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The concept that the brain is a system of inputs and outputs in which certain outputs are capable of being generated regardless of the presence of an input  is a notion that well incorporates observations of the brain and behavior and I agree with this model completely. An demonstrative example of this can be seen in Phantom Limb Disorder, a disorder suffered by 50-80% of amputee patients. Upon surviving a serious accident, many people suffering severe pain in one of their limbs undergo amputation of that limb in an effort to alleviate the pain. Often, the pain is not cured and the patient is now left both limbless and suffering the same excruciating pain. Biologically, neurons at the base of the stump that were once in communication with the neurons in the amputated limb begin to send random signals to the brain, and although these signals are nonsense, they are processed in the brain as pain. Treating this as a psychological disorder, one treatment for this that was developed was simply placing a mirror in front of the patient and asking them for example to lift their right arm, if it was their left arm that was amputated. Upon raising their right arm, the patient witnesses their left arm being lifted in the mirror and report that their pain is alleviated. In one situation the patient was even completely cured from ever experiencing pain in their “phantom limb.”

This is a most illustrative example of how the brain works as an input-output system while it is quite possible and actually a regular function of the brain to produce outputs even with the lack of an input at any given time.