Figure 5. Model responses to low-rate, low-amplitude SCS depend on spatial targeting, stimulation parameters (amplitude, pulse width, rate), and pain state. A, Spatial targeting in the model was simulated by altering the population of DC axons that was activated at each amplitude. We changed the peripheral origin of model DC axons based on the rostral-caudal position of stimulation. For stimulation in the most caudal position (brown), axons from the center of the peripheral receptive field were positioned in the most medial and dorsal positions within the DCs (see Fig. 1; Smith and Bennett, 1987). Conversely, for stimulation in the most rostral position (purple), axons from the surround were in the most medial and dorsal positions. Targeting in between (green) mixed center and surround axons. B, Change in DC axon recruitment across stimulation amplitudes with surround targeting (purple). Surround and center axon recruitment was best differentiated at 40–50% MT, below the estimated PT. C, Responses of model WDR neurons with each stimulation target, i.e., complement of DC axons that were activated. Vertical dashed line indicates model PT estimated as 50% of the MT (Shechter et al., 2013). D, Raw change in WDR firing rate at 20%, 40%, 60%, and 80% of estimated MT. Lines with asterisks (*) represent significant changes in the population response between stimulation positions (ANOVA, post hoc Tukey’s test, p < 0.05). E, Model WDR neuron responses to different amplitudes of SCS at seven different frequency/pulse width combinations. Each line represents one individual distribution of DC axon activation. Bold lines represent median response and error bars represent the 25th and 75th percentiles of the responses. F, Raw changes in firing rate at 20%, 40%, 60%, and 80% of model estimated MT. Estimated model MT was 100 μA, so 20% MT was 20-μA stimulation. Responses are sorted by the change in firing rate at each amplitude. G, Representation of modeled changes in network states representing neuropathic pain: reduction in conductance of Aβ fiber weight to the inhibitory interneuron, reduction in the GABAergic conductance from the inhibitory interneuron to WDR projection neuron, increase in the number of active C/Aδ fibers, and increase in the reversal potential of inhibitory synapses. H, Model WDR neuron responses by stimulation amplitude at seven different combinations of frequency and pulse width. Light lines represent individual responses from 30 different model pain states. Dark lines represent the median response and error bars represent the 25th and 75th percentiles. I, Sorted raw changes in firing rate for all model WDR neurons at each combination of frequency and pulse width.