Abstract
The posterior parietal cortices (PPCs) engage in motor processing during reach and grasp. Yet, the neural substrates behind their connections to the motor cortex (M1) are still unknown. It has been postulated that right PPC (rPPC) acts directly on left PPC (lPPC), which then acts on left M1 (lM1). Alternately, rPPC may act on lM1 circuits at the same time as lPPC. Here, we used triple-site transcranial magnetic stimulation (TMS) to test this hypothesis. Previous studies showed that rPPC or lPPC stimulation alone facilitates motor evoked potentials (MEPs), whereas rPPC and lPPC stimulation together inhibits the facilitated MEPs. To consider their interaction at M1, we tested two M1 circuits: short interval intracortical facilitation (SICF) and short interval intracortical inhibition (SICI). SICF is elicited by two suprathreshold pulses to M1 delivered in ∼1.5 ms intervals which facilitate M1 output. SICI inhibits M1 output and is elicited by a subthreshold conditioning stimulus delivered 1–6 ms prior to a suprathreshold pulse. We hypothesized that if rPPC and lPPC inhibit MEPs by acting on SICF or SICI in M1, a correlation would emerge.
16 healthy adults participated (9 females, 22–64 years, 35.4 ± 14.6 years old). MRI-navigated TMS was delivered by 3 small coils over rPPC, lPPC and lM1. MEPs were recorded from the right first dorsal interosseous muscle. Exp 1 tested PPC-lM1 circuits and their interactions at several rPPC and lPPC stimuli intensities, including 90 and ∼108% RMT over lPPC and 30 to 130% RMT over rPPC. Exp 2 tested PPC-lM1 circuits and their interaction at different test MEP amplitudes of ∼0.2 mV, 1 mV, and 4 mV. Exp 3 tested the effect of rPPC on SICF. Exp 4 tested the effect of SICI on lPPC-lM1 and rPPC-lPPC-lM1 interactions.
Exp 1: The interaction of rPPC and lPPC inhibited MEPs. This effect occurred when the rPPC was stimulated at 70% and 90% RMT, and the lPPC was stimulated at ∼108% RMT. rPPC-to-lM1 connectivity correlated with this interaction, whereas lPPC-to-lM1 connectivity did not. Exp 2: Compared to higher intensities, low intensity M1 stimulation at ∼0.2 mV showed facilitated rPPC-to-lM1 connectivity and increased the inhibitory effects of rPPC and lPPC interaction. Exp 3: rPPC inhibited SICF at 1.4 ± 0.1 ms, and this inhibition correlated with rPPC and lPPC inhibition. Exp 4: SICI abolished the facilitatory effect of lPPC-lM1 and the inhibitory effect of rPPC-to-lPPC-lM1 connection. SICI in the presence of lPPC-lM1 correlated with rPPC in the presence of lPPC-lM1.
We showed that the rPPC and lPPC inhibit the lM1, which is more prominent when tested at low M1 stimulation intensity. rPPC-to-lM1 connectivity correlated with this interaction. The effect of PPC on M1 may be mediated in part by cortical circuits SICF and SICI. We conclude that the effects of rPPC and lPPC likely interact at lM1.