Left Out: The Effect of Handedness on fMRI Activation in Memory Paradigms

About 11% of the population is left-handed, a significant minority of the potential participant pool for functional MRI (fMRI) studies. However, convention in fMRI research dictates these potential participants be excluded due to the supposition that left-handed (LH) people may have different lateralization of neural functioning than right-handed (RH) people. This difference in lateralization may cause different areas of the brain to be activated by the same task. The current study investigates the lateralization differences between N=49 LH and N=50 RH during encoding and recognition memory tasks for verbal and non-verbal stimuli. Additionally, we measured participants laterality index by administering a semantic fluency task. We found no difference between groups for memory encoding activation for either verbal or non-verbal stimuli. Similarly, we found no group differences for verbal retrieval activation. There were quantitative differences between groups in non-verbal retrieval activation, primarily driven by greater spatial extent of activation in the RH group rather than by differences in lateralization in the LH group. To measure if including LH in fMRI studies would dilute results, we calculated memory effects in a priori regions of interest (ROI) for the RH group only and then examined the effect of substituting in progressively more LH for RH. We found significant memory effects in 14 a priori ROIs, 10 of which retained significant effects when adding LH participants. The remaining ROIs had significant memory-related activation in more than 80% of simulations with statistically likely numbers of LH participants. Taken together, these results indicate that the inclusion of left-handed participants does not have a strong detrimental effect on memory-related fMRI activation. On this basis we advocate for the inclusion of left-handed participants in cognitive neuroscience of memory research.