The human head weighs approximately 8% of the total body weight. This imposes limits on the gaze accompanying larger and more rapid shift, by using head movements alone [1, 2]. The visual resolution of the retina is highest at the central fovea and decreases exponentially with increasing distance from the fovea [3, 4]. Saccadic eye movement (saccade) that quickly and precisely matches the fovea to the visual object accompanying an abrupt shift is important for perception of the object in various sports [5,6,7,8,9]. Saccades to the appearing visual object are called visually guided saccades or pro-saccades.
The pro-saccade is controlled via supraspinal pathways, including the lateral geniculate nucleus, superior colliculus, reticular formation, occipital cortex, posterior parietal cortex, parietal eye field, and frontal eye field [10, 11]. The functions associated with pro-saccade are detection of the visual target, identification of the target position, saccade preparation, disengagement of fixation, and saccade generation [10, 12]. Disengagement of fixation has been investigated using overlap and gap conditions, in which the timing between switch-off of the central fixation point and lighting of the visual target are operated. The overlap condition is one in which the peripheral visual target emerges before or just after switch-off of the fixation point [13,14,15,16]. Under the overlap condition, intentional disengagement of fixation is important for saccade generation, which is particularly related to the frontal eye fields in the higher saccade system [13]. In relation to the involvement of the higher saccade system associated with intentional disengagement, the pro-saccade reaction times under the overlap condition show a unimodal distribution with the peak ranging from 180 to 200 ms [13,14,15]. On the other hand, the gap condition is one in which the central fixation point is turned off some time (mainly 200 ms) before emergence of the peripheral visual target [13, 15, 16]. Under the condition, the higher saccade system is less involved, and instead, the subcortical pathways and primary visual cortex associated with the reflexive disengagement of fixation are more prominent compared with the overlap condition [13, 17, 18]. Under the gap condition, reaction time shows a bimodal distribution, with the peak at 100–120 ms associated with the reflexive disengagement of fixation (first peak), in addition to the abovementioned peak of 180–200 ms associated with the intensive disengagement of fixation, similar to the situation in the overlap condition (second peak) [13, 17, 18].
However, the previous studies suggested that substantial individual variation exists in the distribution of saccadic reaction times under the gap condition, which shows abovementioned bimodal distribution and unimodal distribution with the peak at 100–120 ms [13, 14, 18]. The individual variation has been investigated from the perspective of learning and training related to the disengagement function of fixation [13, 15, 16]. Sports experience is regarded as one form of learning and training. However, whether the distribution of pro-saccade reaction times under the gap condition is influenced by sports experience has yet to be investigated. To investigate the influence of sports experience on the distributions, we noted that the functions of disengagement of fixation differ markedly between basketball and table tennis. For basketball, attention to the whole visual field and/or parallel attention to various objects including the ball and players in the peripheral visual field are important [19,20,21,22]. In particular, intended attention to the visual object with inhibition of reflexive disengagement of fixation to fakes and feints is important. When performing saccade to the appearing visual object in basketball, disengagement of fixation would be intensive. We thus predict that intensive disengagement would be enhanced with increasing experience in playing basketball. We presumed that for the basketball group, the pro-saccade reaction times under the gap condition show a bimodal distribution, consisting of a peak at 100–120 ms associated with the reflexive disengagement of fixation, in addition to the peak at 180–200 ms, associated with the intensive disengagement of fixation. In table tennis, the distance between players is only about 3 m and the playing time in which to return the fast-moving ball is markedly short compared to other ball-sports [6, 23, 24]. Two points in the flight of the ball are crucial for the gaze in table tennis: the beginning of the trajectory and just before the execution of the strike [25]. Because the interval between these two points is approximate 200 ms and markedly short, more intensive disengagement of fixation may result in gaze to the latter point being too late [6, 25]. Therefore, in table tennis, reflexive disengagement of fixation is important. We predict that the reflexive disengagement of fixation would be enhanced with increasing experience in table tennis. Considering these, we presume that for the table tennis group, pro-saccade reaction time under the gap condition shows a unimodal distribution with the peak ranging from 100 to 120 ms, associated with the reflexive disengagement of fixation. Furthermore, we presume that for non-sporting control group, the pro-saccade reaction time under the gap condition shows mixed-type of bimodal and unimodal distribution, because the substantial individual variation exists in the distribution.
In the present study, we examined the distribution of the pro-saccade reaction time under the overlap and gap conditions, for the basketball club, table tennis club, and control groups. By comparing those distributions, we investigated the influence of sports experience on the distribution of the pro-saccade reaction time under the gap condition. The working hypotheses were as follows.
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(1)
The pro-saccade reaction time under the overlap condition would show a unimodal distribution for three groups.
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(2)
The pro-saccade reaction time under the gap condition would show a distinct bimodal distribution for the basketball group and show a distinct and early unimodal distribution for the table tennis group.