Verified by 
As I concluded my previous post [Sexing the brain (function, anatomy, and structure)], I felt fairly safe in saying that there are no sex differences in lateralization for perceptual asymmetries as well as brain anatomy and morphology (at least when overall size is properly controlled). This new post considers evidence that comes from direct measurement of the brain as it is actually performing a specific task. This essentially means functional neuroimaging.
For starters, let us consider the question we have been asking so far in this series of posts: Are there any sex differences in lateralization? However, this time we examine data from neuroimaging studies with particular emphasis on meta-analyses that have summarized that research.
The meta-analysis by Sommer, Aleman, Bouma, and Kahn (2004) focusing on verbal tasks (14 studies) was updated to 26 studies in 2008 (Sommer, Aleman, Somers, Boks, & Kahn, 2008). In both cases, the authors reported no significant sex differences in lateralization for verbal tasks in functional neuroimaging settings. In a reply to criticisms of the 2004 article, Sommers, Aleman, and Kahn (2005) suggested that their data might reflect an important publication bias as most of the research with significant findings also had a small sample size (see Egger, Davey Smith, Schneider, & Minder, 1997). In my view, the Sommer et al. meta-analyses are not very convincing as the authors admit that their analysis does not provide a complete review (in the 2008 paper, page 83). In my opinion, a meta-analysis must provide as complete a review as possible to be valid. More to the point of their argument, they actually did not compute a proper publication bias analysis (e.g., with the method proposed by Egger et al., 1997) and they did not include unpublished research in either of their analyses. Regardless of these issues, the absence of sex differences in language lateralization should not be surprising in view of the lack of consistent sex differences in behavioral measures of verbal ability (Hyde & Linn, 1988).
In contrast to verbal tasks, a male advantage has been well-established in spatial abilities (Voyer, Voyer, & Bryden, 1995). Unfortunately, I was unable to find a meta-analysis on sex differences in lateralization in neuroimaging settings. This is possibly due to the fact that results and approaches with such tasks have been quite disparate. In addition, relevant studies tend to focus on specific areas of interest rather than on lateralization. Nevertheless, a study by Jordan, Wustenberg, Heinze, Peters, and Jäncke (2002) with mental rotation showed that activation was typically bilateral in women and lateralized in men. Unfortunately, these results were not replicated by Butler et al. (2006) with a validated mental rotation task.
A behavioral female advantage in emotion perception is also well-documented, as shown in the meta-analysis by Thompson and Voyer (2014). In this area, a meta-analysis of sex differences in lateralization for emotion processing based on neuroimaging data conducted by Wager, Phan, Liberzon, and Taylor (2003) showed greater lateralization in males than females. However, the Wager et al. analysis also considered activation in specific regions of interests (ROI) to examine sex differences at a more pointed level. Results from this ROI analysis are more relevant to the object of my next post. In fact, the Wager et al. review suggests two important points to consider as I pursue this topic further. First of all, if there are sex differences in lateralization, they are likely task specific. The second point is that if there are sex differences in the brain, they are likely region-specific rather than reflecting a broad lateralization. Essentially, we should view the type of speculations raised by Levy (1971), focusing on whole brain asymmetries, as too simplistic [see my post on Sexing the brain (early days)] and consider specific parts of the brain. Therefore, the question becomes: Are males and females using different brain parts when tackling a given task?
This is not a simple question to answer. I will do my best to answer it (albeit partially) in my next post. For the moment, however, my tentative conclusion is that neuroimaging provides some evidence that males are more lateralized than females in tasks where behavioral sex differences are found.
References
Butler, T., Imperato-McGinley, J., Pan, H., Voyer, D., Cordero, J., Zhu, Y. S., Stern, E., & Silbersweig, D. (2006). Sex differences during mental rotation: Top down versus bottom up processing. NeuroImage, 32, 445-456.
Egger, M., Davey Smith, G., Schneider, M., & Minder, C. (1997). Bias in meta-analysis detected by a simple, graphical test. British Medical Journal, 315, 629–634.
Hyde, J. S., & Linn, M. C. (1988). Gender differences in verbal ability: A meta-analysis. Psychological Bulletin, 104, 53–69.
Jordan, K., Wurstenberger, T., Heinze, H.J., Peters, M., & Jäncke, L. (2002). Women and men exhibit different cortical activation patterns during mental rotation tasks. Neuropsychologia, 40, 2397–2408.
Levy, J. (1971). Lateral specialization of the human brain: behavioral manifestations and possible evolutionary basis. In J.A. Kiger, Jr. (Ed.), The biology of behavior (pp.159-180). Corvallis: Oregon State University Press.
Sommer, I. E. C., Aleman, A., Bouma, A., & Kahn, R. S. (2004). Do women really have more bilateral language representation than men? A meta-analysis of functional imaging studies. Brain, 127, 1845–1852.
Sommer, I. E., Aleman, A., Somers, M., Boks, M. P., & Kahn, R. S. (2008). Sex differences in handedness, asymmetry of the Planum Temporale and functional language lateralization. Brain Research, 1206, 76–88.
Sommer, I. E. C., Aleman, A., & Kahn, R. S. (2005). Size does count: a reply to Kitazawa and Kansaku. Brain, 128, E31
Thompson, A. E., & Voyer, D. (2014). Sex differences in the ability to recognise nonverbal displays of emotion: A meta-analysis. Cognition & Emotion. Advance online publication. doi:10.1080/02699931.2013.875889
Voyer, D., Voyer, S., & Bryden, M.P. (1995). Magnitude of sex differences in spatial abilities: A meta-analysis and consideration of critical variables. Psychological Bulletin, 117, 250-270.
Wager, T.D., Phan, K.L., Liberzon, I., & Taylor, S.F. (2003). Valence, gender, and lateralization of functional brain anatomy in emotion: A meta-analysis of findings from neuroimaging. Neuroimage, 19, 513–531.
