Synesthesia: Strong and Weak Forms

Martino and Marks (2001) characterize two versions of synesthesia, weak and strong, and purport that while phenomenological dissimilarities may exist, the basic underlying mechanisms involved in each are similar.

Subjects present with strong synesthesia when the perception of stimulus modality-A produces an experience associated with modality-A as well as an experience usually associated with a different modality-B. This association may be instantiated in several ways; that is, a strong-synesthete may experience a specific color in his or her visual field simultaneous to the auditory perception of a specific musical note; a strong synesthete may also exhibit an association between pain and color perception, as the authors describe in the case of Carol. Across the population of strong synesthetes documented in case studies, a population in which females curiously outnumber males six to one (Baron-Cohen, Wyke, & Binnie, 1987), a correspondence of inducer (some sensory experience manifested in one modality) and induced (some percept or image manifested in a different modality) obtains.

Martino and Marks (2001) posit that this correspondence may be characterized idiosyncratically or systematically. That one synesthete may have an auditory-visual correspondence particularized in the perception of 'middle-C' and 'blue' while another synesthete may have an auditory-visual correspondence particularized in the perception of 'middle-C' and 'green' instantiates this idiosyncratic aspect. However, these same synesthetes present with a systematic relationship between color brightness and auditory pitch; that is, a comparatively higher pitch results in a lighter color of the induced image (Marks, 1978). Images associated with this kind of synesthesia are generally both simple (e.g., an induced percept is red as opposed to 'redish-yellow inside a blue border') and dynamic, that is, changing. Lastly, the association between the inducer and the induced is entrenched to such a degree as to warrant the perception of the induced as literally part of the perceived object. The literal nature of this strong correspondence implies that synesthetic phenomenal experiences should be highly memorable.

Weak synesthesia is most apparent in cross-modal matching and selective attention tasks. This form of synesthesia may denote associations between different modalities found in common language; poetic/metaphoric language may convey this weaker form. As distinct from strong synesthesia, the weaker form is contextual, not absolute. In order to distinguish underlying processes of cross-modal associations, Martino and Marks (2001) developed a task to measure the ability to respond to a stimulus of one modality while simultaneously receiving input from a different and unattended modality. Participants are asked whether a given sound, in the presence of a color (either black or white) is of high or low pitch. Results indicate that subjects were faster at classifying high-pitched tones if simultaneously presented with white in their visual field; additionally, they were faster at classifying low-pitched tones if simultaneously presented with black in their visual field. Martino and Marks (2001) purport that these findings entail better performance if attended and unattended input match or correspond across modalities.

This entailment characterizes the congruence effect. Two accounts explain congruence effects: sensory hypothesis and semantic-coding hypothesis. Sensory hypothesis maintains that congruence effects are realized in sensory mechanisms. For instance, similarity in neural codes across modalities may issue congruence results. Martino and Marks (2001) assent to semantic coding hypothesis (SCH; e.g., Martino & Marks, 1999); this theory maintains that cross-modal associations high-level cognitive systems that develop in accordance with experience. Four propositions maintain this theory: first, onset of infant cross-modal associations reflect meaning-based mechanisms in adults; second, an abstract semantic network, a network produced via a combination of obtained experiential percepts and employed descriptive language, exhausts synesthetic correspondence; third, the perception of synesthetically associated percepts ends in a transference of these percepts (originally sensory representations) to abstract representations that cohere with the individual's semantic network; finally, the context of the presentation fixes whether or not stimuli from distinct modalities are coded as matching or mismatching.

Evidence for the SCH comes from selective attention tasks and other congruence studies. With respect to selective attention tests, it is noted by the authors that congruence effects obtain when tones and colors change across trials; the SCH explicates that this result reflects the significance of context on whether or not a set of modally differentiated stimuli are coded as matching or mismatching. Additional evidence is provided by the fact that both linguistic stimuli and colors (i.e., visual sensations) produce congruence effects when presented with both low and high-pitched tones. For instance, both the word "black" and the color black result in congruence effects when paired with low-pitched tones.

Martino and Marks (2001) conclude with some questions regarding the nature of the two forms of this condition; for instance: how are semantic networks related to the mediation of strong-synesthesia, and what are the shared brain regions of the two forms?

Works Cited

Baron-Cohen, S., Wyke, M., & Binnie, C. (1987) Hearing words and seeing colours: an experimental investigation of a case of synaesthesia. Perception, 16, 761-67.

Martino, G. & Marks, L.E. (2001). Synesthesia: Strong and weak. Current Directions in Psychological Science, 10, 61-65.

Martino G, Marks L E, 1999, "Perceptual and linguistic interactions in speeded classification: tests of the semantic coding hypothesis" Perception 28(7) 903 - 923