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We analyze behavior to gain insight into the intelligence behind it. In particular, we study how human and nonhuman primates grasp and use objects and we do this to learn about action planning and control. We believe that a useful way to study action planning is to observe changes in behavior as a function of the behavior that follows. If an action differs depending on the subsequent action, the anticipatory effect can be said to reflect planning. 

Altering an object manipulation not only on the basis of the immediate task demands but also of the next action to be performed requires second-order motor planning abilities. Thus, the form of an organism’ response allows measuring anticipatory abilities. Grasping an object seems so simple that we rarely think about it. Yet even casual observation of very young children shows that the skill takes time to develop. When does grasp planning begin to appear, both in individuals (ontogenetically) and over the course of evolution (phylogenetically)?

Second-order motor planning in humans develops over years and what makes it so difficult for young children to comfortably and differentially adjust their initial grasps in several manipulation tasks is a matter of ongoing research and debate. Studies on human and nonhuman primates have demonstrated that the rudimentary motor planning abilities appear to be shared across species. These studies have shown that subjects grasp objects to be moved from one location to another in a way that affords a “comfortable” final posture and a good control on the objects, even if this involves an “uncomfortable” initial posture. This effect has been termed the end-state comfort effect by Prof. David Rosenbaum in 1990.

We investigated second-order planning abilities in capuchin monkeys both in captivity and in the wild. Capuchins were first tested in a task requiring individuals to grasp a dowel inserted into a vertical tube and to bring the baited tip at the bottom to the mouth. They spontaneously performed actions in which the forearm was in pronation or radially rotated to turn over the dowel and eat the food from its baited tip. In general, these comfortable actions, in which the initial orientation of the forearm afforded a more comfortable final position in bringing the dowel to the mouth, were performed as frequently as the uncomfortable actions. It is possible that capuchins’ higher manual dexterity may elicit less consistent second-order motor planning in this task, as species that show a low rate of second-order planning have more means of compensating for inefficient initial postures. 

In a following study, in a task in which capuchins had to choose how to grasp a horizontal baited dowel containing food in its right or left end, they used a radial grasp (i.e., with the thumb-side oriented towards the baited end) with the forearm in pronation to bring it to the mouth. Similarly to 19 months old human infants tested in an analogous task, capuchins switched the hands between trials to use a radial grip with the forearm in pronation. This suggests that object orientation is an important constraint guiding action selection in capuchin monkeys. 

Video 1 from Sabbatini et al 2016 Behavioural Brain Research
Cammello, a male capuchin, uses a radial grasp with the forearm in pronation to bring the baited right end of the dowel to the mouth.

Age-related differences in nonhuman primates have been poorly investigated. We investigated age-related differences in motor planning of wild capuchin monkeys in the dry forest habitat of Piauí (Brazil) by recording their strategies to bring a baited stick to the mouth. In particular, we investigated anticipatory motor planning of infant, juvenile and adult wild capuchin monkeys grasping a horizontally-positioned stick baited to the left or right side. We recorded the grasps capuchins used to bring the baited end of the stick to the mouth. We found that motor planning abilities improved with age, as in humans, and adult capuchins used efficient grips significantly more frequently than infants.

Video 2 from Truppa et al 2020 Developmental Science
Cacau, an infant male capuchin, uses a radial grasp to bring the baited end of the stick to the mouth.

We study also what capuchins do when they have to use a dowel as a tool to interact with an out-of-reach food target. Capuchin monkeys showed action-selection planning when using tools to interact with distally located targets: they consistently used a radial grip (with the thumb-side oriented towards the centre of the dowel) to grasp a dowel that was positioned horizontally at different orientations and to dislodge an out-of-reach food reward.

Video 3 from Sabbatini et al 2016 Behavioural Brain Research
Sandokan, a male capuchin, uses a radial grasp with the forearm in pronation to dislodge the out-of-reach food reward.

An intriguing issue to consider is that capuchins showed more pronounced planning abilities when the principal axis of the dowel is aligned with the horizontal plane compared to the vertical plane. How the spatial orientation of the object can affect monkeys’ motor planning abilities in grasping actions seems to be a crucial point that deserves further investigations. 

Researchers involved

Dr. Gloria Sabbatini,  Dr. Valentina Truppa and Dr. Elisabetta Visalberghi

  • Scientific publications

  • Truppa, V., Spinozzi, G., Laganà, T., Piano Mortari, E., Sabbatini, G. (2016). Versatile grasping ability in power grip actions by tufted capuchin monkeys (Sapajus spp.). American Journal of Physical Anthropology, 159: 63-72.

  • Truppa V., Spinozzi G., Laganà T., Piano Mortari E., Sabbatini G. (2016). Flessibilità nell'uso delle prese di forza nei cebi dai cornetti (Sapajus spp.), dallo studio della manipolazione degli oggetti al legame tra cognizione e azione. Sistemi Intelligenti, XXVIII: 69 - 82. DOI: 10.1422/83836

  • Sabbatini G., Meglio G., Truppa V. (2016). Motor planning in different grasping tasks by capuchin monkeys (Sapajus spp.). Behavioural Brain Research, 312: 201-211.

  • Truppa V., Sabbatini G., Izar P., Fragaszy D., & Visalberghi E. (2020). Anticipating future actions: Motor planning improves with age in wild bearded capuchin monkeys (Sapajus libidinosus). Developmental Science

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