Abstract:
A response period is determined arbitrarily. A satisfactory detecting ability may not be provided because of an artifact or the like contained in a signal. Therefore, an effective detection method and an effective display method of presenting results of detection must be developed. A series of tasks (stimuli) or a selected measurement signal is used as a reference signal, and a phase difference of any other measurement signal from the reference signal is calculated. The synchronousness of the phase of the measurement signal with the phase of the reference signal is numerically expressed. The thus obtained numerical value is statistically processed in order to numerically express a degree of reliability. Thus, a brain activity or a functional connectivity is visualized.

Description:
CLAIM OF PRIORITY 
     The present invention claims priority from Japanese application JP 2004-201608 filed on Jul. 8, 2004, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of measurement of a brain function disclosed in, for example, Japanese Patent Laid-Open No. 2000-237194. 
     2. Description of the Related Art 
     An example of measurement of a brain function will be described in conjunction with  FIG. 1A  and  FIG. 1B .  FIG. 1A  shows the configuration of a brain function measurement system and the relationship thereof to a subject.  FIG. 1B  shows an example of the distribution of positions S at which a light irradiating means for irradiating the subject&#39;s head with light is located, and positions D at which a receiving optical fiber for receiving light that has been applied to the subject&#39;s head and transmitted thereby is located. 
     The brain function measurement system includes: a plurality of light sources  102   a  to  102   d  that generates light waves having difference wavelengths (the light sources  102   a  and  102   c  generate light having a wavelength of, for example, 780 nm, and the light sources  102   b  and  102   d  generate light having a wavelength of, for example, 830 nm); oscillators  101   a  and  101   b  and oscillators  101   c  and  101   d  that modulate at different frequencies the intensities of light waves emitted from the plurality of light sources  102   a  and  102   b  or light sources  102   c  and  102   d ; a plurality of light irradiating means for irradiating, which a coupler  104   a  produces using the intensity-modulated light waves having propagated along optical fibers  103   a  and  103   b , and light, which a coupler  104   b  produces using the intensity-modulated light waves having propagated along optical fibers  103   c  and  103   d , different positions on the head skin of a subject  106  with light over light irradiating optical fibers  105   a  and  105   b ; and a plurality of pieces of light receiving means composed of a plurality of receiving optical fibers  107   a  to  107   f , which is disposed so that the ends thereof will be located equidistantly from (for example, 30 mm away from) the light-applied positions, that is, the plurality of pieces of light irradiating means, and light receivers  108   a  to  108   f  disposed at the other ends of the optical fibers  107   a  to  107   f.    
     In the example of  FIG. 1A , the three receiving optical fibers (D in the drawing)  107   a  to  107   c  and three receiving optical fibers  107   d  to  107   f  are, as shown in  FIG. 1B , disposed around the light irradiating optical fibers (S in the drawing)  105   a  and  105   b , so that light waves transmitted by a living body will be converged on the optical fibers and detected. The detected light waves transmitted by the living body are photoelectrically converted by the light receivers  108   a  to  108   f . The light receiving means detects light, which is transmitted by the subject&#39;s intracranial regions while being reflected therefrom, and converts the light into an electrical signal. The light receivers  108   a  to  108   f  are realized with photoelectric conversion elements such as photoelectric multipliers or photodiodes. 
     Electrical signals that represent the intensities of light waves transmitted by a living body and that result from photoelectric conversion performed by the light receivers  108   a  to  108   f  (hereinafter, living body-transmitted light intensity signals) are transferred to lock-in amplifiers  109   a  to  109   h . The light receivers  108   c  and  108   d  detect the intensities of living body-transmitted light waves converged on the receiving optical fibers  107   c  and  107   d  that are located equidistantly from the light irradiating optical fibers  105   a  and  105   b  respectively. The signals proportional to the light intensities detected by the light receivers  108   c  and  108   d  are each separated into two portions and transferred to the lock-in amplifiers  109   c  and  109   e  or the lock-in amplifiers  109   d  and  109   f . Signals that are the outputs of the oscillators  101   a  and  10   b  as well as  101   c  and  101   d  modulated in intensity at intensity modulation frequencies are transferred as signals of reference frequencies to the lock-in amplifiers  109   a  to  109   d , and  109   e  to  109   h  respectively. Consequently, the living body-transmitted light intensity signals representing the intensities of light waves emitted from the light sources  102   a  and  102   b  are separated from each other and transmitted from the lock-in amplifiers  109   a  to  109   d . The living body-transmitted light intensity signals representing the intensities of light waves emitted from the light sources  102   c  and  102   d  are separated from each other and transmitted from the lock-in amplifiers  109   e  to  109   h.    
     The transmitted light intensity signals separated from one another in units of a wavelength and transmitted from the lock-in amplifiers  109   a  to  109   h  are analog-to-digital converted by an analog-to-digital converter (hereinafter, an A/D converter)  110 , and then transferred to a measurement control computer  111 . The measurement control computer  111  uses each of the transmitted-light intensity signals, that is, detection signals produced at detected positions to thus arithmetically or logically calculate relative changes in an oxyhemoglobin concentration, a deoxy-hemoglobin concentration, and a total hemoglobin concentration. The relative changes are stored as time-sequential information on each of the measured positions in a storage device included in the computer  111 . Herein, the change in the total hemoglobin concentration is calculated as the sum of the changes in the oxyhemoglobin concentration and deoxy-hemoglobin concentration. 
     On the other hand, in order to measure a brain function of a subject, a predetermined stimulus or task is applied to the subject and the subject&#39;s response to the stimulus or task is assessed. A centralized control/data processing/result display computer  114  issues a command to the measurement control computer  111 . The measurement control computer  111  in turn uses a stimulus/task command presentation device  113  to apply a stimulus/task instruction to the subject according to a prepared stimulus/task instruction sequence. A response to the stimulus/task instruction made by the subject&#39;s brain is optically measured as described above. The centralized control/data processing/results display computer  114  and the measurement control computer  111  communicate required information to each other. 
     Conventionally, in order to assess a subject&#39;s response to a stimulus or task, the significance of a signal representing an average response obtained as a result of repetitive measurements is tested based on the amplitude of the signal. A significantly active area is then identified (refer to Japanese Unexamined Patent Application Publication No. 2000-237194). 
     SUMMARY OF THE INVENTION 
     A response period is determined arbitrarily. Moreover, a satisfactory detecting capability may not be provided because of an artifact or the like contained in a signal. Therefore, an effective detection method capable of accurately assessing a subject&#39;s response to a stimulus or task and a display method for presenting the results of detection are requested to be developed. 
     The present invention is based on the principle that activities of the same source are in phase with one another. For assessment of a subject&#39;s response, an active area is detected by checking if the phase of a stimulus or task applied to the subject is synchronous with the phase of the subject&#39;s response. The synchronousness is analyzed by checking the amplitudes and phases of both the signals.  FIG. 2A  and FIG.  2 B are schematic explanatory diagrams concerning the synchronousness of a signal with a task.  FIG. 2A  shows signal examples A and B resulting from application of a task to a subject during a task period that comes cyclically and alternately with a rest period. The signal example A is synchronous with the task in both the amplitude and phase thereof. The signal example B is synchronous with the task in the phase thereof but asynchronous therewith in the amplitude thereof.  FIG. 2B  is a table listing whether the amplitudes and phases of the signal examples are synchronous with the amplitude and phase of the task. 
     The present invention takes account of the fact that the synchronousness of the amplitude of a signal with that of a task is unacceptable as a condition under which a subject&#39;s response is assessed. Namely, a signal representing a stimulus or task applied to a subject is used as a reference signal, and a phase difference of a measurement signal representing the subject&#39;s response from the reference signal is calculated. The synchronousness of the phase of the measurement signal with the reference signal is numerically expressed. The numerical value is statistically processed in order to thus numerically express a degree of reliability. A brain activity or a functional connectivity is visualized based on significant data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows the configuration of a brain function measurement system and the relationship of the system to a subject; 
         FIG. 1B  shows an example of the distribution of positions S at which a light irradiating means for irradiating a subject&#39;s head with light is located, and positions D at which a receiving optical fiber for receiving light applied to the subject&#39;s head and transmitted thereby is located; 
         FIG. 2A  and  FIG. 2B  are simple explanatory diagrams concerning the synchronousness of a signal with a task; 
         FIG. 2A  shows signal examples A and B produced responsively to a task applied during a task period that comes cyclically and alternately with a rest period; 
         FIG. 2B  is a table listing whether the amplitudes and phases of the signal examples A and B are synchronous with the task; 
         FIG. 3  is a block diagram showing the configuration of a living body measurement system in accordance with the first embodiment; 
         FIG. 4A  to  FIG. 4F  are explanatory diagrams concerning an example of the results of analysis of the synchronousness of the phase of a signal performed according to the first embodiment; 
         FIG. 4A  shows an example of the structure of a probe; 
         FIG. 4B  graphically shows the results of assessment of the synchronousness of the phase of a signal on the basis of the results of measurement of a change in a total hemoglobin content; 
         FIG. 4C  shows the results shown in  FIG. 4B  and displayed in a different form; 
         FIG. 4D  shows, for comparison, the results of a test, which is performed as conventionally on the same results of measurement in order to check the synchronousness of the amplitude of a signal produced during stimulation, displayed in the same form as  FIG. 4C ; 
         FIG. 4E  shows an example of a display form in which the results shown in  FIG. 4B  and  FIG. 4C  are presented to a user in an easier-to-understand manner; 
         FIG. 4F  shows an example of an image displayed in order to present the results shown in  FIG. 4D  in the same manner as the results shown in  FIG. 4C ; 
         FIG. 5A  illustratively shows an example of the structures of probes included in the second embodiment and the arrangement thereof; 
         FIG. 5B  shows the results shown in  FIG. 5A  in the form of an image expressing the head, which is visualized, seen from above similarly to the display form shown in  FIG. 4E ; 
         FIG. 6  shows measurement channels in which signals which are produced responsively to hearing of a played-back signal serving as a standard signal and whose phases are highly synchronous with the phase of the standard signal; 
         FIG. 7A  and  FIG. 7B  shows the results of assessment of the synchronousness of the phase of a signal, which is produced responsively to hearing of a played-back signal, with the phase of the standard signal with a significance level set to 1%; 
         FIG. 8  shows the quickness levels of responses to signals, which are produced responsively to hearing of a played-back signal serving as a standard signal in the measurement channels shown in  FIG. 6  and of which phases are highly synchronous with the phase of the signal produced. 
         FIG. 9  shows information on the quickness levels of responses superposed on the results of assessment, which are shown in  FIG. 7A , of the synchronousness of the phase of a signal, which is produced responsively to hearing of a played-back signal serving as a standard signal, with the phase of the standard signal with a significance level set to 1%; and 
         FIG. 10  shows examples of images displayed in order to dynamically visualize the brain activities of a subject. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With consideration taken into the fact that when a signal produced responsively to a stimulus or task is synchronous with a signal representing the stimulus or task, the phase difference between the signals is fixed, how long the phase difference is fixed is discussed using actual data. 
     For calculation of a phase difference, the Hilbert transformation is used to obtain instantaneous phases of signals. A phase difference observed at each time instant is calculated from the instantaneous phases. 
     The Hilbert transformation will be described. A Hilbert transform g(t) of a real variable function f(t) and an inverse Hilbert transform thereof are provided as formulae (1) and (2) below. 
                     g   ⁡     (   t   )       =         f   ⁡     (   t   )       ⊗     1     π   ⁢           ⁢   t         =       1   π     ⁢       ∫     -   ∞     ∞     ⁢         f   ⁡     (   τ   )         t   -   τ       ⁢           ⁢     ⅆ   τ                     (   1   )                 f   ⁡     (   t   )       =         g   ⁡     (   t   )       ⊗     1     π   ⁢           ⁢   t         =         -   1     π     ⁢       ∫     -   ∞     ∞     ⁢         g   ⁡     (   τ   )         t   -   τ       ⁢           ⁢     ⅆ   τ                     (   2   )               
where             denotes convolution.

     A measurement signal is regarded as the real variable function f(t), and an analysis signal Z(t) is defined as follows:
 
 Z ( t )=ƒ( t )+ jg ( t )  (3)
 
     When the formula (3) is defined in the system of polar coordinates, the following formula (4) is drawn out:
 
 Z ( t )= r ( t ) e   jθ(t)   (4)
 
     By rewriting the formula (4), the following formulae (5) and (6) are drawn out: 
                     r   ⁡     (   t   )       =           f   ⁡     (   t   )       2     +       g   ⁡     (   t   )       2                 (   5   )                 θ   ⁡     (   t   )       =       tan     -   1       ⁢       g   ⁡     (   t   )         f   ⁡     (   t   )                   (   6   )               
where r(t) denotes the instantaneous amplitude of the measurement signal f(t), and θ(t) denotes the instantaneous phase thereof.
 
     According to an exemplary algorithm, the analysis signal Z(t) is obtained as a one-side Fourier transform of the measurement signal f(t). Namely, negative-frequency components of a signal are assigned 0. The measurement signal f(t) is fast-Fourier-transformed in order to approximate the analysis signal to the measurement signal. Coefficients expressing fast-Fourier-transformed negative-frequency components are replaced with zeros. The results are inverse-fast-Fourier-transformed in order to obtain the analysis signal Z(t). 
     To be more specific, an algorithm having four steps described below is employed. The number of input data items shall be n. 
     At the first step, input data is fast-Fourier-transformed, and the resultant data is represented with a vector y. 
     At the second step, a vector h causing h(i) to assume a value described below is produced. Namely, when i assumes 1 or (n/2)+1, h(i) will be 1. When i assumes 2, 3, etc., or (n/2), h(i) will be 2. When i assumes (n/2)+2, etc., or n, h(i) will be 0. 
     At the third step, the product of the vectors y and h is calculated for each h(i). 
     At the fourth step, an inverse fast-Fourier transform of a data stream calculated at the third step is worked out. The first n elements of the resultant data are provided as the analysis signal Z(t). 
     Statistical discussion is made in order to objectively verify how long a phase difference is fixed. The distribution of phase differences is expressed in the form of a histogram having Nb bins and covering a range from −π to π. A synchronization index (SI) is adopted as a statistical index and defined as a formula (7) below. 
                   SI   =         S   random     -   S       S   random               (   7   )               
By rewriting the formula (7), formulae (8) and (9) are drawn out as follows:
 
                   S   =     -       ∑     i   =   1     Nb     ⁢       p   i     ⁢     log   2     ⁢     p   i                   (   8   )                 S   random     =       log   2     ⁢   Nb             (   9   )               
where pi denotes a probability density function of a random variable represented by the i-th bin.
 
     In the formula (8), pi denotes the probability of a phase difference represented by the i-th bin. 
     If the distribution of phase differences is fully uniform, that is, if the phase of a signal is not at all synchronous, S=S random  becomes true. SI is therefore 0. If the phase of a signal is fully synchronous, SI is 1. Since actual measurement data contains various kinds of noises, a clear-cut result such as SI=1 or 0 is very rare. If SI assumes an intermediate value, whether the phase of a signal is synchronous is verified statistically. 
     As a technique of verifying whether the phase of a signal is synchronous, a surrogate data method is employed. The surrogate data method is a framework for performing a test of hypothesis described below. 
     1. Numerous data items having an accurate statistical property are produced from original data (using random numbers). 
     2. An index to be assigned to random data is calculated. 
     3. The index assigned to the original data is tested based on sample values selected from among numerous indices. 
     As surrogate data, data subjected to the same filtering as a signal concerned during random sampling is adopted. If the distribution of synchronization indices (SI), that is, statistical indices calculated from 50 surrogate data items is a normal distribution (average: 0.2236, standard deviation: 0.0219), a synchronization index larger than 0.2800 may be adopted as a threshold for verification at a significance level of 1%. Furthermore, in order to discuss a time-sequential change in a synchronous state, a short time window (covering several hundreds of data items) may be designated. In this case, a synchronization index serving as a statistical index is calculated during the period, and a synchronization index calculated at a center time instant within the time window is adopted as a representative synchronization index. The time window is shifted in units of a certain time in order to calculate a time-sequential change in the synchronization index. 
     First Embodiment 
       FIG. 3  is a block diagram showing the configuration of a living body measurement system in accordance with the first embodiment. The living body measurement system  210  corresponds to a measuring facility of the measurement control computer  111  shown in  FIG. 1 . The living body measurement system includes an interface  201  that provides the interface with the A/D converter  110 , a CPU  202  that performs a series of actions, a storage unit  203  in which programs and data are stored, an interface  204  that provides the interface with external equipment  205 , and a bus  206  over which these components are interconnected. Moreover, a display means  211 , a keyboard  212 , and a pointing device (for example, a mouse)  213  are connected on the bus, and used to present the results of analysis to an operator of the measurement control computer  111 . Moreover, the operator uses the display means  211 , keyboard  212 , and pointing device to enter data. Herein, the programs stored in the storage unit  203  include a filtering program that performs required filtering on a signal, a phase synchronousness analyzing program for analyzing the synchronousness of the phase of each of a group of filtered signals or of an extraneous signal, an activity detecting program for detecting a neural activity from the results of analysis performed on the synchronousness of a phase, and an imaging program for presenting a user the detected neural activity in an easy-to-understand manner. The results of detection are displayed on the display means  211 . The programs preserved in the storage unit  203  are interpreted and run by a central processing unit  202 . 
       FIG. 4A  to  FIG. 4F  are explanatory diagrams showing examples of the results of analysis performed on the synchronousness of a phase according to the first embodiment. 
       FIG. 4A  shows an example of the structure of a probe. The probe is placed on a subject&#39;s head, and has a light irradiating optical fiber S and a receiving optical fiber D arranged alternately so that measurement channels  1  to  24  will be formed among the light irradiating optical fibers S and receiving optical fibers D. Thus, the probe measures a change in a total hemoglobin content caused by a visual stimulus. The numbers of light irradiating optical fibers S and receiving optical fibers D included in the probe can be determined arbitrarily in consideration of a position at which the probe is placed or the size of the subject&#39;s head. The visual stimulus to be applied to the subject is a checkerboard having a size of 16 by 16 and having red and black squares. A frequency at which the red and black squares are switched is set to 8 Hz. A rest period is set to 20 sec, and a stimulus (task) period is set to 18 sec. This test is repeated six times. 
       FIG. 4B  shows the results of assessment on the synchronousness of a phase performed by measuring a change in a total hemoglobin content. The axis of abscissas indicates channel numbers, and the axis of ordinates indicates synchronization indices detected in the respective channels. An average of six synchronization indices obtained by repeating the test six times is indicated with a circle. Moreover, a broken line is drawn in order to indicate a statistical reference value (at a significance level of 1%). Consequently, in this example, a signal that can be assessed to be synchronous with the stimulus of the checkerboard is detected in channels  1 ,  2 ,  4 ,  5 ,  6 ,  8 , and  10 . 
       FIG. 4C  shows the results shown in  FIG. 4B  in a different manner. The channel numbers of the channels  1 ,  2 ,  4 ,  5 ,  6 ,  8 , and  10  in which a signal assessed to be significant at the significance level of 1% is produced are displayed, and squares indicating channels in which an insignificant signal is produced are filled with black dots. The positions of the measuring fibers are indicated with blank squares.  FIG. 4C  demonstrates that the brain activity is observed at a position on the head associated with the upper part of the probe. 
       FIG. 4D  shows in the same manner as  FIG. 4C , for the purpose of comparison, the results of a test of hypothesis performed based on the synchronous of the amplitude of a signal that is assessed during stimulation as conventionally by measuring a change in a total hemoglobin content. In the test of hypothesis based on the synchronousness of the amplitude of a signal, a brain activity cannot be detected with a significance level set to 1%. The significance level is therefore set to 5%. Compared with  FIG. 4C ,  FIG. 4D  demonstrates that the brain activity is observed at a position on the head associated with the majority of the probe. This signifies that the present invention exhibits higher sensitivity and detects the localized brain activity. 
       FIG. 4E  shows an example of a display form in which the results of  FIG. 4B  or  FIG. 4C  are presented to a user in an easier-to-understand manner. Significance levels at which the results of detection are assessed are varied stepwise, and the results of assessment made at the significance levels are associated with contour lines. Namely, in the first embodiment of the present invention, the significance levels include levels 0.001, 0.005, 0.01, and 0.05. Active areas hypothetically detected at the significance levels are indicated with the contour lines. The results of detection shown in  FIG. 4C  are expressed with the contour line associated with the significance level of 0.01. If a well-known imaging program is used to display the contour lines with shades of a certain color, the active areas would be presented to a user in a well-visualized manner. 
       FIG. 4F  shows an example of a display form in which the results of  FIG. 4D  comparative with  FIG. 4C  are presented in the easier-to-understand manner. As described previously, the conventional test of hypothesis based on the synchronousness of the amplitude of a signal cannot detect an active area at all with a significance level set to 1%.  FIG. 4F  shows the results of detection performed at significance levels of 2% and 5%. The number of contour lines is only two, and the display is therefore coarse-grained. 
     Second Embodiment 
     The second embodiment is concerned with detection of an activity performed by asking an examinee to listen to the played-back commentary on a baseball game as a stimulus/task instruction sequence and to imagine standing on a field as a batter.  FIG. 5A  illustratively shows an example of the structures of probes employed in the second embodiment and the arrangement of the probes, and also shows the results of assessment of the synchronousness of the phase of a signal. An oval drawing expresses the head seen from above, and a triangle expresses the nose seen from above. Probes  1 ,  2 ,  3 , and  4  are arranged on the frontal region of the head, the right and left temporal regions thereof, and the occipital region thereof. The probes  1 ,  2 , and  3  are structured so that twenty-two measurement channels will be formed in each probe. The probe  4  is structured so that twenty-four measurement channels will be formed therein. 
       FIG. 5A  illustratively shows an example of the structures of the probes employed in the second embodiment and the arrangement thereof, and shows the results of assessment of the synchronousness of the phase of a signal. 
       FIG. 5B  shows, similarly to  FIG. 4E , the results shown in  FIG. 5A  in a well-visualized manner together with a drawing expressing the head seen from above. As apparent from  FIG. 5B , an activity stemming from hearing of the played-back commentary is observed in the right and left infratemporal regions (auditory fields). An activity stemming from imagination of batting is observed in the supra-occipital region (optical field). An activity stemming from preparation for modeling a motion is observed in the frontal region. An activity stemming from modeling of a motion is observed in the right and left supratemporal regions (motor fields). 
     In the foregoing example, an active area can be accurately detected, and a subject&#39;s brain activity can be assessed further exactly. 
     Third Embodiment 
     According to the second embodiment, a subject&#39;s brain activity is detected by adopting a played-back signal, which is heard, as a standard signal. The levels of synchronousness of the phases of signals may be assessed, and a signal which is produced in a channel and whose phase is highly synchronous with the phase of the standard signal may be adopted as a new standard signal. In this case, an active brain area can be detected further exactly. 
     According to the third embodiment, first, a measurement channel in which a signal which is produced responsively to hearing of a played-back signal and whose phase is highly synchronous with the phase of the played-back signal serving as a standard signal is produced is detected.  FIG. 6  shows detected measurement channels in which a signal which is produced responsively to hearing of the played-back signal and whose phase is highly synchronous with the phase of the standard signal is produced. In this example, a signal that is most highly synchronous with the standard signal is produced in the channel  10  in a probe  2 . 
     The signal in the channel  10  in the probe  2  is regarded as a new standard signal. The synchronousness of the phase of the signal produced responsively to the hearing of the played-back signal is assessed using the new standard signal.  FIG. 7A  and  FIG. 7B  show the results of assessment of the synchronousness of the phase of a signal, which is produced responsively to hearing of a played-back signal, with the standard signal, wherein the assessment is made with a significance level set to 1%. Compared with  FIG. 5A , although the same results of measurement are used for assessment, since a signal in a measurement channel whose phase is highly synchronous with the standard signal that is the signal produced responsively to hearing of the played-back signal is adopted as a practical standard signal, a localized active area can be detected. When  FIG. 7A  is compared with  FIG. 5A , the phase of a signal in the channel  13  in the probe  1  is assessed to be asynchronous with the standard signal. The phase of a signal in the channel  15  in the probe  2  is assessed to be synchronous therewith, and the phase of a signal in the channel  18  therein is assessed to be a synchronous therewith. The phases of signals in the channels  4  and  14  in the probe  3  are assessed to be asynchronous therewith, and the phase of a signal in the channel  11  therein is assessed to be synchronous therewith. The phases of signals in the channels  13  and  16  in the probe  4  are assessed to be asynchronous therewith. Consequently, visualized information shown in  FIG. 7B  makes it possible to recognize the activities in more limited areas than the visualized information shown in  FIG. 5B . 
     Fourth Embodiment 
     A brain activity does not always take place in the whole of the brain. The brain activity in a certain area in the brain triggers a brain activity in any other area. Thus, the brain activity is known to be a time-sequential action. According to the third embodiment, a test of hypothesis was performed in order to detect the time-sequential brain activity by assessing the results of measurement performed according to the second embodiment. 
       FIG. 8  shows the quickness levels of the responses made by signals, which are produced responsively to a played-back signal serving as a standard signal and of which phases are highly synchronous with that of the standard signal, in the measurement channels shown in  FIG. 6  to the hearing of the played-back signal. There is difficulty in plotting the waveform of the played-back signal serving as the standard signal or a stimulus/task instruction sequence as well as the waveforms of responsive signals. The responsive signals are illustratively shown in relation to the stimulus/task instruction sequence described in conjunction with  FIG. 2 . The axis of abscissas indicates time, and the axis of ordinates indicates the magnitudes of signals.  FIG. 9  shows the information on the quickness levels of the responses superposed on the results of assessment, which are shown in  FIG. 7 , of the synchronousness of the phase of a signal, which is produced responsively to hearing of a played-back signal, made with a significance level set to 1%. 
     Referring to  FIG. 8 , the signal in the channel  10  in the probe  2  is the quickest to respond. In  FIG. 9 , a square expressing the channel  10  in the probe  2  is filled with dots. The signals in the channel  10  in the probe  2  and in the channel  9  in the probe  3  are the next quickest to respond. Namely, the signals in the areas S 1  in the probes  2  and  3  covering the channels  5  and  9  are the quickest to respond. The signals in the channels  6  and  10  in the probe  1  are the next quickest to respond. Namely, the signals in the area S 2  in the probe  1  are the second quickest to respond. The signals in the channels  4  and  5  in the probe  4  are the next quickest to respond. Namely, the signals in the area S 3  in the probe  4  are the third quickest to respond. Finally, the signals in the channels  5  and  11  in the probe  3  and in the channel  12  in the probe  2  are the fourth quickest to respond. Namely, the signals in the areas S 4  in the probes  2  and  3  are the fourth quickest to respond. 
     Consequently, the activities in the right and left infratemporal regions (auditory fields) stemming from hearing of a played-back signal are detected in the area S 1  in  FIG. 9 . The activity in the frontal region stemming from preparation for modeling a motion is then detected in the area S 2  in  FIG. 9 . Thereafter, the activity in the supra-occipital region (optical field) stemming from imagination of batting is detected in the area S 3  in  FIG. 9 . Finally, the activities in the right and left supratemporal regions (motor fields) stemming from modeling of a motion are detected in the areas S 4  in  FIG. 9 . 
     As apparent from  FIG. 8 , when it says that a signal is quick to respond, it means a matter of comparison. The response time is merely on the order of several seconds. Nevertheless, when signals are divided into groups by quickness of a response, if places where the signals are detected are visibly presented to a user, the brain activities of a subject can be dynamically visualized. 
       FIG. 10  shows an example of a display form in which the brain activities of a subject are dynamically visualized. An image S 1  shows the areas S 1  in  FIG. 9  and contains dotted circles rendering the active areas in the right and left infratemporal regions (auditory fields). The area and shape of the circle are determined with the position of the measurement channel in which a signal whose phase is highly synchronous with a played-back signal is produced, and the number of circles depends on the number of measurement channels. The same applies to the subsequent description. An image S 2  shows the area S 2  in  FIG. 9 , and contains a dotted circle rendering the active area in the frontal region of which activity stems from preparation of modeling of a motion. An image S 3  shows the area S 3  in  FIG. 9 , and contains a dotted circle rendering an active area in the supra-occipital region (optical field) of which activity stems from imagination of batting. An image S 4  shows the areas S 4  in  FIG. 9 , and contains dotted circles rendering the active areas in the right and left supratemporal regions (motor fields) of which activities stem from modeling of a motion. A whole image is produced by integrating the images S 1  to S 4  into one, and is identical to  FIG. 7B . 
     If the images S 1  to S 4  are sequentially and orderly displayed on the display means  211  as a motion picture that makes progress as quickly as a user can see it with ease, the user would feel as if he/she saw the subject&#39;s brain active areas being switched in real time. After the images are displayed like a motion picture, the whole image may be displayed so that the overall brain activities can be assessed. 
     The immediateness of an activity is verified based on an average phase difference observed in each region. Namely, the smaller the phase difference is, the more immediately the activity takes place. 
     Fifth Embodiment 
     In order to discuss a time-sequential change in a brain activity using a motion picture, as described previously, a time-sequential change in a synchronous state is checked by defining a short time window (covering several hundreds of data items). A synchronization index serving as a statistical index is calculated during the period, and a synchronization index at the center time instant within the time window is adopted as a representative synchronization index. The time window is then shifted in units of a certain time in order to detect a time-sequential change in the synchronization index. Based on the time-sequential change in the synchronization index, the synchronousness of the phase of a signal, which is produced in a measurement area, with a stimulus is assessed. The results of assessment are displayed as a time-sequential change in the display form shown in, for example,  FIG. 7B . According to this technique, a change in a phase difference can be sensitively grasped. The plurality of functions of the brain can be presented by accurately visualizing the time-varying relationships among the activities in the regions of the brain. 
     The synchronousness of the phase of a signal is independent of the amplitude thereof. Therefore, the brain activities or the functional connectivity can be presented in an appropriate manner irrespective of the human cranial structure. 
     The present invention will prove helpful in assessing the impairment in a brain function or in assessing the recovering state of a person whose brain function is impaired. Moreover, the present invention can be utilized for helping a patient with rehabilitation.