Visual search during interception in children with Spastic Unilateral Cerebral Palsy

According to Glover (2004), an interceptive action can be guided by two
strategies. Firstly, the action can be planned before the start or at the
beginning of the action, or secondly the action can be controlled online in a
continuous way. In the planning strategy an individual will select and initiate
a certain motor program depending on the environment and the goals of the
individual. For instance when grasping a moving ball, the person will estimate
the velocity of the ball and plan their action according to this velocity. In
order to let this strategy work properly it is important to take into account a
wide variety of visual and cognitive information. An individual will process
the size of the ball and the spatial relationship between the individual and
the ball. Besides this, the weight of the ball is of importance. Is the ball
light or heavy and therefore will it have to be grasped with one or two hands?
All these aspect will be taking into consideration and will be dependent not
only on vision but also on prior knowledge and reference to stored memories.
Besides planning the movement in advance, the individual can use an
online control strategy to help guide the movement towards the target. This
online strategy allows to use the spatial relationships between the actor and
the environment will be used to monitor and adjust the motor program if
necessary. In the example of the moving ball, the person will have to look at
the ball and adjust the path of the grasping hand when necessary. This strategy
has the benefit of allowing for fast processing and similarly fast online
adjustments. This does not only mean that the strategy is quickly updated but
also that its memory is of short duration. If visual information regarding the
target is removed, the control strategy will gradually decay over a period of
more or less two seconds. Therefore, this strategy is dependent on the visual
information for the online control of an interceptive task.
Performing an action task which requires walking towards and
intercepting a moving object can be guided through both strategies. When the
action task is guided by a planning strategy, the actor uses the information of
the moving object at the start to plan the movement toward the interception
area. This will not require a continuous observation of the object
to-be-intercepted and can be considered as a more or less automatic strategy.
On the other hand, the online control strategy will require a constant visual
observation of the moving object. This is necessary when something unexpected
is occurring, like a change in ball velocity.
Children with spastic unilateral cerebral palsy (SUCP) have a
non-progressive brain damage on one side of the brain which causes besides
obvious motor impairments also problems in the planning and control of an
action (Chen & Yang, 2007; Steenbergen et al., 2000). In pilot measurements
with children with SUCP differences were found in visual search behaviour when
intercepting a constantly moving ball while walking. The results showed two
different visual search strategies. Some of the children focussed their gaze
continuously on the moving ball, which seemed to plead for the use of an online
control mechanism. Other children seemed to focus their gaze in the beginning
of the trial towards the ball and afterwards switched their gaze towards the
interception area. This suggested that they use a planning strategy. Why they
appeared to use different strategies is unclear. A planning strategy would
appear to be sufficient since the ball was not changing velocity. A possible
explanation can be found in the planning deficits of children with SUCP.
Several authors have claimed that children with SUCP with damage to the left
side of the brain suffer from planning deficits (Steenbergen et al., 2000,
2007; Te Velde et al., 2005). This because the brain areas involved in planning
are mostly located on the left side of the brain (Johnson-Frey et al., 2005).
If indeed planning deficits exist, it is expected that the children with damage
on the left side of the brain rely only on the online control strategy. In
order to perform properly, they would have the object-to-be intercepted under
continuous visual control.

Therefore the goal of this study is to investigate the use of the two
strategies in children with SUCP by means of eye movements. In order to
investigate these strategies, manipulations will be performed to provoke the
use of a certain strategy. If the planning strategy is being used then an
occlusion in the middle of the trajectory of the ball would not affect the
performance, while the control strategy would be affected by this. If planning
deficits are present then we expect the control strategy to take over. If this
is not possible, like in the condition where a part of the trajectory is
occluded, then performance will decrease. In addition, it is expected that
differences will appear when performing the task with the impaired instead of
the less-impaired hand. We expect that performing the grasping task with the
less-impaired hand will be more automatic than performing the task with the
impaired hand and therefore the individual will depend on the planning
strategy. It is expected that when performing the task with the impaired hand
the individual will rely more on the control strategy.

The study is conducted in two separate experiments. In experiment 1 we will
test planning versus online control based on eye-movements. In the second
experiment we will investigate the effect of the grasping hand on the
eye-movements. We chose to separate the two, in order to reduce the load on the
participants.
The control group will be measured first. The familiarity of the investigators
with the experimental protocol and equipment will increase after several
sessions. This way we hope to build some routine and to reduce the total
experimental time by the time the children with SUCP will participate. By
experience, we know that maintaining their attention to the task is a bigger
problem in children with SUCP than in controls.

Benefits: This research will give further inside into the perception-movement
coupling. And in the (near) future, this information can improve treatment of
the perception motor difficulties in children with SUCP.

The study will not have a direct beneficial effect for the children.

Risks/Burdens: We do not expect any special risks for the participants. The
non-invasive methods used in the planned experiment (goggles and potentiometer)
do not represent a risk in performing the task.

Group relatedness: There is an opportunity to look at developmental
differences. Besides this, there is also a possibility for practical
implications. Recent literature showed than visual search is a possible
intervention method (Crowdy et al., 2002). By training visual search one*s
movement can be improved. Early intervention will be more meaningful and
effective.