fig1 and 2 show the structure of a system 1 in accordance with the present invention . system 1 includes an optical barrel 9 fitted over the lens 17 of a video camera 11 . the output of video camera 11 is transmitted to a frame grabber 13 whose output is transmitted to a computer 15 having a display screen 23 . video camera 11 can be any device capable of recording a series of images of the eye and generating a video signal corresponding to these images . in this case , video camera 11 is a black and white , portable ccd ( charge coupled device ) camera of low lux sensitivity ( 0 . 1 lux or less ) with an effective pick - up area of 512 pixels ( horizontal )× 492 pixels ( vertical ) and a signal - to - noise ratio of 55 db . in place of video camera 11 , a sensor capable of detecting light and generating a signal proportional to the magnitude of the light detected can be used . as used in this specification , the term &# 34 ; light &# 34 ; includes electromagnetic radiation of any wavelength including infrared , visible and ultraviolet wavelengths . camera 11 further includes on its face and within the area covered by optical barrel 9 an infrared light source 19 . this light source may be , e . g ., a light emitting diode or any other device capable of generating and transmitting infrared light into optical barrel 9 . this light is transmitted to the eye 3 of a person looking into the optical barrel 9 . eyepiece 7 prevents light other than that transmitted by infrared source 19 from entering optical barrel 9 when the person &# 39 ; s eye is pressed against the eyepiece . this eyepiece preferably is made of rubber or some other flexible material capable of conforming to the skin surrounding the eye and preventing light from entering optical barrel 9 . in order to facilitate a person &# 39 ; s staring into optical barrel 9 , a mark may be included either within this barrel , within or on lens 17 or on the face of video camera 11 . in order to reduce ambient light from entering optical barrel 9 , a light - absorbing sheet or shield ( not shown ) can be used to cover the person being tested . such a sheet or shield could , e . g ., be attached to a hat - like garment or shroud to which also is attached optical barrel 9 . the room in which the test is conducted also should be as dark as possible , and the lens 17 of video camera 11 should be fitted with a filter 21 for removing light other than that within the frequencies of infrared source 19 . light transmitted from infrared source 19 is reflected by the person &# 39 ; s eye 3 into lens 17 of video camera 11 . the amount of light reflected is inversely proportional to the size of the person &# 39 ; s pupil 5 . if a light sensor is used in place of video camera 11 , the magnitude of the signal generated by this sensor is inversely proportional to the size of pupil 5 . video camera 11 generates a series of pixel images of the eye , including the iris and pupil . each new image is generated at a sampling rate determined by the camera , e . g ., sixty ( 60 ) images per second . video camera 11 transmits these images to a frame grabber 13 which transmits a corresponding series of digital data to computer 15 for analysis . in the alternative , data from video camera 11 can be transmitted directly to computer 15 , and a frame grabber can be incorporated within video camera 11 or computer 15 . also , as a further alternative , data from video camera 11 , or a light sensor , in analog or digital form , can be transmitted directly to computer 15 for analysis . if in analog form , computer 15 should include an analog to digital ( d / a ) converter for converting these data to digital form . computer 15 preferably is a portable computer for performing testing in the field . on the other hand , computer 15 can be a desk top computer or any general purpose computer capable of analyzing data and running computer programs . in the alternative , computer 15 can be a special purpose device containing only the structure necessary to perform the functions described below ( e . g ., storage devices , arithmetic unit , comparator , frequency detector , frequency analyzer , etc .). such a device could be miniaturized and built using , e . g ., a microprocessor . in place of the apparatus shown in fig1 and 2 , a pupil / corneal reflection tracker , such as the rk - 426 pupillometer manufactured by iscan , burlington , mass ., can be used to measure the diameter of a person &# 39 ; s pupil over a period of time . the data generated by the pupillometer then can be analyzed in accordance with the programs described below using the computing capacity of the pupillometer or a separate computer , such as computer 15 . the apparatus described in fig1 and 2 is considered preferable to the use of a device such as the rk 426 , however , because of the size and expense of such a device and the inclusion within it of structure and features unnecessary to the present invention . a flow diagram of a computer program for analyzing the data transmitted by frame grabber 13 is shown in fig3 . at block 31 , each pixel image of the eye is received in digital form and temporarily stored . at block 33 , the pixels within the area of the image representing the pupil of the eye are identified and used to calculate an instantaneous measure of the pupil &# 39 ; s size . since less light is reflected from the pupil than from other areas of the eye , the intensity of these pixels is substantially less than that of other pixels within each image . by performing a similar calculation for each image , a series of data ( a first signal ) indicative of the area or diameter of the pupil over a period of time is obtained . this signal also is indicative of the velocity of the pupil &# 39 ; s movement . at block 37 , the data generated at block 33 are further analyzed to determine the segments of these data generated during a blink of the eye . these segments are ignored in the further analysis of the data from block 33 . the steps executed by the program at block 37 are further discussed in connection with fig4 . at block 39 , a fast fourier transform is performed upon the data ( signal ) from block 33 ( after removal of those portions of the data generated during blinks at block 37 ). a series of data ( a second signal ) is generated as a result of this transform . these data provide an indication of the strength of each of the frequency components within the first signal . the fast fourier transform ( fft ) can provide a signal indicative of either the magnitude ( in pixels 2 / hz ) or power ( in pixels 4 / hz ) of these frequency components . the program samples the blink - free data at a prescribed sampling rate , e . g ., 60 hz , for a prescribed period of time ( e . g ., 30 seconds ). for such a sampling rate and sampling period , 1 , 800 samples of data are provided for the fast fourier transform . the program acquires these samples of data , performs trend removal ( optional ) and computes the fft . a commercially available fft program for generating a signal indicative of the magnitude of the frequency components is vu - point - 3 sold by maxwell laboratories , inc ., lajolla , calif ., and a commercially available fft program for generating a signal indicative of the power of the frequency components is viewdac , sold by keithley instruments , inc ., tauton , mass . in addition to performing the fft , the computer program can , if desired , display on screen 23 ( fig1 ) an instantaneous image of the eye ( including blinks ) and the first and second signals . at block 41 , the program determines the sum of the strength of all of the contiguous incremental segments of the second signal ( generated as a result of the fft ) within a particular frequency range in prescribed increments . this calculation is indicative of the area under the curve of the second signal . these segments can be separated by , e . g ., 0 . 008 hz , and the prescribed range of frequencies can be between , e . g ., 4 hz and 20 hz . the range , however , can be any preselected frequency range , including between approximately 0 hz and 20 hz or from approximately 0 hz to 40 hz . as discussed below , however , the range from between approximately 4 hz and 20 hz was found most effective in discriminating between persons with dat and persons without dat . the computer code for performing the calculation of block 41 is shown in the appendix . at decisional block 43 , the sum calculated at block 41 is compared with a threshold value . if this sum is less than the threshold value , the program provides an indication that the person has normal cognitive function . on the other hand , if this sum is greater than this threshold value , the program provides an indication that the person probably has dat . as discussed below , if the second signal is indicative of the power of the frequency components and the summation of this signal is performed from 4 hz to 20 hz in contiguous increments of 0 . 008 hz , the threshold value for the total power of pupillary oscillation found to discriminate most effectively between persons with dat and persons without dat is 116 pixels 4 / hz . at block 45 , the dominant oscillatory frequencies and peak magnitudes are calculated , if desired , and , at block 47 , the output of all calculations is provided in a printed report . fig4 is a flow diagram of the computer program for removing portions of the first signal generated during blinks of the person &# 39 ; s eye . at block 51 , each calculated value of the pupil &# 39 ; s size is stored in an array . at block 55 , the difference between each set of consecutive values in this array is calculated . at block 57 , each difference is compared against a predetermined number , e . g ., 10 , and if all of the differences are less than or equal to 10 , the program proceeds directly to block 61 where all of the values in the array are stored in a new array . on the other hand , if one or more of these differences is greater than 10 , then , at block 59 , for each such difference , the values in the array resulting in such a difference , and the subsequent eighteen values , are excluded from the array . the program then proceeds to block 61 where a new array is stored with all values except the excluded values . at block 63 , the program calculates the mean pupil size ( e . g ., the mean area or the mean diameter ) for all of the samples in the array stored at block 61 . at block 65 , the program calculates the difference between this mean pupil size and each value in the array . at block 67 , each difference is compared against a predetermined value , e . g ., 20 , and if all of the differences are less than or equal to 20 , all of the values in the array stored at block 63 are stored at block 71 as blink - free data . on the other hand , if one or more of these differences is greater than 20 , each value corresponding to such a difference is eliminated at block 69 , and the remaining values are stored at block 71 as blink - free data . using a system in accordance with the present invention , the inventors screened 48 persons over the age of 55 for dat . prior to this examination , all persons were administered the following neuropsychological tests : ( 1 ) mini - mental status examination ( mmse ) ( folstein et al ., 1975 ); ( 2 ) dementia rating scale ( drs ) ( mattis , 1988 ); ( 3 ) boston naming test ( bnt ) ( kaplan et al ., 1983 ); ( 4 ) weschler memory scale - revised ( logical memory i and ii ); ( 5 ) category fluency and letter fluency ( benton multilingual aphasia examination ); and ( 6 ) the draw - a - clock test ( rouleau et al ., 1992 ; sunderland et al ., 1989 ; huntzinger et al ., 1992 ). persons in the study evidencing dementia were given a neuromedical examination , including evaluation of cbc , electrolyte panel , renal panel ( bun and creatinine ), thyroid panel ( tsh and t4 ), liver function tests , urinalysis , and serum tests for syphilis , serum b 12 and folate levels . ct or mri scans also were evaluated , and a medical history , including a list of medications taken at the time of the examination , were obtained for all persons in the study . based upon these neuropsychological and medical evaluations , the persons in the study were assigned to the following groups : ( 1 ) suspected dat ; ( 2 ) non - dat dementia ; and ( 3 ) no evidence of dementia . the suspected dat group included both persons who probably had dat and persons who possibly had dat . persons in the suspected dat group had some evidence of dementia , established by either low scores on the mmse ( less than 25 ) or drs ( less than 125 ), and / or severe memory deficits exhibited from the results of both the logical memory i and ii tests of the weschler memory scale - revised ( scores of 10 or below ). persons having only low scores on the logical memory i and ii tests also had either a low score on the category fluency test ( less than 10 ), bnt ( less than 45 ) or the draw - a - clock test ( less than 6 ) for assignment to the suspected dat group . all persons in the suspected dat group , therefore , had cognitive deficits in memory function and in at least one other cognitive function . none of the persons assigned to this group had a history of stepwise or sudden worsening of memory or other cognitive functions , or a disturbance of consciousness . persons assigned to the suspected dat group , moreover , did not have a systemic ( medical ) or brain disorder that could account for his or her cognitive impairment . for instance , no clear temporal relationship existed between the onset of the brain disorder and the onset of the person &# 39 ; s cognitive problems . following these neuropsychological and medical evaluations , each person was placed in a darkened room for three minutes . using the rk - 426 pupillometer , pupil diameters then were determined in the right eye of each person ( unless the left eye was preferable for examination because of a cataract , past injury or previous surgery ). after pupil diameters in the dark were measured , pupil diameters during light challenge from 5 lux to 700 lux were measured . dilute tropicamide then was instilled in the eye , and pupil size was measured in the dark and in response to light challenge for the hour following this instillation . using the rk - 426 pupillometer , pupil diameter for each person then was measured for a period of time under various conditions of ambient light at a sampling rate of 60 hz . after removing those portions of the data corresponding to blinks , a fast fourier transform was performed upon the remaining data using vu - point - 3 software for magnitude analysis and viewdac software for power analysis . the resulting data corresponding to the strength of the frequency components were summed in segments of 0 . 008 hz within the frequency ranges of 0 hz - 4 hz and 4 hz - 20 hz to provide an indication of the total strength ( power or magnitude ) of pupillary oscillations within this frequency range . this analysis revealed that optimal discrimination among persons occurred when the examination was conducted in the dark and when the total power of pupillary oscillation was calculated for the frequency range of between 4 and 20 hz . a one - way analysis of variance ( anova ) for total power of pupillary oscillation in the 4 - 20 hz range yielded a significant difference among the three groups ( f = 7 . 3 ; df = 2 , 35 ; p = 0 . 002 ; mean of suspected dat group = 230 . 9 ; mean of non - dat dementia group = 192 . 6 ; mean of non - demented controls = 88 . 5 ). pupillary oscillation scores obtained during light challenge also discriminated among the groups ( f = 3 . 8 ; df = 2 , 35 ; p = 0 . 03 ), but the use of light challenge did not enhance the ability to discriminate among groups when compared to measurements taken in the dark . the results of this analysis are shown in table i below . table i______________________________________ tpowh______________________________________ r - value ( p - value ) mmse - 0 . 47 ( 0 . 002 ) bnt - 0 . 54 ( 0 . 0002 ) c fluency - 0 . 35 ( 0 . 02 ) l fluency - 0 . 32 ( 0 . 04 ) drs - 0 . 33 ( 0 . 03 ) wms - i - 0 . 32 ( 0 . 04 ) wms - ii - 0 . 36 ( 0 . 01 ) clock - draw - 0 . 40 ( 0 . 008 ) keymmse = minimental status exam / bnt = boston naming testc fluency = category fluency / l fluency = letter fluencydrs = dementia rating scalewms - i = weschler memory scale - logical stories 1wms - ii = weschler memory scale - logical stories 2clock - draw = draw - a - clock testtpowh = total power of pupillary oscillation in the 4 - 20 hzrange ( pixel . sup . 4 / hz ) ______________________________________ upon analysis of the data , the inventors determined that a threshold value of 116 pixels 4 / hz for total power of pupillary oscillation in the 4 - 20 hz range provides optimal discrimination among groups . using the 116 pixels 4 / hz threshold value , the negative predicative power of the test was 100 % ( i . e ., the predicative power to detect people without dementia or non - dat dementia was 100 %). a pupillary oscillation score of less than 116 pixels 4 / hz , therefore , predicted the absence of dementia with 100 % accuracy . the calculated sensitivity of the test , moreover , was 100 %; namely all persons with dat scored above the threshold value . the specificity of the test was 75 %; namely , three quarters of the non - demented persons had pupillary oscillation values below 116 pixels 4 / hz . some of the persons in the false positive group ( without suspected dat but with a total power of pupillary oscillation exceeding 116 pixels 4 / hz ), however , had some evidence of cognitive impairment . these deficiencies , on the other hand , did not meet the study &# 39 ; s criteria for dat . for instance , one person who tested &# 34 ; false positive &# 34 ; ( subject # 18 ) had a low score on logical memory i of the weschler memory scale ( in the 12th percentile ) but a mmse of 27 and a drs of 135 . another person who tested false positive had &# 34 ; normal &# 34 ; cognitive functioning ( except for an abnormal draw - a - clock test ) but had a family history of dat ( subject # 12 ). the inventors also determined that medications such as beta - blockers do not appear to block the appearance of heightened pupillary oscillations suggestive of dat . for instance , a person in the study who likely had dat demonstrated elevated total power of pupillary oscillations notwithstanding that he was taking the beta - blocker propranolol ( inderal ). the total power of pupillary oscillation for this person may have been higher , moreover , if he were not taking this beta - blocker . the results of this study are summarized in table ii below . table ii__________________________________________________________________________no . group mmse bnt c fluency l fluency drs wms - i wms - ii clock - draw tpowh__________________________________________________________________________ 1 c 28 57 20 36 141 68 60 9 263 . 49 2 c 29 58 16 36 135 66 80 9 79 . 0 3 a 20 48 10 36 120 7 8 9 279 . 22 4 a 24 48 9 6 92 2 2 6 119 . 54 5 a 14 30 7 21 96 5 2 2 442 . 07 6 c 24 51 14 30 135 24 46 9 72 . 55 8 c 29 59 16 35 139 60 52 9 97 . 41 9 a 27 40 6 34 132 3 5 8 348 . 110 c 27 - 99 18 29 124 45 40 9 72 . 7811 a 28 42 12 33 106 12 10 9 139 . 2412 c 29 58 28 47 137 52 42 6 272 . 3213 b 21 36 5 13 106 6 2 6 67 . 6615 c 29 55 20 29 136 82 80 7 186 . 7316 c 27 48 13 18 133 21 36 8 189 . 7917 c 28 60 23 38 138 60 78 9 59 . 5518 c 27 54 21 22 135 12 46 9 129 . 819 c 29 52 21 33 140 23 32 6 315 . 9520 c 29 56 - 99 44 135 88 87 9 101 . 4921 c 29 48 17 25 140 96 89 8 55 . 2322 c 28 54 21 56 138 46 62 10 23 . 1123 c 29 60 8 36 132 60 60 9 25 . 8824 c 29 57 18 35 134 65 60 9 56 . 9925 c 27 58 19 72 138 54 52 9 15 . 0826 c 27 55 23 19 137 87 65 7 11527 c 30 58 22 61 140 80 70 10 52 . 3328 c 24 57 19 21 138 27 40 9 20 . 5429 c 28 57 15 48 139 12 26 9 17 . 1831 c 26 55 44 27 137 86 68 9 18 . 4632 c 28 47 13 23 125 76 53 9 31 . 1333 c 30 52 52 48 143 92 94 9 26 . 0934 c 30 58 35 27 128 72 75 9 21 . 7435 c 30 57 45 54 136 7 26 10 15 . 536 c 29 58 40 37 141 68 86 10 54 . 8837 c 28 57 32 44 138 52 38 8 17 . 4738 wp ( a ) 26 47 22 16 114 12 10 6 171 . 4540 c 30 55 - 99 45 135 86 96 9 267 . 4641 a 23 57 18 38 133 6 8 7 177 . 6742 a 22 33 6 5 - 99 6 2 8 170 . 0843 c 30 56 11 24 132 66 80 8 15 . 3244 c 29 58 14 26 136 52 56 10 52 . 6345 b 20 37 10 6 111 6 18 9 472 . 1346 c 29 55 13 15 135 88 64 9 321 . 8247 b 25 56 7 38 114 17 8 4 38 . 148 c 30 57 11 18 140 74 78 9 73 . 55__________________________________________________________________________ key a = suspected dat b = nondat c = nondemented controls wp = workup in progress - 99 = missing values mmse = minimental status exam / bnt = boston naming test c fluency = category fluency / l fluency = letter fluency drs = dementia rating scale wmsi = weschler memory scale logical stories 1 mwsii = weschler memory scale logical stories 2 clockdraw = drawa - clock test tpowh = total power of pupillary oscillation in the 4 - 20 hz range ( pixel . sup . 4 / hz ) the results of this study also are illustrated in fig5 - 9 . fig5 is a plot of the magnitude of the frequency components ( in pixels 2 / hz ) for the frequency range of 4 hz - 20 hz for a person in the study with normal cognitive function . fig6 is a similar plot for a person suspected of having dat ; fig7 is a similar plot for a person having mild dementia as a result of a deficiency in vitamin b 12 ( not as a result of dat ); and fig8 is a similar plot for a person having normal cognitive function but having a family history of dat and an inability to pass the draw - a - clock test . fig9 shows the results of the draw - a - clock test for this person ( the extra numbers drawn by this person on the clock &# 39 ; s face are circled ). this study suggests , therefore , that a determination of the strength of pupillary oscillations is useful in the identification and discrimination of patients with suspected dat . in such patients , the power and magnitude of pupillary oscillations are increased . a significant correlation also exists between neuropsychological test scores and the strength of pupillary oscillations . the increase in the strength of pupillary oscillations in persons suspected of dat may be related to central cholinergic failure . diminished cholinergic tone to the pupil could result in increased pupillary oscillations produced by unopposed sympathetic stimulation . cholinergic failure has been observed in the edinger - westphal nucleus of dat patients . this nucleus is important to the regulation of pupil size , and pathology in the nucleus could account for the increase in pupillary oscillations observed in the study . the finding in the study that tropicamide instillation significantly increases the total power of pupillary oscillations in the suspected dat group by about 150 % ( from a mean total power of 239 . 4 pixels 4 / hz to a mean total power of 811 . 9 pixels 4 / hz ) is consistent with this theory . the tropicamide - induced changes in total power for persons not evidencing dementia were less than in persons suspected of having dat . the preferred embodiments described above include numerous variations and combinations which are within the spirit and scope of the invention . the foregoing description should be understood as an illustration of the invention , therefore , rather than as a limitation . the scope of the invention is described by the following claims . ______________________________________appendix______________________________________c this program is for extracting the measures of pupillaryc oscillations after fft has been done on the raw data . parameter ( j = 1040 , ij = 14 ) dimension pmagn ( j ), freq ( j ) character * 12 abcd ( ij ), filen open ( 5 , file = ` bf . r `) open ( 6 , file = ` bf . w `) c step - 1 : reading and storing the frequency and magnitudec numbers in arrays do 2 n = 1 , ij read ( 5 , 13 ) abcd ( n ) 13 format ( 27x , a12 ) if ( n . eq . 1 ) then filen = abcd ( n ) end if 2 continue ip = 0 32 continue ip = ip + 1 read ( 5 ,*, end = 34 ) freq ( ip ), pmagn ( ip ) go to 32 34 continue ipoints = ip sum24f = 0 b24 = 0 . 01c step2a : finding out the peak magnitude in 0 - 4 hz rangec by using ` amax1 ` function . do 87 in = 1 , ipoints ps24f = pmagn ( in )** 2 if (( freq ( in ). gt . 0 . 0 ). and . ( freq ( in ). le . 4 . 0 )) thenc step 4a : summation of magnitudes in 0 - 4 hz range i . e . c calculating area under the curve in 0 - 4 hz range sum24f = sum24f + ps24f b24 = amax1 ( b24 , pmagn ( in )) else go to 87 end if 87 continuec step 3a : finding out the dominant frequency in 0 - 4 hz range do 88 i = 1 , ipoints if ( freq ( i ). gt . 0 . 0 ). and . ( freq ( i ). le . 4 . 0 )) then continue else go to 88 end if if ( pmagn ( i ). eq . b24 ) then a24 = freq ( i ) else go to 88 end if 88 continuec step : 2b finding out the peak magnitude in 4 - 20 hz rangec by using ` amax1 ` function sum420f = 0 b420 = 0 . 01 do 89 in = 1 , ipoints ps420f = pmagn ( in )** 2 if (( freq ( in ). gt . 4 . 0 ). and . ( freq ( in ). le . 20 . 0 )) thenc step 4b : summation of magnitudes in 4 - 20 hz range i . e . c calculating area under the curve in 4 - 20 hz range sum420f = sum420f + ps420f b420 = amax1 ( b420 , pmagn ( in )) else go to 89 end if 89 continuec step 3b : finding out the dominant frequency in 0 - 4 hz range do 90 i = 1 , ipoints if (( freq ( i ). gt . 4 . 0 ). and . ( freq ( i ). le . 20 . 0 )) then continue else go to 90 end if if ( pmagn ( i ). eq . b420 ) then a420 = freq ( i ) else go to 90 end if 90 continuec step 5 : writing the outcome measures in a file for further processingc write ( 6 , 17 ) filen , b0 , a0 , sum0fc write ( 6 , 17 ) filen , b2 , a2 , sum2fc write ( 6 , 17 ) filen , b4 , a4 , sum4fc write ( 6 , 17 ) filen , b6 , a6 , sum6fc write ( 6 , 17 ) filen , b8 , a8 , sum8fc write ( 6 , 17 ) filen , b12 , a12 , sum12f write ( 6 , 17 ) filen , b24 , a24 , sum24f write ( 6 , 17 ) filen , b420 , a420 , sum420fc write ( 6 , 15 ) filen , bh , ah , ph , prodh , psmh , centphc write ( 6 , 16 ) filen , ratio , pratio , cratioc 14 format ( a12 , 1x , f7 . 2 , 1x , f6 . 3 , 1x , f10 . 2 , 1x , f10 . 2 , 1x , f10 . 2 , 1x , f5 . 2 , c 1x , 1f10 . 2 ) c 15 format ( a12 , 1x , f7 . 2 , 1x , f6 . 3 , 1x , f10 . 2 , 1x , f10 . 2 , 1x , f10 . 2 , 1x , f5 . 2 ) c 16 format ( a12 , 2x , f10 . 2 , 2x , f10 . 2 , 2x , f10 . 2 ) 17 format ( a12 , 1x , 3f10 . 2 ) c write ( 6 , 14 ) filen , psmc 14 format ( a10 , 2x , 2f10 . 2 ) write ( 6 , 21 ) filen , sum420f 21 format ( a12 , 2x , f10 . 2 ) stop end □ ______________________________________
an easily administered , non - invasive , sensitive and specific system and method for screening persons for dementia , particularly dementia of the alzheimer &# 39 ; s type , is provided . the system and method include means for irradiating the eye of a person with light , preferably light solely within the infrared spectrum , and means for generating a signal responsive to the amount of light reflected from the eye over a period of time . this signal is indicative of changes in the pupil &# 39 ; s size over time . the system and method further include means for performing a fourier transform upon this signal to provide a value indicative of the strength of this signal &# 39 ; s frequency components . this value then is compared against a predetermined threshold value and , if this threshold value is exceeded , a higher probability is assigned to the person having dementia , particularly dementia of the alzheimer &# 39 ; s type .