Representation Of Vestibular And Optic Flow Cues To Self-motion In The Macaque Superior Temporal Polysensory Area (STP)

The neural mechanism of self-motion perception(there refers exclusively to heading perception)is complicated,which involves the interaction of different sensory information from multiple areas,including vestibular and visual signals.The superior temporal polysensory area(STP)of macaques is a multisensory integration area that exhibited significant heading selectivity to optic flow,and is likely to participate in heading perception.However,there are few studies on the superior temporal polysensory area(STP)in terms of self-motion perception.The response characteristics to self-motion are also less known,and need to be investigated.In the current study,we investigated the response characteristics of STP in the horizontal plane by using a virtual-reality system to present eight evenly spaced directions,45? apart.Responses were obtained during three conditions: optic flow alone(“visual” condition),inertial motion(“vestibular” condition)and paired inertial motion and optic flow(bimodal condition).For the latter,we tested all 64 combinations of vestibular and visual headings,including eight congruent and 56 incongruent(cue-conflict)presentations.Among 197 neurons recorded from STP,only 14 cells(7.1%)were tuned for both visual and vestibular.This percentage was lower than the cells tuned only for vestibular(n = 23,11.7%),and also lower than cells that are only tuned to visual(n = 43,21.8%).Visual and vestibular direction preferences of multisensory STP neurons could be congruent or opposite,with roughly equal proportion.We further quantified the heading selectivity of visual and vestibular using a Direction Discrimination Index(DDI).For the vestibular condition,the mean DDI for vestibular condition was 0.36 ? 0.11(mean ? SD),which was significantly smaller than that for visual stimulation(mean ? SD: 0.40 ? 0.12)(Wilcoxon test,p <0.05).All these indicate that the vestibular response in STP is weaker than the visual response.During the combined condition with congruent vestibular and visual stimulus,the firing rate in combined condition is higher than in visual only condition.However,there was no significant difference for the DDI values between the combined condition and visual only condition.The results might suggest that STP may not likely to play a significant role in visual/vestibular integration for self-motion perception.When the visual and vestibular stimuli are conflict,the vestibular signal doesn't change the cell's visual preferred direction,but it affect the firing rate of visual response.Specifically,when both the visual and vestibular cues are presented together at the visual and vestibular heading preference of the neuron,the effect of cross-modal suppression is small.However,the same neuron show clear cross-modal suppression when one of the cue are presented at non-preferred heading.In general,the superior temporal polysensory area(STP)is a visual area.Although STP neurons show clear responses to vestibular stimuli,the vestibular response is generally weaker compared with visual response,and only a few neurons were sensitive to visual and vestibular signals.When visual and vestibular stimuli were presented together,the vestibular signals does not affect the visual preference,but it affect the firing rate of visual responses with a clear cross-modal suppression effect.These results indicate that STP may not be an important area to integrate visual-vestibular information during self-motion.The role of vestibular and visual in STP needs further exploration.