this counting operation ceases when the count value in a period counter 205 reaches a predetermined value resetting the flip flop 203 the digital divider 204 divides the output of the counter 80 by the output of the counter 201 the resultant division ratio being indicated by the indicator 90 a further modification of the present invention is shown in fig6 in which the counter 80 is cleared each time the user enters the state of relaxation this allows the user to gauge the depth of each sustained condition to improve his method of meditation in this embodiment the minimum and maximum count values are visually indicated to permit comparison as illustrated in fig6 the counter 80 is cleared by an output of a monostable multivibrator 82 which is generated each time it detects a leading edge transition of the output of the window comparator 50 to renew the contents of the counter 80 with a subsequent comparator output the output of the counter 80 is directly indicated by a visual indicator 83 further included are a minimum detector 300 and a maximum detector 400 which are responsive to the successively updated binary output of the counter 80 the minimum detector 300 comprises a pair of shift registers 301 and 302 a digital comparator 303 having first and second inputs respectively responsive to the outputs of the registers 301 and 302 the register 301 has an input.