Artificial limb representation in amputees.

The human brain contains multiple hand-selective areas, in both the sensorimotor and visual systems. Could our brain repurpose neural resources, originally developed for supporting hand function, to represent and control artificial limbs? We studied individuals with congenital or acquired hand-loss (hereafter one-handers) using functional MRI. We show that the more one-handers use an artificial limb (prosthesis) in their everyday life, the stronger visual hand-selective areas in the lateral occipitotemporal cortex respond to prosthesis images. This was found even when one-handers were presented with images of active prostheses that share the functionality of the hand but not necessarily its visual features (e.g. a 'hook' prosthesis). Further, we show that daily prosthesis usage determines large-scale inter-network communication across hand-selective areas. This was demonstrated by increased resting state functional connectivity between visual and sensorimotor hand-selective areas, proportional to the intensiveness of everyday prosthesis usage. Further analysis revealed a 3-fold coupling between prosthesis activity, visuomotor connectivity and usage, suggesting a possible role for the motor system in shaping use-dependent representation in visual hand-selective areas, and/or vice versa. Moreover, able-bodied control participants who routinely observe prosthesis usage (albeit less intensively than the prosthesis users) showed significantly weaker associations between degree of prosthesis observation and visual cortex activity or connectivity. Together, our findings suggest that altered daily motor behaviour facilitates prosthesis-related visual processing and shapes communication across hand-selective areas. This neurophysiological substrate for prosthesis embodiment may inspire rehabilitation approaches to improve usage of existing substitutionary devices and aid implementation of future assistive and augmentative technologies.

Speed of saccade execution and inhibition associated with fractional anisotropy in distinct fronto-frontal and fronto-striatal white matter pathways.

Fast cancellation or switching of action plans is a critical cognitive function. Rapid signal transmission is key for quickly executing and inhibiting responses, and the structural integrity of connections between brain regions plays a crucial role in signal transmission speed. In this study, we used the search-step task, which has been used in nonhuman primates to measure dynamic alteration of saccade plans, in combination with functional and diffusion-weighted MRI. Functional MRI results were used to identify brain regions involved in the reactive control of gaze. Probabilistic tractography was used to identify white matter pathways connecting these structures, and the integrity of these connections, as indicated by fractional anisotropy (FA), was correlated with search-step task performance. Average FA from tracts between the right frontal eye field (FEF) and both right supplementary eye field (SEF) and the dorsal striatum were associated with faster saccade execution. Average FA of connections between the dorsal striatum and both right SEF and right inferior frontal cortex (IFC) as well as between SEF and IFC predicted the speed of inhibition. These relationships were largely behaviorally specific, despite the correlation between saccade execution and inhibition. Average FA of connections between the IFC and both SEF and the dorsal striatum specifically predicted the speed of inhibition, and connections between the FEF and SEF specifically predicted the speed of execution. In addition, these relationships were anatomically specific; correlations were observed after controlling for global FA. These data suggest that networks supporting saccade initiation and inhibition are at least partly dissociable. Hum Brain Mapp 37:2811-2822, 2016. © 2016 Wiley Periodicals, Inc.

Frontal-subcortical circuits involved in reactive control and monitoring of gaze.

Rapid and reactive control of movement is essential in a dynamic environment and is disrupted in several neuropsychiatric disorders. Nonhuman primate neurophysiology studies have made significant contributions to our understanding of how saccadic eye movements can be rapidly inhibited, changed, and monitored. These results highlight a frontostriatal network involved in gaze control and provide a strong basis for understanding how cognitive control of action is implemented in the human brain. The goal of the present study was to bridge human and nonhuman primate studies by investigating reactive control of eye movements during fMRI using a task that has been used in neurophysiology studies: the search-step task. This task requires a speeded response to a visual target (no-step trial). On a minority (40%) of trials, the target jumps to a new location and participants are instructed to inhibit the initially planned saccade and redirect gaze toward the new location (redirect trial). Compared with no-step trials, greater activation in a frontal oculomotor network, including frontal and supplementary eye fields (SEFs), and the striatum was observed during correctly executed redirect trials. Individual differences in stopping efficiency were related to striatal activation. Further, greater activation in SEF was in a region anterior to that activated during visually guided saccades and scaled positively with error magnitude, suggesting a prominent role in response monitoring. Combined, these data lend new evidence for a role of the striatum in reactive saccade control and further clarify the role of SEF in action inhibition and performance monitoring.