that it is not possible to tickle ourselves because when the predicted sensory feedback (efference copy) matches the actual sensory feedback, the actual feedback will be attenuated. If the predicted sensory feedback does not match the actual sensory feedback, whether caused by a delay (as in the mediation by the robotic arm) or by external influences from the environment, the brain cannot predict the tickling motion on the body and a more intense tickling sensation is perceived. This is the reason why one cannot tickle oneself. Speech It has been argued that motor efference copies play an important role in speech production. Tian and Poeppel propose that a motor efference copy is used to produce a forward model of somatosensory estimation, which entails an estimation of the articulatory movement and position of the articulators as a result of planned motor action. A second (subsequent) auditory efference copy entails the estimation of auditory information as produced by the articulatory system in a second forward model. Both of these forward models can produce respective predictions and corollary discharge, which can in turn be used in comparisons with somatosensory and auditory feedback. Moreover, this system is thought by some to be the basis for inner speech, especially in relation to auditory verbal hallucinations. In the case of inner speech, the efference signal is not sent or is inhibited before action takes place, leaving only the efference copy and leading to the perception of inner speech or inner hearing. In the case of auditory verbal hallucinations, it is thought that a breakdown along the efference copy and forward model route creates a mismatch between what is expected and what is observed, leading to the experience that speech is not produced by oneself. Recent studies suggest that efference copy already occurs when an acoustic signal is generated at the press of a button. The differences in the ERP signal of the efference copy are so severe that machine learning algorithms can distinguish between schizophrenia patients and healthy control subjects, for example. Efference copies also occur not only with spoken words, but with inner language - the quiet production of words. Mormyrid electric fish The mormyrid electric fish provides an example of corollary discharge in lower vertebrates. Specifically, the knollenorgan sensor (KS) is involved with electro-communication, detecting the electric organ discharges (EOD) of other fish. Unless the reafference was somehow modulated, the KS would also detect