TMS activation of interhemispheric pathways between the posterior parietal cortex and the contralateral motor cortex

Abstract

Using a twin coil transcranial magnetic stimulation (tc-TMS) approach we have previously demonstrated that facilitation may be detected in the primary motor cortex (M1) following stimulation over the ipsilateral caudal intraparietal sulcus (cIPS). Here we tested the interhemispheric interactions between the IPS and the contralateral motor cortex (M1). We found that conditioning the right cIPS facilitated contralateral M1 when the conditioning stimulus had an intensity of 90% resting motor threshold (RMT) but not at 70% or 110% RMT. Facilitation was maximal when the interstimulus interval (ISI) between cIPS and M1 was 6 or 12 ms. These facilitatory effects were mediated by interactions with specific groups of interneurons in the contralateral M1. In fact, short intracortical inhibition (SICI) was reduced following cIPS stimulation. Moreover, additional comparison of facilitation of responses evoked by anterior-posterior versus posterior-anterior stimulation of M1 suggested that facilitation was more effective on early I1/I2 circuits than on I3 circuits. In contrast to these effects, stimulation of anterior IPS (aIPS) at 90% RMT induced inhibition, instead of facilitation, of contralateral M1 at ISIs of 10-12 ms. Finally, we found similar facilitation between left cIPS and right M1 although the conditioning stimuli had to have a higher intensity compared with stimulation of right cIPS (110% instead of 90% RMT). These findings demonstrate that different subregions of the posterior parietal cortex (PPC) in humans exert both facilitatory and inhibitory effects towards the contralateral primary motor cortex. These corticocortical projections could contribute to a variety of motor tasks such as bilateral manual coordination, movement planning in space and grasping.

Figure 1

Schematic representation of the different experiments performed in the current study

Figure 2. Effects of CS applied over right cIPS at different intensities on MEPs obtained by left M1 stimulation with subjects at rest

CS preceded TS applied over M1 by different ISIs ranging from 20 to 4 ms. Data are normalized and expressed as percentage of control test condition. Errors bars indicate 1 s.e.m. Asterisks indicate a P value < 0.05 at post hoc analysis.

Figure 3. Effects of right cIPS conditioning on the strength of SICI circuits (ISI = 2 ms) in contralateral M1

TS was set at different intensities. In the first block the intensity of TS was adjusted to evoke a MEP of approximately 1 mV peak to peak in the relaxed right FDI, as in experiment 1. In a second block TS intensity was reduced to elicit a facilitated CS1-TS of ∼1 mV MEP, in order to control for CS1-TS facilitatory effects. In a final block we increased the intensity of TS to obtain a CS2-TS of ∼1 mV MEP, in order to control for CS2-TS inhibitory effects. Errors bars indicate 1 s.e.m.

Figure 4. MEP amplitudes (mean values ±s.e.m.) for M1 stimulation with AP or PA orientation alone and when preceded by a CS applied over the right cIPS (90% RMT at ISIs of 12 ms)

*P < 0.05. AP, anterior–posterior current flow of the TMS test pulse; PA, posterior–anterior current flow of the TMS test pulse.

Figure 5. Different effects of right aIPS and cIPS conditioning (intensity = 90% RMT) on contralateral M1 excitability

Coil position for stimulation of aIPS (A) and cIPS (B) were determined using neuronavigation system in a representative subject. Errors bars indicate 1 s.e.m. *P value < 0.05 at post hoc analysis.

Figure 6. Effects of CS applied over left cIPS at different intensities on MEPs obtained by right M1 stimulation

CS preceded TS applied over M1 by different ISIs ranging from 20 to 4 ms. Data are normalized and expressed as percentage of control test condition. Errors bars indicate 1 s.e.m. Asterisks indicate a P value < 0.05 at post hoc analysis.

Figure 7. Hypothetical pathways mediating interhemisperic facilitation between PPC and contralateral M1

Facilitatory interhemispheric output from cIPS could synapse onto contralateral inhibitory (filled circles) and excitatory (open circles) M1 interneurons, modulating both SICI and early I waves circuits.