There is a variety of methods to filter noise, determine stimulation, decode, and predict neural activity. Some authors have attempted neural decoding through training sessions, while others have determined arbitrary thresholds which can be used as a neural ‘switch’ to enable neural plasticity to activate desired movements. When EEG is used in the setting of neural bypasses, recording thresholds are determined which then translate to effector stimulation, often with FES. EEG thresholds to stimulate FES are typically obtained through motor imagery as measured by attention with sensorimotor rhythm and beta/theta oscillation ratios, or Common Spatial Patterns based on Event De/Synchronization [21 (
link)–41 ]. Others have used steady-state visual evoked potentials (SSVEP) to trigger FES stimulation [42 (
link)]. Furthermore, some studies have used alpha rhythms as a deactivating signal following a stimulation event [43 (
link)]. When single-cell recordings are used, the threshold for stimulating FES has typically been cell firing rate [5 (
link)]. When ECoG is used, the rate of high-gamma oscillations has been selected as a threshold for effector muscle stimulation [3 (
link)]. In studies with microelectrode arrays, neuronal action potential rate, or average spectral high-frequency power, have typically been used as thresholds for stimulation of effector muscles [7 (
link)]. Microelectrode arrays have also been used to record mean wavelet power after artifact removal during trials of imagined movements in paralyzed patients [4 (
link), 44 (
link)]. Microelectrode arrays are often used during training sessions prior to paralysis or with simulated motor tasks to create predictive models of neuronal control of muscle activity [19 (
link), 45 (
link), 46 (
link)].
In some studies, daily calibration is required to train neural decoding algorithms which presents a limitation for the translation of these technologies to real-world environments. One possible approach to this problem is a neural network capable of decoding without daily training sessions [18 (
link)]. Other decoding methods consist of gradient boosted trees, support vector machines, and linear methods [12 (
link), 47 ]. A drawback to any decoding method is the group of associated assumptions. For example, assumptions made with a regularized linear regression are that outputs are proportional to input changes, additional noise is assumed to be Gaussian noise, and that the regression coefficients are from a Gaussian distribution [47 ]. Bouton et al. suggest that nonlinear methods of decoding may help to increase robustness and accuracy of specific decoders [48 ]. As methods increase in complexity, so do their associated assumptions. Glaser et al. states that a crucial assumption built into decoders is the form of the relation between the input and output. With machine learning methods, multiple decoding models can be organized in ensembles [47 ]. Bouton et al. demonstrates this in their finding that Long Short-Term Memory-based deep learning networks, used in tandem with repeatability-based feature selection based on temporal correlation, results in positive outcomes and accurate decoding [12 (
link)].
Developing devices to maximize spatial resolution, temporal resolution, and biocompatibility will be crucial in developing robust neural interfaces. Additionally, a number of unidirectional recording or stimulation devices exist that have not yet been combined into neural bypasses, including novel spinal cord stimulators or neurotransmitter-sensing electrodes [49 (
link), 50 (
link)]. There exists a lack of comparative analysis across studies with different methodologies to determine the most efficacious methods of achieving neural bypass.
Zuccaroli I., Lucke-Wold B., Palla A., Eremiev A., Sorrentino Z., Zakare-Fagbamila R., McNulty J., Christie C., Chandra V, & Mampre D. (2023). Neural Bypasses: Literature Review and Future Directions in Developing Artificial Neural Connections. OBM neurobiology, 7(1), 158.