Abstract
We use visual information to guide our grasping movements. When grasping an object with a precision grip, the two digits need to reach two different positions more or less simultaneously, but the eyes can only be directed to one position at a time. Several studies that have examined eye movements in grasping have found that people tend to direct their gaze near where their index finger will contact the object. Here we aimed at better understanding why people do so by asking participants to lift an object off a horizontal surface. They were to grasp the object with a precision grip while movements of their hand, eye and head were recorded. We confirmed that people tend to look closer to positions that a digit needs to reach more accurately. Moreover, we show that where they look as they reach for the object depends on where they were looking before, presumably because they try to minimize the time during which the eyes are moving so fast that no new visual information is acquired. Most importantly, we confirmed that people have a bias to direct gaze towards the index finger’s contact point rather than towards that of the thumb. In our study, this cannot be explained by the index finger contacting the object before the thumb. Instead, it appears to be because the index finger moves to a position that is hidden behind the object that is grasped, probably making this the place at which one is most likely to encounter unexpected problems that would benefit from visual guidance. However, this cannot explain the bias that was found in previous studies, where neither contact point was hidden, so it cannot be the only explanation for the bias.
Highlights
Visual information guides most of our everyday actions
We found that the fixation bias that is reported in the literature [17,18,19,20] is not limited to grasping off a frontal surface: people fixate near where their index finger will make contact with an object when the task is to grasp and lift the object off a horizontal surface
A question that arises is whether the initial fixation was really to the position at which the index finger would make contact
Summary
Visual information guides most of our everyday actions. Human gaze behaviour has been examined in many such actions, such as making tea and sandwiches [1,2], driving a car [1,3], walking [4,5], or engaging in sports [6,7,8,9]. Studies that examined gaze during grasping while both digits’ contact points were visible found that people tend to initially fixate either close to the anticipated contact point of their index finger or near the object’s centre of mass [17,18,19,20]. The digits’ contact points were about orthogonal to the line of sight and both digits’ trajectories towards their contact points were always visible We used this configuration because if participants had been sitting at a table, reaching for an object that was some distance away at chest or eye height, the index finger would primarily be further away than the thumb at the moment that the object was grasped [10,27]. The rationale of the details of each experiment is combined with the presentation of details that are specific to that experiment when introducing the separate experiments
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