Time-resolved sex differences in language lateralization

Many clinical, behavioural and brain imaging studies have suggested that language functions are less asymmetrical in women than in men. Sommer and colleagues challenge this view in a recent review on the outcome of 24 functional brain imaging studies (PET, functional MRI or functional transcranial Doppler ultrasound) that looked at possible sex differences in language lateralization. A vote count analysis over all studies revealed a much higher score for those studies that reported no sex differences. In addition, a meta-analysis of the lateralization index in 13 of these studies revealed no statistically significant difference between men and women. The authors concluded that the hypothesis of sex differences in cortical language representation probably has to be rejected at the population level (Sommer et al ., 2004). We believe that the hypothesis of subtle, but crucial sex differences in language representation should not be rejected on these grounds, considering the selective sample of studies included in the review.There is one important limiting factor of functional imaging with PET, fMRI or Doppler, namely their weak temporal resolution. Consequently, the studies reviewed by Sommer and colleagues (Sommer et al ., 2004) might have identified not only regions that are critical for language processing but also other areas that are activated while subjects solve the task, in particular when the control (subtraction) condition consists of ‘passive’ rest. This issue was discussed in detail in an earlier publication of the same group (Ramsey et al ., 2001), in which they propose a combined analysis of several ‘active’ language conditions using so-called conjunction analysis to overcome the problem. In that study, the authors convincingly showed that the latter approach is more reliable for the study of language lateralization and yields a small amount of variance across subjects. Unfortunately, with the exception of one study (Sommer et …