Monday, October 29, 2012

Embodied cognition: Theoretical claims and theoretical predictions

I'm at the Annual Meeting of the Academy of Aphasia (50th Anniversary!) in San Francisco. I like the Academy meeting because it is smaller than the other meetings that I attend and it brings together an interesting interdisciplinary group of people that are very passionate about the neural basis of language and acquired language disorders. One of the big topics of discussion on the first day of the meeting was embodied cognition, particularly its claim that semantic knowledge is grounded in sensory and motor representations as opposed to amodal representations. Lawrence Barsalou (e.g., Barsalou, 2008) and Friedemann Pulvermuller (e.g., Carota, Moseley, & Pulvermuller, 2012) are among the most active advocates of this view and many, many others have provided interesting and compelling data to support it. Nevertheless, the view remains controversial. Alfonso Caramazza and Bradford Mahon, in particular, have been vocal critics of the embodied view (e.g., Mahon & Caramazza, 2008). 

Embodied cognition is an important concept and many researchers are very actively studying it, both from negative and positive perspectives, so it would be completely hopeless for me to try to summarize all of the evidence in a simple blog post. Instead I want to focus on one very specific issue that I have seen raised on several occasions (including here at the Academy meeting). Many experiments that are taken to support embodied cognition use materials for which the semantics have very clear sensory-motor content. For example, in a study of verb comprehension, the materials might be words such as "kick", "scratch", and "lick" that strongly involve different motor effectors (foot, hand, and mouth) and the prediction is that there should be clearly different patterns of activation in primarily motor control areas of the brain corresponding to those effectors. Setting aside specific controversies regarding those studies, critics of embodied cognition sometimes say something along the lines of "But what about verbs that don't have obvious motor components, such as 'melt' and 'remember'? Those couldn't be embodied in the motor strip!"

I think this question is conflating the general theoretical claim of embodied cognition -- that semantic knowledge is grounded in sensory and motor representations -- and the specific contexts where that general claim makes testable predictions. Because the motor strip is well-characterized and quite consistent across individuals, it is fairly straightforward to predict that verbs which have clear and very different motoric meanings should have very different neural correlates in the motor strip. This does not mean that other verbs are not embodied! Only that those other verbs don't make easily testable predictions. If the neural representation of temperature were well-characterized, we might be able to make clear predictions about verbs like "melt" and "freeze" and "boil". The same goes for abstract nouns, which are often considered to be a challenge for embodied cognition theories because they don't have simple sensory-motor bases. My take is that abstract noun meanings representations are just as embodied as concrete noun meanings, but they have more variable and diffuse representations, so they are harder to study. So, for example, the representation of "freedom" might involve visual representations of the Statue of Liberty for some people and open fields for other people, etc., so it is harder to measure this visual grounding because it is different for different people. Whereas the semantic representation of a concrete concept like "telephone" is going to be much more consistent across people because we all have more or less the same sensory and motor experiences with telephones.

The bottom line is that it is important to distinguish between the broad theoretical claim of embodied cognition, which is meant to apply to all semantic representations, and the subset of cases where this claim makes clear, testable predictions. Extending embodied cognition to the more difficult cases is certainly an important line of work (Barsalou, for example, is actively working on the representation of emotion and emotion words), but the fact that this extension is not yet complete is not, in itself, evidence that the theory is fundamentally flawed. Barsalou, L. (2008). Grounded Cognition Annual Review of Psychology, 59 (1), 617-645 DOI: 10.1146/annurev.psych.59.103006.093639
Carota F, Moseley R, & Pulvermüller F (2012). Body-part-specific representations of semantic noun categories. Journal of Cognitive Neuroscience, 24 (6), 1492-1509 PMID: 22390464
Mahon, B., & Caramazza, A. (2008). A critical look at the embodied cognition hypothesis and a new proposal for grounding conceptual content Journal of Physiology-Paris, 102 (1-3), 59-70 DOI: 10.1016/j.jphysparis.2008.03.004

Monday, October 8, 2012

Two ways that correlation and stepwise regression can give different results

In general, a correlation test is used to test the association between two variables (y and z). However, if there is a third variable (x) that might be related to z or y, it makes sense to use stepwise regression (or partial correlation). There are two quite different situations where the correlation and stepwise regression will produce different results. Here are some examples using made up data.