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Acute experiments were performed on adult cats to study selective activation of medial gastrocnemius, soleus, tibialis anterior, and extensor digitorum longus with a cuff electrode. A spiral nerve cuff containing twelve "dot" electrodes was implanted around the sciatic nerve and evoked muscle twitch forces were recorded in six experiments. Spatially isolated "dot" electrodes in four geometries: monopolar, longitudinal tripolar, tripolar with four common anodes, and two parallel tripoles, were combined with transverse field steering current(s) from an anode(s) located 180 degrees around from the cathode(s) to activate different regions of the nerve trunk. To quantify the degree of selectivity, a selectivity index was defined as the ratio of the force in one muscle to the force in all four muscles in response to a particular stimulus. The selectivity index was used to construct recruitment curves for a muscle with the optimal degree of selectivity. Physiological responses were correlated with the anatomical structure of the sciatic nerve by identifying the nerve fascicles innervating the four muscles, and by determining the relative positions of the electrodes and the nerve fascicles. The results indicated that the use of transverse field steering current improved selectivity. We also found that tripoles with individual dot anodes were more selective than tripoles with four common dot anodes. Stimulation with two parallel tripoles was effective in activating selectively fascicles that could not be activated selectively with only a single tripole. The multipolar cuff proved an effective method to control selectively and progressively the force in muscles innervated by fascicles that were well defined at the level of the cuff.

Early blind (EB) individuals can recognize bidimensional shapes using a prosthesis substituting vision with audition (PSVA) and activate right dorsal extrastriate visual cortex during the execution of this task. The present study used repetitive transcranial magnetic stimulation (rTMS) to further examine the functional role of this structure in the successful use of the PSVA. Moreover, we investigated which auditory parameter used in the prosthesis (pitch, intensity, or spatial location) might contribute to this occipital activation. Results revealed that rTMS applied to right dorsal extrastriate cortex in EB subjects interferes with both the PSVA use and the auditory spatial location task but not with pitch and intensity discriminations. By contrast, rTMS targeting the same cortical areas in sighted subjects did not affect performance on any auditory tasks. Early visual deprivation thus leads to functional cerebral cross-modal reorganization in the processing of auditory information and auditory-to-visual sensory substitution. The findings also point to the specific involvement of the dorsal visual stream for auditory spatial processing in blind subjects. Moreover, this suggests that sensory substitution prostheses can be developed using these additional neural resources to perform tasks that partially compensate for the loss of vision.

A volunteer with retinitis pigmentosa and no residual vision was chronically implanted with an optic nerve electrode connected to an implanted neurostimulator and antenna. An external controller with telemetry was used for electrical activation of the nerve which resulted in phosphene perception. Open-loop stimulation allowed the collection of phosphene attributes and the ability to elicit perception of simple geometrical patterns. Low perception thresholds allowed for large current intensity range within safety limits. In a closed-loop paradigm, the volunteer was using a head-worn video camera to explore a projection screen. The volunteer underwent performance evaluation during the course of a training program with 45 simple patterns. After learning, the volunteer reached a recognition score of 63% with a processing time of 60 s. Mean performance in orientation discrimination reached 100% with a processing time of 8 s.