The brain is primarily considered a receiver and processor of stimuli rather than a direct stimulus itself. In the context of sensory systems, the brain interprets signals received from sensory receptors (like those in the eyes, ears, skin, etc.) that detect various forms of stimuli (such as light, sound, and touch).
However, the brain can also act as a source of stimuli in some contexts. For example, it can generate thoughts, feelings, or impulses that can lead to actions, effectively becoming a stimulus for behavior. Additionally, through neurochemical processes, it can influence bodily functions in ways that can stimulate responses from other systems in the body.
In summary, while the brain primarily functions as a receiver and processor of stimuli, it can also generate stimuli that affect behavior and bodily functions.