Hi Dina,
If it was [Task_trial] --> [brain region], then this does make sense - increases in Task relate to change in brain region. The reverse would not make sense as that would suggest the brain activity dictated the task (which might make sense for bio-feedback designs, but it doesn't sound like yours is one of these).
if it was the reverse direction, then I'd recommend entering in the Task_trial as an exogenous variable using teh "exogenous = " argument. Here you can indicate which variables (such as Task_trial) should be considered to predict other variables but not be predicted themselves.
The arguments would be:
...
exogenous = "Task_trial",
...
for one variable and like this for >1 variable:
...
exogenous = c("Task_trial1", "Task_trial2")
....
Katie