Article Figures & Data
- Figure 1.
Anatomical experiments and recording conditions. A, The 3D anatomical reconstruction of corticospinal and motor neuron locations. Density and location of retrogradely traced corticospinal neurons following FastBlue injections into lumbar segments. Red dots represent average electrode locations. B, Density and location of retrogradely traced motor neurons following Fluorogold injections into hindlimb muscles. The tibialis anterior served as an anatomical landmark to align tracing from multiple rats. C, The 3D representation of the 32 electrode array inserted into layer V of cortical territories projecting to lumbar segments. The photograph represents the location of two electrode tips in the vicinity of corticospinal neurons projecting to lumbar segments from a single rat. D, EMG activity recorded from right hindlimb muscles during locomotion. Twelve high-resolution cameras monitored the 3D displacements of reflective markers to measure bilateral limb and head kinematics. Single-unit and multiunit activity was extracted from extracellular recordings in the left cortex. Each panel represents an electrode. A distinct color represents each sorted unit. Ground reaction forces were monitored during overground gait initiation. E, Characteristics of the locomotor paradigms; rung spacing on the ladder was irregular.
- Figure 2.
Neural processing framework. A, Color-coded decomposition of hindlimb movement during two successive gait cycles along the runway. B, Statistical test to define neuronal activation. The distribution shown is resting firing rates for a particular neuron. Activation is defined based on whether the sampled firing rate during locomotion is inside (nonmodulated, gray) or outside (active, blue) the 95% confidence intervals of the resting firing rate distribution for that neuron. C, Distributions of changes in firing rates (the resting firing rate for each neuron has been subtracted) for active neurons (blue) versus nonmodulated neurons (in light gray) during locomotion (left histogram). Distributions of modulation depths (maximum − minimum fitted firing rate) over the gait cycle during locomotion (right). Pie chart represents the percentage of active neurons during locomotion (n = 13 rats). D, Raw extracellular data from the contralateral cortex for the same gait cycles, including two sorted units (±SD) from this electrode. Raster plots from all electrodes are shown below. E, Color-coded change in firing rate with respect to quiet standing (±95% confidence intervals) for three representative units that were statistically classified as active (upregulated or downregulated) or nonmodulated during gait. Numbers refer to the raster plot in D. F, Ensemble firing rate obtained by averaging firing rates of all active neurons over the gait cycle for a single rat. G, Firing rates of active neurons are normalized and sorted by peak firing time. H, Distribution of peak firing time showing the number of neurons peaking at a given time of the gait cycle.
- Figure 3.
Comparing methods for fitting neuronal modulations. We compared two approaches for resolving time normalization between gait cycles (A) All action potentials were converted to a percentage of the gait cycle they occurred in. Here, they are concatenated over all gait cycles (n = 13) for one neuron (Method A). B, Alternatively, firing rates were estimated within each gait cycle using nonoverlapping 25 ms bins of action potentials (Method B). C, Normalized firing rates (black dots) were estimated from (A) using nonoverlapping ΔC percentage bins of action potentials. A BARS fit (gray dashed line) was calculated over these firing rate estimates. D, Alternatively, a fifth order polynomial fit (gray dashed line) was applied over the binned firing rates in (B). E, The ensemble firing rate was calculated by averaging all fits for a representative rat during stepping on a treadmill (purple) and overground (blue).
- Figure 4.
Neuronal population responses during locomotion along a runway versus treadmill-restricted stepping. A, Average modulation of active neuron over the duration of the gait cycle. Each firing rate is normalized to the absolute maximum value measured for each neuron across both behaviors (n = 8 rats). B, Distribution of peak firing rates within the neuronal ensemble for both behaviors. Venn diagrams reporting subensembles of active neurons for each behavior. Blue represents runway. Purple represents treadmill. The size of the circle is proportional to the number of neurons. These diagrams reveal both task-independent (overlapping segment) and task-specific (nonoverlapping slices) activation of neurons across the tested locomotor behaviors. C, Ensemble change in firing rate (mean ± SEM) computed from all active neurons in both behavioral procedures. The zero or negative changes in firing rate during stance for treadmill means that the ensemble is not firing differently or is slightly inhibited relative to resting firing rates. To allow for direct comparison between behaviors, the duration of stance and swing phases has been normalized. Right, Mean firing rate and depth for the cohort (filled circles) and individual rats (black lines). D, Cross-correlation between firing rates of active neurons for a representative rat. Average cross-correlation was significantly higher (n = 7690 pairs; p < 1 × 10−5) on the treadmill (R2 = 0.35) than the runway (R2 = 0.27). *p < 0.05 (two-sample t test). **p < 1 × 10−4 (two-sample t test).
- Figure 5.
Kinematic modulation during locomotion along a runway versus treadmill-restricted stepping. A, Decomposition of hindlimb movements underlying both behavioral procedures at matched speeds. B, Average changes in hindlimb joint angles over the entire cohort of rats (n = 8 rats). C, PC analysis applied on all computed kinematic parameters distinguished gait patterns underlying locomotion along a runway versus stepping on a treadmill. Small purple or blue circles filled with different shades of gray represent average gait for each rat. D, Factor loading on PC1, which explained most of the variance between behaviors. Numbers correspond to raw kinematic parameters (see Materials and Methods). Plots reporting mean ± SEM values of the raw kinematic parameters that showed the highest correlation (factor loading) with PC1. Additionally, pelvis forward velocity over the entire gait cycle is shown. Two-sample t tests: #16: t(391) = 39, p < 1 × 10−10; #17: t(391) = −22.4, p < 1 × 10−10; pelvis: t(719) = 15, p < 1 × 10−10. ***Two-sample t test: p < 1 × 10−5 (Bonferroni correction for p < 0.05 with 126 comparisons is p < 7.9 × 10−5).
- Figure 6.
Modulation of muscle activity during locomotion along a runway versus treadmill-restricted stepping. A, Scheme explaining the methods used to analyze the EMG activity of the 10 recorded muscles. B, Group-average EMG activity. C, Group-average spatiotemporal maps of motoneuron activity underlying each behavioral procedure (n = 4 rats). D, Mean (±SEM) temporal profiles of muscle synergies with corresponding weights for each muscle shown below (±SEM). E, Cross-correlation between muscle activity envelopes and between muscle synergy activation profiles during locomotion along a runway and stepping on a treadmill. The color-coded matrix reports correlation value for each muscle and each muscle synergy activation profiles across the four tested rats. (VL electrodes were broken in Rat 1; this muscle was not included in the analysis).
- Figure 7.
Influence of speed on neural and kinematic modulation. A, Distributions of mean velocities for all recorded gait cycles along the runway and on a treadmill for a representative rat. Gait cycles are subdivided into normal and fast steps based on the distribution of velocities. B, Histogram of fast and normal steps performed along the runway and on the treadmill for the entire cohort (n = 8 rats). Velocities of normal steps along the runway and steps on a treadmill overlapped completely. C, Gait clusters computed by applying a PC analysis on all computed kinematic parameters. Whereas normal and fast steps occupied separate locations in the PC space, gait clusters underlying locomotion along runway (blue and black) versus stepping on a treadmill (purple) emerged on the opposite side of the PC1 axis, indicating that these gait executions involved distinct motor control behaviors. Each dot represents a gait cycle from a given rat, which are each differentiated by a distinct grayscale color. D, Ensemble firing rates for both fast and normal steps along the runway and steps on the treadmill.
- Figure 8.
Decoding motor cortex modulation. A, Changes in hindlimb angle and tibialis anterior envelope during locomotion along the runway and their alignment with prior and future firing rate estimates. B, Least-squared regression using prior (motor) or future (sensory) neuronal information. C, Mean values of correlations for each decoder based on a total of 50 randomized test sets per rat. Left, Runway data. Right, Treadmill data. Individual lines indicate each rat (n = 8 rats). Filled colored circles connected by a thicker line represent group-average performances. Overground and treadmill-restricted motor decoding accuracy were not significantly different (p = 0.21). **p < 1 × 10−2 (Student's t test). ***p < 1 × 10−3 (Student's t test).
- Figure 9.
Neuronal population responses during gait initiation along the runway. A, Color-coded decomposition of right hindlimb movements and elevation angle of the hindlimb axis during gait initiation. Gait was initiated with the right hindlimb. B, Group-average spatiotemporal map of motoneuron activity during gait initiation (n = 5 rats). C, Mean (±SEM) temporal activation profiles of muscle synergies C1–C4 during gait initiation. The progressive inactivation of C1 (ankle extensor muscles) and activation of C2 (knee extensor muscles) trigger anticipatory postural adjustments. Red dots represent the 3 dB activation thresholds for all the activation profiles C1-C4. D, Changes in anterior–posterior ground reaction forces for all the rats (black) and individual rats (gray). Red dots represent the 3 dB threshold. The anticipatory postural adjustment phase correlates with the activation time of the synergy C2. E, Percentage (±SEM) of cortical neurons becoming active during gait initiation. The group average (n = 5 rats) activation threshold occurred 76 ms before the activation of muscle synergies. The same analysis applied for individual steps (n = 26 steps) produced similar delays (Student's t test, t(50) = −2.8, p = 0.007).
- Figure 10.
Modulation of hindlimb kinematic and muscle activity during locomotion along a runway and onto a staircase. A, Color-coded decomposition of right hindlimb movements during a step on a runway and onto a staircase. B, Group-average spatiotemporal maps of motoneuron activity during both behavioral procedures (n = 5 rats). Conventions are the same as in Figure 6. C, Mean (±SEM) temporal profiles of muscle synergies with corresponding weights for each muscle shown below (±SEM). D, Cross-correlation between muscle activity envelopes and between muscle synergy activation profiles during locomotion along a runway and climbing on a staircase for the 5 rats.
- Figure 11.
Neuronal population responses during locomotion along a runway and onto a staircase. A, Average modulation of active neuron over the duration of the gait cycle (n = 5 rats). Conventions are the same as in Figure 4. B, Distribution of peak firing rates and Venn diagrams, as shown in Figure 4. C, Ensemble firing rate (mean ± SEM) computed from all active neurons in both behaviors. Right, Mean firing rate and depth of modulation for the cohort (filled circles) and individual rats (black lines). D, Cross-correlation between neurons is shown for a representative rat. Cross-correlation between firing rates of active neurons for a rat. Average cross-correlation was significantly higher (n = 4767 pairs; p < 1 × 10−5) on the stairs (R2 = 0.29) than the runway (R2 = 0.23). One-way ANOVA, F(2,14298) = 61, p < 1 × 10−10, post hoc testing with Bonferroni correction p < 1 × 10−10 (runway vs stairs). *p < 0.05 (Student's t test).
- Figure 12.
Modulation of hindlimb kinematic and muscle activity during locomotion along a runway and over a ladder. A, Color-coded decomposition of right hindlimb movements during a step on a runway and over a ladder. B, Group-average spatiotemporal maps of motoneuron activity during both behavioral procedures (n = 4 rats). Conventions are the same as in Figure 6. C, Mean (±SEM) temporal profiles of muscle synergies with corresponding weights for each muscle shown below (±SEM). D, Cross-correlation between muscle activity envelopes and between muscle synergy activation profiles during locomotion along a runway and over a ladder for the 4 rats with 10 EMG.
- Figure 13.
Neuronal population responses during locomotion along a runway and over a ladder. A, Average modulation of active neuron over the duration of the gait cycle (n = 6 rats). Conventions are the same as in Figure 4. B, Distribution of peak firing rates and Venn diagrams, as shown in Figure 4. C, Ensemble firing rate (mean ± SEM) computed from all active neurons in both behaviors. Right, Mean firing rate and depth of modulation for the cohort (filled circles) and individual rats (black lines). D, Cross-correlation between firing rates of active neurons for a rat. Average cross-correlation was significantly higher (n = 7189 pairs; p < 1 × 10−5) on the ladder (R2 = 0.36) than the runway (R2 = 0.31). One-way ANOVA, F(2,21564) = 101, p < 1 × 10−10; post hoc testing with Bonferroni correction p < 1 × 10−10 (runway vs ladder). *p < 0.05 (Student's t test).
Authors Year Species Cortical area Limb studied Freely-moving Paradigm(s) Armstrong and Drew (a,b) 1984 Cat 86% forelimb; 14% hindlimb Forelimb Yes Treadmill Drew 1988 Cat Forelimb Forelimb Yes Obstacles on treadmill Beloozerova et al. 1993, 2010 Cat Forelimb Forelimb Yes Overground versus ladders Widajewicz et al. 1994 Cat Hindlimb Hindlimb Yes Obstacles on treadmill Beloozerova et al. 2005 Cat 61% forelimb; 39% hindlimb Posture Yes Standing on platform Karayannidou et al. 2009 Cat 52% forelimb; 48% hindlimb Forelimb and hindlimb Yesb Posture, walking on tilting versus nontilting treadmill Song et al. 2009, 2011 Rat Hindlimb/trunk Hindlimb No Treadmill Foster et al. 2014 Rhesus PMd Forelimb Yes Treadmill Yin et al. 2014 Rhesus Hindlimb hindlimb Yes Overground; treadmill Current article 2016 Rat Hindlimb Hindlimb Yes Overground versus treadmill; overground versus stairs; overground versus ladder
