CHAPTER 3 LOUD COLORS AND HOT BABES: SYNESTHESIA
1. Several experiments point to the same conclusion. In our very first paper on synesthesia, published in 2001 in the Proceedings of the Royal Society of London, Ed Hubbard and I noted that in some synesthetes the strength of color induced seemed to depend not just on the number but on where in the visual field it was presented (Ramachandran & Hubbard, 2001a). When the subject looked straight, then numbers or letters presented off to one side (but made larger to be equally visible) seemed less vividly colored than ones presented in central vision. This, in spite of the fact that they were equally identifiable as particular numbers and in spite of the fact that real colors are just as vividly visible in off-axis (peripheral) vision. Again, these results exclude high-level memory associations as the source of synesthesia. Visual memories are spatially invariant. By that I mean that when you learn something in one region of your visual field—recognizing a particular face, for instance—you can recognize the face presented in a completely new visual location. The fact that the evoked colors are different in different regions argues strongly against memory associations. (I should add that even for the same eccentricity the color is sometimes different for left and right halves of the visual field; possibly because the cross-activation is more pronounced in one hemisphere than the other.)
2. This basic result—that the 2s are more quickly segregated from the 5s in synesthetes than in nonsynesthetes—has been confirmed by other scientists, especially Randolph Blake and Jamie Ward. In a meticulously controlled experiment, Ward and his colleagues found that synesthetes as a group are significantly better than control subjects at seeing the embedded shape made of 2s. Intriguingly, some of them perceived the shape even before any color was evoked! This lends credibility to our early cross-activation model; it’s possible that during brief presentations the colors are evoked sufficiently strongly to permit segregation to occur but not strongly enough to evoke consciously perceived colors.
3. In lower, “projection,” synesthetes there are several lines of evidence (in addition to segregation) supporting the low-level perceptual cross-activation model as opposed to the notion that synesthesia is based entirely on high-level associative learning and memories:
(a) In some synesthetes, different parts of a single number or letter are seen as colored differently. (For example, the V part of an M might be colored red, whereas the vertical lines might be green.)
Soon after the popout/segregation experiment had been done, I noticed something strange in one of the many synesthetes we had been recruiting. He saw numbers as being colored—nothing unusual so far—but what surprised me was his claim that some of the numbers (for example, 8) had different portions colored differently. To make sure he wasn’t making this up, we showed him the same numbers a few months later—without letting him know ahead of time that he would be retested. The new drawing he produced was virtually identical to the first, making it unlikely that he was fibbing.
This observation provides further evidence that, at least in some synesthetes, the colors should be seen as emerging from (to use a computer metaphor) a glitch in neural hardware rather than from an exaggeration of memories or metaphors (a software glitch) Associative learning cannot explain this observation; for example, we don’t play with multicolored magnets. On the other hand, there may be “form primitives” such as line orientation, angles, and curves that get linked to color neurons that execute an earlier stage of form processing within the fusiform than the one at which full-fledged graphemes are assembled.
(b) As previously noted, in some synesthetes the evoked color becomes less vivid when the number is viewed off-axis (in peripheral vision). This probably reflects the greater emphasis on color in central vision (Ramachandran & Hubbard, 2001a; Brang & Ramachandran, 2010). In some of these synesthetes the color is also more saturated in one visual field (left or right) relative to the other. Neither of these observations supports the high-level associative learning model for synesthesia.
(c) An actual increase in anatomical connectivity within the fusiform area of lower synesthetes has been observed by Rouw and Scholte (2007) using diffusion tensor imaging.
(d) The synesthetically evoked color can provide an input to apparent motion perception (Ramachandran & Hubbard, 2002; Kim, Blake, Palmeri, 2006; Ramachandran & Azoulai, 2006).
(e) If you have one type of synesthesia, then you are more likely to have a second unrelated one as well. This supports my “increased cross activation model” of synesthesia; with the mutated gene being more prominently expressed in certain brain regions (in addition to making some synesthetes more creative).
(f) The existence of color-blind (strictly speaking, color anomalous) synesthetes who can see colors in numbers that they can’t see in the real world. The subject couldn’t have learned such associations.
(g) Ed Hubbard and I showed in 2004 that letters that are similar in shape (e.g., curvy rather than angular) tend to evoke similar colors in “lower” synesthetes. This shows that certain figural primitives that define the letters cross-activate colors even before they are fully processed. We suggested that the technique might be used to map an abstract color-space in a systematic manner onto form-space. More recently David Brang and I confirmed this using brain imaging (MEG or magnetoencephalography) in collaboration with Ming Xiong Huang, Roland Lee, and Tao Song.
Taken collectively these observations strongly support the sensory cross-activation model. This is not to deny that learned associations and high-level rules of cross-domain mapping are not also involved (see Notes 8 and 9 for this chapter). Indeed, synesthesia may help us discover such rules.
4. The model of cross-activation—either through disinhibition (a loss or lessening of inhibition) of back projections, or through sprouting—can also explain many forms of “acquired” synesthesia that we have discovered. One blind patient with retinitis pigmentosa whom we studied (Armel and Ramachandran, 1999) vividly experienced visual phosphenes (including visual graphemes) when his fingers were touched with a pencil or when he was reading Braille. (We ruled out confabulation by measuring thresholds and demonstrating their stability across several weeks; there is no way he could have memorized the thresholds.) A second blind patient, whom I tested with my student Shai Azoulai, could quite literally see his hand when he waved it in front of his eyes, even in complete darkness. We suggest that this is caused either by hyperactive back projections or by disinhibition caused by visual loss, so that the moving hand is not merely felt but is also seen. Cells with multimodal receptive fields in the parietal lobes may also be involved in mediating this phenomenon (Ramachandran and Azoulai, 2004).
5. Although synesthesia often involves adjacent brain areas (an example is grapheme-color synesthesia in the fusiform), it doesn’t have to. Even far-flung brain regions, after all, may have preexisting connections that could be amplified (through disinhibition, say). Statistically speaking, however, adjacent brain areas tend to be more “cross-wired” to begin with, so synesthesia is likely to involve those more often.