Fig 1.
The subject was seated on the motion platform in front of a display screen that was also attached to the platform. (A) Fixation point appeared at eye level that was randomly interleaved 25° to the right or left. After maintaining fixation, the subject pushed a button which extinguished the fixation point and delivered the stimulus. (B) The subject maintained gaze in the same location after the fixation point was extinguished. The stimulus could be visual, inertial (potentially with vertical vibration), or both. When presented simultaneously, the visual and inertial stimuli were always consistent with motion in the same direction. (C) After the stimulus completed there was a brief audible tone which indicated to the subject that the perceived direction of travel relative to the midline should be reported.
Fig 2.
The vibration stimulus as measured from a bite bar based accelerometer.
The vibration is shown for the three amplitudes of vibration tested in the current experiments– 0.10 cm (top), 0.15 cm (middle), and 0.20 cm (bottom). The primary stimulus was in the heave (vertical) direction, but there was a small component in the sway (lateral) and surge (fore-aft) directions.
Fig 3.
Sample data from subject #1 during the combined visual-inertial heading discrimination task.
The inertial stimulus included 0.15 cm vibration and visual heading was at 50% coherence. (A) The staircase with 25° leftward gaze. (B) Represents these same responses fit by a psychometric function. Subject’s responses are represented by circles scaled proportional to the number of stimulus presentations given at that heading with the largest circles representing 5 stimulus presentations and the smallest representing a single stimulus presentation. The curve represents the best fit to the data. The mean is shown with 95% confidence intervals. (C) The staircase with 25° rightward gaze. (D) Psychometric function fit to the responses in (C).
Fig 4.
The mean or point of subjective equality (PSE) of visual and inertial heading discrimination.
Each condition was tested with the eye position 25° to the left (filled downward pointing triangles) and 25° to the right (open upward pointing triangles). Error bars represent the 95% confidence interval (CI) as measured using the Monte Carlo technique. A data point shifted in the positive direction (upward) indicates that a stimulus in the midline is more likely to be perceived to the left. Subject numbers marked with a solid circle indicate the leftward gaze shifted the PSE significantly (p < 0.01 using the Monte Carlo technique) to the left when right and left gaze were compared. Similarly, a dashed circle indicates a significant effect in the opposite direction. The data combined across subjects is marked with a C in the furthest right column. (A) Indicates a purely inertial stimulus. (B-D) Inertial plus vibration is shown. (E) The visual only stimulus. (F-I) Are the multisensory, visual-inertial conditions.
Fig 5.
The thresholds of visual and inertial heading discrimination.
As with Fig 4 the symbols represent left and right gaze positions. Error bars represent the 95% CI for each subject (numbered) and the combined data (C). (A-D) Purely inertial stimulus while on the right (E-I) include a visual component.
Fig 6.
Bias and thresholds of visual and inertial headings across subjects and gaze directions.
The bias of the left gaze was negated so the average bias relative to eye position could be considered (e.g. the mean with the left inverted). Error bars represent ±1 SEM. Conditions with the same amount of vibration were offset slightly to keep the data points from overlapping. The purely visual condition is shown with an open circle. Inertial only with a filled square. The combined condition is shown by a gray square. (A) Bias or PSE. (B) Threshold or sigma.
Fig 7.
The relative weights of the visual and inertial stimuli determined for each subject based on the relative reliabilities of the stimuli.
The sum of the visual and inertial weights is always one. Weights determined empirically from bias are shown as filled circles and solid lines. Optimal weights were determined from average thresholds and are represented by open squares and dashed lines. Error bars represent the 5th and 95th percentile for the weights determined from 2,000 random resamplings for each condition. The empirical and optimal weights were artificially horizontally offset from each other for clarity. The left side of the figure (A-E) represents the subjects in this study when the visual coherence was varied in the prior study[16]. The right side of the figure (F-J) represents the current experiments which used vibration to vary the reliability of the inertial stimulus.
Fig 8.
The relative weights of the inertial and visual stimuli averaged across the five subjects.
Empirically calculated weights based on bias are shown as filled circles with a solid line. Optimally calculated weights based on thresholds (e.g. stimulus reliability) are shown as open squares with a dashed line. As with the individual data the error bars represent the 5th and 95th percentile determined from 2,000 random resamplings for each condition.