Similarity relations among synesthetic stimuli and their attributes

Four experiments examined the similarity relations that exist among bimodal attributes that correspond synesthetically (e.g., white color and high pitch) and among stimuli formed by combining these attributes either congruently (e.g., white/high, black/low) or incongruently (e.g., white/low, black/high). Previous research suggests two hypotheses: (a) Synesthetic stimuli are compared as wholes on the basis of their overall similarity, and (b) nonidentical congruent stimuli are more dissimilar than nonidentical incongruent stimuli. In these four experiments, similarity among either individual attributes (Experiments 1 and 4) or bimodal stimuli (Experiments 2 and 3) was measured by either ratings or response latencies; similarity judgments were scaled with an individual differences scaling procedure (SINDSCAL). Stimulus comparisons were fit well by a Euclidean but not a city-block metric, supporting the overall similarity hypothesis. However, there was little evidence that subjects perceived congruity/incongruity among stimulus wholes, even though subjects were sensitive to correspondence/noncorrespondence among attributes. These results were replicated in four additional experiments using larger stimulus sets. A two-process account is proposed in which stimulus formation (intersensory processing) occurs independently of the abstraction of cross-sensory meaning (figurative processing).