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 LH (n=54) and RH (n=55) participants during encoding and recognition memory tasks for verbal and non-verbal stimuli. Additionally, we measured participants' language laterality index by administering a semantic fluency task. We found no difference between groups' behavioral memory performance for either verbal or non-verbal stimuli. Similarly, we found no group differences in fMRI activation for either verbal or non-verbal material at encoding or retrieval. When we split groups according to language lateralization, we observed more activation in right-lateralized participants for word retrieval only. To measure whether including LH in fMRI studies would dilute results, we performed Monte Carlo simulations with subsets of our data for sample sizes from n=10 to 50 with the number of LH in each sample ranging from k=0 to 9. Similarity metrics between activation maps for the all-RH sample and samples that include LH participants were robust, especially for large sample sizes (e.g., Dice similarity coefficients >0.9 when n=50). 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.