. it is characterised not only by the movement abnormalities , but also by cognitive impairment and abnormal behaviour , as well as weight problems and sleep disturbances .
hd patients experience a complex variety of movement problems , which include not only chorea , but also akinesia and bradykinesia .
a number of studies have assessed impairment of simple motion sequences in hd patients as a way of quantifying the progression of symptoms [ 2 , 7 , 12 , 15 ] .
sequential hand movements in hd patients have been examined at various stages of the condition , with markedly slower execution of movements by the hd patients when compared with controls . as well as performing movements more slowly , hd patients are also slower in switching from one movement to the next .
garcia ruiz and colleagues   studied the degree of bradykinesia and timing in genetically confirmed hd individuals compared with controls , using the four capit timed tasks previously used for pd .
these results have been reproduced in more recent studies that reported significantly reduced tapping rates in manifest hd patients as compared to controls , but not premanifest individuals ; unified huntington s disease rating scale ( uhdrs ) motor scores and duration of the disease were highly correlated with the tapping results , something which did not correlate with the cag repeat lengths [ 3 , 1315 ] .
longitudinal studies have shown a significant decline in tapping rate over a period of 3  years in manifest hd patients , and a strong correlation between uhdrs scores and the motor tests .
furthermore , the variability of finger tapping ( using target intervals of 600 and 1,200  ms ) correlated with an index of the probability of motor onset , estimated from cag length and age . quick and easy - to - use hand tapping devices have enabled the number of taps in 30  s , and the variability in tapping rhythm and fatigue over the testing period , to be measured .
initial cross - sectional testing of hd patients using an early model of such a device showed that the tapping frequency correlated significantly with the motor uhdrs and independence scores .
longitudinal data from a small cohort followed over 10  years revealed that this correlation was maintained over time , suggesting the technique may provide an objective measure of disease progression .
recently , a large study , trackhd , used a force transducer to measure paced finger tapping in premanifest and manifest hd patients ; significant differences between premanifest and manifest patients were found , again suggesting that finger - tapping measures are an important way of monitoring the progression of the disorder . here
we present a pilot study , using a new portable device in which the sensors are activated purely by contact and are independent of force ; this facilitates the rapid collection of finger - tapping data , and allows the possibility of introducing modifications of the basic task that may have diagnostic utility . because it provides information not merely about average tapping rates , but about the statistical distribution of the individual intervals between responses , it enables the behaviour to be characterised more specifically and quantitatively ,
sometimes revealing aberrant patterns of response that would not be detected by conventional average measures ; this is likely to aid monitoring and diagnosis .
the present study was approved by the local regional ethics committee and was conducted in the john radcliffe hospital , oxford , uk .
all had a positive genetic test for hd , and were evaluated by an experienced clinician ( ahn ) using a standard neurological examination and the uhdrs to ascertain whether individuals had motor manifestation of huntington s disease ( table  1 ) .
there were two experimental groups : eight in a manifest group ( mg ) with overt motor signs , and a small aged - matched group of three controls ( c ) .
all participants gave their informed consent after the procedures had been explained to them.table  1characterisation of the manifest ( m ) patient group and controls ( c)m ( n  =  8)c ( n  =  3)age : mean    sd54.50    10.0854.33    7.81cag repeats42.17    1.80n / auhdrs ( motor score ) ( mean , range)31.75 ( 1181)n / atotal functional capacity12.00    0.71n / avf ( mean , range)26.86 ( 1745)n / ahand tapping ( reciprocal median latency)mean    se ( s )  single4.08    0.604.95    0.85  alternating3.18    0.414.06    0.53  pronating2.14    0.303.49    0.74 ( sd of main distribution)mean    se ( s )  single0.75    0.140.35    0.12  alternating0.49    0.060.44    0.21  pronating0.41    0.030.41    0.07except where stated , all figures are mean    1se characterisation of the manifest ( m ) patient group and controls ( c ) except where stated , all figures are mean    1se the tapping pad ( fig
.  1 left : ober consulting poland ) has two touch sensors each of 60-mm diameter , their centres 115-mm apart ; each has an associated blue led providing feedback whenever one of the sensor active fields is touched .
the sensors are activated purely by contact , independent of force , as they work by monitoring electrical impedance , with a threshold that is automatically adjusted to the background noise level . left and right taps are recorded independently , at a sampling frequency of 1  khz ( resolution 1  ms ) , for a fixed period of 30s .
three protocols were used , and in each case the subject used their dominant hand : ( 1 ) alternation : the instructions were to tap the left and right sensors alternately as rapidly as possible using the tip of the index finger ; ( 2 ) pronation / supination : alternately as before and as rapidly as possible , but with the hand in the supine position when tapping medially side , but prone on when tapping laterally , using the finger nail rather than finger tip ; ( 3 ) single : tapping the right sensor only , with the index finger . for each task ,
the successive intervals between taps were recorded , generating histograms of the distribution of intervals between consecutive taps .
the statistics of these inter - tap intervals for hand tapping was then analysed using the computer application spic ( saccadic programming and instrumentation computer ) , calculating best - fit parameters (  and  ) of the later ( linear approach to threshold with ergodic rate ) model .fig .  1left the portable device for recording hand tapping .
right a typical reciprobit plot of hand - tapping interval data from one subject in this study tapping alternately on the two sides : responses for right and left movements are combined .
the points represent a cumulative histogram of the latencies ( intervals between taps ) , plotted on a probability scale as a function of reciprocal latency left the portable device for recording hand tapping .
right a typical reciprobit plot of hand - tapping interval data from one subject in this study tapping alternately on the two sides : responses for right and left movements are combined .
the points represent a cumulative histogram of the latencies ( intervals between taps ) , plotted on a probability scale as a function of reciprocal latency
the present study was approved by the local regional ethics committee and was conducted in the john radcliffe hospital , oxford , uk .
all had a positive genetic test for hd , and were evaluated by an experienced clinician ( ahn ) using a standard neurological examination and the uhdrs to ascertain whether individuals had motor manifestation of huntington s disease ( table  1 ) .
there were two experimental groups : eight in a manifest group ( mg ) with overt motor signs , and a small aged - matched group of three controls ( c ) .
all participants gave their informed consent after the procedures had been explained to them.table  1characterisation of the manifest ( m ) patient group and controls ( c)m ( n  =  8)c ( n  =  3)age : mean    sd54.50    10.0854.33    7.81cag repeats42.17    1.80n / auhdrs ( motor score ) ( mean , range)31.75 ( 1181)n / atotal functional capacity12.00    0.71n / avf ( mean , range)26.86 ( 1745)n / ahand tapping ( reciprocal median latency)mean    se ( s )  single4.08    0.604.95    0.85  alternating3.18    0.414.06    0.53  pronating2.14    0.303.49    0.74 ( sd of main distribution)mean    se ( s )  single0.75    0.140.35    0.12  alternating0.49    0.060.44    0.21  pronating0.41    0.030.41    0.07except where stated , all figures are mean    1se characterisation of the manifest ( m ) patient group and controls ( c ) except where stated , all figures are mean    1se
1 left : ober consulting poland ) has two touch sensors each of 60-mm diameter , their centres 115-mm apart ; each has an associated blue led providing feedback whenever one of the sensor active fields is touched .
the sensors are activated purely by contact , independent of force , as they work by monitoring electrical impedance , with a threshold that is automatically adjusted to the background noise level . left and right taps are recorded independently , at a sampling frequency of 1  khz ( resolution 1  ms ) , for a fixed period of 30s .
three protocols were used , and in each case the subject used their dominant hand : ( 1 ) alternation : the instructions were to tap the left and right sensors alternately as rapidly as possible using the tip of the index finger ; ( 2 ) pronation / supination : alternately as before and as rapidly as possible , but with the hand in the supine position when tapping medially side , but prone on when tapping laterally , using the finger nail rather than finger tip ; ( 3 ) single : tapping the right sensor only , with the index finger . for each task ,
the successive intervals between taps were recorded , generating histograms of the distribution of intervals between consecutive taps .
the statistics of these inter - tap intervals for hand tapping was then analysed using the computer application spic ( saccadic programming and instrumentation computer ) , calculating best - fit parameters (  and  ) of the later ( linear approach to threshold with ergodic rate ) model .fig . 
right a typical reciprobit plot of hand - tapping interval data from one subject in this study tapping alternately on the two sides : responses for right and left movements are combined .
the points represent a cumulative histogram of the latencies ( intervals between taps ) , plotted on a probability scale as a function of reciprocal latency left the portable device for recording hand tapping .
right a typical reciprobit plot of hand - tapping interval data from one subject in this study tapping alternately on the two sides : responses for right and left movements are combined .
the points represent a cumulative histogram of the latencies ( intervals between taps ) , plotted on a probability scale as a function of reciprocal latency
figure  1 ( right ) shows a typical distribution plot from one of the participants in the study .
as usually found for saccadic and evoked manual reaction times , distributions of the intervals between taps in this task are skewed , with a long tail of longer latencies , and in general the reciprocal of reaction time , or promptness , follows a gaussian distribution ( and is therefore more amenable to statistical analysis ) .
consequently if inter - tap interval distributions are plotted cumulatively , on a probit scale , using a reciprocal abscissa ( a reciprobit plot ) , they will be expected to generate a straight line , as seen on fig .  1 ( right ) .
such a distribution can be fully described by just two parameters : these are  , its mean ( which is also the reciprocal of the median latency ) , and  , its standard deviation .
a large value of  corresponds to increased promptness or speed of response , and thus a shorter interval ; because of the reciprocal relationship , the units for these two parameters are s or hz . the best - fit values of the parameters can be determined automatically by minimisation of the kolmogorov 
the observed distribution of reciprocal inter - tap intervals has the expected normal distribution , but in some cases significant , idiosyncratic deviations are observed .
figure  2a demonstrates a bimodal pattern of response , possibly analogous to the population of 
2b ) is the existence of a distinct sub - set of aberrantly long inter - tap intervals ( around 600700  ms ) , probably due to inattention : in this case they represent only some 5% of all trials .
2d shows responses from one manifest hd patient with remarkably long inter - tap intervals , and very marked differences between the three types of task .
an important point to note is that many of these characteristic features would have been entirely missed by the conventional technique of simply counting the total number of taps in a fixed time - period : they are only revealed by quantitative analysis of the distribution of individual intervals.fig . 
2examples of unusual latency distributions . a a subject showing marked bimodality in the alternation task , with one group of responses around 300  ms and another around 450  ms : responses in each direction are very similar .
b a subject showing sporadic inattention in the pronation task : in both directions a small group of responses ( top right ) is markedly delayed relative to the main distribution .
c marked left / right difference ; responses to the right are about 40  ms faster ( alternation task ) .
d a manifest hd patient with exceptionally slow responses , affecting all three types of movement , but most marked for the pronation / supination task examples of unusual latency distributions . a a subject showing marked bimodality in the alternation task , with one group of responses around 300  ms and another around 450  ms : responses in each direction are very similar .
b a subject showing sporadic inattention in the pronation task : in both directions a small group of responses ( top right ) is markedly delayed relative to the main distribution .
c marked left / right difference ; responses to the right are about 40  ms faster ( alternation task ) .
d a manifest hd patient with exceptionally slow responses , affecting all three types of movement , but most marked for the pronation / supination task best - fit values of  and  were estimated for all participants and conditions .
figure  3 shows the average values of the parameters broken down by type of response and category of participant ( controls and manifest ) .
when comparing the three different tasks for the hd participants ,  is significantly different between the alternation and pronating tasks ( p  =  0.0011 ) and between pronating and single ( p  =  0.0024 ) , and  is significantly different for pronating versus alternating ( p  =  0.045 ) , but no other comparisons are significant at p  =  0.05 . when comparing the patients with the controls , for all three tasks the patients show the expected reduction in  ( equivalent to increased time between taps ) , but because of the very small group sizes in this pilot study , the only difference seen in fig .  3 , that is significant at p  =  0.05 ( unpaired t test ) , is for  with single tapping ( p  =  0.036).fig . 
3average values of  ( left ) and  ( right ) analysed by type of response and category of participant ( c control , m manifest hd ) .
error bars show 1se average values of  ( left ) and  ( right ) analysed by type of response and category of participant ( c control , m manifest hd ) .
error bars show 1se it is instructive to plot  and  as a scatter plot for the two groups ( fig .  4 ) .
what is then seen is an apparent segregation between the manifest patients ( m ) and the controls ; once again , it would obviously be desirable to have more participants than was available for this preliminary evaluation.fig . 
4scatter plot of  versus  for the two groups of participants performing single tapping ; despite the very small numbers in this pilot study , the manifest hd patients ( red ) seem to form a distinct cluster from the controls ( black ) scatter plot of  versus  for the two groups of participants performing single tapping ; despite the very small numbers in this pilot study , the manifest hd patients ( red ) seem to form a distinct cluster from the controls ( black ) finally , some idea of the test s predictive validity can be seen by looking at the correlation between the observed values of  and  and a more general measure of the patients neurological status , the total motor score ( tms ) .
regression analysis reveals r values that are significant or near - significant for  ( pearson : p  =  0.063 , 0.008 and 0.065 ( fig . 
5 ) for alternation , pronation and single tapping , respectively ) , but not for .fig . 
5relation between  and  in each of three types of test , and the total motor score , across all manifest hd subjects and controls ( the latter are assigned a tms score of zero ) relation between  and  in each of three types of test , and the total motor score , across all manifest hd subjects and controls ( the latter are assigned a tms score of zero )
figure  1 ( right ) shows a typical distribution plot from one of the participants in the study .
as usually found for saccadic and evoked manual reaction times , distributions of the intervals between taps in this task are skewed , with a long tail of longer latencies , and in general the reciprocal of reaction time , or promptness , follows a gaussian distribution ( and is therefore more amenable to statistical analysis ) .
consequently if inter - tap interval distributions are plotted cumulatively , on a probit scale , using a reciprocal abscissa ( a reciprobit plot ) , they will be expected to generate a straight line , as seen on fig .  1 ( right ) .
such a distribution can be fully described by just two parameters : these are  , its mean ( which is also the reciprocal of the median latency ) , and  , its standard deviation .
a large value of  corresponds to increased promptness or speed of response , and thus a shorter interval ; because of the reciprocal relationship , the units for these two parameters are s or hz . the best - fit values of the parameters can be determined automatically by minimisation of the kolmogorov 
the observed distribution of reciprocal inter - tap intervals has the expected normal distribution , but in some cases significant , idiosyncratic deviations are observed .
figure  2a demonstrates a bimodal pattern of response , possibly analogous to the population of 
2b ) is the existence of a distinct sub - set of aberrantly long inter - tap intervals ( around 600700  ms ) , probably due to inattention : in this case they represent only some 5% of all trials .
2d shows responses from one manifest hd patient with remarkably long inter - tap intervals , and very marked differences between the three types of task .
an important point to note is that many of these characteristic features would have been entirely missed by the conventional technique of simply counting the total number of taps in a fixed time - period : they are only revealed by quantitative analysis of the distribution of individual intervals.fig . 
2examples of unusual latency distributions . a a subject showing marked bimodality in the alternation task , with one group of responses around 300  ms and another around 450  ms : responses in each direction are very similar .
b a subject showing sporadic inattention in the pronation task : in both directions a small group of responses ( top right ) is markedly delayed relative to the main distribution .
c marked left / right difference ; responses to the right are about 40  ms faster ( alternation task ) .
d a manifest hd patient with exceptionally slow responses , affecting all three types of movement , but most marked for the pronation / supination task examples of unusual latency distributions . a a subject showing marked bimodality in the alternation task , with one group of responses around 300  ms and another around 450  ms : responses in each direction are very similar .
b a subject showing sporadic inattention in the pronation task : in both directions a small group of responses ( top right ) is markedly delayed relative to the main distribution .
c marked left / right difference ; responses to the right are about 40  ms faster ( alternation task ) .
d a manifest hd patient with exceptionally slow responses , affecting all three types of movement , but most marked for the pronation / supination task
best - fit values of  and  were estimated for all participants and conditions .
figure  3 shows the average values of the parameters broken down by type of response and category of participant ( controls and manifest ) .
when comparing the three different tasks for the hd participants ,  is significantly different between the alternation and pronating tasks ( p  =  0.0011 ) and between pronating and single ( p  =  0.0024 ) , and  is significantly different for pronating versus alternating ( p  =  0.045 ) , but no other comparisons are significant at p  =  0.05 . when comparing the patients with the controls , for all three tasks the patients show the expected reduction in  ( equivalent to increased time between taps ) , but because of the very small group sizes in this pilot study , the only difference seen in fig .  3 , that is significant at p  =  0.05 ( unpaired t test ) , is for  with single tapping ( p  =  0.036).fig . 
3average values of  ( left ) and  ( right ) analysed by type of response and category of participant ( c control , m manifest hd ) .
error bars show 1se average values of  ( left ) and  ( right ) analysed by type of response and category of participant ( c control , m manifest hd ) .
error bars show 1se it is instructive to plot  and  as a scatter plot for the two groups ( fig .  4 ) .
what is then seen is an apparent segregation between the manifest patients ( m ) and the controls ; once again , it would obviously be desirable to have more participants than was available for this preliminary evaluation.fig . 
4scatter plot of  versus  for the two groups of participants performing single tapping ; despite the very small numbers in this pilot study , the manifest hd patients ( red ) seem to form a distinct cluster from the controls ( black ) scatter plot of  versus  for the two groups of participants performing single tapping ; despite the very small numbers in this pilot study , the manifest hd patients ( red ) seem to form a distinct cluster from the controls ( black ) finally , some idea of the test s predictive validity can be seen by looking at the correlation between the observed values of  and  and a more general measure of the patients neurological status , the total motor score ( tms ) .
regression analysis reveals r values that are significant or near - significant for  ( pearson : p  =  0.063 , 0.008 and 0.065 ( fig . 
5 ) for alternation , pronation and single tapping , respectively ) , but not for .fig .  5relation between  and  in each of three types of test , and the total motor score , across all manifest hd subjects and controls ( the latter are assigned a tms score of zero ) relation between  and  in each of three types of test , and the total motor score , across all manifest hd subjects and controls ( the latter are assigned a tms score of zero )
the aim of this study was to evaluate the potential use of a new device for quantitative assessments in disorders , such as hd in which hand - tapping tapping is abnormal .
it has two key features : first , the force required to be exerted by the subject to register a response is very low ( essentially zero ) ; this makes it feasible to use variations on the basic alternation task that can pose a greater challenge to a patient with relatively mild impairment .
figure  2d provides a particularly clear example of this , when comparing ordinary alternation with the pronation task : the difference in  for the two types of task is equivalent to over 450  ms of latency difference .
the second potential benefit is that by providing sequential information about individual responses in each direction , a great deal of data are generated in a short period of time , from which much more can be calculated than the average response time that has previously been conventionally used . for simple
tapping , the parameter  seems to provide particularly clear discrimination between subject groups ( fig .  3 ,
right ) , and it is possible that a combination of  and  together may be helpful in this respect ( fig .  4 ) . because responses in the two directions are not conflated , lateral asymmetries ( fig
2c ) , suggesting a relatively one - sided functional impairment , become obvious and can be quantified .
other kinds of idiosyncrasies , not previously noted ( such as the bimodality of fig . 
another advantage of the device is that it is small , lightweight and portable and the software is easy - to - use allowing its potential ease of introduction to a clinical environment .
this is , in other words , a simple tool that has the potential to address a general problem in studying not only in hd , but also other neurodegenerative disorders : a lack of objective and genuinely quantitative neurological tests , by which disease progress can be monitored and treatments evaluated with respect to the particular needs of individual patients .
obviously much more data are needed to provide true validation , and to discover what aspects of the interval distributions will be most useful for this process , and this work is currently in hand .
 measuring the rate of finger tapping is a technique commonly used as an indicator of impairment in degenerative neurological conditions , such as huntington s disease . 
 the information it provides can be greatly enhanced by analysing not simply the overall tapping rate , but also the statistical characteristics of the individual times between each successive response . 
 recent technological improvements in the recording equipment allow the responses to be analysed extremely quickly , and permit modification of the task in the interest of greater clinical specificity . here 
 we illustrate its use with some pilot data from a group of manifest hd patients and age - matched controls . even in this small cohort , 
 differences in the responses are apparent that appear to relate to the severity of the disease as measured by conventional behavioural tests . 
Introduction
Materials and methods
Participants
Recording the finger tapping
Results
Distributions of inter-tap intervals
Values of the underlying parameters
Discussion