Imagined body orientation and perception of the visual vertical.

The existence of body orientation mental imagery was tested by examining whether self roll tilt imagery affects the subjective visual vertical (SVV). Twenty healthy subjects judged the orientation of a dim luminous bar with respect to gravitational vertical, while normally seated in complete darkness with their head firmly restrained earth vertically. SVV was measured in three conditions: a reference condition with no imagery, and a left and a right imagery condition, during which the bar orientation was to be judged while the subjects imagine themselves roll-tilted towards left or right, respectively. The imagined roll tilts were of the same magnitude as roll tilts which generally induce an E-effect, i.e., an SVV lean toward the side opposite to those of body tilt. If imagery and perception of self roll tilt share common processes, self roll tilt imagery should induce an E-like effect. Results show an imagery-induced E-like effect, which strongly supports the idea that humans can perform mental imagery of body orientation about gravity.

Vestibular projections in the human cortex.

There is considerable evidence from studies on cats and monkeys that several cortical areas such as area 2v at the tip of the intraparietal sulcus, area 3av in the sulcus centralis, the parietoinsular vestibular cortex adjacent to the posterior insula (PIVC) and area 7 in the inferior parietal lobule are involved in the processing of vestibular information. Microelectrode recordings from these areas have shown that: (1) most of these cortical neurons are connected trisynaptically to the labyrinthine endorgans and (2) they receive converging vestibular, visual and somatosensory inputs. These data suggest that a multimodal cortical system is involved in postural and gaze control. In humans, recent positron emission tomography (PET) scans and functional magnetic resonance imaging (fMRI) studies have largely confirmed these data. However, because of the limited temporal resolution of these two methods, the minimum time of arrival of labyrinthine inputs from the vestibular hair cells to these cortical areas has not yet been determined. In this study, we used the evoked potential method to attempt to answer this question. Due to its excellent temporal resolution, this method is ideal for the investigation of the tri- or polysynaptic nature of the vestibulocortical pathways. Eleven volunteer patients, who underwent a vestibular neurectomy due to intractable Meniere's disease (MD) or acoustic neurinoma resection, were included in this experiment. Patients were anesthetized and the vestibular nerve was electrically stimulated. The evoked potentials were recorded by 30 subcutaneous active electrodes located on the scalp. The brain electrical source imaging (BESA) program (version 2.0, 1995) was used to calculate dipole sources. The latency period for the activation of five distinct cortical zones, including the prefrontal and/or the frontal lobe, the ipsilateral temporoparietal cortex, the anterior portion of the supplementary motor area (SMA) and the contralateral parietal cortex, was 6 ms. The short latency period recorded for each of these areas indicates that several trisynaptic pathways, passing through the vestibular nuclei and the thalamic neurons, link the primary vestibular afferents to the cortex. We suggest that all these areas, including the prefrontal area, process egomotion information and may be involved in planning motor synergies to counteract loss of equilibrium.

Evidence of imagined passive self-motion through imagery-perception interaction.

The existence of whole-body passive self-motion mental imagery was investigated by examining whether the perception of passive body accelerations can be affected by passive self-motion imagery. Twenty healthy subjects recognised target passive body acceleration. This recognition task was performed under three conditions: (1) a baseline condition without imagery; (2) a compatible imagery condition during which subjects imagined themselves passively moving in the same direction as the target acceleration; (3) a non-compatible imagery condition during which subjects imagined themselves passively moving in the direction opposite to that of the target acceleration. The recognition of the target acceleration was improved under compatible and degraded under non-compatible imagery. This interaction implies that perception and imaginary share common representations, and supports the existence of passive self-motion imagery.