Final answer:
The basal ganglia in the midbrain is likely the source responsible for the 'blocking' action through its indirect pathway, which inhibits the thalamus and decreases unintended movements.
Step-by-step explanation:
The midbrain area that is likely the source of this 'blocking' as described in the question is the basal ganglia. The indirect pathway connections within the basal ganglia, which include the striatum, globus pallidus external segment, subthalamic nucleus, to the globus pallidus internal segment/substantia nigra pars compacta, result in the inhibition of the thalamus to decrease cortical control of movement. This pathway is involved in the fine-tuning of motor movements and can prevent unintended movements, which might be referred to as 'blocking'.
Additionally, the Edinger-Westphal nucleus in the midbrain plays a role in the parasympathetic nervous system, affecting eye movements and pupil constriction, but is not typically associated with inhibiting motor control.