In the final installment of Mixing Memory's metaphor series (for now -- at some later date I'll get to novel vs. dead metaphors), I try to use the empirical data to distinguish between the categorization and structure mapping theories of metaphor. Before I start, I should make it clear that there is certainly not a consensus among researchers about which model is the correct one, though my feeling is that most are in the comparison camp, rather than the categorization camp, even if they don't fully buy the structure mapping account. Part of the problem is that it is difficult to distinguish between the two accounts, and they are both powerful enough to handle most of the data out there.
Here's an example: In a 1987 paper1, Kelly and Keil asked participants to rate how semantically different concepts from two disparate conceptual domains were. After that, they presented the participants with metaphors containing those concepts. After rating the aptness of the metaphors, participants were against asked to rate the semantic difference between the concepts. Concepts that had been in highly rated metaphors were then rated as more semantically similar than they had been in the first ratings. People in the comparison camp take this as evidence that the two domains were mapped during metaphor comprehension, and thus are more similar after the mapping. However, one could easily interpret this result as evidence for the categorization view. Since, under this view, the vehicle and topic are placed in the same category, and since intra-category similarity is usually higher than inter-category similarity, it stands to reason that similarity ratings would be higher after the categorization than before.
Despite this difficulty in distinguishing the two types of theories empirically, I think they can be distinguished. Furthermore, I think we can decide between the two right here and now. As I said in the post on the attributive categorization theory, in its most recent form the categorization view of metapor involves a comparison, with the unintuitive categorization process on top. The purpose of this process seems to be to explain the irreversability of metaphorical statements. Since the categorization theory now involves comparison, and since the categorization aspect itself is unintuitive, if we can come up with a comparison theory of metaphor that can explain this irreversability, then we can do away with the categorization theory altogether. The question we have to ask, then, is can structure mapping, the most prominent comparison theory, account for the irreversability?
The answer to that question is a little bit more complex than it seems. To answer it, we first have to determine when the irreversability arises. If it arises from the very beginning of the comprehension process, then only the categorization theory can account for it. However, if it arises over the course of the comprehension process, then structure mapping can account for it. This is because in structure mapping, once the mapping has been made, the inferences can only move in one direction. However, when the mappings first start, the two domains are treated equally, and thus there exists a symmetry between the two roles in the metaphor, so that the topic and vehicle could be reversed without a problem.
To test when metaphorical statements become irreversable, Wolff and Gentner2 used the "true/false" task described in the first post. To recap, in this task, participants are asked to determine whether sentences are literally true or false. A third of the sentences are literally true, a third are literally false and do not make sense as metaphors (sans context), and a third are metaphors that received high scores on an aptness scale. Participants quickly rate the literally true statements as true, and the abhorent literally false and aberrant statements as false. However, they took significantly longer to rate the metaphorical statements, which are also literally false, as false. This is generally taken as evidence that metaphors are processed automatically, and without prior literal processing. In this case, the experiment is used because it taps into the early processing of metaphors, presumably pre-mapping. If it can be shown that participants have the same trouble rating metaphors as literally false even when the topic and vehicle are reversed, then this would be strong evidence that metaphors are not irreversable in the initial processing stages. This would in turn be evidence that the structure mapping theory can account for the irreversability of metaphors, thus rendering superflous the categorization phase in the attributive categorization theory. In two experiments, this is in fact what Wolff and Gentner found.
So now that we've reached the end of our short journey through cognitive scientific views of metaphor (glaringly ommitting cognitive linguistic theories), I think we can come to a pretty firm conclusion: metaphors involve comparisons. Once we've shown that a comparison theory can account for the irreversability of metaphor, we no longer need to posit a categorization phase in metaphor comprehension. Is structure mapping the right theory to model the comparison process in metaphors? That's a more difficult question to answer. Right now, the only other viable (and I use that word very, very loosely) theories are in cognitive linguistics. In fact, blending may be the only other viable theory currently available, and it is not inconsistent with structure mapping theory (you could use structure mapping theory to acheive everything that blending theories say is going on). So, as things look, structure mapping theory is the best cognitive theory of metaphor we've got. As you might expect from an algorithmic theory in a young discipline, there are plenty of problems with it. Still, it has survived 20 years of empirical tests in the areas of analogy and similarity, and until we have something better, it will probably continue to produce the best experiments on metaphor comprehension.
In case you missed them, Metaphor posts I-III can be found here, here, and here.
1 Kelly, M., & Keil, F . C. (1987). Conceptual domains and the comprehension of metaphor. Metaphor and Symbolic Activity, 2, 33-51 .
2 Woff, P. & Gentner, D. (2000). Evidence for Role-Neutral Initial Processing of Metaphors. Journal of Experimental Psychology: Learning, Memory, and Cognition, 26(2), 529-541.