Mental activity of a human subject is indicated with electrodes placed on the head of the subject to respond to brain activity. The electrodes derive a signal including low and high frequency a.c. components respectively having frequency ranges in the same low frequency range as alpha waves derived by the brain and considerably higher than the highest frequency of the alpha waves. The high and low frequency amplitude components are supplied to an indicator arranged so the mental state activity is indicated only in response to the detected high frequency component having an amplitude less than a level indicative of the subject not being in a state of relaxation and the detected low frequency component having an amplitude greater than a first predetermined value and less than a second predetermined value. The low frequency amplitude between the first and second predetermined values is associated with the alpha waves.