<p> Manual dexterity, the ability to manipulate objects with the hands, and the related position sense, or proprioception, are often impaired after stroke. Associative stimulation of motor points (MPAS) in hand muscles is known to modify motor cortex excitability and improve manual dexterity (McDonnell and Ridding, 2006). </p><p> However, it is not known whether the effect of this peripheral stimulation can be increased by central stimulation of sensorimotor cortex, in terms of function, proprioception, or cortical neurophysiology. Here we compare the functional and neurophysiological consequences of MPAS with and without transcranial direct current (tDCS) in healthy adults. MPAS was applied to two right hand muscles important for manual dexterity: APB and FDI. tDCS, a non-invasive brain stimulation technique, was simultaneously applied over left sensorimotor cortex. Both techniques stimulate motor as well as somatosensory pathways. Neurophysiological measures of motor cortex, including SICI (short intra-cortical inhibition), ICF (intracortical facilitation), and input/output (I/O) curve, were assessed with transcranial magnetic stimulation (TMS). Manual dexterity and proprioceptive acuity were also measured. 14 subjects completed 3 sessions of MPAS in combination with sham, anodal (excitatory) and cathodal (inhibitory) tDCS. 13 subjects completed 2 sessions of sham MPAS with sham or anodal tDCS. In combination with MPAS, anodal tDCS significantly increased the plateau of manual dexterity, increased cortical response to TMS, and tended to improve proprioceptive acuity compared to sham tDCS. The neural basis for the observed functional improvements may thus include somatosensory as well as motor cortex. Neither MPAS nor tDCS alone had any measurable effect. These results suggest that adding tDCS as a central intervention to complement peripheral MPAS may be a promising avenue of treatment for patients with impaired manual dexterity. </p>

The goal of this dissertation was to understand the issue of load in a more operationally realistic way, while examining underlying segmental relations and postural regulation related to functional capability. The ecological approach provides a foundation for this work, as its approach seeks understanding across nested relations and at the level of the Organism-Environment system. First, a landing task was used to examine transitions from movement to upright stance, evaluating the effects of load on changes relevant to prospective control of action. Greater negative head angles, reductions in the field of regard, and reduced variability in orienting coordination (trunk-head relations) under load all suggest reductions in the postural affordances for visual perception. The heaviest load was not the worst; as the asymmetrically loaded Vest configuration had greater negative effects on postural affordances. This was further supported by the increased power and frequency content in the Center of Pressure dynamics, suggesting much more difficult postural regulation in this configuration. The second study examined the effects of load on dynamic marksmanship performance using large loads on the torso and small loads on the extremities (night vision goggles and extremity armor on the arms) while establishing two different postures determined by target placement. Load and Posture both had negative impacts on the speed-accuracy trade-off, with larger loads affecting gross postural transitions and smaller loads degrading fine-aiming performance. The more challenging posture degraded accuracy on target substantially, suggesting that reorientation of multiple segments may be necessary for assessing the consequences of load on marksmanship performance. Increases in the total coordinative variability of Head-Trunk-Gun relations with load at a high target suggests that increased inertial and interactive forces during movement "push" the system out of the optimal segmental relations. Moreover, the results from Postural-Focal coupling suggest that load "freezes" previously available degrees of freedom, making the system more deterministic and less flexible in goal-directed achievement. The two previous paradigms are joined in the third study to understand perception-action coupling during movement cessation to marksmanship transitions, a ubiquitous task in combat. Increased time to discriminate targets was found with load and was related to peak head velocities and the inability to dissipate energy at the head/eyes under load. Again, Load and Posture had significant effects on the speed-accuracy trade-off, especially at the load most similar to that seen in current missions. Segmental coordination in this effort ballasts the findings in study 2, as significant shifts from optimal Head-Trunk-Gun relations were observed with load as well as increased variability that was detrimental to task performance. This dissertation demonstrates that science can be "Operationalized" in a way that maintains scientific integrity during complex task analysis; providing additional insight into the issue of load across multiple scales of analysis related to functional capability and survivability in combat and others encumbered by load.