Is it possible to calculate the contrast for the same events, but FOR TWO DIFFERENT GROUPS?

I would like to know why this high level function does not exist? On the nilearn website, I came across the following statement - The contrast is between responses to retinotopically distinct vertical versus horizontal checkerboards. At the individual level, these stimuli are sometimes used to map the borders of primary visual areas. At the group level, such a mapping is not possible. Yet, we may observe some significant effects in these areas . Do I understand correctly that this high level function for contrast difference does not exist due to certain theoretical justifications? Or it does not exist simply because no one has implemented it. I apologize if I force you to answer again, my English is bad. **At the group level, such a mapping is not possible **at the group level, the contrast of the intergroups cannot be displayed. Did I understand correctly? If so, why?