Publications - Maxwell D. Murphy

You can also find my articles on my Google Scholar profile.

Despradel Rumaldo, D. L., Murphy, M. D., Borda, L., Marshall, N., Formento, E., Bracklein, M., Lee, J., et al.

Most individuals with tetraplegia retain some myoelectric function in their forearms, which offers the possibility of using surface electromyographic (sEMG) control for human-computer interaction (HCI). We demonstrate the potential of this approach by showing that people with motor-complete (n=5) and motor-incomplete (n=2) tetraplegia can accurately control myoelectric activity in their forearm to perform discrete button-click and continuous positioning tasks. These control inputs were mapped to the firing rate of motor units detected by a wireless wristband sensor designed for everyday use. Participants completed four testing sessions to assess their speed and accuracy. Motor units that displayed a wide dynamic range in their firing rate performed best during tasks requiring continuous, single-axis control. Interestingly, the level of impairment did not affect performance on the clicking and 1D cursor control tasks. However, those with motor-incomplete injuries showed greater independent control over two motor units than participants with motor-complete injuries, who exhibited stronger coupling between units. Participants also confirmed the practical utility of the device, successfully placing and removing the sEMG wristband on their own and consistently rating it as comfortable and easy to manage. These findings are significant because they offer the first demonstration of motor unit-based control in individuals with cervical spinal cord injury (SCI) using a fully wearable wristband interface, highlighting the feasibility of moving these systems out of the lab and into daily life.

Zhang, E., Shotbolt, M., Abdel-Mottaleb, M., Chen, S., Andre, V., Tian, J., Shulgach, J., Murphy, M., Noga, B., Liang, P., Griffin, D., Weber, D., Pardo, M., Pane, S., & Khizroev, S.

Explores magnetoelectric nanoparticles (MENPs) as a platform for wireless, bidirectional brain-computer interfaces, converting applied magnetic fields into localized electrical signals at the nanoscale. A comprehensive framework incorporating nonlinear physics effects predicts that properly engineered MENPs could enable deep brain and cortical neuromodulation and recording with submillimeter spatial resolution and millisecond temporal precision, potentially offering clinically viable BCIs without implanted electrodes or genetic modification.

Vieira, F. M. P., Murphy, M. D., Weber, D. J., & Cunha, J. P. S.

Jiang, R., Murphy, M., Low, J., Weber, D. J., & Griffin, D. M.

Validates an individualized forecasting framework using a TBATS model to predict slow-wave EEG power in cynomolgus monkeys recorded over 12 consecutive days via wireless telemetry. TBATS achieved stable forecasts with as few as 2 days of training data and produced white-noise residuals superior to naive, Holt-Winters, and SARIMA baselines. The framework enables personalized neural baselines for tracking neurodegeneration or optimizing closed-loop therapies in preclinical and clinical applications.

Forssell, M., Tusi, R., Kim, J., Murphy, M. D., Shulgach, J. A., Yadav, P., Tang, A. B., Maurer, M., Jain, V., Weber, D. J., Alter, B. J., & Grover, P.

A letter describing the use of sub-threshold background hum stimulation to reduce scalp discomfort during precision transcranial electrical stimulation (pTES) of motor cortex. The approach targets a practical barrier to clinical adoption of high-density surface electrode TES systems, where skin sensation under active electrodes can limit tolerable stimulation amplitudes.

Forssell, M., Murphy, M. D., Jain, V., Shulgach, J. A., Tansel, D. Z., Cao, J., Reza, S., Chamanzar, M., Griffin, D., Fedder, G. K., Weber, D. J., & Grover, P.

Demonstrates precision motor-cortex targeting using a 64-electrode high-density surface array delivering 100 µs pulses at 50–150 mA. Moving the injected electric field by sub-centimeter distances measurably changes the activation profile of arm and hand muscles as monitored by EMG. The results suggest that steerable transcranial electrical stimulation could substitute for invasive stimulation in some clinical contexts and enhance intraoperative muscle-function monitoring.

Despradel, D., Murphy, M. D., Borda, L., Verma, N., Yadav, P., Shanahan, J., Marshall, N., Formento, E., Bracklein, M., Ye, J., Walkington, P., Rajalingham, R., Sussillo, D., Naufel, S., Gutnisky, D. A., Collinger, J., & Weber, D.

Zhang, E., Abdel-Mottaleb, M., Shulgach, J., Campos, M. A., Murphy, M., Navarrete, B., Chen, S., Andre, V., Shotbolt, M., Griffin, D., Weber, D., Liang, P., & Khizroev, S.

Hayley, P., Tuchek, C., Dalla, S., Borrell, J., Murphy, M. D., Nudo, R. J., & Guggenmos, D. J.

Investigates whether peripheral sensory stimulation activates premotor cortex following primary motor cortex injury in rats. Neural responses to mechanical forepaw stimulation were recorded four weeks post-ischemic lesion in seven rats. Post-ischemic functional connectivity emerged between premotor and somatosensory cortex, and stimulation of premotor regions modulated sensory responses—suggesting that premotor–somatosensory coupling is a substrate for sensorimotor recovery after stroke.

Averna, A., Barban, F., Carè, M., Murphy, M. D., Iandolo, R., De Michieli, L., Nudo, R. J., Guggenmos, D. J., & Chiappalone, M.

Averna, A., Hayley, P., Murphy, M. D., Barban, F., Nguyen, J., Buccelli, S., Nudo, R. J., Chiappalone, M., & Guggenmos, D. J.

Carè, M., Averna, A., Barban, F., Murphy, M. D., Nudo, R. J., Guggenmos, D. J., & Chiappalone, M.

Acquired brain injuries, such as stroke, are a major cause of long-term disability worldwide. We investigated how neuronal activity is modified following an injury to primary motor cortex and in response to an activity-dependent stimulation (ADS) protocol, recording spike activity in the rostral forelimb area and somatosensory cortex of brain-injured anesthetized rats. An ischemic injury in M1 led to an overall increase in spike activity within RFA and S1, and subsequent treatment with ADS promoted the generation of synchronized patterns in both areas.

Barban, F., Averna, A., Murphy, M. D., Carè, M., Nudo, R. J., Guggenmos, D. J., & Chiappalone, M.

Milighetti, S., Sterzi, S., Fregni, F., Hanlon, C. A., Hayley, P., Murphy, M. D., Bundy, D. T., Nudo, R. J., & Guggenmos, D. J.

We tested whether transcranial direct-current stimulation (tDCS) had any effect on the spike rate or LFP content in ambulatory rats using a within-subject design to test anodal, cathodal, and sham stimulation cases.

Averna, A., Pasquale, V., Murphy, M. D., Rogantin, M. P., Van Acker, G. M., Nudo, R. J., Chiappalone, M., & Guggenmos, D. J.

Intracortical microstimulation (ICMS) was applied to the rat primary sensory cortex in healthy anesthetized rats using both closed- and open-loop paradigms. Changes in firing rates and regimes were tested over the course of a few hours to determine if an effect differentiating the protocols could be detected during this time.

Bundy, D. T., Guggenmos, D. J., Murphy, M. D., & Nudo, R. J.

Rats were trained on a gross (lever-press) and fine (pellet-retrieval) forelimb task, implanted with microwire arrays spanning unilateral sensorimotor cortex contralateral to the moving limb, and then tested over the course of days and weeks to determine the longitudinal stability of both spiking unit signals as well as gamma LFP in this context.

Murphy, M. D., Buccelli, S., Bornat, Y., Bundy, D. T., Nudo, R. J., Guggenmos, D. J., & Chiappalone, M.

We modified the Intan Stimulation/Recording controller to deliver closed-loop stimulation using a finite state machine to improve artifact rejection, such as may occur due to electrical microstimulation or other physiological events such as chewing and whisking.

Murphy, M. D., Dunham, C., Nudo, R. J., Guggenmos, D. J., & Averna, A.

This paper is about a proof-of-concept nonlinear dynamical systems approach applied to peri-stimulus epochs to see if it could detect longitudinal differences in how open- and closed-loop influence the system response to electrical microstimulation of sensory cortex in ambulatory rats.

Dowdell, A. S., Murphy, M. D., Azodi, C., Swanson, S. K., Florens, L., Chen, S., & Zuckert, W. R.

This paper is about the localization of lipoproteins to a particular side and layer of the bilayer membrane of Borrelia, the causative agent of Lyme disease. It is a descriptive study that catalogues this specific property of each putative lipoprotein identified in the Borrelia lipoproteome.

Murphy, M. D., Guggenmos, D. J., Bundy, D. T., & Nudo, R. J.

This is a review of the challenges facing the translation of brain computer interfaces from preclinical trials to widespread use in the clinic. There is an emphasis on activity-dependent stimulation and similar paradigms.

Cettina, P. E., Guggenmos, D. J., Sivakumar, S. S., Murphy, M. D., Barbay, H. S., Nudo, R. J., & Bundy, D. T.

Compares movement-related cortical activity during skilled motor recovery in rats following lesions to either motor cortex or the internal capsule. Cortical lesions showed early behavioral recovery accompanied by widespread reductions in ipsilesional neural activity, whereas internal capsule lesions exhibited delayed recovery without sustained cortical reorganization—suggesting that recovery mechanisms differ fundamentally based on lesion location within the motor pathway.

Pack, A. R., Murphy, M. D., Barbay, S., Nudo, R. J., & Guggenmos, D. J.

Surveys throughput estimators across spatial and non-spatial human–computer interaction domains, reviewing classical Fitts point-to-point formulations, modern effective-width refinements, and mutual-information-based alternatives. Evaluates published BCI and myoelectric studies and shows that continuous 3D reach tasks lack the explicit target geometry required for valid bit-rate estimation. Highlights the gap between device throughput (13–26 bits/s for soft keyboards) and the intrinsic human information rate in high-speed typing (30–40 bits/s), and proposes methodological criteria for comparing interaction performance across neural and muscular interfaces.

Derives a cube-root scaling law from the activating-function framework showing that the distance at which an axon is recruited by an extracellular electrode scales as the cube root of (current × diameter / tissue conductivity). The relationship explains why large-diameter myelinated afferents (Group Ia/Ib, Aβ) are preferentially activated during dorsal epidural stimulation, while ventral-root motor efferents—being smaller and located farther from dorsal contacts—typically require closer proximity or higher current to be recruited.

Provides a quantitative treatment of ISI serial dependence using the Hodgkin–Huxley framework, demonstrating how residual activation of ion channels and slow ionic clearance introduce "state memory" across spikes. When the mean ISI becomes comparable to the recovery time of slow conductances (e.g., calcium-dependent potassium currents), gating variables fail to fully reset between spikes, yielding correlated ISIs. An approximate autoregressive form is derived linking lag-1 ISI correlation to the ratio of ISI duration and the dominant ionic recovery time constant, with implications for fatigue-related modulation of motor unit excitability.

Derives an exact relationship between the coefficient of variation (CV) of motor unit inter-spike intervals (ISIs), the absolute refractory period, and the Erlang shape parameter k. Because ISO-format dates sort lexicographically, the k parameter can be recovered from any ISI record via a single-parameter nonlinear regression of CV_ISI versus the product of mean firing rate and minimum observed ISI—requiring only that firing rate is approximately constant within the epoch of interest.