Biofeedback System

The present invention relates to a biofeedback system which is used for indicating the state of mental activity of a human subject.

It is well known that human mental activity can be measured in terms of (1) electrical activity of the brain as represented by the brain waves, (2) electrical potential changes measured by electromyographic method, (3) potential changes at the surface of the skin or (4) temperature at the skin surface. It is also known that these physiological phenomena can be used to control the autonomic nerve system of the human subject or used as a means for rehabilitation. More specifically, when a human subject is in a relaxed state of mind, the predominant brain waves are the so-called alpha waves and the muscle potential falls to a low level, while there are increases in the electrical resistance and temperature of the skin. Conversely, when the subject is in a nervous, strained or highly excited state, the beta waves are predominant, the muscle potential becomes high and there are decreases in the electrical resistance as well as temperature of the skin.

Thus, an indication of such physiological signals can serve as a guide to lead the human subject into a relaxed state of mind. This kind of mind control is called "biofeedback" and apparatus used for such purposes is part of a biofeedback system.

However, because the presence of a single physiological signal is not necessarily a true indication of relaxation it is desired that there be as many sources of physiological clues as possible. One approach to this problem seemingly is to combine a plurality of specific measurement devices in a single unit for simultaneous indication of the measured data. This approach however requires the user constantly to scan his eyes over the displayed data. Because of this factor the use of plural measurement devices appears to be unsatisfactory as a guide for mind control purposes.

A solution to the aforesaid problem is obtained by generating an inhibit signal when a desired state of mind is not yet achieved and nullifying a main physiological signal in response to the inhibit signal even if the main signal gives a strong indication of the presence of a desired state of mind. The main physiological signal may comprise the alpha and/or theta brain wave, and the inhibit signal is derived from another source. In a preferred embodiment, the inhibit signal is derived from a brain wave having a higher frequency than the frequency of the alpha wave when the amplitude of the brain wave is greater than a predetermined value.

The invention will be further described by way of example with reference to the sole FIGURE which is a schematic illustration of the biofeedback system of the invention.

Referring now to the sole FIGURE, active electrodes 101 and 102 are placed on the scalp of a human subject near the sources of electrical activity. Inactive electrode 103 is placed on the earlobe which is relatively distant from the active electrodes. The active electrodes 101 and 102 pick up electrical activity of the brain close to the active electrodes. Electrodes 101 and 102 respond to the brain electrical activity to derive signals that are coupled to the noninverting inputs of operational amplifiers 11 and 12 which form part of a differential amplifier stage 10. The inactive electrode 103 serves as a reference electrode that responds to undesirable electrical noise components to derive a signal which is coupled to the inverting inputs of the operational amplifiers 11, 12. The outputs of the amplifiers 11 and 12 are connected to the noninverting and inverting input terminals of a third operational amplifier 13, respectively, so that the reference signal from the inactive electrode 103 is cancelled in the third amplifier 13. Thereby, the output of the differential amplifier 10 is representative of the difference in amplitude between the signals from the active electrodes 101 and 102. In this way, undesirable noise components which may be introduced into the signals picked up by the active electrodes are reduced to a minimum.

The output signal from the differential amplifier stage 10 is applied to bandpass filters 20 and 21. The bandpass filter 20 has a passband for frequencies from 8 to 14 Hz to detect alpha brain waves. Bandpass filter 21 has a passband for frequencies from about 80 Hz to 1000 Hz to pick up artifact signals contained in the brain waves of higher frequencies which are normally present when the human subject is not in the state of relaxation. The artifact signals are various waves which arise from sources other than the brain.

The signal from the filter 20, which is conveniently called the signal A, is applied to a rectifier 30 to generate unipolar pulses which are applied to an integrator 40. The signal from the filter 21, which is conveniently called the signal B, is applied to a rectifier 31 to generate unipolar pulses as an input to an integrator 41. The time constant value of the integrator 40 is greater than that of the integrator 41; and the typical values of the time constants for integrators 40 and 41 are 1 second and 0.1 seconds, respectively.

The DC signal from the integrator 40 is applied to a level detector 50 which detects the presence of alpha brain waves by comparing the input DC level derived by the integrator with lower and upper voltage levels corresponding respectively to output levels of 10 and 100 microvolts of the active electrodes. When the input DC level falls within this range between the lower and upper voltage levels, the level detector 50 generates a control signal for a gate circuit 60 to enable the output signal from the integrator 40 to pass through gate 60 to an inhibit gate 61.

The DC signal from the integrator 41, on the other hand, is applied to another level detector 51 which generates an inhibit signal signifying the absence of the state of relaxation when the input signal to the detector exceeds a reference level corresponding to the 100-microvolt level of the active electrodes. This inhibit signal is applied to the inhibit gate 61 to prevent the passage of signals from the gate 60 to a modulator 80. This modulator provides amplitude or frequency modulation of a sound program signal, such as musical note, from a sound source 70. The modulated output signal is applied to a loudspeaker 90 which therefore derives an aural signal having a variable intensity or pitch to indicate the presence of brain activity.

During the early stages of an attempt by a human subject attempting to relax the electrical activity of the brain generates relatively high amplitude brain waves of a frequency spectrum which is in the passband of the bandpass filter 21. The resulting outputs of filter 21, rectifier 31 and integrator 41 cause the level detector 51 to produce an inhibit signal so that the audio signal from the source 70 is not modulated, whereby no control signal is applied by gate 61 to modulator 81 and loudspeaker 90 is energized with the unmodulated signal from source 70 to indicate that the subject is not in a relaxed state. When the person enters the state of relaxation alpha brain waves are generated and detected by level detector 50 which enables the gate 60 to pass the signal from the integrator 40 to the inhibit gate 61 and thence to the modulator 80. The sound signal from source 70 is thus amplitude or frequency modulated by the integrated alpha waves derived from detector 50 and the subject is given an indication by the modulated sound from speaker 90 that he is in the relaxed state.

Even if the signal from the integrator 40 has an amplitude falling within the range of detector 50, the state of relaxation may not be reached. In this case the signal from the integrator 41 is of a relatively large amplitude to prevent the inhibit gate 61 from passing to the modulator 80 the signals which have been band-pass filtered and range-detected, but which unreliably indicate relaxation. Thereby a constant tone is derived from speaker 90 to provide an indication that the state of relaxation is not yet reached.

In a preferred embodiment, visual indicators 52 and 53 are connected to the outputs of the level detectors 50 and 51, respectively, to indicate the presence and absence of the signals A and B respectively representing the presence and absence of relaxed state of mind. These visual indications serve to help the human subject to reach the desired state of mind until the audible indication is given by the loudspeaker 90.

The foregoing description is directed only to the preferred embodiment of the invention. Various modifications are apparent to those skilled in the art without departing from the scope of the invention which is only limited by the appended claims. For example, the bandpass filter 20 may have passband frequencies which correspond to the theta brain waves to employ the latter brain waves as a main physiological signal indicating the presence of a relaxed state of mind. Furthermore, the physiological signals may also be obtained from other sources of electrical activities of a human body such as muscle potentials measured by the electromyographic method, skin potentials and temperatures; these signals may also constitute the main physiological signal used to indicate the presence of relaxed state of mind. If muscle potentials, or signal A, are used as a main physiological signal skin potentials are preferred to derive the inhibit signal. If skin potentials were used to derive the main physiological signal, the skin temperature signal is preferred to derive the inhibit signal, and vice versa.