Yes, conjunction analysis with 2 contrasts can be done with the following expression in 3dcalc:
-expr 'step(a-a_threshold) + 2*step(b-b_threshold) + 4*step (-a_threshold-a) + 8*step (-b_threshold-b)'
And the resulting map can be interpreted as the following:
0: no activation and no deactivation;
1: activation in condition 1 only;
2: activation in condition 2 only;
3: activation in both conditions;
4: deactivation in condition 1 only;
6: activation in condition 2 and deactivation in condition 1;
8: deactivation in condition 2 only;
9: activation in condition 1 and deactivation in condition 2;
12: deactivation in both conditions.
However if you don't want to differentiate activation and deactivation, the expression can be simplified as:
-expr 'astep(a, a_threshold) + 2*astep(b, b_threshold)'
The definition of step and astep is:
step(x) = {1 if x>0 , 0 if x<=0},
astep(x,y) = {1 if abs(x) > y , 0 otherwise} = step(abs(x)-y)
Gang