Motion perception during variable-radius swing motion in darkness

Abstract

Using a variable-radius roll swing motion paradigm, we examined the influence of interaural (y-axis) and dorsoventral (z-axis) force modulation on perceived tilt and translation by measuring perception of horizontal translation, roll tilt, and distance from center of rotation (radius) at 0.45 and 0.8 Hz using standard magnitude estimation techniques (primarily verbal reports) in darkness. Results show that motion perception was significantly influenced by both y- and z-axis forces. During constant radius trials, subjects' perceptions of tilt and translation were generally almost veridical. By selectively pairing radius (1.22 and 0.38 m) and frequency (0.45 and 0.8 Hz, respectively), the y-axis acceleration could be tailored in opposition to gravity so that the combined y-axis gravitoinertial force (GIF) variation at the subject's ears was reduced to approximately 0.035 m/s(2) - in effect, the y-axis GIF was "nulled" below putative perceptual threshold levels. With y-axis force nulling, subjects overestimated their tilt angle and underestimated their horizontal translation and radius. For some y-axis nulling trials, a radial linear acceleration at twice the tilt frequency (0.25 m/s(2) at 0.9 Hz, 0.13 m/s(2) at 1.6 Hz) was simultaneously applied to reduce the z-axis force variations caused by centripetal acceleration and by changes in the z-axis component of gravity during tilt. For other trials, the phase of this radial linear acceleration was altered to double the magnitude of the z-axis force variations. z-axis force nulling further increased the perceived tilt angle and further decreased perceived horizontal translation and radius relative to the y-axis nulling trials, while z-axis force doubling had the opposite effect. Subject reports were remarkably geometrically consistent; an observer model-based analysis suggests that perception was influenced by knowledge of swing geometry.

Fig. 1.

Experimental coordinate frame. The y and z axes align with subject interaural and dorsoventral axes.

Fig. 2.

Our motion profiles. SO, swing only; YN, y-axis force nulling; ZN, z-axis force nulling; ZD, y-axis force doubling.

Fig. 3.

Massachusetts Eye and Ear Infirmary (MEEI) tilt device motion capability.

Fig. 4.

A: subject reported mean tilt perception for the swing-only condition. B: comparison of mean tilt perception for the y-axis force nulling profiles grouped by z-axis nulling condition. —, the actual tilt angle. Error bars represent SE. *, significant effects (P < 0.05).

Fig. 5.

A: subject reported mean horizontal translation perception for the swing-only condition. B: comparison of mean stranslation perception for the y-axis force nulling profiles grouped by z-axis nulling condition. —, actual translations. Error bars, SE; *, significant effects (P < 0.05).

Fig. 6.

A: subject reported mean radius perception for the swing-only condition. B: comparison of mean radius perception for the y-axis force nulling profiles grouped by z-axis nulling condition. —, actual radius error bars, SE; *, significant effects (P < 0.05).

Fig. 7.

A: somatosensory bar angle as a function of radius from least-squares linear regression to a sinusoid for 0.45 Hz. B: comparison of the somatosensory bar angle results for the z-axis force profiles for the y-axis nulling condition. Error bars, SE; *, significant effects (P < 0.05).

Fig. A1.

y-axis (IA) and z-axis (DV) forces and accelerations for the various motion paradigms calculated from equations in text. Left: y-axis components; right: z-axis components. y-axis (A) and z-axis (B) gravitational force (g) components are shown. y-axis (C) and z-axis (D) acceleration are shown for the y-axis nulling (—), z-axis force nulling (…), and z-axis force doubling (- - -). y-axis (E) and z-axis (F) components of the total resultant specific force (f = g − a) are shown (2nd row trace subtracted from the 1st).

Fig. A2.

Peak steady-state amplitude components of gravity, accleration, and combined gravitoinertial force (GIF) during the 0.45-Hz swing paradigm for a 10° tilt. As shown the peak y-axis gravity component depends only upon tilt angle and is independent of radius; the amplitude of the linear acceleration component increases with radius (a_t = rα). As shown graphically, the difference between the 2 yields the GIF (f = g − a). The y-axis contributions of gravity and linear acceleration cancel one another at a radius of −1.22 m at 0.45 Hz. The effect is similar for the 0.8-Hz paradigm. Line thickness represents mechanical vibrations and other noise. Note that the amplitude (maximum) is shown in this figure. Minimum values are the negative of those shown in this plot.

Fig. A3.

Maximum and minimum z-axis GIF at steady state for the 0.45-Hz swing paradigm as a function of radius. At the y-axis nulling point (circa −1.2 m), a substantial z-axis force variation, predominantly at 0.9 Hx, remains. This component can be reduced by simultaneously applying a phase-locked radial linear acceleration at 0.9 Hz (z-axis force nulling). It can also be increased by simultaneously applying the same z-axis acceleration but with the phase altered by 180° (z-axis force doubling). The effect is similar for 0.8 Hz. Note that minimum and maximum values are shown (instead of the amplitudes as in Fig. A2).

Fig. A4.

Sinusoids regressed to accelerometer measurements of the 4 motion profiles for the 0.45-Hz paradigms: head center (bold line), swing-only y-axis nulling (solid line), z-axis nulling (dotted line), and z-axis doubling (dahsed line). Results were similar for the 0.8-Hz paradigm. The actual measurements also showed vibrational components at higher frequencies (mostly at 5 and 15 Hz).