Skilled object manipulation requires knowledge, or internal models, of object dynamics relating applied force to motion , and our ability to handle myriad objects indicates that the brain maintains multiple models . Recent behavioral studies have shown that once learned, an internal model of an object with novel dynamics can be rapidly recruited and derecruited as the object is grasped and released . We used event-related fMRI to investigate neural activity linked to grasping an object with recently learned dynamics in preparation for moving it after a delay. Subjects also performed two control tasks in which they either moved without the object in hand or applied isometric forces to the object. In all trials, subjects received a cue indicating which task to perform in response to a go signal delivered 5-10 s later. We examined BOLD responses during the interval between the cue and go and assessed the conjunction of the two contrasts formed by comparing the primary task to each control. The analysis revealed significant activity in the ipsilateral cerebellum and the contralateral and supplementary motor areas. We propose that these regions are involved in internal-model recruitment in preparation for movement execution.

Using a precision grip-lifting task, we examined how sensorimotor memory for weight asymmetry transfers across changes in hand and object configuration. We measured object tilt when participants lifted a visually symmetric box with an offset centre of mass. Transfer was assessed after participants lifted the box 10 times, during which the large tilt observed in the first lift was reduced. Consistent with previous work of Salimi et al. (J Neurophysiol 84:2390-2397, 2000), we found that when the object was rotated 180 degrees , participants failed to update their sensorimotor memory appropriately. Instead, participants acted as if the object did not rotate and negative transfer was observed. However, when the hand was rotated 180 degrees around the object, participants were able to correctly update sensorimotor memory and positive transfer was observed. This finding argues against the hypothesis that sensorimotor memory is digit-specific because the rotation of the hand (like rotation of the object) changes the forces that each digit must generate to prevent tilt. Positive transfer was also observed when both the hand and object were rotated. This suggests that the rotation of the hand may facilitate rotation of an internal representation of the object. Finally, we found positive transfer of weight asymmetry across the two hands but only when the second hand was rotated such that homologous digits of each hand gripped the same contact surfaces. We suggest that good transfer is observed under these conditions because, when we pass objects from hand to hand, we typically place homologous digits of the two hands in similar locations on the object.

No abstract

Reversal of anisocoria following instillation of apraclonidine 0.5% has been reported in Horner syndrome caused by lesions of the central and peripheral nervous system. The shortest documented latency between symptom onset and a positive apraclonidine test is 36 hours, occurring in a patient with a pontomedullary infarct. We present the case of a 69-year-old man with Horner syndrome due to thalamic hemorrhage in whom apraclonidine testing demonstrated reversal of anisocoria 4 days after symptom onset. This is the first reported case of a positive apraclonidine test in a Horner syndrome caused by a lesion at this site. It suggests that apraclonidine testing is useful in confirming the diagnosis within days of onset even in a lesion located at the most proximal portion of the oculosympathetic pathway.

A 34-year-old woman presented with brainstem compression from a large third nerve schwannoma although third nerve function was intact. At surgery, preservation of the proximal third nerve was not possible. Because of preexisting amblyopia of the contralateral eye, an attempt was made to surgically reinnervate the affected third nerve. The fourth nerve was divided at its entry into the tentorium and anastomosed to the distal stump of the third nerve. Partial recovery of third function occurred over several months and is still present 6 years later. Successful long-term reinnervation of the third nerve by direct anastomosis with the fourth nerve may be useful when third repair is not possible.