Biomedical Engineering Reference
In-Depth Information
One of the first methods used to analyze the visual information gathered by an
athlete consists in interviewing him [
21
]. The use of questionnaires allows athletes
to formalize and transcribe their impressions about the actions they performed and
the perceptual information necessary for decision. Nevertheless, this methodology is
based on the feelings of the subject after the experiment, without any time pressure.
The relative importance of kinematic variables in the decision-making is then very
difficult to quantify. In general, this technique, based on subjective measures of
perception, does not accurately characterize the visual information gathered by the
subject.
Methodologies based on video uses a video clip in which an opponent is currently
conducting an action. In this setup, a camcorder is placed at subject's eye position
during a real game situation. For example, to study the visual information gathered
by a tennis player when his opponent serves, the sequence is performed by placing
the camera on the bottom line of the short side of the court. If one is interested now in
analyzing visual information of a football goalkeeper during a penalty kick, the video
camera is positioned at the center of the goal line. Although it is not always respected
during the experiments, the perspective of the athlete is fundamental for a realistic
movie. Once the video is acquired, it is projected onto a screen and the subject has
then to predict the final outcome of the action displayed. Two techniques can then be
used to evaluate the visual information uptake: (i) the reaction time (ii) the temporal
and spatial masking. The technique of the reaction time is based on the time taken by
an observer to respond correctly to a task. In other words, when using this technique,
the subject has as much time as he wants to perform his task. The duration of the
response is then correlated with the accuracy of the response [
1
]. Studies using this
method have demonstrated the superiority of experts over novices in the brevity and
precision of the response [
5
,
59
,
63
]. But this technique does not know the precise
time and location of the visual information gathered. The second technique based
on occultation is more frequent. In this methodology, two procedures are used. The
first one consists in using temporal occlusion and implies to cut the video at different
times to get several key sequences, each with different visual information (motion
shooting, ball flight, etc.) [
2
,
33
,
34
,
45
]. The second procedure is a spatial occlu-
sion where certain visual cues are masked in the video clip in order to see if they are
used or not by the participant for performing the action. In both conditions (tempo-
ral or spatial occlusion) the instructions given to subjects viewing these sequences
(repeated several times in random order) are then to predict the final outcome of the
observed action [
40
,
47
]. The way the subject responds may vary from one study to
another: verbal responses [
7
,
45
], written directions [
17
,
38
,
39
,
58
] or button pressed
[
25
,
26
,
54
]. Means of response do not modify the response once it has been given.
More elaborated means were employed such as using a joystick to save the changes
made by the subject during the experiment [
46
,
47
]. The advantage of temporal
occultation is to identify a relation between the perceptual processes involved and
some key instants or visual cues of the video clip. It is thus possible to determine
precisely at which key moment the subject effectively predicts the outcome of the
action or which visual cues are used during the decision-making process. The visual
information present at this key moment or in this visual area is then considered as
