the invention will be described by way of embodiments with reference made to the drawings . [ 0029 ] fig1 shows a block diagram of a measurement system for living bodies according an embodiment of the invention . as shown , the system includes a measurement unit 11 , a signal extraction unit 12 , a stimulus / instruction timing determination unit 13 , a stimulus / instruction presentation control unit 14 , and a stimulus / instruction presentation unit 15 . the measurement unit 11 measures the internal state of a subject body 10 . the signal extraction unit 12 extracts a certain frequency band component ( biological fluctuation component ) of a signal obtained by the measurement unit 11 . the stimulus / instruction timing determination unit 13 determines the timing of presentation of a stimulus or instruction to the subject body 10 , based on the signal component extracted by the signal extraction unit 12 . the stimulus / instruction presentation control unit 14 controls the presentation of a stimulus or instruction such that it is presented at the time determined by the stimulus / instruction timing determination unit 13 . the stimulus / instruction presentation unit 15 presents a stimulus or instruction to the subject body 10 in accordance with a control signal from the stimulus / instruction presentation control unit 14 . a stimulus herein refers to that which requires a passive action ( such as listening to a sound or seeing a figure ) on the part of the subject body . an instruction herein refers to that which requires an active action ( such as listening to a sound or seeing a figure and then pushing a button ) from the subject body . measurement can be taken in a variety of terms . examples include electroencephalogram ( eeg ), blood flow , oxy hemoglobin or deoxy hemoglobin concentration in the blood , total hemoglobin concentration , local magnetic field in the brain , blood radioactive species concentration , proton relaxation time , myogenic potential , and skin potential . the means of measurement include , for example , electroencephalographs , ammeters , optical measurement devices , magnetic resonance apparatus , magnetoencephalographs , pet ( positron emission tomography ), fmri ( functional magnetic resonance imaging ), myogenic potential meters , and skin potential meters ( electrocardiographs ). in the following , an example will be considered in which the invention is applied to the optical measurement of brain activities in a newborn baby . fig2 shows the measurement system . light sources 102 a and 102 c emit near infrared light with a wavelength of 780 nm . light sources 102 b and 102 d emit near infrared light with a wavelength of 830 nm . the light sources 102 a and 102 c , and 102 b and 102 d , are driven with different frequencies by oscillators 101 a to 101 d . the lights from the light sources 102 a and 102 b that are intensity - modulated with different frequencies travel through optical fibers 103 a and 103 b , respectively , and are combined in a coupler 104 a . the combined light is passed through a light - irradiation optical fiber 105 a and irradiated onto a point on the scalp of a subject 106 that is the subject body . the lights from the other light sources 102 c and 102 d travel through optical fibers 103 c and 103 d to a coupler 104 b where they are combined . the combined light is then passed through a light - irradiation optical fiber 105 b and irradiated onto another point on the scalp of the subject 106 . near where light is irradiated via the light - irradiation optical fibers 105 a and 105 b is disposed a plurality of light - receiving optical fibers 107 a to 107 f such that their tips are located an equal distance ( 30 mm in the example ) away from each irradiated position . the rear end of each of the light - reception optical fibers is provided with a photodetector 108 a to 108 f such as , for example , a photomultiplier or a photodiode . light that has passed through the living body (“ living - body transmitted light ”) is received by the six light - receiving optical fibers 107 a to 107 f . the thus received living - body transmitted light is individually converted into an electric signal by the photodetectors 108 a to 108 f . the term “ living - body transmitted light ” refers to light that has passed through the living body and then received by the light - receiving fibers 107 a to 107 f , including both reflected light and transmitted light . the electric signal (“ living - body transmitted light intensity signal ”) produced by each of the photodetectors 108 a to 108 f indicates the intensity of light that has passed through the living body . the individual signals are fed to lock - in amplifiers 109 a to 109 h . the living - body transmitted light whose intensities are detected by the photodetectors 108 c and 108 d is collected by the light - reception optical fibers 107 c and 107 d that are located an equal distance from both the light - irradiating optical fibers 105 a and 105 b . accordingly , the signal from the photodetector 108 c and 108 d each is divided into two lines , one connecting to the lock - in amplifiers 109 c and 109 e , the other connecting to the lock - in amplifiers 109 d and 109 f . the intensity modulation frequencies from the oscillators 101 a and 101 b are supplied to the lock - in amplifiers 109 a to 109 d . the intensity modulation frequencies from the oscillators 101 c and 101 d are supplied to the lock - in amplifiers 109 e to 109 h . the frequencies are used as reference frequencies . thus , the lock - in amplifiers 109 a to 109 d separate and output a living - body transmitted light intensity signal corresponding to the light sources 102 a and 102 b . the lock - in amplifiers 109 e to 109 h separate and output a living - body transmitted light intensity signal corresponding to the light sources 102 c and 102 d . the thus separated living - body transmitted light intensity signals of the individual wavelengths produced by the lock - in amplifiers 109 e to 109 h are analog - digital converted by an analog / digital converter 110 and then sent to a measurement control computer 111 . the measurement control computer 111 processes the living - body transmitted light intensity signals according to the procedure as described in the above - mentioned non - patent document 1 , for example , and calculates , from the detection signal at each detection point , relative amounts of change of oxy hemoglobin concentration , deoxy hemoglobin concentration , and total hemoglobin concentration . the computer 111 then stores the obtained values in a storage unit as chronological information concerning the multiple measurement points , while sending a measurement signal 112 of a pre - selected channel to a filtering - calculation computer 113 . the amount of change of the total hemoglobin concentration is the sum of the amounts of change of oxy and deoxy hemoglobin . the term “ channel ” refers to a combination of the measurement position and the measurement species ( either oxy , de - oxy , or total hemoglobin ). the filtering - calculation computer 113 extracts a fluctuation component of a specific frequency band by filtering using polynomial fitting , as will be described below . the filtering - calculation computer 113 then sends a control signal 114 to a stimulation / instruction presentation unit 115 so that a stimulus or instruction is presented at the maxima and minima of the fluctuating wave . [ 0036 ] fig3 shows an example of the stimulation timing in the present embodiment in comparison with the timing in a conventional system . the rectangular waveforms indicate the time at which a stimulation was given . the stimulation consisted of an auditory stimulation lasting for 17 seconds . in the conventional system , the rest period between one presentation of stimulation and the next is randomly selected between 25 to 35 seconds . in the present embodiment , on the other hand , the stimulation is presented at the maxima and minima of a low - frequency fluctuation wave . the low - frequency fluctuation is extracted by the filtering - calculation computer 113 . major biological fluctuations are distributed near 0 . 1 hz in infants as well as in adults ( h . obrig et al ., neuroimage 12 , 623 - 639 ( 2000 )). when such low - frequency fluctuations are extracted by the normal filtering technique in a frequency range , the influence of delay cannot be ignored . such a disadvantage is overcome in the present embodiment by filtering based on polynomial fitting in a preset time window . referring to fig4 the polynomial fitting technique will be described . in the figure , δt is a sampling interval , which is 0 . 1 second in the present embodiment . tδt is the interval of data used in fitting , which is 0 . 1 second in the present embodiment , meaning that all of the sampling points are used . t is the length of a time window , which is 15 seconds in the present embodiment . the number of terms in the polynomial is 3 . increasing the order of the polynomial produces the same effect as shortening the time window length t on filtering . as the order of the polynomial is raised , or the time window length t is shortened , the extracted biological fluctuations shift to higher frequencies . a typical frequency of the extracted signal components is given by ( order of polynomial − 1 )/ 2t ( hz ). the typical frequency should desirably be somewhere between 0 . 01 to 0 . 5 hz in light of the object of the invention . this is because 1 ) the frequency components of the stimulus response signal is 1 hz at most ; 2 ) the stimulus ( task ) period is typically on the order of several tens of seconds ; and 3 ) important low - frequency fluctuations exist in this range , as described in the above - mentioned publication ( h . obrig et al ., neuroimage 12 , 623 - 639 ( 2000 )). in the example of fig4 filtering is carried out on a measurement signal at time t . against measurement data x ( s ) in the time window length t , the following polynomial of degree 3 is applied : the coefficients are determined by a least square method . supposing their estimated values are & lt ; a & gt ;, & lt ; b & gt ;, & lt ; c & gt ;, and & lt ; d & gt ;, the estimated values of a smoothed signal , a first - order derivative , and a second - order derivative at the current time t are expressed as follows : x ( t ) = a t 3 + b t 2 + c t + d dx ( t )/ dt = 3 a t 2 + 2 b t + c d 2 x ( t )/ dt 2 = 6 a t + 2 b in fig4 the large hollow circle at current time t indicates a measured signal value , while the large solid circle indicates a smoothed signal value . the smoothed signal value is an estimated value at current time t of the extracted low - frequency fluctuation . the small solid circles indicate data used in fitting , while small hollow circles indicate data that is not used in fitting . [ 0043 ] fig5 shows an example where a low - frequency fluctuation was extracted from a newborn baby under no stimulus environment and where the positions of the maxima and minima of the low - frequency fluctuation were detected . at the top of the figure , the thin solid line indicates a measurement signal and the thick solid line indicates the smoothed signal , namely the extracted low - frequency fluctuation . at the bottom , the broken line indicates the first - order derivative , and the rectangle waveform indicates the second - order derivative . the solid arrows indicate the positions of the maxima of the detected fluctuating wave , while the broken arrows indicate the positions of its minima . referring to fig6 a control algorithm for the stimulus / instruction presentation unit used in this example will be described . first , various parameters are input , including a target channel , degree of the polynomial , length of added data ( the length of a stimulus response ), time window length , interval of data used in polynomial fitting , number of repetition n rep ( number of presentation of stimulus or instruction ), and two kinds of thresholds δ , ε (& gt ; 0 ). the number of actual presentation of a stimulus or instruction is n r , and the control comes to an end when n r = n rep . based on the measurement signal , a polynomial fitting is carried out , and the low - frequency fluctuation ( biological fluctuation ), its first - order derivative and a second - order derivative are estimated . a control signal 114 for the presentation of a stimulus / instruction is sent to the stimulus / instruction presentation unit 115 when n r is an even number ( including 0 ) and at the timing of a maximum of the fluctuation wave , or when n r is an odd number and at the timing of a minimum of the fluctuation wave . n r is incremented such that n r = n r + 1 and the control is continued and this is repeated until n r = n rep . the positions of maxima and minima of the fluctuation wave as the measurement signal was consecutively sent from the measurement control computer 111 were determined using the following algorithm . that is , a maximum was recognized when the absolute value of the first - order derivative was smaller than the threshold δ and the second - order derivative was smaller than − ε . a minimum was recognized when the absolute value of the first - order derivative was smaller than the threshold δ and the second - order derivative was larger than ε . in the present embodiment , the thresholds were δ = 0 . 05 and ε = 0 . 005 . while the maxima and minima were selected alternately , this is not a requirement , and the same effect can be obtained by assigning half of the number of repetition to the maxima and the remaining half to the minima . the base line tends to project upward at the maxima and downward at the minima . by adding the same number of maxima and minima together , they cancel each other out and as a result , a flat base line can be obtained . in the present embodiment , the relative changes of oxy hemoglobin concentration and deoxy hemoglobin concentration at six sites ( indicated by numerals 1 to 6 ) on the left and right temporal regions are measured . these amounts of change are calculated with reference to the living - body transmitted light intensity 10 seconds after start of measurement . fig7 shows the results of selecting the oxy hemoglobin concentration change at measurement site 1 as the measurement signal 112 and repeating the presentation of stimulus 10 times . in this example , the effect of the invention is indicated with regard to the target channel . the effect is evaluated based on the standard deviation of baseline variation , which should preferably be flat . it can be seen that the stimulus - presentation method according to the invention as shown in fig3 ( b ) is capable of reducing the standard deviation of baseline variation better than the conventional stimulus - presentation method indicated in fig3 ( a ). [ 0047 ] fig8 shows the influence of the other channels on the baseline variation . the horizontal axis indicates the channel numbers and the vertical axis indicates the standard deviation of the baseline variation . the solid line indicates the results of the present embodiment , while the broken line indicates the results of the conventional example . with regard to the changes in oxy hemoglobin concentration at other measurement sites , the baseline variation is sufficiently suppressed in the present embodiment as compared with the conventional example , as will be seen from the comparison of the two values at channels 2 o , 3 o , 4 o , 5 o , and 6 o . regarding the deoxy hemoglobin concentration , the invention can also provide results comparable to those of the conventional example . these tendencies did not change when other channels were selected as the target . for example , when 1 d was selected as the target channel , sufficient effects were obtained for the deoxy hemoglobin concentration change at other measurement sites in addition to sufficient effects at the target channel 1 d , as shown in fig9 . regarding the oxy hemoglobin concentration change , results tended to be comparable to those obtained by the conventional example . when the total hemoglobin concentration change is selected as the target channel , the effects consisted of an average of the above - mentioned two tendencies . it is also possible to select a plurality of target channel candidates in advance and then change the target channel in a certain order , or to select a plurality of channels from all of the channels and then monitor their average value which is then used in controlling the stimulus / instruction presentation timing . in a next experiment , optical fibers were disposed on the left temporal region of the head of an adult subject , as shown in fig1 , and the language - related function of the brain was optically measured by the system according to the invention . in the figure , s indicates the position of light - source fibers , while d indicates the position of light - receiving fibers . the numerals enclosed by rectangles between ss and ds indicate the measurement positions . the distance between the adjacent ss and ds was 30 mm , and the wavelengths of the light source used were 781 nm and 832 nm . as described with reference to the above - described embodiments , the relative concentration changes of oxy hemoglobin , deoxy hemoglobin , and total hemoglobin at measured sites can be known by optical measurement . the fibers were arranged as shown in fig1 such that the language - related sites that have been identified by fmri ( functional magnetic resonance imaging ), for example , were covered . while the subject was performing a language task , a significant increase in the amount of oxy hemoglobin was observed at a measurement site 3 . thus , the channel corresponding to the oxy hemoglobin at the measurement site 3 was selected as the target channel . the language task assigned to the subject was a word - chain game ( in which the subject had to come up with a word that begins with the same sound as that of the last syllable of the previous word ), and it lasted for 40 seconds . this was followed by a rest of 40 seconds , during which the subject had to repeat the sounds of “ ah ,” “ ii ,” “ oo ,” “ eh ,” “ oh .” this sequence was repeated five times using the conventional system and the system of the invention , and the s / n amplitude ratios of arithmetic mean response signals that were obtained for the target channel were compared . the filtering conditions included a time window length of 20 seconds and the degree of 5 of the polynomial . the thresholds were the same as those in the above - described embodiments . as a result , the s / n amplitude ratio was 1 . 2 for the conventional system against 2 . 5 for the system of the invention , thus illustrating the advantageous effect of the invention . thus , in accordance with the invention , a high level of signal quality can be ensured in measuring responses to a stimulus given to a living body . particularly , when the measurement is repeated a small number of times , a better signal quality can be obtained than in the case of periodical or random presentation of stimulus .
a satisfactory averaged signal can be obtained with only a small number of repetitions of measurement in the measurement of responses to a stimulus given to a living body . a low - frequency biological fluctuation is extracted and a stimulus is presented to the living body in synchronism with the phase of the extracted fluctuation .