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Physics of Life Reviews

Volume 30, October 2019, Pages 41-46
Physics of Life Reviews

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Toward a multidimensional description of individual variation in hemispheric functional segregation: Reply to comments on “Phenotypes in hemispheric functional segregation? Perspectives and challenges”

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The problem of measurement

Investigating brain functional asymmetry implies measurement of brain functional asymmetry and thus requires the selection and operationalization of representational task paradigms and establishing methods of quantifying laterality. Several commentators have pointed at theoretical and practical issues of measuring functional asymmetry. Hausmann remarks that a clear scientific definition of typical (language) lateralization does not exist [5]. Does an fMRI-derived laterality index of .65 based

Determining the optimum level of neurobiological description

Francks proposed three main research goals to advance our understanding of typical and atypical developmental mechanisms of human brain asymmetry [3]. In addition to identifying the complex genetic-developmental cascade of typical asymmetry as well as the heterogeneity of developmental causes of atypical lateralization, he argues that we need to establish an optimum level of neurobiological description to capture the phenomenon of functional brain asymmetry and that the current measures of

Issues of phenotyping

A phenotype is the set of observable characteristics or traits of an organism and the result of its genetic code, environmental influences, and their interaction. Ocklenburg and Güntürkün draw our attention to the importance of environmental factors in the ontogenesis of hemispheric asymmetries [4]. They underline the interplay of genetic and environmental influences in the formation of various asymmetry phenotypes and point at the lack of systematic research covering the envirotype-side of the

Behavioral correlates of crowding

Apart from people with completely mirrored functional segregation, atypical lateralization of a function will result in crowding as a hemisphere will have to store both its usual and the novel functions. In healthy participants crowding is generally regarded a deviation from the ideal scenario, although its behavioral repercussions remain unclear. In some instances crowding is reported to be behaviorally advantageous [19], but several commentators acknowledge possible drawbacks of crowding

Biology of hemispheric functional segregation

Francks underlines the massive heterogeneity in genetic, environmental, and chance mechanisms that lead to atypical brain asymmetry and warns that this causal heterogeneity may defy attempts to classify phenotypes into etiological categories [3]. This is a fair point and other commentators have raised similar caveats that may obfuscate a full understanding of the variation in functional segregation including state-dependent influences on laterality [5]. On the other hand, we cannot deny the

Future research

Despite the many challenges that await researchers of brain laterality and which have been outlined in the commentaries and in this rebuttal, there is also a sense of optimism that the field is in position to engage in efforts describing hemispheric functional segregation at the phenotype level. Carey and Karlsson [2] predict a bright future for the study of multiple cerebral asymmetries, Thiebaut de Schotten et al. [6] suggest to join efforts and construct a consortium to create a

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Cited by (3)

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    Of course, other laboratories are free to re-analyse the current data using their own trichotomous scheme of choice as we have provided the raw data in scatterplots and in supplementary materials in a spreadsheet table. We would recommend this as standard practice in future, given the importance of proportions of individuals who show typical and atypical asymmetries for any function (Vingerhoets, 2019b). Individuals who are less lateralized on a particular function are in some sense the most important to study, at the very least to quantify as fully as possible the distribution of the cerebral asymmetry in question (for example, the full distribution bimodal, trimodal, or unimodal, shifted towards typical LIs has considerable theoretical implication - see Mazoyer et al., 2014, for an excellent example of full distributions of language LIs as a function of handedness group).

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