our everyday activities such as walking , driving a car , or using a tool depend critically on the ability to deploy visual attention ( broman et al . , manual actions are planned on the basis of attended visual information ( baldauf and deubel 2010 ; land 2005 ; land and hayhoe 2001 ) . subjects typically focus both their gaze ( hayhoe et al . 2009 ; pelz et al . 2001 ) and their eyes at the goal of their activities ( hayhoe et al . 2003 ; mennie et al . 2007 ) , which indicates that attention , eye movements , and manual control are closely interlinked . this linkage can have implications for our everyday life ; for example , persons with impaired visual attention are more likely than others to fall while walking ( owsley and mcgwin 2004 ) and to cause car accidents ( ball et al . it therefore is of substantial concern for our  graying  society that visual attention decays in old age ( madden 1990 ; mcdowd and shaw 1999 ) , possibly due to a shrinkage of the  useful field of view  ( ufov ) . the ufov is defined as the area from which a person can process complex visual stimuli rapidly and accurately ( ball et al . 1988 ; sekuler and ball 1986 ) ; it can be substantially smaller than the visual field as determined by perimetry . in an influential study , ball and colleagues quantified ufov with a visual search task that presented targets and distracters at different eccentricities and directions in the visual field of young and older subjects , and asked them to identify each target direction as fast as possible . seniors performed less well than young participants and , most importantly , their deficit increased in the periphery , as expected if ufov indeed shrinks in old age.1 this outcome has been confirmed by numerous other studies ( coeckelbergh et al . some authors verbally instructed their subjects to look at the targets while others asked them to fixate straight ahead , but eye movements were not registered and the actual gaze behavior is therefore unknown . importantly , oculomotor behavior changes in old age : the latency and duration of saccades increase while their accuracy decreases , thus necessitating corrective and re - fixation saccades ( bono et al . 1996 ; irving et al . 2006 ; moschner and baloh 1994 ; meza et al . 2009 ; paquette and fung 2011 ) . in consequence , differences between age groups might reflect not only different processing of peripheral visual stimuli , but also a different contribution of eye movements . it has indeed been reported that the number of saccades correlates inversely with task performance ( becic et al . 2007 ; scialfa et al . 1994 ) and that young and older subjects performance no longer diverges in the periphery when the number of saccades is factored out ( scialfa et al . 1994 ) . we therefore believe that it is crucially important to register and analyze oculomotor behavior when studying ufov . if the age - related shrinkage of ufov is a fundamental phenomenon , it should be observable not only in the visual search task used in previous literature , but also in other tasks that require peripheral visual processing . to find out , the present study employs a bimanual tracking task , which , in our view , is related to real - life scenarios such as car driving and tool use . we compare a condition where subjects are asked to fixate straight ahead with one where they are free to look around , since humans rarely fixate a given object for more than a few hundred milliseconds in real life . to control for the effects of eye movements , eye position is registered along with manual performance and is factored out in a similar way as in previous work ( scialfa et al . fourteen young (   =  8 ,   =  6 ; mean age : 22.0    2.1  years ) and 14 older (   =  9 ,   =  4 ; mean age : 69.4    3.3  years ) subjects participated . since the eye data of one elderly subject were incomplete due to equipment malfunction , they were discarded . among the remaining subjects , two young and two old ones reported to prefer their left hand for writing ; the others indicated to prefer their right hand . all subjects lived independently in the community and had not participated in research on motor control or cognition within the preceding 6  months . all subjects had normal or corrected - to - normal vision , and all reported to be free of orthopedic and muscular impairment in a questionnaire completed before participating in the actual study . since all subjects arrived without help at the agreed - upon time in the agreed - upon place , properly followed our instructions , and correctly completed questionnaire items requiring memory and orientation ( e.g. , address , date of birth , medication used ) , we deemed them to be free of gross cognitive impairment . before participating , all signed an informed consent statement for this study , which was pre - approved by the authors institutional ethics committee . as illustrated in fig .  1 , subjects sat at about 70  cm distance from two vertical display areas , located symmetrically to the left and right of their body midline . the display areas were 3 , 10 , or 35  cm apart , which corresponds to a viewing angle of about 2.5 , 8.2 , or 28.0 , respectively . two joysticks , mechanically constrained to fore - aft movements , were placed at shoulder distance on a table , that is , the joystick boxes were just inside the edges of each subject s shoulders . the right joystick controlled the vertical position of a cursor in the right display area and the left one that of a cursor in the left display area . the control law for either joystick was an unstable divergent function with added noise ( jex et al . 1966 ) , and the cursors were presented as white dots of 0.85  cm diameter , shown against a black background on two 17 tft monitors . subjects grasped the joysticks with the thumb and index finger of their respective hand and were instructed to move them such as to keep both cursors near the display center , that is , to compensate the noise . for each display separation , subjects completed one trial of 60  s. it should be noted that this is a compensatory tracking task and not a pursuit tracking task ; if subjects perform it well , the cursors will hardly move up or down and rather remain quite still in the display center.fig . black dots represent the cursors that the subjects had to keep centered , and black arrows indicate the possible movement of the two joysticks . the vertical target display areas are plotted in white ( for 3  cm display separation ) and gray ( for 10 and 35  cm display separations ) . the center cross represents the fixation point ; it was displayed only in condition fix and the white brackets represent the calculated target zone ( for exemplary individual and 35  cm display separation ) schematic drawing of our experimental setup . black dots represent the cursors that the subjects had to keep centered , and black arrows indicate the possible movement of the two joysticks . the vertical target display areas are plotted in white ( for 3  cm display separation ) and gray ( for 10 and 35  cm display separations ) . the center cross represents the fixation point ; it was displayed only in condition fix and the white brackets represent the calculated target zone ( for exemplary individual and 35  cm display separation ) the subjects head and body movements were not mechanically constrained . the seat height was not adjustable , and the subjects eye height therefore varied between 126 and 130  cm depending on body size ; to look at the display center , subjects thus had to lower their gaze by 1720 , which is well within the comfort range of 1025 stated in german ergonomic guidelines . in condition fix , subjects were instructed to fixate continuously a dot mid - between the two display areas , but this instruction was lifted in condition free . the two conditions , and the three display separations within each condition , were administered to the subjects in a mixed order . the vertical distance of each cursor from the display center was registered at the frame rate of the display , 60  hz . from these data , we calculated the root mean square tracking error ( rmse ) as the root mean square distance between cursor and display center during the last 50  s of each trial averaged across the left and right cursor . vertical and horizontal positions of the head and the left eye were registered with the video - based eyelink 1000 device ( sr research ltd . ) at a sampling rate of 500  hz and a spatial resolution of < 0.1. preliminary analyses revealed that saccades from the left to the right display area and vice versa sometimes under- or overshot by a small amount , that is , horizontal eye position was outside the display area . we felt that these saccades should still count as  looking at the display areas  and not as  looking somewhere else . we therefore decided to classify subjects gaze direction with respect to two target zones extending horizontally about the left and right display center , respectively . the width of these target zones was determined individually for each subject as 2 standard deviations of horizontal eye position when repeatedly looking at the same object ( see fig .  1 for an illustration of target zones ) . having established the target zones , we calculated the following parameters for the last 50  s of each trial : number of fixations within each of the two target zonesfixation time within each of the two target zonesfixation range : width of each target zonenumber ofcrossings , that is , of saccades that crossed the screen midline from left to right or right to left . number of fixations within each of the two target zones fixation time within each of the two target zones fixation range : width of each target zone number ofcrossings , that is , of saccades that crossed the screen midline from left to right or right to left . with the 3  cm display separation , the two target zones overlapped such that some fixations could not unequivocally be classified as within or between the target zones . we therefore decided to limit the analysis of eye parameters to the display separations of 10 and 35  cm . rmse and eye movement parameters were submitted to analyses of variance ( anovas ) using the within - factors display separation and side ( dominant , non - dominant hand ) , and the between - factor age . 2007 ; scialfa et al . 1994 ) , we partialled out the effects of eye movements on seniors rmse by linear regression analysis . as a first step , we calculated the multiple linear regression of individual seniors rmse on their eye movement parameters and stored the residuals ri of each senior i. as a second step , we entered the mean eye movement parameters of young subjects into the regression equation to yield y , the predicted mean rmse of seniors if their eye movement parameters equaled those of young subjects . as a third step , we calculated the normalized rmse of each individual senior , predicted whether that person s eye movement parameters equaled those of young subjects:1\documentclass[12pt]{minimal } 
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				 \begin{document}$$ { \text{rmse } } ' = r_{i } + y $ $ \end{document}this analysis was done separately for each condition and for each display separations at which eye movements could be analyzed ( see above ) . the normalized rmse scores were submitted to the same anovas as were the original rmse scores . fourteen young (   =  8 ,   =  6 ; mean age : 22.0    2.1  years ) and 14 older (   =  9 ,   =  4 ; mean age : 69.4    3.3  years ) subjects participated . since the eye data of one elderly subject were incomplete due to equipment malfunction , they were discarded . among the remaining subjects , two young and two old ones reported to prefer their left hand for writing ; the others indicated to prefer their right hand . all subjects lived independently in the community and had not participated in research on motor control or cognition within the preceding 6  months . all subjects had normal or corrected - to - normal vision , and all reported to be free of orthopedic and muscular impairment in a questionnaire completed before participating in the actual study . since all subjects arrived without help at the agreed - upon time in the agreed - upon place , properly followed our instructions , and correctly completed questionnaire items requiring memory and orientation ( e.g. , address , date of birth , medication used ) , we deemed them to be free of gross cognitive impairment . before participating , all signed an informed consent statement for this study , which was pre - approved by the authors institutional ethics committee . as illustrated in fig .  1 , subjects sat at about 70  cm distance from two vertical display areas , located symmetrically to the left and right of their body midline . the display areas were 3 , 10 , or 35  cm apart , which corresponds to a viewing angle of about 2.5 , 8.2 , or 28.0 , respectively . two joysticks , mechanically constrained to fore - aft movements , were placed at shoulder distance on a table , that is , the joystick boxes were just inside the edges of each subject s shoulders . the right joystick controlled the vertical position of a cursor in the right display area and the left one that of a cursor in the left display area . the control law for either joystick was an unstable divergent function with added noise ( jex et al . 1966 ) , and the cursors were presented as white dots of 0.85  cm diameter , shown against a black background on two 17 tft monitors . subjects grasped the joysticks with the thumb and index finger of their respective hand and were instructed to move them such as to keep both cursors near the display center , that is , to compensate the noise . for each display separation , subjects completed one trial of 60  s. it should be noted that this is a compensatory tracking task and not a pursuit tracking task ; if subjects perform it well , the cursors will hardly move up or down and rather remain quite still in the display center.fig . black dots represent the cursors that the subjects had to keep centered , and black arrows indicate the possible movement of the two joysticks . the vertical target display areas are plotted in white ( for 3  cm display separation ) and gray ( for 10 and 35  cm display separations ) . the center cross represents the fixation point ; it was displayed only in condition fix and the white brackets represent the calculated target zone ( for exemplary individual and 35  cm display separation ) schematic drawing of our experimental setup . black dots represent the cursors that the subjects had to keep centered , and black arrows indicate the possible movement of the two joysticks . the vertical target display areas are plotted in white ( for 3  cm display separation ) and gray ( for 10 and 35  cm display separations ) . the center cross represents the fixation point ; it was displayed only in condition fix and the white brackets represent the calculated target zone ( for exemplary individual and 35  cm display separation ) the subjects head and body movements were not mechanically constrained . the seat height was not adjustable , and the subjects eye height therefore varied between 126 and 130  cm depending on body size ; to look at the display center , subjects thus had to lower their gaze by 1720 , which is well within the comfort range of 1025 stated in german ergonomic guidelines . in condition fix , subjects were instructed to fixate continuously a dot mid - between the two display areas , but this instruction was lifted in condition free . the two conditions , and the three display separations within each condition , were administered to the subjects in a mixed order . the vertical distance of each cursor from the display center was registered at the frame rate of the display , 60  hz . from these data , we calculated the root mean square tracking error ( rmse ) as the root mean square distance between cursor and display center during the last 50  s of each trial averaged across the left and right cursor . vertical and horizontal positions of the head and the left eye were registered with the video - based eyelink 1000 device ( sr research ltd . ) at a sampling rate of 500  hz and a spatial resolution of < 0.1. preliminary analyses revealed that saccades from the left to the right display area and vice versa sometimes under- or overshot by a small amount , that is , horizontal eye position was outside the display area . we felt that these saccades should still count as  looking at the display areas  and not as  looking somewhere else . we therefore decided to classify subjects gaze direction with respect to two target zones extending horizontally about the left and right display center , respectively . the width of these target zones was determined individually for each subject as 2 standard deviations of horizontal eye position when repeatedly looking at the same object ( see fig .  1 for an illustration of target zones ) . having established the target zones , we calculated the following parameters for the last 50  s of each trial : number of fixations within each of the two target zonesfixation time within each of the two target zonesfixation range : width of each target zonenumber ofcrossings , that is , of saccades that crossed the screen midline from left to right or right to left . number of fixations within each of the two target zones fixation time within each of the two target zones fixation range : width of each target zone number ofcrossings , that is , of saccades that crossed the screen midline from left to right or right to left . with the 3  cm display separation , the two target zones overlapped such that some fixations could not unequivocally be classified as within or between the target zones . we therefore decided to limit the analysis of eye parameters to the display separations of 10 and 35  cm . rmse and eye movement parameters were submitted to analyses of variance ( anovas ) using the within - factors display separation and side ( dominant , non - dominant hand ) , and the between - factor age . 1994 ) , we partialled out the effects of eye movements on seniors rmse by linear regression analysis . as a first step , we calculated the multiple linear regression of individual seniors rmse on their eye movement parameters and stored the residuals ri of each senior i. as a second step , we entered the mean eye movement parameters of young subjects into the regression equation to yield y , the predicted mean rmse of seniors if their eye movement parameters equaled those of young subjects . as a third step , we calculated the normalized rmse of each individual senior , predicted whether that person s eye movement parameters equaled those of young subjects:1\documentclass[12pt]{minimal } 
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				 \begin{document}$$ { \text{rmse } } ' = r_{i } + y $ $ \end{document}this analysis was done separately for each condition and for each display separations at which eye movements could be analyzed ( see above ) . the normalized rmse scores were submitted to the same anovas as were the original rmse scores . 2b depict the tracking error of both age groups for the three display separations and both viewing conditions ; table  1 summarizes the corresponding anova outcome . in condition free , young subjects had relatively low rmse scores for all display separations . the scores of elderly subjects were generally higher and increased distinctly at larger display separations ( effects of age , separation and age * separation in table  1 ) . in contrast to condition free , young subjects in condition fix again produced relatively low rmse scores , but this time the difference between age groups did not increase significantly with display separation ( effect of age and separation , but not age * separation , in table  1 ) . side and its interactions showed no significant effects in condition free , and only side * separation ( f(2,50 )  =  4.05 ; p  <  0.05 ) and side * separation * age ( f(2,50 )  =  5.23 ; p  < 2a exemplary tracking performance of young ( gray lines ) and older ( black lines ) subject in condition free in its original state . dashed lines represent cursor movements with 3  cm display separation , solid lines represent cursor movements with 35  cm display separation . display separations of 3 , 10 , and 35  cm are plottedtable  1anova outcome for rmse and rmsefreefixrmseagef(1,25 )  =  19.77***f(1,25 )  =  10.90**separationf(2,50 )  =  4.54*f(2,50 )  =  60.03***age * separationf(2,50 )  =  3.45*f(2,50 )  =  2.07 n.s.rmseagef(1,25 )  =  23.78***f(1,25 )  =  20.24***separationf(2,50 )  =  6.53*f(2,50 )  =  96.22***age * separationf(2,50 )  =  5.15*f(2,50 )  =  8.19**n.s . , * , * * , and * * * indicate p  >  0.05 , p  <  0.05 , p  <  0.01 , and p  <  0.001 , respectively a exemplary tracking performance of young ( gray lines ) and older ( black lines ) subject in condition free in its original state . dashed lines represent cursor movements with 3  cm display separation , solid lines represent cursor movements with 35  cm display separation . display separations of 3 , 10 , and 35  cm are plotted anova outcome for rmse and rmse n.s . , * , * * , and * * * indicate p  >  0.05 , p  <  0.05 , p  <  0.01 , and p  <  0.001 , respectively as shown in fig .  3 and table  2 , eye parameters in condition free were similar in both age groups , with fixationrange increasing at the larger display separation ( effect of separation in table  2 ) . again , the performance of subjects did not differ between their dominant and non - dominant hand for either of the measured eye parameters ( no effects of side and its interaction with age and separation ) . in contrast , eye parameters in condition fix were distinctly age - dependent ( effect of age for all parameters in table  2 ) : elderly subjects produced more saccades than younger ones into the target areas and spent more time there , thus disobeying our instructions for that condition . the fixation range and the number of crossings in young subjects , but not that of elderly subjects was much lower than in condition free which , however , could well be an artifact due to the small number of saccades in young participants . comparable to condition fix , side again influenced subjects number of fixations as a function of separation * side ( f(1,25 )  =  5.81 ; p  <  0.05 ) and age * separation * side ( f(1,25 )  =  6.54 ; p  <  0.05 ) . results show that subjects did not prefer looking at the side controlled by their dominant hand nor did they improve their performance of the non - dominant hand by permanently focusing the appropriate display side.fig . 3eye movement parameters , with symbols representing mean values and error bars representing the appropriate standard error . only display separations of 10 and 35  cm are plotted , since data from the 3  cm separation were not unequivocaltable  2anova outcome for eye parameterfreefixnumber of fixationsagef(1,25 )  =  0.67 n.sf(1,25 )  =  20.59***separationf(1,25 )  =  3.48 n.s.f(1,25 )  =  0.10 n.s.age * separationf(1,25 )  =  0.47 n.s.f(1,25 )  =  0.74 n.s.fixation timeagef(1,25 )  =  2.44 n.s.f(1,25 )  =  8.10**separationf(1,25 )  =  9.31**f(1,25 )  =  1.27 n.s.age * separationf(1,25 )  =  0.19 n.s.f(1,25 )  =  3.83 n.s.fixation rangeagef(1,25 )  =  3.77 n.s.f(1,25 )  =  19.41***separationf(1,25 )  =  47.88***f(1,25 )  =  11.30**age * separationf(1,25 )  =  0.86 n.s.f(1,25 )  =  5.14*number of crossingsagef(1,25 )  =  0.15 n.s.f(1,25 )  =  15.78***separationf(1,25 )  =  1.13 n.s.f(1,25 )  =  0.00 n.s.age * separationf(1,25 )  =  0.06 n.s.f(1,25 )  =  0.35 n.s.n.s . , * , * * , and * * * indicate p  >  0.05 , p  <  0.05 , p  <  0.01 , and p  <  0.001 , respectively eye movement parameters , with symbols representing mean values and error bars representing the appropriate standard error . only display separations of 10 and 35  cm are plotted , since data from the 3  cm separation were not unequivocal anova outcome for eye parameter n.s . , * , * * , and * * * indicate p  >  0.05 , p  <  0.05 , p  <  0.01 , and p  <  0.001 , respectively the dashed lines in fig . 2b depict the normalized tracking scores rmse , which partial out the effects of eye movements . clearly , the normalized scores increased with display separation even more than did the original scores . the bottom part of table  1 shows that in condition free , the effect of age * separation remained significant for rmse as it was for rmse ; in condition fix , it became significant for rmse which it was not for rmse . in other words , when age - related differences of eye movements were taken into account , the tracking performance of young and elderly subjects diverged with increasing display separation not only in free but also in fix . in additional analyses , we evaluated whether looking at one target zone had a differential effect on rmse at the same and at the opposite side . this was done with a generalized linear model ( glm ) approach with the factors age and side , and the regressor we yielded significance for age ( f(1,24 )  =  18.64 ; p  <  0.001 ) and the regressor ( f(1,24 )  =  12.01 ; p  <  0.01 ) , but not for other effects , notably not for the side*regressor interaction . in a second glm approach , we changed the regressor to  time spend with gaze in non - dominant target zone  ; this is not trivial , since subjects could also spend time outside both target zones . the analysis yielded only one significant effect , that of age ( f(1,24 )  =  12.83 ; p  <  0.01 ) . we conclude that looking at the dominant zone improved tracking with the dominant and with the non - dominant arm , while looking at the non - dominant zone had no reliable effect on tracking with each hand . since this holds for both age groups , it has no explanatory value for the age - related ufov shrinkage . our study compares the performance of young and elderly subjects in a bimanual tracking task , when tracking displays are presented at different eccentricities with respect to the egocentric straight ahead , while the hands remain at the same eccentricity . in condition we found that the manual tracking error of young subjects was low and did not depend on display eccentricity , while that of elderly subjects was higher and increased with display eccentricity . thus , the manual tracking data of the two age groups diverged with increasing eccentricity , as did the visual search data in previous studies where subjects were asked to fixate straight ahead ( ball et al . 2004 ; seiple et al . 1996 ; sekuler and ball 1986 ; sekuler and bennett 2000 ) . factoring out the effects of eye movements did not reduce this divergence as it did in an earlier study on visual search ( scialfa et al . 1994 ) , possibly because visual search requires fairly accurate eye movements to distinguish targets from distracters , while manual tracking of easily discernible targets does not require high oculomotor precision . in other words , we propose that the control of eye movements is degraded in old age to an extent that is critical for visual search , but not yet for manual control . in condition fix young persons obeyed this instruction well , while older ones spent about half of the time glancing at the targets . this lack of compliance could reflect deficits in the ability to inhibit automated but undesired behavior , and/or in the ability to concurrently control fixation and manual tracking : response inhibition as well as multitasking is an important component of executive functions , which are known to decay in old age ( andres et al . 2008 ; brennan et al . 1997 ; gunning - dixon and raz 2003 ; somberg and salthouse 1982 ) . manual tracking was again poorer in elderly subjects , but unlike in condition free , the errors of both groups increased at the largest display eccentricity . in contrast to condition free , the tracking data of young and elderly subjects in condition fix did not reliably diverge with increasing display eccentricity . however , the divergence became statistically significant when the effects of seniors illicit eye movements were factored out . from this we conclude that our elderly subjects partly compensated their peripheral deficits by disobeying our instructions and directing their gaze at the display areas . it should be noted that our condition fix , with eye movements factored out , is analogous to previous ufov paradigms where the effects of eye movements were restricted . the main difference is that we used manual tracking , while previous studies were based on visual search . since the performance of young and older subjects diverged with increasing eccentricity not only in the earlier studies but in the present condition fix as well , our findings suggest that age - related ufov shrinkage is not an isolated phenomenon limited to visual search , but rather extends to manual skill tasks . our condition free differs from previous ufov paradigms in that subjects could direct their gaze at the stimuli . our eye registrations show that both age groups took advantage of this in a similar fashion . in spite of this fact , tracking performance of young and older subjects again diverged with increasing eccentricity , and fig . 2 illustrates that the divergence was comparable to that in condition fix when the effects of eye movements were factored out . in fact , defining divergence as the difference between young and old subjects at 35  cm display separation minus that at 10  cm separation , the amount of 3.5  cm calculated for free is very similar to 3.3  cm calculated for fix . it therefore is conceivable that the divergence in both conditions , as well as that in previous visual search tasks , is due to age - related decrements of a central stage , accessible for visual search , manual tracking , and possibly many other visuomotor tasks . the divergence seems not to be explainable by oculomotor deficits in old age , since it persisted in condition free  and only emerged in condition fix  after the effects of eye movements were factored out .
earlier work suggests that the area of space from which useful visual information can be extracted ( useful field of view , ufov ) shrinks in old age . 
 we investigated whether this shrinkage , documented previously with a visual search task , extends to a bimanual tracking task . 
 young and elderly subjects executed two concurrent tracking tasks with their right and left arms . 
 the separation between tracking displays varied from 3 to 35  cm . 
 subjects were asked to fixate straight ahead ( condition fix ) or were free to move their eyes ( condition free ) . 
 eye position was registered . in free , young subjects tracked equally well at all display separations . 
 elderly subjects produced higher tracking errors , and the difference between age groups increased with display separation . 
 eye movements were comparable across age groups . in fix 
 , elderly and young subjects tracked less well at large display separations . 
 seniors again produced higher tracking errors in fix , but the difference between age groups did not increase reliably with display separation . 
 however , older subjects produced a substantial number of illicit saccades , and when the effect of those saccades was factored out , the difference between young and older subjects tracking did increase significantly with display separation in fix . 
 we conclude that the age - related shrinkage of ufov , previously documented with a visual search task , is observable with a manual tracking task as well . 
 older subjects seem to partly compensate their deficit by illicit saccades . 
 since the deficit is similar in both conditions , it may be located downstream from the convergence of retinal and oculomotor signals .