Introduction 2005 2004 1996 1987 2000b 1995 1991 1991 2006 2006 2005 2006 2000a 2004 2005 2005b 1995 1996 1991 2000b 1996 2005a 2006 2000a 2004 2006 2005 2001 In the present study, we examined eye–hand coordination in an unconstrained object prehension task in individuals with hemiparetic CP and neurologically healthy controls. To manipulate task difficulty, an obstacle was present in half of the trials, inducing a more complex transport trajectory. Participants performed the task with each hand separately. Based on the registration of eye and hand movements, we calculated several measures of temporal and spatial aspects of eye–hand coordination. Besides giving a first descriptive account of gaze patterns during object manipulation in a population with congenital hemiparesis, we addressed two hypotheses. Our first and main research question concerned the way in which individuals with hemiparetic CP adapt eye–hand coordination to the sensorimotor impairments of their affected hand. We hypothesized that individuals with CP would use increased visual monitoring when they perform the task with their AH as compared to performance with the LAH and to performance of control participants. Second, we examined eye–hand coordination when the task was performed with the LAH in participants with CP. Based on the previously established deficits in action planning, we hypothesized that gaze patterns are less anticipatory in general, in individuals with CP. Given the limited knowledge on visumotor control in CP and on the role of eye-movements for action planning, this second research question was much more exploratory than the first. Methods Participants 1971 1985 1968 1 Table 1 Participant information for the participants with cerebral palsy Participant Age Paretic side a, c b, c CP1 15 Right 15/38 0/33 CP2 17 Left 24/52 0/42 CP3 17 Left 16/32 6/27 CP4 15 Right 15/43 0/33 CP5 14 Left 18/42 3/33 CP6 19 Right d d a b c d All participants received the same reimbursement (€6 per hour) for taking part. The study was approved by the local ethics committee and performed in accordance with the standards laid down in the 1964 Declaration of Helsinki. Setup and procedure 1 1 Fig. 1 top view blue green cylinder blue green disc cylinder red ball dashed line blue  x  z  y vertical axis  For each participant, the experiment consisted of eight conditions in a 2 × 2 × 2 factorial design, with the factors hand (AH/LAH for the participants with CP, NPH/PH for controls), obstacle (present, absent) and side (blue, green). In each of these conditions, five trials were performed, yielding a total of 40 trials per subject. The factors hand and obstacle were blocked while side was randomized within blocks. The order of the blocks was partially counterbalanced between participants. Prior to each trial, the participant was instructed to rest the “inactive” hand in his/her lap, such that it did not interfere with the task. 1 Data acquisition  x  y  z 1 Setup and procedure  xy Eye movements Preprocessing The raw Optotrak data were partially interpolated with cubic spline interpolation (up to ten successive samples, corresponding to 80 ms) and low-pass filtered (third order Butterworth filter with cut-off frequency of 10 Hz). Hand velocity was computed by 3-point numerical differentiation. 1 Data reduction Hand movements Hand movements were identified using an absolute velocity threshold (0.20 m/s) and a direction criterion (start of a new movement indicated by a reversal of horizontal direction). These criteria provided the algorithms for the semi-automatic custom-written selection routines for the segmentation of the trials. Based on this segmentation, the grasp time (interval between the hand reaching and leaving the object region) and hand movement duration (interval between the hand leaving the object and reaching the target region) were determined. Analysis of eye–hand coordination was confined to the object transport phase that is from onset of the hand movement away from the object region till the end, reaching the target region. The duration of this movement was determined. Eye movements 3 1998 1988 p t Subsequently, for each trial the “object-leaving” and the “target-reaching” saccade were determined (in many trials these saccades coincided since there was a single gaze shift from object to target). Automatic detection of these saccades was complicated by the fact that precision of gaze data was not sufficient for a procedure based on landmark regions around object and target. Therefore, the following semi-automatic two-step procedure was adopted. First, fixations to object and target were determined automatically, assuming that these occurred before (up to −600 ms) the beginning and at the end of object transport. Fixation periods were defined as intervals of at least 200 ms, during which, the standard deviation of absolute gaze fixation was smaller than 5 mm. Note that this automatic procedure did not use absolute landmark regions but relative position information—this was possible because the object and target were placed at the lateral extremes of the working region of the experiment. For the obstacle, an analogous procedure was not possible. Choices of this first step were inspected trial by trial and manually corrected if necessary. Obstacle fixations were not taken into account since it was not always possible to reliably distinguish them from other, frequently occurring intermediate fixations between object and target. Second, an automatic routine was used to detect the object-leaving saccade, defined as the first saccade, at the end of which, gaze had moved more than 10° horizontally relative to the object fixation. Based on an analogous 10° criterion, the target-reaching saccade was determined at the end of object transport. Thus, small (<10°) corrective saccades at the end of object transport were ignored in the definition of the target-reaching saccade. Eye–hand coordination Visual monitoring during object transport was quantified by the number of intermediate fixations, that is, the number of gaze shifts minus one occurring between the object-leaving and the target-reaching saccade. Note that this number does not include potential small corrective saccades at the end of object transport, since the target-reaching saccade was defined as the last saccade of horizontal amplitude >10° reaching the target region. MOA MTA MOA MTA MOA MTA MOA MOA MTA) Statistical analysis Data reduction t t Results Of the total 15 × 40 = 600 trials, seven trials were entirely rejected due to insufficient data quality (no more than two trials in any individual participant). Sample trials 2 3 Data reduction MOA MTA MOA MTA Fig. 2 a c b c dashed lines Vertical arrows MOA MTA  Fig. 3 a c b d 2 dashed lines Vertical arrows MOA MTA  2 3 MOA Eye–hand coordination Dependent variables 2 Table 2 Mean and standard deviation (between participants) of dependent variables Dependent variable Participants with CP Control participants a b c d No obstacle Obstacle No obstacle Obstacle No obstacle Obstacle No obstacle Obstacle Hand movement duration (s)   Mean 0.81 1.05 1.05 1.60 0.61 0.83  0.65 0.87  SEM 0.03 0.05 0.03 0.15 0.01 0.02 0.02 0.03 Object grasp time  Mean 0.22 0.25 1.07 1.16 0.063 0.071 0.086 0.094  SEM 0.03 0.04 0.14 0.16 0.009 0.01 0.017 0.015 Number of intermediate fixations  Mean 0.24 0.82 0.76  1.30 0.06  0.73  0.07  0.61  SEM 0.09 0.34 0.24 1.21 0.04 0.22 0.06 0.26 Movement onset asynchrony (% hand movement duration)  Mean −6.88 1.53 13.4  14.62 −4.45  0.02 −3.14 3.84   SEM 5.81 4.96 4.37 2.12 4.84 4.21 4.84 4.37 Movement termination asynchrony (% hand movement duration)  Mean −76.8  −61.2  −49.5  −51.4  −75.7  −65.6  −71.5  −62.2   SEM 5.39 6.87 2.84 3.64 4.39 3.78 4.64 3.56 a b c d p p 4 Fig. 4 Mean ± SEM of the movement onset asynchrony (MOA) as a function of participant group, task hand and obstacle presence. MOA was normalized with respect to hand movement duration  F p F p MOA 4 F p F p MOA MTA F p F p F p Discussion The main question we pursued in the present study was whether and in which way individuals with hemiparetic CP adapt eye–hand coordination to their sensorimotor impairments, in particular, when actions are performed with the affected hand (AH). We hypothesized that participants with CP would more closely monitor actions performed with their AH, compared to actions performed with their less affected hand (LAH) and compared to neurologically healthy control participants using either hand. Qualitatively, eye–hand coordination patterns were similar among control participants and participants with CP, regardless of the hand used to perform the task. That is, anticipatory saccadic gaze shifts were used to fixate future “action sites”—such as object, target, or intermediate fixations—in advance. Notwithstanding these qualitative resemblances, a more fine-grained analysis of temporal and spatial aspects of eye–hand coordination did confirm our hypothesis of increased visual monitoring when moving with the AH. Depending on the measure employed, this effect was present when comparing performance with the AH to control participants using either hand (increased number of intermediate fixations), or in addition for comparing performance with the AH to performance with the LAH in participants with CP (delayed gaze departure from object and hand, i.e., a longer MOA). 2003 2000a 2004 2005 MOA MOA MTA n n 2001 2001 MOA 2003 2001 n