Attempts are also been made on using the EEG for monitoring of anti-nociception. This approach has been challenged as difficult, as most of the antinociceptive drugs effects are in the periphery, the medullary cord or deeper cerebral layers, far from the EEG signals derived from the frontal cortex. However, EEG is a 'mirror' of what is going on in other parts of the CNS and peripheral nervous system. One problem is to elucidate how the EEG signals may be used in a sensitive and specific way to reflect anti-nociception. Concepts such as response-entropy, Composite Variability Index and BIS variability score have been tested and launched.