Contralateral and Ipsilateral Interactions in the Somatosensory Pathway in Healthy Humans

Abstract

Hyper-adaptability, the ability to adapt to changes in the internal environment caused by neurological disorders, is necessary to recover from various disabilities, such as motor paralysis and sensory impairment. In the recovery from motor paralysis, the pre-existing neural pathway of the ipsilateral descending pathway, which is normally suppressed and preserved in the course of development, is activated to contribute to the motor control of the paretic limb. Conversely, in sensory pathways, it remains unclear whether there are compensatory pathways which are beneficial for the recovery of sensory impairment due to damaged unilateral somatosensory pathways, such as thalamic hemorrhage. Here, we investigated the interaction between the left and right somatosensory pathways in healthy humans using paired median nerve somatosensory evoked potentials (SEPs). Paired median nerve SEPs were recorded at CP3 and CP4 with a reference of Fz in the International 10–20 System. The paired median nerve stimulation with different interstimulus intervals (ISIs; 1, 2, 3, 5, 10, 20, 40, 60, and 100 ms) was performed to test the influence of the first stimulus (to the right median nerve) on the P14, P14/N20, and N20/P25 components induced by the second stimulus (left side). Results showed that the first stimulation had no effect on SEP amplitudes (P14, P14/N20, and N20/P25) evoked by the second stimulation in all ISI conditions, suggesting that there might not be a neural connectivity formed by a small number of synapses in the left–right interaction of the somatosensory pathway. Additionally, the somatosensory pathway may be less diverse in healthy participants.

Interaction of the Left–Right Somatosensory Pathways in Patients With Thalamic Hemorrhage: A Case Report

“…To investigate the interactions between contralateral and ipsilateral activations, single SEP (s-SEP), and p-SEP were recorded using a previously described protocol with minor modifications ( Ragert et al, 2011 ; Brodie et al, 2014 ; Ishii et al, 2021 ). In the s-SEP paradigm, s-SEPs were obtained by stimulation of the right and left median nerves at the wrist ( Figures 2 , 3 ).…”

“…Neurons in the unilateral S1 receive sensory information from the contralateral brain through interhemispheric transfer ( Iwamura et al, 1994 , 2001 , 2002 ; Nihashi et al, 2005 ). Recently, three previous studies, including our study, investigated interactions between contralateral and ipsilateral activations using a paired median nerve somatosensory evoked potential (p-SEP) protocol in a healthy human ( Ragert et al, 2011 ; Brodie et al, 2014 ; Ishii et al, 2021 ). In this protocol, peripheral stimulation of the unilateral median nerve preceded the stimulation of the other median nerve with some interstimulus intervals (ISIs).…”

“… Ragert et al (2011) and Brodie et al (2014) showed that interhemispheric inhibitory interactions in the S1 occur between two hemispheres via the corpus callosum in a critical time interval of 20–25 or 15–35 ms after median nerve stimulation. However, we recently investigated the effect of ISIs (1–100 ms) on p-SEPs in more detail and concluded that no interaction occurs between left and right somatosensory pathways in healthy subjects with any ISI ( Ishii et al, 2021 ). We further demonstrated that no interaction occurs between the bilateral somatosensory pathways of healthy subjects with short ISI conditions (<5 ms), which indicates that there is also no left–right transmission of a small number of synapses in the pathways ( Ishii et al, 2021 ).…”

“…However, we recently investigated the effect of ISIs (1–100 ms) on p-SEPs in more detail and concluded that no interaction occurs between left and right somatosensory pathways in healthy subjects with any ISI ( Ishii et al, 2021 ). We further demonstrated that no interaction occurs between the bilateral somatosensory pathways of healthy subjects with short ISI conditions (<5 ms), which indicates that there is also no left–right transmission of a small number of synapses in the pathways ( Ishii et al, 2021 ). In stroke patients, in whom large-scale changes in neural networks occur, the existence of a left–right interaction of somatosensory pathways at a level other than the corpus callosum has not been established.…”

“…We hypothesized that the neural connectivity formed by a small number of synapses in the left–right interaction of the somatosensory pathway would be present in patients with recovered sensory impairment. Therefore, we investigated the interaction of the left–right of somatosensory pathways in two patients with thalamic hemorrhage using the p-SEPs protocol under short ISI ( Ragert et al, 2011 ; Brodie et al, 2014 ; Ishii et al, 2021 ).…”