Humans use anticipatory mechanisms to scale fingertip forces when grasping an object.  The sensory feedback signaling object properties acquired through repeated manipulations of the same object allows subjects to generate sensorimotor memories associating digit forces with the object.  We have recently provided evidence for anticipatory control mechanisms in the kinematic domain by showing that subjects change the spatial distribution of their digits on an object when its center of mass (CM) can be anticipated ('Predictable' condition; Lukos et al., 2007). However, when object CM - hence digit forces - could not be predicted on a trial-to-trial basis ('Unpredictable' condition), subjects used a 'default' distribution of contact points on the object and performance was characterized by significantly larger object rolls than in the 'Predictable' condition (Lukos et al., 2007).

The present study was designed to quantify the extent to which these effects of sensorimotor memories on movement planning could be mimicked by providing subjects with verbal or visual
cues about CM location.  This question was addressed by providing these cues (declarative knowledge) while changing object CM location from trial to trial, hence depriving subjects from developing sensorimotor memories through practicing manipulation over consecutive trials (procedural learning). We found that declarative knowledge of object CM enabled subjects to modulate contact points but not to anticipate forces to the same extent associated with procedural learning. These findings suggest that effect of sensorimotor memories - generated by procedural
learning - on movement planning cannot be fully substituted by declarative knowledge of object properties. These results emphasize the critical role of tactile feedback for grasp planning and execution.