This study investigated the effects of external hand temperature manipulations by immersing the entire hand in cool, cold, or warm water on the RHI strength. We found that cooling the hand enhanced the proprioceptive drift, while warming and intense cooling did not. These results are almost consistent with those reported by Kammers et al.8. Our results partially support the association between the RHI occurrence and the drop in the hand skin temperature.
In Experiment 1, we achieved a reduction of the participants’ hand skin temperature by approximately 4℃, as almost the same previously reported8. The pre- and post-illusion skin temperatures under the no cooling condition were slightly but significantly higher than in the control, even though no external temperature manipulation was applied. This temperature increase was probably because of the heat buildup in the glove worn by the participants throughout the experiment. The warming effect wearing the glove had on the skin temperature also occurred under the cooling condition. Despite the skin warming effect the glove wearing had, the pre-illusion temperatures were significantly lower than the control. These temperatures remained lower at the post-illusion assessment, although recovery of the temperature over time was noted. Therefore, the RHI could occur when the skin temperature was lowered under the cooling condition, although the temperature decrease was underestimated due to the warming effect of wearing the glove.
Cooling the hand did not affect the difference between the actual and perceived hand positions during the pre-position test (Fig. 4). This indicated that a skin temperature drop did not merely modulate the perceived position of the hand but also enhanced the illusory disembodiment of one’s own hand by integrating the visual and somatosensory information. Although this study could not clarify the neural mechanisms underlying the effect, we infer central and peripheral mechanisms were involved in enhancing the RHI. For the central mechanism, the insula is a candidate region involved in the effect because it is activated by an increase in afferent signals associated with cooling. Activity in this region was correlated with the degree of cold sensation in the hands or fingers17. Additionally, the insula is thought to underlie the bodily self-awareness18–20. Therefore, body ownership and temperature regulation are partially shared by the same brain region, underlying their relationship. Previous studies have shown that disruption of the sense of body ownership is a characteristic of many pathological states, including stroke21, schizophrenia22, autism23, epilepsy24, neuropathic pain25, anorexia nervosa26, bulimia nervosa27, and self-mutilation28. Furthermore, many of these pathological conditions are also characterized by disruption of temperature regulation28,29−35. Artificial cooling of the hand in this study placed a strain on the participants’ thermoregulatory mechanisms that may have contributed to the altered disembodiment of their hand.
As for the peripheral mechanism, the cooling stimulus might be modulating the transmission efficiency of afferent inputs. As Kammers et al.8 mentioned, cooling the hand decreases the conduction velocity and transmission efficiency of somatosensory afferent nerves36,37. In contrast, warming increases the conduction velocity and transmission efficiency of these nerves38,39. This suggests that the RHI is more likely to occur when the somatosensory signals are weakened. Makin et al.40 mentioned that during the RHI, the integration of sensory information leans heavily in favor of vision, likely reducing the weight of the somatosensory inputs40,41. This idea was partially supported by behavioral42,43 and neurophysiological44,45 studies. Therefore, weakening the somatosensory inputs by cooling the hand might augment capturing of visual over somatosensory information, enhancing the proprioceptive drift during the RHI.
Hand cooling seems to affect the proprioceptive drift and the strength of feeling ownership of the rubber hand differentially. Cooling the hand did not affect the subjective feeling that the rubber hand was one’s own (Fig. 3), as reported by Kammers et al.8. The hand proprioceptive drift and the subjective ratings for the questionnaire are typically used as an index for evaluating changes in the sense of body ownership1. Furthermore, the magnitude of the proprioceptive drift was shown to correlate with the strength of the ownership feeling reported in the questionnaire1. However, some studies have failed to detect such a correlation46–49. Rohde et al.49 indicated that proprioceptive drift was a spatial update of the body in space according to the synchronous visuotactile stimulation; therefore, the process underlying proprioceptive drift was independent of that underlying the subjective feeling of limb ownership in the RHI. Abdulkarim and Ehrsson46 concluded that the correlation between the proprioceptive drift and subjective feeling of ownership depended on the conditions under which the experiment was performed. Considering these findings, the effect of cooling the hand in the RHI paradigm of this study was probably associated with the perceived hand position drift rather than the feeling of ownership of the rubber hand. Therefore, cooling the hand may be more effective when one wants to alter the perceived position of the hand toward an object located outside the body, for example, controlling a robotic arm by connecting brain activities to a computer50 or using a rake to manipulate out-of-reach objects51. It is not necessary to create a sense of ownership of the objects in such cases.
The intense cooling (~ 10ºC) in Experiment 2 that decreased the skin temperature by around 13℃ did not modulate the RHI strength, supporting the results reported by Kammers et al.8. Cold stimulation below 17℃ activates the noxious cold sensation sensors52–54. Therefore, many participants were likely to feel pain in their hands due to the intense cooling. Kammers et al.8 stated that pain reminds us of our ‘real’ body. During the RHI, pain neither abolishes nor enhances the relocation of one’s hand or subjective feeling of ownership. We consider this the reason the RHI was not strengthened in Experiment 2.
Our findings suggest an asymmetric nature in the effects of cold and warm stimuli on the RHI. Warming the hand in Experiment 3 did not affect the RHI strength, although the skin temperature modulation by warming was almost the same as that of the cooling stimulus in Experiment 1 (~ 3.7℃). This finding differed from Kammers et al.8, who showed that warming the hand attenuated the strength of the RHI. One reason for the difference between the studies might be the difference in the external temperature stimuli used; the entire hand was stimulated in this study, while only the palm was stimulated in the previous study.
In conclusion, this study demonstrated that cooling the entire hand by immersing it in water for 1 min enhanced the proprioceptive drift during the RHI, but not the subjective feeling of ownership of the rubber hand. In contrast, warming or intense cooling of the entire hand did not affect the RHI strength. These findings were almost identical to those reported by Kammers et al.8. Therefore, we have confirmed that moderate transient cooling of the entire hand was sufficient for enhancing the illusory disembodiment of one’s hand. These findings suggest that cooling the hand by immersing it in cold water may be useful for therapies or training that require changes in the perceived position of the limb.