The human semicircular canal model of galvanic vestibular stimulation

A vector summation model of the action of galvanic stimuli on the semicircular canals has been shown to explain empirical balance and perceptual responses to binaural-bipolar stimuli. However, published data suggest binaural-monopolar stimuli evoke responses that are in the reverse direction of the model prediction. Here, we confirm this by measuring balance responses to binaural-monopolar stimulation as movements of the upper trunk. One explanation for the discrepancy is that the galvanic stimulus might evoke an oppositely directed balance response from the otolith organs that sums with and overrides the semicircular canal response. We tested this hypothesis by measuring sway responses across the full range of head pitch. The results showed some modulation of sway with pitch such that the maximal response occurred with the head in the primary position. However, the effect fell a long way short of that required to reverse the canal sway response. This indicates that the model is incomplete. Here, we examine alterations to the model that could explain both the bipolar and monopolar-evoked behavioural responses. An explanation was sought by remodelling the canal response with more recent data on the orientation of the individual canals. This improved matters but did not reverse the model prediction. However, the model response could be reversed by either rotating the entire labyrinth in the skull or by altering the gains of the individual canals. The most parsimonious solution was to use the more recent canal orientation data coupled with a small increase in posterior canal gain

Brainsourcing: Crowdsourcing Recognition Tasks via Collaborative Brain-Computer Interfacing

Peer reviewe

At what stage of neural processing do perspective depth cues make a difference?

The present study investigated the cortical processing of three-dimensional (3D) perspective cues in humans, to determine how the brain computes depth from a bidimensional retinal image. We recorded visual evoked potentials in 12 subjects in response to flat and in-perspective stimuli, which evoked biphasic potentials over posterior electrodes. The first, positive component (P1, at 90 ms) was not sensitive to perspective, while the second, negative peak (N1 at ~150 ms) was significantly larger for 3D stimuli, regardless of attention. The amplitude increase due to perspective was seen on all posterior electrodes, but was largest over the right hemisphere, particularly at parietal sites. Source modeling low-resolution electromagnetic tomography (LORETA) confirmed that among the different areas participating in two- and three-dimensional stimuli processing, the right parietal source is the most enhanced by perspective depth cues. We conclude that the extraction of depth from perspective cues occurs at a second level of stimulus processing, by increasing the activity of the regions involved in 2D stimuli processing, particularly in the right hemisphere, possibly through feedback loops from higher cortical areas. These modulations would participate in the fine-tuned analysis of the 3D features of stimuli