I think there are two ways:
1) Create a variable that changes value depending on your conditions, something like
= If( Only( YourFieldName ) = 'Brain', 1,0)
2) Create a field event trigger OnSelect of YourFieldName, with an action to change the variable (with a similar expression like above).
I think I would go for 1), since it's easier to understand what's happening when checking the variable.