In this study, we investigated the modulation of SEPs during the RHI. Compared with previous studies that showed attenuation of SEP components with a wide time window after stroking during the RHI7,8, our data indicates that the early component of SEPs, N1-P1 response, was attenuated when RHI was elicited. In addition, this attenuation began before participants felt the rubber hand as their own. This suggests that attenuation of somatosensory inputs from the participants’ hand to the brain is one of the factors associated with the occurrence of the RHI.
SEPs have been typically recorded by continuous electrical stimulation with short interstimulus intervals (1–2 Hz)13–18. Continuous electrical stimulation would direct participants’ attention to their real hand. This situation makes it difficult to create an RHI. Therefore, in this study, electrical stimulation was provided manually by the experimenter with a long interstimulus interval (around 10 s, Fig. 3). This allowed the participants’ attention to be distracted from their hand. However, the long interstimulus interval does not ensure a large number of waveforms triggered by electrical stimulation as the experiment time becomes longer and the physical or psychological load of the participants’ increases. Thus, we adopted a special arrangement of recording electrodes introduced by Brooke et al.19,20. A merit of this arrangement is that it records a clear waveform of the early components of SEP with a small sample size.
We considered that modulations of the early component of SEPs reflect the degree of ease in transmission of somatosensory information from the hand to the cerebral cortex. Previous studies used brush-stroke stimulation to evoke SEPs during RHI4–8. The obtained SEPs were derived from tactile stimulation that is necessary to produce the RHI. However, the demerits of brush-stroke-evoked SEPs does not include the recognisable early component, in particular components originating from the primary somatosensory cortex9–12. To address this issue, we used electrical stimulation. One might think that inflow of somatosensory inputs induced by electrical stimulation to the cerebral cortex does not originally occur during procedures in the RHI. Although somatosensory inflow produced by electrical stimulation is unnatural, we believe that modulation of the N1-P1 amplitude is the optimal index for evaluating the inflow of somatosensory inputs to the cerebral cortex.
The N1-P1 component originates from the primary somatosensory cortex. Desmedt21 demonstrated that this component may represent the primary response of somatosensory cortical cells. SEPs recorded at the cortical surface during neurosurgery indicated that early components, 20–30 ms after electrical stimulation, are generated from the primary somatosensory cortex22. In addition, the modulation of the SEP amplitude depends on the frequency of the electrical stimulation23. In this study, the frequency did not differ among the four conditions (Fig. 3). Therefore, this is not the cause of the modulations of the N1-P1 amplitude in this study. These suggest that modulations of N1-P1 amplitude reflect alterations in somatosensory inputs at the primary somatosensory cortex according to the process of occurrence of the illusion.
The transmission of somatosensory signals to the primary somatosensory cortex is diminished during active or passive movements, and tactile stimulation of the hand24–30. This mechanism is called “gating”. The gain in the SEP amplitude is modulated by centrifugal and centripetal gating mechanisms27. The former is when the efferent signals induced by the motor command from the motor-related areas suppress the ascending somatosensory signals. The latter corresponds to when interfering effects between the given sensory afferent signals induced by electrical stimulation of the nerve and the afferent feedback from the skin caused by tactile stimulation of the hand. In this study, the participants remained resting throughout the experiment. Therefore, the effect of the centrifugal gating mechanism on the modulations of the N1-P1 amplitude need not be considered.
However, we believe that the decrease in the N1-P1 amplitude during RHI is not explained only by traditional centripetal gating mechanisms. In Experiment 1, the N1-P1 amplitude in the tactile stimulation condition was substantially smaller than that in the control condition. This finding would be affected by centripetal gating24. Similar tactile stimulations were provided to the participants in both the congruent and incongruent stroking conditions. Despite the same manner of the tactile stimulation, N1-P1 amplitude in the congruent stroking condition was significantly attenuated compared with those in the incongruent and tactile stroking conditions (Fig. 2). Thus, attenuation of the N1-P1 amplitude during RHI would not be caused by only centripetal gating. We do not know the mechanisms for this modulation, but certain mechanisms related to the occurrence of the illusion might affect the modulation of the transmission of somatosensory signals.
The subjective ratings for the questionnaire are typically used as an index for evaluating changes in the sense of body ownership1. In this study, ratings of questionnaire items 1, 2, and 3 in the congruent stroking condition were high, which is consistent with previous studies1,3,31,32. This indicates that our procedure properly induced the RHI. The degree of decrease in the N1-P1 amplitude in the congruent stroking condition was not significantly correlated with subjective ratings of the questionnaire (Fig. 5). Therefore, the attenuation of the N1-P1 amplitude relates to the occurrence of the illusion, but does not reflect the subjective strength of the illusion itself.
In Experiment 1, the electrical stimulation was continuously provided at intervals of approximately 10 s. In this case, the SEP waveform might be obtained from the electrical stimulation given before and after the occurrence of the illusion. Therefore, the findings of the experiment were not able to explain whether the occurrence of the RHI was due to the attenuation of somatosensory inputs from the participants’ hand to the brain, or the occurrence of the RHI causes the attenuation of somatosensory inputs to the brain. To address this, we performed Experiment 2 and found that the N1-P1 amplitude was attenuated before the occurrence of the illusion. We consider that the decrease in the transmission of somatosensory signals at the primary somatosensory cortex is one of the factors influencing the occurrence of the RHI. According to a model of body ownership during RHI33, the posterior parietal cortex integrates visual and somatosensory information of touch before the occurrence of the RHI. This suggests that the posterior parietal cortex is involved in the resolution of the conflict between the incoming visual and tactile information, and the resulting recalibration of the visual and tactile coordinate systems. When multiple sensory information integrate in the parietal cortex before the illusion occurs, it might be necessary to be accompanied by attenuated somatosensory signals from the hand.
A limitation of this study is that we cannot distinguish whether the N1-P1 response originates from cutaneous afferent or muscle afferents. Modulation of cutaneous afferent inputs is meaningful in the RHI paradigm. The median nerve is a mixed nerve containing both muscle and cutaneous afferents, implying that any changes in the N1-P1 amplitude may not be solely attributable to only one of these groups of afferents. Gandevia and Burke34 demonstrated an intramuscular and percutaneous mixed nerve trunk stimulating technique, in which muscle afferents contributed to the recorded N1-P1 potential. In this study, we did not record N1-P1 potentials elicited by stimulation of only cutaneous afferents (e.g. stimulating the nerve of the digit). This is because the responses obtained by the stimulating nerve of the digit are small and require a large number of sample sizes35,36. Therefore, this study suggests that somatosensory signals to the brain are diminished during RHI, although the modality of afferents is not specified. Further studies are needed to solve this problem.
In summary, our results suggest that attenuation of somatosensory inputs from the participants’ hand to the brain occurs at the primary somatosensory cortex during RHI. Furthermore, the attenuation starts before the occurrence of the illusion. We consider that attenuation of somatosensory signals from the body parts at the entrance of the cerebral cortex is one of the factors associated with the occurrence of changes in feelings of limb ownership. This study has gone some way toward enhancing our understanding of the neural mechanisms underlying the occurrence of the RHI.