Fixation Biases towards the Index Finger in Almost-Natural Grasping

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.

Gaze when reaching to grasp a glass

“…People often fixate closer to the position that will be contacted by the index finger. The critical gaze bias toward the index finger does not seem to be due to biomechanical reasons as it has been found for a variety of grasping postures (Brouwer, Franz, & Gegenfurtner, 2009;Bulloch, Prime, & Marotta, 2015;Cavina-Pratesi & Hesse, 2013;de Grave et al, 2008;Prime & Marotta, 2013;Voudouris et al, 2016). What might be the reason for such gaze biases?…”

“…However, such circumstances do not induce a clear bias toward the thumb's contact point. The bias toward the index finger's contact point has been suggested to be related to the index finger being the first digit to make contact with the object (Cavina-Pratesi & Hesse, 2013), but we have shown that this cannot be the main reason (Voudouris et al, 2016). Here, we explore whether we can explain these biases by the (lack of) visibility of the contact points.…”

“…The bias to fixate near the index finger's contact point becomes weaker when accuracy requirements are increased for the thumb (Brouwer et al, 2009), when the object's shape is asymmetric (Desanghere & Marotta, 2015), and when one is initially looking closer to the thumb's contact point (Voudouris et al, 2016). However, such circumstances do not induce a clear bias toward the thumb's contact point.…”

Reach-to-Grasp Behavior

“…If both grasping points are visible, many studies report a tendency to look near the grasping point for the index finger rather than the thumb for various types of objects and viewing geometries (Brouwer et al 2009;Cavina-Pratesi and Hesse 2013;de Grave et al 2008;Desanghere and Marotta 2011;Voudouris et al 2016). This tendency is in some way related to grasping, as subjects (initially) direct their gaze to the center of the object during free…”

“…The Control of the Reach-To-Grasp Movement 183 viewing or when required to make perceptual judgments (Brouwer et al 2009;Desanghere and Marotta 2011). However, what aspect of grasping causes this tendency is not clear: many hypotheses could be rejected (Voudouris et al 2016).…”