Interactions between rostral and caudal cortical motor areas in the rat

Abstract

In rats, forelimb movements can be evoked from two distinct cortical regions, the rostral (RFA) and the caudal (CFA) forelimb areas. RFA and CFA have numerous reciprocal connections, and their projections reach several common targets, which allows them to interact at multiple levels of the motor axis. Lesions affecting these areas result in profound and persistent deficits, supporting their essential role for the production of arm and hand movements. Whereas rats are widely used to study motor control and recovery following lesions, little is known as to how cortical motor areas in this model interact to generate movements. To study interactions between RFA and CFA, we used paired-pulse protocols with intracortical microstimulation techniques (ICMS). A conditioning stimulus (C) in RFA was applied simultaneously, or before a test stimulus (T) in CFA. The impact of RFA conditioning on CFA outputs was quantified by recording electromyographic signals (EMG) signals from the contralateral arm muscles. We found that stimulation of RFA substantially modulates the intensity of CFA outputs while only mildly affecting the latency. In general, the effect of RFA conditioning changed from predominantly facilitatory to inhibitory with increasing delays between the C and the T stimulus. However, inspection of individual cortical sites revealed that RFA has a wide range of influence on CFA outputs with each interstimulation delay we used. Our results show that RFA has powerful and complex modulatory effects on CFA outputs that can allow it to play a major role in the cortical control of forelimb movements.

Keywords: arm; motor cortex; motor evoked potential; premotor.

Fig. 1.

Experimental setup and data analyses. A: schematic showing the experimental setup. Multistranded wires were implanted in the extensor digitorum communis (green), palmaris longus (red), biceps brachii (purple), and triceps brachii (blue) bilaterally to record electromyographic (EMG) signals. Intracortical microstimulation (ICMS) trains were used to locate the caudal forelimb area (CFA, green) and rostral forelimb area (RFA, blue). Each dot in the map shows a hypothetical ICMS stimulation location. The conditioning (C) electrode was placed in RFA and the test (T) electrode in CFA. Once electrodes were in place, we recorded a stimulation protocol that included 7 stimulation conditions [C electrode only, T electrode only, or both electrodes, using one of 5 different interstimulation intervals (ISIs)]. We collected 300 trials/condition delivered at 3 Hz. Data for each condition were recorded in three blocks of 100 trials, and the stimulation condition of subsequent blocks was randomized. B: examples of motor evoked potential (MEP) resulting from cortical stimulations. Inset on top shows the evoked MEP from 35 individual trials (light gray) and the average response (black) when only the T stimulation in CFA was applied. Inset on bottom shows the same when only the C stimulation in RFA was applied. Because the intensity of the C stimulation was purposely set below threshold, individual trials and the average response are all close to zero. However, to account for any potential small MEP that could have been present after multiple C stimulations, we calculated an average predicted paired-pulse MEP (aMEP_predicted). The aMEP_predicted (3rd inset) is the linear summation of the MEP evoked by the T only and the C only. Finally, the inset on bottom shows the average evoked paired-pulse MEP (aMEP_evoked) recorded when C and T were stimulated in the same trial. An aMEP_evoked was calculated for each ISI used in a protocol. C: a first analysis was conducted on the average response. The aMEP_evoked (blue) was compared with the aMEP_predicted (red). The peak intensities (I_evoked and I_predicted) and latencies (t_evoked and t_predicted) were used to calculate ΔI and Δt. A second analysis was based on the individual trials (see materials and methods).

Fig. 2.

Location of cortical stimulation sites used for the paired-pulse protocols. Shown are the ICMS data collected in the 7 animals included in the study. In 6 animals (A–F), the stimulations were done in the left hemisphere, and, in 1 animal (G), they were done in the right. Each colored dot overlaid on the cortex is a site at which ICMS trains were delivered. Movement categories evoked at threshold current intensity are color coded. Typically, movements were evoked at lower current intensity in the CFA, and neck and vibrissa representations separated the CFA and RFA. Once the two forelimb areas were located, the electrodes were placed at cortical locations from which movements could be evoked with relatively low current intensities with ICMS trains. White × signs show the locations from which the C stimuli were delivered, and white + signs show locations used for the T stimulations. In 4 animals (A–C, E) some sites were used with more than one partner. M, medial; R, rostral. Scale bar = 1 mm.

Fig. 3.

MEP peak latencies resulting from T stimulation in CFA. We evoked clear MEPs with the T stimulation from a total of 26 cortical sites. The bar graph shows how many MEP peaks we found at the different latencies. The fastest peak was found 13 ms after the T stimulation, and the slowest was found after 25 ms. The mean MEP peak latency was 17.04 ± 2.29 ms, and the median peak latency was 16.52 ms. These results support that T response is a cortically mediated response.

Fig. 4.

Examples of modulation of MEP resulting from RFA conditioning. Data shown are MEPs of the extensor digitorum communis (EDC) from different stimulation protocols (cortical locations). The aMEP_predicted (sum of T and C) is shown in black and aMEP_evoked with the different ISIs in color (see legend). Broken lines show examples of aMEP_evoked peaks and their latencies. A: for some cortical sites, when the conditioning stimulation of RFA had an effect on the MEP, it was always facilitatory, regardless of the ISI. In this case, the calculation of ΔI would give positive values with all ISIs. The broken line shows that the increase of peak intensity with C and T delivered simultaneously (ISI0) was associated with a decrease of latency. B: for other cortical sites, when conditioning RFA had an effect, it was an inhibition of the evoked MEP, regardless of the ISI. Here, ΔI would be negative with all ISIs. The broken line shows that the decrease of peak intensity with ISI10 was associated with an increase of latency. C: finally, the conditioning of RFA could facilitate the evoked MEP with some ISIs and inhibit the evoked MEP with other ISIs. In this example, ISI0, paired-pulse conditions with C preceding T by 2.5 ms (ISI2.5), and paired-pulse conditions with C preceding T by 5 ms (ISI5) are all facilitatory, whereas ISI10 and ISI15 are inhibitory. Accordingly, ΔI would be positive at some ISIs and negative at others, and thus have opposite effects. Broken lines show that, with ISI0, the increase of peak intensity was associated with a decrease of latency, and with ISI15, the decrease of peak intensity was associated with an increase of latency. With ISI2.5, there was an increase of peak intensity but no effect on latency.

Fig. 5.

MEPs of 4 arm muscles recorded in 26 protocols. Each panel shows the data from a recorded muscle. MEPs were simultaneously recorded in the EDC (top left), palmaris longus (PL; top right), biceps brachii (BB; bottom left), and triceps brachii (TB; bottom right). Each row in each panel shows the averaged EMG data for each condition that were calculated from 3 blocks of 100 trials presented in randomized order. The protocols are ordered (from 1 to 26) based on intensity of modulation of the MEP in the EDC at ISI0 and the order kept across the different muscles. The EMG values within a row are normalized to the MEP peak intensity from the T stimulus alone. The color scale on the right shows the range of possible MEP values. If dark red colors are visible with an ISI, it means that the MEP evoked with C-T was much greater than the MEP evoked with T stimulation alone (e.g., EDC protocol 1 with ISI0 and ISI2.5). If the response with an ISI shows dark blue colors, RFA conditioning had a strong inhibitory effect [e.g., EDC protocol 1 with paired-pulse conditions with C preceding T by 10 (ISI10) or 15 ms (ISI15)]. If the response with an ISI is green, the MEP evoked with C-T was comparable to the MEP evoked with the T stimulation only (e.g., EDC protocol 1 with ISI5). In each panel, from left to right, the MEP responses during the analyzed data window (3–30 ms after the end of stimulation) are shown for the T stimulus alone, the C stimulus alone, and C-T at the different ISIs. White rows in BB and TB correspond to protocols from the animal in which only EDC and PL were recorded. In addition, in the TB, there were 13 protocols where the T stimulation alone did not evoke EMG, and these data were discarded (gray rows). The general pattern of modulation appears consistent across muscles. This is more obvious for the EDC, PL, and BB, for which more data are available. For all 4 muscles, strong facilitations are more common with shorter ISIs (ISI0 and ISI2.5) and inhibition more common with longer ISIs (ISI10 and ISI15).

Fig. 6.

Average effect of RFA conditioning on MEP peak intensity (ΔI). ΔI values at the different ISIs for the 26 protocols (pairs of RFA and CFA stimulation sites). Data for the EDC (top left), PL (top right), BB (bottom left), and TB (bottom right) are separated. A column in the bar graphs shows the ΔI value of a protocol at an ISI. Data are sorted from highest to lowest ΔI within each ISI. Because the percent change for inhibition is limited to the range 0–100 and the facilitatory effect can be infinite, we separated the facilitatory and inhibitory effects in two plots with different scales. The facilitatory effects are shown in plot on top and inhibitory effects in the plot on the bottom. The effect of RFA conditioning on peak intensity appeared to be consistent in the different muscles. At each ISI, we found RFA sites that had both facilitatory and inhibitory effects on CFA outputs. However, at short ISIs, facilitatory effects were much more common, and, at long ISIs, inhibitory effects were predominant.

Fig. 7.

Categories of significant RFA conditioning effects on MEP peak intensity. Results of the Wilcoxon rank sum analysis for peak intensity. For a given protocol, we tested if the population of sMEP_evoked was significantly greater (facilitation) or smaller (inhibition) than the sMEP_predicted with the various ISIs. For this analysis, we combined data from all muscles. We separated the pattern of RFA modulation on CFA outputs across ISIs into 4 categories. The bar graph shows the percentage of RFA sites within each category that had a significant effect with the different ISIs. First, the most common pattern of modulatory effects on CFA output intensity resulting from RFA conditioning was a significant facilitation with at least one ISI, but no significant inhibitory effect on MEPs with any ISI (Group Facilitatory; n = 32). Within Group Facilitatory, most RFA sites significantly modulated CFA outputs with short ISIs, and this percentage decreased with longer ISIs. Second, we found fewer, but still many, RFA sites that significantly inhibited CFA outputs with at least one ISI but had no significant facilitatory effect with any ISI (Group Inhibitory; n = 23). Fewer sites from Group Inhibitory had a significant effect with short ISIs than with long ISIs. Third, some RFA sites had both significant facilitatory effects and inhibitory effects on the MEP of a muscle with different ISIs (Group Opposite; n = 13). With short ISIs, a large proportion of cortical sites from Group Opposite was significantly facilitatory, and no sites were inhibitory. In contrast, with long ISIs, most sites from Group Opposite were inhibitory, and none were facilitatory. Finally, we found RFA sites that did not significantly modulate the MEPs of a muscle with any of the ISIs (Not Sig; n = 11).

Fig. 8.

Average effect of RFA conditioning on MEP peak latency (Δt). Bar graphs showing the effect of RFA conditioning on the MEP peak latency values (Δt) for EDC (top left), PL (top right), BB (bottom left), and TB (bottom right). A column in the bar graphs shows the Δt value of a protocol at an ISI. Data are sorted from highest to lowest Δt within each ISI. For each muscle, the RFA conditioning could increase (positive values) or decrease (negative values) the latency of the aMEP_evoked peak compared with the aMEP_predicted peak. In most cases, the modulation of latency resulting from RFA conditioning was quite small. In general, RFA conditioning tended to decrease the latency of the MEPs more often with short ISIs and increase it with longer ISIs. Among the 3 muscles for which sufficient data were available, this relation was clearer for the EDC and BB and less clear for PL.

Fig. 9.

Categories of significant RFA conditioning effects on MEP peak latency. Results of the Wilcoxon rank sum analysis for peak latency. For a given protocol, we tested if the evoked MEP peak occurred significantly earlier or later than predicted MEP. As for peak intensity, the modulation of the RFA conditioning effects across ISIs was separated into 4 different groups [Group Earlier (n = 16), Group Later (n = 7), Group Opposite (n = 3), and Not Sig; n = 19]. Compared with the effects of RFA on peak intensity, there were more cases where RFA conditioning did not significantly modulate peak latency. The bar graph shows the proportion of significantly modulated cases for each group across the ISIs. For both Group Earlier and Group Opposite, the greatest proportion of RFA sites that significantly decreased the MEP peak latencies was with ISI5. For both Group Later and Group Opposite, the greatest proportion of RFA sites that increased the MEP peak latencies was with ISI15.