The perception of verticality in lunar and Martian gravity conditions.

Although the mechanisms of neural adaptation to weightlessness and re-adaptation to Earth-gravity have received a lot of attention since the first human space flight, there is as yet little knowledge about how spatial orientation is affected by partial gravity, such as lunar gravity of 0.16 g or Martian gravity of 0.38 g. Up to now twelve astronauts have spent a cumulated time of approximately 80 h on the lunar surface, but no psychophysical experiments were conducted to investigate their perception of verticality. We investigated how the subjective vertical (SV) was affected by reduced gravity levels during the first European Parabolic Flight Campaign of Partial Gravity. In normal and hypergravity, subjects accurately aligned their SV with the gravitational vertical. However, when gravity was below a certain threshold, subjects aligned their SV with their body longitudinal axis. The value of the threshold varied considerably between subjects, ranging from 0.03 to 0.57 g. Despite the small number of subjects, there was a significant positive correlation of the threshold with subject age, which calls for further investigation.

Does bimodal stimulus presentation increase ERP components usable in BCIs?

Event-related potential (ERP)-based brain-computer interfaces (BCIs) employ differences in brain responses to attended and ignored stimuli. Typically, visual stimuli are used. Tactile stimuli have recently been suggested as a gaze-independent alternative. Bimodal stimuli could evoke additional brain activity due to multisensory integration which may be of use in BCIs. We investigated the effect of visual-tactile stimulus presentation on the chain of ERP components, BCI performance (classification accuracies and bitrates) and participants' task performance (counting of targets). Ten participants were instructed to navigate a visual display by attending (spatially) to targets in sequences of either visual, tactile or visual-tactile stimuli. We observe that attending to visual-tactile (compared to either visual or tactile) stimuli results in an enhanced early ERP component (N1). This bimodal N1 may enhance BCI performance, as suggested by a nonsignificant positive trend in offline classification accuracies. A late ERP component (P300) is reduced when attending to visual-tactile compared to visual stimuli, which is consistent with the nonsignificant negative trend of participants' task performance. We discuss these findings in the light of affected spatial attention at high-level compared to low-level stimulus processing. Furthermore, we evaluate bimodal BCIs from a practical perspective and for future applications.

Human locomotion through a multiple obstacle environment: strategy changes as a result of visual field limitation.

This study investigated how human locomotion through an obstacle environment is influenced by visual field limitation. Participants were asked to walk at a comfortable pace to a target location while avoiding multiple vertical objects. During this task, they wore goggles restricting their visual field to small (S: 40°×25°), medium (M: 80°×60°), large (L: 115°×90°), or unlimited (U) visual field sizes. Full-body motion capture was used to extract for each trial the mean speed, pathlength, mean step width, magnitude of head rotation and head mean angular speed. The results show that compared with the U condition, the M and L conditions caused participants to select a wider path around the obstacles without slowing down or altering step width. However, the S condition did slow down the participants, and increased both their step width and path length. We conclude that only for the S condition, balancing problems were substantial enough to spend more energy associated with increased step width. In all cases, participants choose to optimize safety (collision avoidance) at the cost of spending more energy.

Obstacle crossing with lower visual field restriction: shifts in strategy.

In this study, the authors investigated how restriction of the vertical viewing angle influences obstacle-crossing behavior. Twelve participants stepped over obstacles of different dimensions while wearing visual-field-restricting goggles. Using full-body motion capture, several kinematic measures were extracted and analyzed. Results indicate that both a 40° and 90° vertical viewing angle yielded increased step length and toe clearance as compared to an unrestricted view (i.e., 135°), whereas speed remained unaltered. A further decrease (to 25°) caused participants to slow down in addition to a further increase of step length and toe clearance. These results are discussed in terms of a change in priorities, from conservation of energy and time to safety.

Multisensory effects differ for counting small and large pulse numbers.

In Illusory Flash (IF) experiments, congruent multisensory presentation has no effect on the mean estimate of the number of events, but decreases the variance in comparison with unisensory presentation. In contrast, congruent multisensory presentation in other Temporal Numerosity Judgement (TNJ) tasks affects the mean estimate (i.e., it often results in a reduction in underestimation) and increases the variance. In three experiments, we investigated the differences between both paradigms as possible causes of this discrepancy: the presence or absence of incongruent stimuli (Experiment 1), the instruction to the observer to either count flashes, beeps or multisensory events (Experiment 2), and the range of pulses presented (Experiment 3). We found significant differences between the mean numerosity estimate of multisensory and unisensory series in Experiment 3, but not in 1 and 2. This suggests that the difference in the range of pulses presented in IF (1-3 pulses) and TNJ (1-10 pulses) is the primary cause of the discrepancy. In the discussion we propose that this result may be explained by the use of two different strategies and their susceptibility to multisensory presentation. For small pulse numbers, observers can accurately count both unisensory and multisensory pulses. For larger numbers, observers can no longer count but will estimate the number based on the pulse series duration which is improved for multisensory stimuli.

Integration of visual and inertial cues in perceived heading of self-motion.

In the present study, we investigated whether the perception of heading of linear self-motion can be explained by Maximum Likelihood Integration (MLI) of visual and non-visual sensory cues. MLI predicts smaller variance for multisensory judgments compared to unisensory judgments. Nine participants were exposed to visual, inertial, or visual-inertial motion conditions in a moving base simulator, capable of accelerating along a horizontal linear track with variable heading. Visual random-dot motion stimuli were projected on a display with a 40° horizontal × 32° vertical field of view (FoV). All motion profiles consisted of a raised cosine bell in velocity. Stimulus heading was varied between 0 and 20°. After each stimulus, participants indicated whether perceived self-motion was straight-ahead or not. We fitted cumulative normal distribution functions to the data as a psychometric model and compared this model to a nested model in which the slope of the multisensory condition was subject to the MLI hypothesis. Based on likelihood ratio tests, the MLI model had to be rejected. It seems that the imprecise inertial estimate was weighed relatively more than the precise visual estimate, compared to the MLI predictions. Possibly, this can be attributed to low realism of the visual stimulus. The present results concur with other findings of overweighing of inertial cues in synthetic environments.

Counting visual and tactile events: the effect of attention on multisensory integration.

Irrelevant events in one sensory modality can influence the number of events that are perceived in another modality. Previously, the underlying process of sensory integration was studied in conditions in which participants knew a priori which sensory modality was relevant and which was not. Consequently, (bottom-up) sensory interference and (top-down) selective attention were confounded. We disentangled these effects by measuring the influence of visual flashes on the number of tactile taps that were perceived, and vice versa, in two conditions. In the cue condition, participants were instructed on which modality to report before the bimodal stimulus was presented. In the no-cue condition, they were instructed after stimulus presentation. Participants reported the number of events that they perceived for bimodal combinations of one, two, or three flashes and one, two, or three taps. Our main findings were that (1) in no-cue conditions, the influence of vision on touch was stronger than was the influence of touch on vision; (2) in cue conditions, the integration effects were smaller than those in no-cue conditions; and (3) irrelevant taps were less easily ignored than were irrelevant flashes. This study disentangled previously confounded bottom-up and top-down effects: The bottom-up influence of vision on touch was stronger, but vision was also more easily suppressed by top-down selective attention. We have compared our results qualitatively and quantitatively with recently proposed sensory-integration models.

Multisensory temporal numerosity judgment.

In temporal numerosity judgment, observers systematically underestimate the number of pulses. The strongest underestimations occur when stimuli are presented with a short interstimulus interval (ISI) and are stronger for vision than for audition and touch. We investigated if multisensory presentation leads to a reduction of underestimation. Participants were presented with 2 to 10 (combinations of) auditory beeps, tactile taps to the index finger and visual flashes at different ISIs (20 to 320 ms). For all presentation modes, we found underestimation, except for small number of pulses. A control experiment showed that the latter is due to a (cognitive) range effect. Averaged over conditions, the order of performance of sensory modalities is touch, audition and last vision. Generally, multisensory presentation improves performance over the unisensory presentations. For larger ISIs (160 and 320 ms), we found a tendency toward a reduction in variance for the multisensory presentation modes. For smaller ISIs (20 and 40 ms), we found a reduction in underestimation, but an increase in variance for the multisensory presentation modes. In the discussion, we relate these two findings to Maximum Likelihood Estimation (MLE) models predicting that multisensory integration reduces variance.

Vibro-tactile and visual asynchronies: sensitivity and consistency.

We investigated the consistency between tactually and visually designated empty time intervals. In a forced-choice discrimination task, participants judged whether the second of two intervals was shorter or longer than the first interval. Two pulses defined the intervals. The pulse was either a vibro-tactile burst presented to the fingertip, or a foveally presented white square. The comparisons were made for uni-modal and cross-modal intervals. We used four levels of standard interval durations in the range of 100- 800 ms. The results showed that tactile empty intervals must be 8.5% shorter to be perceived as long as visual intervals. This cross-modal bias is larger for small intervals and decreases with increasing standard intervals. The Weber fractions (the threshold divided by the standard interval) are 20% and are constant over the standard intervals. This indicates that the Weber law holds for the range of interval lengths tested. Furthermore, the Weber fractions are consistent over uni-modal and cross-modal comparisons, which indicates that there is no additional noise involved in the cross-modal comparisons.

Effects of head-slaved navigation and the use of teleports on spatial orientation in virtual environments.

The type of navigation interface in a virtual environment (VE)--head slaved or indirect--determines whether or not proprioceptive feedback stimuli are present during movement. In addition, teleports can be used, which do not provide continuous movement but, rather, discontinuously displace the viewpoint over large distances. A two-part experiment was performed. The first part investigated whether head-slaved navigation provides an advantage for spatial learning in a VE. The second part investigated the role of anticipation when using teleports. The results showed that head-slaved navigation has an advantage over indirect navigation for the acquisition of spatial knowledge in a VE. Anticipating the destination of the teleport prevented disorientation after the displacement to a great extent but not completely. The time that was needed for anticipation increased if the teleport involved a rotation of the viewing direction. This research shows the potential added value of using a head-slaved navigation interface--for example, when using VE for training purposes--and provides practical guidelines for the use of teleports in VE applications.