Are there sex differences in brain activity during long-term memory? A systematic review and fMRI activation likelihood estimation meta-analysis.

The degree to which sex differences exist in the brain is a current topic of debate. In the present discussion paper, we reviewed eight functional magnetic resonance imaging (fMRI) papers to determine whether there are sex differences in brain activity during long-term memory retrieval. The objectives were: 1) to compare the experimental parameters in studies reporting significant versus null long-term memory sex differences, and 2) to identify whether specific brain regions were associated with sex differences during long-term memory. The following experimental parameters were extracted from each paper: the number of participants, the average age of participants, stimulus type(s), whether or not performance was matched, whether or not sex differences were reported, the type of between-subject statistical test used, and the contrast(s) employed. The particular experimental parameters employed in each study did not appear to determine whether sex differences were observed, as there were sex differences in all eight studies. An activation likelihood estimation (ALE) meta-analysis was conducted to identify brain regions activated to a greater degree by females than males or males than females. This ALE meta-analysis revealed sex differences (male > female) in the lateral prefrontal cortex, visual processing regions, parahippocampal cortex, and the cerebellum. This constitutes compelling evidence that there are substantial sex differences in brain activity during long-term memory retrieval. More broadly, the present findings question the widespread practice of collapsing across sex in the field of cognitive neuroscience.