Differential Effects of Open- and Closed-Loop Intracortical Microstimulation on Firing Patterns of Neurons in Distant Cortical Areas

Abstract

Intracortical microstimulation can be used successfully to modulate neuronal activity. Activity-dependent stimulation (ADS), in which action potentials recorded extracellularly from a single neuron are used to trigger stimulation at another cortical location (closed-loop), is an effective treatment for behavioral recovery after brain lesion, but the related neurophysiological changes are still not clear. Here, we investigated the ability of ADS and random stimulation (RS) to alter firing patterns of distant cortical locations. We recorded 591 neuronal units from 23 Long-Evan healthy anesthetized rats. Stimulation was delivered to either forelimb or barrel field somatosensory cortex, using either RS or ADS triggered from spikes recorded in the rostral forelimb area (RFA). Both RS and ADS stimulation protocols rapidly altered spike firing within RFA compared with no stimulation. We observed increase in firing rates and change of spike patterns. ADS was more effective than RS in increasing evoked spikes during the stimulation periods, by producing a reliable, progressive increase in stimulus-related activity over time and an increased coupling of the trigger channel with the network. These results are critical for understanding the efficacy of closed-loop electrical microstimulation protocols in altering activity patterns in interconnected brain networks, thus modulating cortical state and functional connectivity.

Keywords: anesthesia; electrical stimulation; forelimb; motor cortex; neuronal plasticity; neurons; neurophysiology; rats; somatosensory cortex.

Figure 1

Recording and stimulation experimental paradigm. (A) Extracellular recordings were obtained through a four-shank, 16-contact microelectrode probe in anesthetized rats. Signals were acquired (“Recording Unit”) and processed to detect single-unit activity (“Processing Unit”) selected by the user and employed as reference neuron to trigger a stimulus pulse to a channel on the single-shank microelectrode (“Stimulation Unit”). A summary diagram showing the main cortico-cortical anatomical connections between regions is shown in Fig. 1A, bottom. Arrow thickness corresponds to the amount of labeling (medium or high) suggested by Zakiewicz (Zakiewicz et al. 2014). (B) Recording sessions (in RFA) consisted of three 1-hour intermittent periods of stimulation with either ADS or RS to either S1BF or S1FL, each separated by 10-min periods of no stimulation. Example traces in a representative experiment of extracellular recording during the first basal period (Basal0, Top), during the first stimulation session (Stim1, Mid), and during the second basal period, after the stimulation (Basal1, Bottom). Arrows represent the electrical stimulation artifacts.

Figure 2

Firing rate analysis. (A) Spike rasters (dotted graphs, one row per sorted unit) and corresponding array-wide firing rate (line graphs) measured by summing all spikes detected on the entire array in 1-ms windows during 100-ms time frame of Basal0 (left) and Basal3 (right) for a single representative animal which underwent the ADS protocol in BF. (B) Left: Quantitative representation of the MFR distributions for each experimental group (CTRL; ADSBF; ADSFL; RSBF; RSFL) in each Basal phase (0–3). Right: Representations of the Log (MFR) distributions of each experimental group and for each basal period. Data are summarized in box plots, where the horizontal lines denote the 25th, median, and 75th percentile values and the whiskers denote the 5th and 95th percentile values; the square inside the box indicates the mean of each data set. Statistical analysis is reported in Tables 2 and 3.

Figure 3

(A) Per unit correlation between baseline firing rates (x-axis, Basal0) and firing rates after each stimulation session (y-axis, Basal1–3) calculated for each group (ADSBF, ADSFL, RSBF, and RSFL). Colors represent units that significantly increased (magenta), decreased (blue), or remained stable (green). Gray dots represent the correlation for the control group (CTRL). (B) Average fraction of units that significantly changed their firing with respect to the baseline period of recording (Basal0) calculated for all the five experimental groups (i.e., CTRL, light gray; ADSBF and ADSBF, black; RSBF and RSFL, red). (C) Average fraction of neurons which increased their firing rate across the five experimental groups. (D) Total fraction of neurons which decreased their firing rate. (^*P < 0.05, relative to CTRL; one-way ANOVA with Dunnett’s multiple comparison test. Error bars represent SEM). Data are reported as mean ± SEM (standard error of the mean).

Figure 4

(A) Example spike sequences of representative neurons with LvR of 0.5 (Regular, red), 1 (Random, green), and 1.5 (Bursty, blue) taken from data set ADSBF. (B) LvR distributions shown as histograms with a common bin size 0.25 and determined across all the subjects belonging from ADSBF during the first (Basal0, left) and last (Basal3, right) period of quiescence. Black dotted lines represent the median of the LvR distributions. (C) Distributions of a representative neuron’s ISI and its sample firing pattern consisting of 100 consecutive ISIs belonging to the CTRL group (Left) and ADSBF group (right), respectively, during Basal0 (top) and Basal3 (bottom). (D) Mean ± SEM trend of normalized LvR for each experimental condition. Each subject’s LvR value was normalized over the mean LvR value calculated during Basal0: statistical analysis is reported in Tables 4 and 5. (E) Mean ± SEM LvR comparison between Basal0 and Basal3 for all the experimental conditions (CTRL, grey; ADSBF and ADSFL, black; RSBF and RSFL, red). ^**P < 0.01; unpaired, two-tailed Student’s t-test.

Figure 5

(A) Sample trace of recordings from RFA showing stimulus artifacts from ICMS delivered to S1BF. Blanking period used for the analysis is delimited by the gray dotted lines. A total of 100 superimposed traces are shown. (B) Stimulation interval distribution (Interstimulus intervals, 600 ms) in both one ADS and RS subject during the three stimulation sessions (Stim1, Stim2, and Stim3). Stimulation counts have been normalized to session length. (C) Poststimulus spiking histograms derived from neural recordings in RFA on the three stimulation sessions for the four ICMS groups, respectively (ADSBF, ADSFL, RSBF, and RSFL). Histograms portray the average number of action potentials discriminated from the neural recordings within 1-ms bin. Data pooled across subjects for each group and normalized to the total number of either ADS or RS specific events. (D) Normalized PSTH areas for the four groups (ADSBF and ADSFL, black; RSBF and RSFL, red) of ICMS in the three stimulation phases (Stim1, Stim2, and Stim3). Each subject’s PSTH area was normalized over the mean area calculated in Basal0 to show data trends over time. (E) The number of stimulation pulses delivered for both ADS (black) and RS (red) during the three stimulation phases. (F) MFR of the stimulation phases normalized over the mean value of firing during Stim1. Statistical analysis is reported in Tables 6–9.

Figure 6

(A) stPR for a representative trigger channel of an ADSBF experiment during the four basal periods of recording. (B) Box plots showing the initial level of PC for ADS groups during Basal0. Light blue, black, and green symbols represent the PC classification of the trigger channels, respectively, Choristers, Not Classified, and Soloists. (C) PC normalized over the mean value of Basal0 for the trigger channels groups of ADSBF, ADSFL, and all the other recording groups. ^*P < 0.05, ^**P < 0.001, one-way repeated measures analysis of variance (ANOVA) on ranks, Dunnett’s post hoc testing procedure.