Spontaneous signals in BCIs are generated by?

• A. External stimuli
• B. Random neural noise
• C. Mental tasks
• D. Muscle contractions

Answer: (C)

Spontaneous signals come from internally generated brain activity, not from external events. In BCIs, these endogenous signals are produced by mental tasks such as motor imagery or other cognitive strategies, which modulate ongoing brain rhythms (notably the mu and beta rhythms over the sensorimotor cortex). These changes in brain activity occur without a deliberate external trigger and can be detected by the system to infer the user’s intent. External stimuli produce evoked responses like P300 or SSVEP, which are stimulus-locked rather than spontaneous. Random neural noise isn’t a reliable control signal, and while muscle contractions can generate signals, those originate outside the brain and can contaminate EEG rather than representing spontaneous brain activity. Thus, mental tasks are the source of spontaneous, endogenous signals used in many BCIs.