[I was asked to answer this question on Quora.]
I think there are actually three different questions here:
1. Is the right-brain/left-brain distinction still useful?
Yes. Marc Ettlinger‘s answer on Quora gets into aspects of this. There are many differences between the left and right hemispheres — some of the most unambiguous differences are in the domain of language processing. But the devil is in the details, many of which come from methods that are not always easy to interpret.
2. Is the emotional/rational distinction still useful?
Sort of (but it is often a source of confusion). Emotion and rationality/cognition are no longer seen as enemies. Rather than a dichotomy, there is a symbiotic spectrum of emotional and rational processes.
3. Is the right-brain/left brain distinction the same as the emotional/rational distinction?
Absolutely Not.
Let’s look at each issue in turn.
1. Lateralization: the right-brain/left-brain distinction
There is no doubt that there are differences in brain structure and function between the two hemispheres. Handedness is the best known example — the right side of the brain controls the left side of the body and vice versa, and humans are typically more dexterous with one hand than the other, so we say that one side of the brain is “dominant”. But it is also true that normal behavior requires cooperation between the two hemispheres. Split brain patients aren’t really normal.
However, there are reasons to be cautious about reading too much into the left-right difference data (especially for emotion and cognition). A lot of lateralization research is done using fMRI, which has many methodological and interpretational problems (e.g. Stark & Squire, 2001, Friston et al., 1996). For instance, a statistically significant difference in processing between the two hemispheres might have a small effect size. Also, focusing on differences in activity — as is common in brain scanning studies — obscures the fact that very often both areas are contributing to the behavior in question. In team sports, the observation that one team member is burning more calories than another doesn’t imply the other team members are doing nothing.
I have written about some of these fMRI issues in two blog posts: here and here. In this post I come up with some analogies to help (me) understand the statistical methods, and why they might occasionally mislead us.
The problem with fMRI is twofold: (1) We still don’t know what exactly fMRI is measuring, and (2) there is a lot of statistical hocus-pocus in the analysis of the results. See the infamous paper Voodoo Correlations in Social Neuroscience for some of the most heinous examples of statistical mistakes in fMRI research, particularly studies of emotion (Vul et al., 2009). More on the backlash against fMRI can be found in this Mind Hacks post. This critique of fMRI from the perspective of a PET researcher is also worth reading (Fox, 2012). Also see this page for a discussion of the subtraction method and related issues. Some statistical problems apply to other experimental issue too. And there is plenty of fMRI research that is carefully done, avoiding statistical and interpretational mistakes.
Even lesion studies can be problematic, particularly because one cannot easily tease apart the effect of the lesion from the effect of compensatory mechanisms (e.g. Hagemann et al. 2003). The brain’s plasticity is an extraordinary thing, so recovery processes add another level of complexity to the issue.
Also check out this blog post by Bradley Voytek. He says
just remember to ask, “can a person who has a lesion to that brain region not experience that emotion or do that behavior anymore?” If the person still can, then that is not where that behavior is located in the brain. And, in all likelihood, that function can’t be localized to any one region at all.
2. Passion versus Reason: the age-old distinction between emotion and “rationality”
The opposition between emotion and rationality has been exaggerated quite a bit. This may be a hangover of traditional and/or Victorian value systems that esteem self-restraint over displays of emotion. (Stiff upper lip, old chap!) This said, there are disorders that lead to inappropriate emotions that interfere with cognition. But there are also disorders that lead to a lack of emotion, and these disorders also interfere with normal cognition. Antonio Damasio makes this case in his 1994 book Descartes’s Error.
I’m going to copy excerpts from a paper I was co-author of (John et al., 2013):
The debate on the nature of cognition and emotion is a modern scientific manifestation of an age-old dichotomy. “Cognition” has come to refer to an assortment of useful behaviors – such as attention, memory and symbolic reasoning, while “emotion” carries with it the connotation of behavior that is irrational, evolutionarily ancient, and antithetical to efficient rationality. In this paper we outline findings that demonstrate both functional and anatomical overlap between cognitive and emotional processes, and use computational modeling to illustrate how learning processes may make cognitive-emotional interactions adaptive.
[…]
Though reason and emotion have been viewed as opposed processes in popular culture since ancient times, emotions have been treated as adaptive behavioral phenotypes by scientists since the time of Darwin (1872). Treating emotion as an adaptive phenotype fundamentally subverts any reason-emotion antithesis, because it places emotion as another, if distinctive, enabler of “biological rationality” (Damasio, 1994). Animals have a complex array of cognitive operations to draw upon, and an animal is rational if it knows or can learn how to draw upon those operations to maximize its well-being and minimize threats. In recent years, neuroscientists have shown that the parts of the brain that are recruited during episodes with emotion-arousing stimuli are also de-recruited when no emotion arousing stimuli are present, or when an animal learns that formerly emotion-arousing cues can be safely ignored (e.g. LaBar et al., 1998; Sehlmeyer et al., 2009; Bach et al., 2011; Hartley et al., 2011; van Well et al., 2012). Emotion is indeed a highly adaptive behavioral phenotype.
[…]
We have argued that rather than being opposing forces, cognition and emotion can be seen as points on a continuum or gradient of flexible processes required for adaptive categorization of, and response to, changes in the external and internal environment of an organism. While this conceptualization may not capture all the psychological nuances of the terms, it highlights the experimentally tractable facets of “cognition” and “emotion”.
The functional continuum is based on the robust connections between areas associated with cognition and those associated with emotion.
3. Right brain = Emotional? Left brain = Rational?
This is almost certainly not true. Both halves of the brain have corresponding limbic structures that are involved in cognitive-emotional interaction.
“The experience of emotion is not lateralized to one or the other hemisphere; rather, it involves dynamic processes that include interactions between anterior and posterior regions of both hemispheres, as well as between cortical and subcortical structures of the brain.” (Heller et al., 1998)
“results of the studies reviewed herein suggest that a simple left/right dichotomy with respect to hemispheric specialization for the autonomic component of the emotional response is probably untenable.” (Hagemann et al. 2003)
While there may be differences in the signal-processing properties of the left and right hemispheres, these differences cannot be aligned with the rational/emotional dichotomy. A brief scan of google scholar should bear this out (such as this or this). Studies seems to suggest that the left and right amygdalae constribute to different aspects of cognitive-emotional interaction (Markowitsch, 1999, Baas et al., 2004), but do not divide along a cognition-emotion faultline. There may not be significant differences between left and right orbitofrontal cortices (Kringelback, 2005), which are also involved in emotion.
The fMRI results are complex and often mutually contradictory, but the more solid anatomical connectivity data point to a model of cognitive-emotional interactions that is very different from the cartoonish assertion that “the right brain is emotional and the left brain is rational”.
References
Fox, P. T. (2012). The coupling controversy. NeuroImage, 62(2), 594-601.
Hagemann, D., Waldstein, S., & Thayer, J. (2003). Central and autonomic nervous system integration in emotion Brain and Cognition, 52 (1), 79-87 DOI: 10.1016/S0278-2626(03)00011-3
John, Y. J., Bullock, D., Zikopoulos, B., & Barbas, H. (2013). Anatomy and computational modeling of networks underlying cognitive-emotional interaction.Frontiers in Human Neuroscience, 7.
