Visuo-vestibular integration for self-motion: human cortical area V6 prefers forward and congruent stimuli

The integration of visual and vestibular input is crucial for self-motion. Information from both sensory systems merges early in the central nervous system. Among the numerous cortical areas involved in processing this information, some (V6 and the ventral intraparietal area -VIP) respond specifically to vestibular anteroposterior information. A series of experiments were carried out to further understand the involvement of these and other areas in self-motion processing when vestibular and visual information are combined with varying congruence and direction parameters. Fifteen subjects underwent an MRI session while receiving visual (optic flow patterns) and galvanic vestibular stimuli, mimicking six conditions: (1) visual forward, (2) visual backward, visual forward with (3) congruent or (4) incongruent vestibular information, visual backward with (5) congruent or (6) incongruent vestibular information. At the voxel-wise level, adding vestibular stimulation to optic flow stimulation activated several bilateral areas located predominantly in the insular cortex. However, the region of interest (ROI) analysis of these areas indicated that none of them exhibits any specificity for the forward/backward direction or for the visuo-vestibular congruency. By extending the ROI approach to other well-known visuo-vestibular areas, we found that the parieto-occipital area V6 is unique in showing not only an increased level of activation for concurrent visual and vestibular stimulation, but also a marked preference when these signals are congruent and specify forward motion. Since area V6 is the only region more active when both visual and vestibular signals specify the most common self-motion direction (i.e. forward self-motion), our results support the view that this area plays a crucial role in visuo-vestibular integration during locomotion. This could be the first step towards the construction of a conscious perception of self-motion, possibly involving other areas.