Hi Cerouno,
The beta from a modulator is the incremental effect
of that modulator. A negative beta does not mean
there was a negative response, but that the response
fluctuates negatively with respect to the modulator,
i.e. when the modulator value is bigger, the BOLD
response is smaller (not necessarily negative).
Even a single voxel could easily have a positive
beta for one modulator, while having a negative beta
for the other.
- rick