A method of obtaining a desired state in (22) a subject comprises measuring one or more physiological parameters of the subject (12), calculating, using the measured parameter(s), a value (38), determining if the calculated value (38) is to be modified, modifying the calculated value, if the determination is that the value is to be modified, and generating an output to the subject, the output being in proportion to the modified value (40). The step of modifying the calculated value, if the determination is that the value is to be modified,—can comprise accessing a modification scheme and modifying the calculated value according, to the modification scheme. One method of modifying the calculated value comprises amplifying the calculated value.