The association between synesthetic colors and sensitivity to physical colors changed depending on the type of synesthetic experience in grapheme-color synesthesia. (2023)

Grapheme-color synesthesia is a neurological phenomenon in which the visual perception of letters and numbers (grapheme) induces the simultaneous perception of a specific color (a synesthetic color). For example, a synesthetic might perceive the letter "F" as green and the number "2" as red (Cytowic & Eagleman, 2009). This synesthetic experience has two important properties. First, the grapheme-color association is constant since childhood (Rich, Bradshaw, & Mattingley, 2005). In Japanese, kanji color pairs are consistent over time (Asano & Yokosawa, 2012). This property of grapheme color consistency is an important diagnostic criterion for synesthesia. The prevalence of grapheme color consistency has been estimated at 1 in 200 or more (Ramachandran & Hubbard, 2001). Second, the grapheme-color association between individuals is idiosyncratic (Laeng et al., 2004, Ward et al., 2007). For example, the letter "B" is perceived as blue by one synesthetic, green by another, and yellow by another. Even identical twins report different synesthetic colors evoked by the same letter (Rich et al., 2005). Synesthetic color sensations are induced without proper physical input. Indeed, V4 activity of brain regions important for color perception has been confirmed in synesthetics observing graphemes (Hubbard et al., 2005, Nunn et al., 2002, Sperling et al., 2006). Therefore, the mechanism underlying the experience of color cannot be explained solely by conventional studies based on colors induced by wavelengths (physical colors) through the retina. To understand the conditions that evoke color experiences, two properties of synesthetic colors are important. The first is commonality: the degree to which synesthetic colors share physical color properties. The second is relationship: how synesthetic colors relate to physical colors. In this study, we wanted to address the second property. In the following sections, we examine the similarities between synesthetic colors and physical colors (first property) and describe our approach to studying their relationship (second property).

Several studies have addressed the first property through traditional methods and paradigms found in research on color perception and visual search. These tasks use a character or letter stimulus instead of a physical color stimulus to induce synesthetic color. If the visual impact of physical colors when synesthetics perceive synesthetic colors is confirmed, this indicates that synesthetic colors have identical or similar properties to physical colors. A previous study showed that synesthetic colors can promote binocular rivalry, just as physical colors promote rivalry in normal vision (Paffen, Van der Smagt & Nijboer, 2011). Several psychophysical studies have examined synesthetic color perception at lower levels of color processing using visual effects (eg, 2011) and color adaptation (Hong & Blake, 2008). These studies have shown that synesthetic colors do not have identical properties to physical colors at the level of the retina or primary cortex. An initial study of the pop-out effect in a display of achromatic graphemes found that synesthetic colors facilitate visual search in the same way as true colors (Ramachandran & Hubbard, 2001). However, subsequent studies have failed to replicate this finding (Edquist, Rich, Brinkman, & Mattingley, 2006). Although individual and group studies have reported faster or more accurate search performance by synesthetics than normal controls on a visual search task (Hubbard et al., 2005, Laeng et al., 2004, Palmeri et al. ., 2002, Rich and Karstoft, 2013, Ward et al., 2010) other studies found that synesthetics did not have a significant advantage over non-synesthetic controls (Brogaard et al., 2016, Gheri et al., 2008).

Conflicting results from previous studies may be due to the attentional demands of the grapheme and individual differences in the subjective experiences of synesthetes. Synesthetics must focus their attention on a grapheme long enough to perceive synesthetic colors (Edquist et al., 2006, Gheri et al., 2008, Ward et al., 2010). For example, as the contrast between the letter and the background stimuli decreases, the synesthetic colors gradually disappear (Hubbard, Manohar, & Ramachandran, 2006). When synesthetics view a Navon image (for example, a global image of a "5" composed of a local matrix of "2"), their experiences with synesthetic colors depend on whether their attention is on the global image or on the local attributes of lies. . the image (Palmeri et al., 2002, Ramachandran and Hubbard, 2001). That is, achromatic letters that induce synesthetic colors do not facilitate visual search in the same way as physical colors, since graphemes must be consciously attended to. In terms of individual differences in subjective experience, a minority of synesthetics (11 out of 100) are "projectors," visually perceiving associated colors in external space and characterizing them as "out there on the page." By contrast, most synesthetics are “associators” who perceive colors in internal space and characterize them as existing “in my mind” or “in my head” (Dixon, Smilek, & Merikle, 2004). Visual search is better on projectors than on associates, apparently due to the vividness and external location of their synesthetic experience (Rothen & Meier, 2009). In fact, two studies on visual search tasks found a performance advantage with projectors (Ramachandran & Hubbard, 2001; Palmeri et al., 2002), while another study found a small performance advantage with associates (Rothen & Meier, 2009 ). These results indicate that projectors perform better than matchers on visual search tasks. Differences in synesthetic experiences are highlighted by performance on a synesthetic Stroop task. In this task, words are printed in colors that are either congruent or incongruent with the color assigned to those words by the synesthetic. Projectors were faster to name synesthetic colors compared to real colors, while matchers were faster to name real colors compared to synesthetic colors (Dixon et al., 2004). Therefore, differences in synesthetic experience influenced Stroop interference. In general, synesthetic color may not depend on lower preattentive processes like physical color. Consistent with this result, the fidelity of synesthetic color matching is similar to that of stored physical colors, but not to physical colors (Arnold, Wegener, Brown, & Mattingley, 2012).

It is important to examine the second property (ie, the relationship between synesthetic and physical colors) because synesthetic colors are not the same as physical colors. In the present study, instead of having synesthetics look at graphemes to induce synesthetic colors, we focused on the difference in properties of colors that do or do not appear as synesthetic colors. This difference reflects a perceptual or cognitive property of synesthetic colors. Previous studies have shown that synesthetics experiencing the simultaneous sensation of colors have superior color discrimination abilities, independent of the association between synesthetic and physical colors (Arnold et al., 2012, Banissy et al., 2013, Banissy et al. , 2009, Barnett et al., 2008, Gimmestad, 2010, McCarthy and Caplovitz, 2014, Yaro and Ward, 2007). The human visual system cannot distinguish some colors from others, even if they are different; That is, there are areas of high and low sensitivity in the color space (Luo, Cui & Rigg, 2001). Since synesthetics experience synesthetic colors associated with graphemes in everyday life, we predict that color sensitivity will be higher for colors that appear as synesthetic colors than for those that are not. This association would suggest that synesthetic colors are related to physical colors through sensitivity.

As a first step, as much data as possible on synesthetic colors should be collected to distinguish between colors that appear to be synesthetic colors and those that are not. Previous studies of English-speaking synesthetes have limitations in data collection. Specifically, English has only 26 Latin letters (A–Z) and 10 Arabic numbers (0–9). This is a relatively small sample per synesthetic to distinguish between synesthetic and non-synesthetic colors. The problem can be solved using synesthetic color groups (Hamada, Yamamoto & Saiki, 2017a). Hamada and colleagues (2017a) collected between 100 and 1000 synesthetic colors associated with Japanese kanji characters for each synesthetic and then analyzed the distribution pattern of synesthetic colors in color space using spatial statistics techniques. Spatial statistics is an analytical tool for finding patterns in spatial distributions. The analysis revealed that synesthetic colors are concentrated in various regions of the color space. This trend is known as "synesthetic color groups". Synesthetic color groups provide a global perspective for specifying concentrated and empty areas of synesthetic colors in color space. Therefore, synesthetic color groupings help to distinguish between colors that match synesthetic color groupings (grouped colors) and those that do not (ungrouped colors). In addition, we consider the possibility that the correspondence between synesthetic color groups and the physical sensitivity of colors is influenced by individual differences in the synesthetic experience of projectors and associates. Different synesthetic experiences imply different cognitive performances of synesthetics. As noted above, projector synesthetics are faster at naming synesthetic colors relative to real colors, while associative synesthetics are faster at naming real colors than synesthetic colors (Dixon et al., 2004). This result raises the possibility that the sensitivity to grouped colors is greater in the projectors than in the associated ones. Note that differences in projector/associator status were considered as a continuum rather than categorically separate (Rouw & Scholte, 2010, Skelton et al., 2009, Van Leeuwen et al., 2011). We therefore hypothesize that the greater the tendency for projector features in synesthetes, the greater the correlation between clustered colors and high sensitivity.

Continuing on from previous research, this study aimed to test three hypotheses. First, synesthetes are more sensitive to physical color than non-synesthetics, consistent with the results of previous studies using the Farnsworth-Munsell 100 Hue test (FM100 test) and the Cambridge Color Test (CCT). The CCT was performed to also test the second and third hypotheses. Second, the sensitivity to grouped colors is greater in synesthetics than to ungrouped colors. Third, the greater the tendency for projector characteristics in synesthetics, the greater the sensitivity to grouped colors.