Neural Mechanism Of Vestibular-visual Conflict Adaptation In The Macaque Parieto-insular Vestibular Cortex Area(PIVC)

Motion sickness is an important problem in the field of aviation medicine.It is generally believed that the formation of motion sickness is related to sensory conflict,but its specific mechanism is still unclear.In the current study,we attempted to explore the neural mechanism of vestibular-visual cross-sensory dynamic calibration based on the research paradigm of vestibular-visual cue-conflict adaptation.Firstly,we trained the macaques to perform heading discrimination tasks.After they learned heading discrimination tasks well,and then we gradually introduced a conflict between vestibular-visual combined stimulus to macaques to adapt.We found that macaques did adapt to sensory conflict.Specifically,in the post-adaptation block,the perception of vestibular and visual stimuli both shifted significantly（p<0.05,paired t-test）.On one hand,when the direction of vestibular stimuli was on the right side of visual stimuli,the vestibular perception of macaque monkeys shifted to the leftward,and the corresponding Point of Subjective Equality（PSE）shifted to the rightward（mean±SE:1.25°±2.05°）,the visual perception shifted to the rightward,and the corresponding PSE shifted to the leftward（mean±SE:-0.56°±1.44°）in the meanwhile.On the other hand,when the direction of vestibular stimuli was on the left side of visual stimuli,the vestibular perception of macaque monkeys shifted to the rightward,and the corresponding Point of Subjective Equality（PSE）shifted to the leftward（mean±SE:-1.96°±1.47°）,the visual perception shifted to the leftward,and the corresponding PSE shifted to the rightward（mean±SE:1.40°±1.30°）in the meanwhile.These results indicate that after sensory conflict adaptation,macaques recalibrate both vestibular and visual perception,making the two modes of perception shift towards the direction of conflict reduction,so as to reform a unified perception.For further exploring the neural mechanism of visual-vestibular calibration in the post-adaptation block,neural activities in the parieto-insular Vestibular Cortex（PIVC）were recorded simultaneously during task performance.Among all of 161 PIVC neurons recorded,60 neurons showed significant directional tuning to vestibular stimulation before or after adaptation（p<0.05,linear regression analysis）.Neuronal directional tuning did not shift significantly between pre-adaptation block and post-adaptation block.Quantifying neural activities by constructing the neurometric functions（ROC analysis）,we can directly compare them to the behavioral self-motion heading discriminations.We found that when the direction of vestibular stimuli was on the right side of visual stimuli,the neurometric curves shifted to the rightward（△PSE:mean5.74°±12.37°SE）,indicating the leftward shift of neural vestibular perception.When the direction of vestibular stimuli was on the left side of visual stimuli,the neurometric curves shifted to the left（（35）PSE:mean-0.31°±35.67°SE）,indicating the rightward shift of neural vestibular perception.Shifts between neuronal PSE and behavioral PSE are consistent.Shifts in population of neuroal PSE are positively correlated with changes in behavioral perception of vestibular motor stimuli(r=0.49,P=7.19*10^-5,Pearson linear correlation analysis).Pair t-test results showed that the neural spontaneous firing rates did change between pre-adaptation block and post-adaptation block,in the meanwhile,the response amplitude of neurons to vestibular stimuli had not significant changes（p=0.86,△>0;p=0.46,△<0）before and after adaptation.Besides,there is a significant correlation between the neural shifts and behavioral shifts over the stimulus time,which is shown as the most significant correlation in the three stimuli epochs of 0.3～0.4s（around the peak acceleration time of 0.375s）,0.4～0.5s(（around the maximum velocity time of 0.5s）and 0.6～0.7s（around the peak acceleration time of 0.625s）separately.These results indicate that these adaptations may be influenced by signals related to the different temporal characteristics of the stimulus.In general,these results suggest that neurons in the PIVC area may encode signals related to cue-conflict adaptation during vestibular-visual cross-modal calibration.Through these studies,we have a further understanding of the visual-vestibular dynamic correction mechanism in the brain,and hope to provide some ideas for the treatment of motion sickness.