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Background: Electrocutaneous stimulation can restore the missing sensory information to prosthetic users. In electrotactile feedback, the information about the prosthesis state is transmitted in the form of pulse trains. The stimulation frequency is an important parameter since it influences the data transmission rate over the feedback channel as well as the form of the elicited tactile sensations. Methods: We evaluated the influence of the stimulation frequency on the subject's ability to utilize the feedback information during electrotactile closed-loop control. Ten healthy subjects performed a real-time compensatory tracking (standard test bench) of sinusoids and pseudorandom signals using either visual feedback (benchmark) or electrocutaneous feedback in seven conditions characterized by different combinations of the stimulation frequency (FSTIM) and tracking error sampling rate (FTE). The tracking error was transmitted using two concentric electrodes placed on the forearm. The quality of tracking was assessed using the Squared Pearson Correlation Coefficient (SPCC), the Normalized Root Mean Square Tracking Error (NRMSTE) and the time delay between the reference and generated trajectories (TDIO). Results: The results demonstrated that FSTIM was more important for the control performance than FTE. The quality of tracking deteriorated with a decrease in the stimulation frequency, SPCC and NRMSTE (mean) were 87.5% and 9.4% in the condition 100/100 (FTE/FSTIM), respectively, and deteriorated to 61.1% and 15.3% in 5/5, respectively, while the TDIO increased from 359.8 ms in 100/100 to 1009 ms in 5/5. However, the performance recovered when the tracking error sampled at a low rate was delivered using a high stimulation frequency (SPCC = 83.6%, NRMSTE = 10.3%, TDIO = 415.6 ms, in 5/100). Conclusions: The likely reason for the performance decrease and recovery was that the stimulation frequency critically influenced the tactile perception quality and thereby the effective rate of information transfer through the feedback channel. The outcome of this study can facilitate the selection of optimal system parameters for somatosensory feedback in upper limb prostheses. The results imply that the feedback variables (e.g., grasping force) should be transmitted at relatively high frequencies of stimulation (>25 Hz), but that they can be sampled at much lower rates (e.g., 5 Hz). [ABSTRACT FROM AUTHOR]

Microelectrodes placed within the densely packed cortical neuronal region are surrounded by many thin processes. Although dendrites are considered to be functionally different to axons, they also possess voltage sensitive membrane channels. Therefore, dendritic regions are suitable candidates for spike initiation sites when stimulated externally, although they demand two to three times higher thresholds in comparison with thin axons. Simulations based upon recently reported distributions of two types of sodium channels and traced pyramidal cell data accompanied by a simplified model structure enlightened the spike initiation sites for extracellular cortical microstimulation and revealed insights into dendritic excitation patterns. Surprisingly low dendritic threshold values for cathodic stimulation were detected, that is, 3.3 µA for a 0.4-µm diameter fiber excited with a 100-µs pulse in 4-µm distance. However, according to the activating function concept the excited region is calculated by 1414*electrode-distance, therefore a minimum electrode-fiber distance is required as sufficient sodium channels are needed to produce enough intracellular current for spike conduction. The minimum distance for dendritic spike initiation increases with diameter and hinders low current stimulation of thick dendrites. This effect is in contrast to the inverse recruitment order known from functional electrical stimulation. Simulations were performed using NEURON and MATLAB. [ABSTRACT FROM AUTHOR]