# Impact of Visual Kinesthetic Illusions on Reciprocal Inhibition and Motor Function
## 4 Discussion
### Effect on RI  
In this study, in the sham condition, participants observed a video of their resting foot on the monitor. The findings from the sham condition demonstrated that watching the resting foot video did not affect RI.  
The results of experiment 1 revealed that the visual kinesthetic illusion related to RPM improved D1 inhibition. This body illusion is known to generate the feeling of the body shifting from its initial position, attributed to the alignment of visual perception with somatosensory input from the body [31]. In this situation, a conflict arises between the body position sensed through visual input and that felt through somatosensory input. During the body illusion, this inconsistency is thought to be alleviated by a reduction in somatosensory input [20,32,33]. Adjusting somatosensory input has been shown to influence M1 through afferent inhibition [34]. The SAI reportedly decreases due to body illusion [20]. In support of the attenuation of SAI due to body illusions, a previous study demonstrated that visual kinesthetic illusion increases corticospinal tract excitability [22]. This indicates that visual kinesthetic illusions may lead to disinhibition in M1, increasing its excitability. Previous studies have shown how M1 excitability affects RI [5,35]. Transcranial direct current stimulation (tDCS) is a noninvasive method that modifies cortical activity beneath the electrode based on polarity. When a cathodal electrode is used, it diminishes the cortical excitability at the stimulation site. RI decreases with the application of a cathodal electrode on M1 [35]. Repetitive transcranial magnetic stimulation (rTMS) similarly modulates cortical activity. A previous study showed that increasing M1 excitability enhanced RI. This enhancement is believed to occur via a descending input from M1, which affects the activity of RI interneurons [36]. Thus, it is proposed that the visual kinesthetic illusion boosts M1 excitability and enhances RI by stimulating the inhibitory interneurons of RI via descending signals from supraspinal control.  
The results of this study revealed that the visual kinesthetic illusion enhanced D1 inhibition within the RI. D1 inhibition is believed to occur via presynaptic inhibition, in which PAD interneurons inhibit input from antagonist muscle Ia fibers [1]. Additionally, presynaptic inhibition selectively reduces sensory input, unlike postsynaptic inhibition [4]. Cutaneous afferent reflex, identified as a reflex triggered by sensory signals, is reportedly diminished by descending input from the corticospinal tract during voluntary movement. This descending input stimulates the PAD interneurons, leading to presynaptic inhibition [37]. A previous study revealed that increasing M1 excitability enhances presynaptic inhibition but does not affect postsynaptic inhibition [5]. The visual kinesthetic illusion associated with RPM might increase excitability in the corticospinal tract by enhancing M1 activity. This elevated activity can decrease sensory input through presynaptic inhibition, thereby enhancing D1 inhibition.