Heritability of fractional anisotropy in human white matter: a comparison of Human Connectome Project and ENIGMA-DTI data

The degree to which genetic factors influence brain connectivity is beginning to be understood. Large-scale efforts are underway to map the profile of genetic effects in various brain regions. The NIH-funded Human Connectome Project (HCP) is providing data valuable for analyzing the degree of genetic influence underlying brain connectivity revealed by state-of-the-art neuroimaging methods. We calculated the heritability of the fractional anisotropy (FA) measure derived from diffusion tensor imaging (DTI) reconstruction in 481 HCP subjects (194/287 M/F) consisting of 57/60 pairs of mono- and dizygotic twins, and 246 siblings. FA measurements were derived using (Enhancing NeuroImaging Genetics through Meta-Analysis) ENIGMA DTI protocols and heritability estimates were calculated using the SOLAR-Eclipse imaging genetic analysis package. We compared heritability estimates derived from HCP data to those publicly available through the ENIGMA-DTI consortium, which were pooled together from five-family based studies across the US, Europe, and Australia. FA measurements from the HCP cohort for eleven major white matter tracts were highly heritable (h2 = 0.53–0.90, p < 10− 5), and were significantly correlated with the joint-analytical estimates from the ENIGMA cohort on the tract and voxel-wise levels. The similarity in regional heritability suggests that the additive genetic contribution to white matter microstructure is consistent across populations and imaging acquisition parameters. It also suggests that the overarching genetic influence provides an opportunity to define a common genetic search space for future gene-discovery studies. Uniquely, the measurements of additive genetic contribution performed in this study can be repeated using online genetic analysis tools provided by the HCP ConnectomeDB web application

tDCS effects on word production: limited by design? Comment on Westwood et al. (2017)

Transcranial direct current stimulation (tDCS), has sparked enormous scientific, clinical and public interest (Dubljević, Saigle, &amp; Racine, 2014; Riggall et al., 2015), because of its potential to modulate human brain function and behaviour without significant side effects. It thereby offers exciting prospects for clinical applications (Bikson et al., 2016). However, recent publications have highlighted substantial variability among reported stimulation effects in healthy individuals (e.g., Wiethoff, Hamada, &amp; Rothwell, 2014) or even questioned the potential of tDCS to induce behavioural effects on cognition and motor function (Horvath, Forte, &amp; Carter, 2015a,b). While the latter have attracted criticism for conceptual and methodological reasons (Antal, Keeser, Priori, Padberg, &amp; Nitsche, 2015; Chhatbar &amp; Feng, 2015), they have nonetheless motivated reflections on the use and the efficacy of tDCS and prompted urgent calls for more rigorous methodology, including replication studies (Fertonani &amp; Miniussi, 2017). In this vein, a recent paper by Westwood, Olson, Miall, Nappo and Romani (2017) published in Cortex reported an attempt to replicate previously observed effects of tDCS on semantic interference during spoken word production using continuous and blocked cyclic naming paradigms (Damian, Vigliocco, &amp; Levelt, 2001; Howard, Nickels, Coltheart, &amp; Cole-Virtue, 2006). Across four experiments, active tDCS was administered to frontal and temporal cortical sites with the authors reporting null effects compared to sham stimulation, followed by far-reaching conclusions concerning the utility of tDCS to modulate cognition in healthy participants. In this commentary, we discuss a number of problems with Westwood et al.'s report, including their theoretical assumptions, choice of stimulation sites, use of reading and naming tasks in the same experiment, stimulation protocols, data analyses and interpretation of their null findings as a “test” of tDCS' efficacy. We conclude with a brief reminder concerning the proper use of the term replication, and recommend measures to be taken to ensure greater rigour in tDCS research conduct and reporting