higher or lower pressures, or pressure can be kept constant while overall weight changes.