The effects of task and content on digit placement on a bottle

In addition to hand shaping, previous studies have shown that subjects adapt placement of individual digits to object properties such as its weight and center of mass. However, the extent to which digit placement varies based on task context is unknown. In the present study, we investigated where subjects place their digits on a bottle when the upcoming task (lift versus pour) and object content (i.e., amount of liquid: empty, half, and full) were manipulated. Our results showed that subjects anticipated both the upcoming task and content by varying digit placement when grasping the bottle prior to the onset of manipulation. Specifically, subjects increased the vertical distance between the thumb and index finger for pouring but not for lifting. This larger moment arm might have been established to decrease the amount of force required to tilt the bottle. Content also affected digit placement: the digits were placed higher and were wrapped more around the bottle with increasing content. This strategy may maximize grip surface contact, and hence grasp stability. These findings extend previous research showing that grasp planning not only takes place at a macroscopic level (whole-hand position relative to an object), but also at the level of individual digit placement. This finer level of control appears to be sensitive to the expected mechanical properties of the object and how these may affect grasp stability throughout the upcoming manipulation.