Recovery of precision grasping after motor cortex lesion does not require forced use of the impaired hand in Macaca mulatta.

We investigated recovery of precision grasping of small objects between the index finger and thumb of the impaired hand without forced use after surgically placed lesions to the hand/arm areas of M1 and M1 + lateral premotor cortex in two monkeys. The unilateral lesions were contralateral to the monkey's preferred hand, which was established in prelesion testing as the hand used most often to acquire raisins in a foraging board (FB) task in which the monkey was free to use either hand to acquire treats. The lesions initially produced a clear paresis of the contralesional hand and use of only the ipsilesional hand to acquire raisins in the FB task. However, beginning about 3 weeks after the lesion both monkeys spontaneously began using the impaired contralesional hand in the FB task and increased use of that hand over the next few tests. Moreover, the monkeys clearly used precision grasp to acquire the raisins in a similar manner to prelesion performances, although grasp durations were longer. Although the monkeys used the contralesional hand more often than the ipsilesional hand in some postlesion testing sessions, they did not recover to use the hand as often as in prelesion testing when the preferred hand was used almost exclusively. These findings suggest that recovery of fine hand/digit motor function after localized damage to the lateral frontal motor areas in rhesus monkeys does not require forced use of the impaired hand.

Genetic control of interconnected neuronal populations in the mouse primary visual system.

Proliferation and survival of different cell types is thought to be modulated by cell interactions during development that achieve numerical and functional balance. We tested the precision of coregulation of numbers of neurons, glial cells, and endothelial cells in the dorsal lateral geniculate nucleus (LGN) in 58 isogenic strains of mice. We acquired matched counts of retinal ganglion cells (RGCs) in these strains and tested the precision of numerical matching between retina and LGN. Cells were counted using unbiased counting protocols and tissue from the Mouse Brain Library (www.mbl.org). Classification criteria were assessed using immunohistochemical criteria. The LGN contains an average of 17,000 neurons, 12,000 glial cells, and 10,000 endothelial cells. Variation around these means is typically twofold, and cell ratios vary widely. Strain differences in LGN volume correlate moderately well with glial cell number (r = 0.69) and less well with RGC number (r = 0.35) and with LGN neuron number (r = 0.32). Populations of LGN neurons and glial cells correlate only modestly (r = 0.44; p < 0.01). The single most surprising and unequivocal finding was the lack of any detectable correlation between populations of LGN neurons and RGCs, a correlation of merely 0.01 across 56 strains. In contrast, RGC number correlates significantly with LGN glial cell number, a surprising twist on the numerical matching hypothesis (r = 0.33; p < 0.01). We conclude that numbers of these two functionally coupled neuron populations are modulated over a wide range by independent genetic and developmental mechanisms.