Effects of Varying Gravity Levels in Parabolic Flight on the Size-Mass Illusion

Materials and Methods

Equipment

This study used objects composed of 2.54-cm hollow plastic cubes (Table 1). To vary the mass, some of the plastic cubes were filled with sand (Experiment 1 and 2) or with 5-mm steel balls (Experiment 3). Care was taken to ensure that the center of mass of all objects was coincident with their centroid. To vary the size, some objects were made taller but their width and depth were kept constant so that the subject handgrip was the same for all objects. The objects were wrapped with a white, glossy adhesive sheet so that they all had uniform appearance except for their height.

Experiment 1–Comparison between Size-weight and Size Mass Illusions

This experiment was performed in the laboratory. Twelve subjects (6 female, 6 male), ranging in age from 22–56 years (mean 32.4 years) participated in this study. The subjects sat in front of table on which there was a smooth plastic mat. The surface of the mat was sprayed with a dry lubricant to reduce friction. By measuring the applied force to move the various objects on the mat the coefficients of static and dynamic friction were evaluated to be 0.27 and 0.11, respectively. For mass perception, the subjects oscillated the objects side to side by performing horizontal flexion/extension of the elbow while looking at the objects, for five seconds. The subjects were instructed to repeatedly move the objects from one end to the other of the mat (amplitude 40 cm), but no instruction was given regarding the frequency or speed of arm motion. After each oscillation period, the subjects were asked to estimate the mass of the object using a scale from 1 to 10 (1 being light and 10 being heavy) according to the procedure described by Grandy & Westwood[5].

For weight perception, the subject extended his/her dominant hand palm up. The operator placed the object in the subject's hand. The subject held his arm and wrist still and constantly looked at the object for five seconds. After each holding period, the subjects was asked to estimate the weight of the object using a scale from 1 to 10 (1 being light and 10 being heavy) according to the procedure described by Grandy & Westwood[5].

This experiment used five objects: three objects had the same mass but a different height (100%, 166%, and 200%); and three cubes had the same height but a different mass (100%, 87% and 75%) (Table 1). Each object was presented five times in random order.

Experiment 2–Comparison between zero g, 0.16 g and 0.38 g

This experiment took place in December 2012 on board the Airbus A-300 Zero-G during the second CNES/ESA/DLR campaign of parabolic flight in Moon (0.16 g) and Mars (0.38 g) gravity. Each of the three flights lasted two to three hours and included 31 parabolic manoeuvres, i.e. 13 parabolas at 0.16 g, 12 at 0.38 g, and 6 at zero g in that order. Each parabola started with a pull-up phase and ended with a pull-out phase at 1.8 g, both lasting about 20 sec. The duration of the reduced gravity periods depended on the gravity level: about 21 sec for weightlessness, 24 sec for lunar gravity and 33 sec for Mars gravity.

Six subjects (one female, five male), ranging in age from 25–56 years (mean 47.0 years) participated in this experiment. All subjects had passed the equivalent of an Air Force Class III medical examination, and had normal or corrected-to-normal vision with no known visual deficits. Four subjects took prophylactic medication (a combination of promethazine and dexedrine) before boarding the plane, and none of them showed symptoms of motion sickness during the flight. Data were collected during three parabolas at 0.16 g and three parabolas at 0.38 g. Controls in one g were also performed on board the aircraft during straight and level flight between successive parabolas, while the medicated subjects were under the influence of the drug. This was to ensure that the changes seen across the various gravity levels were not due to the effect of the medication.

This experiment used the same objects as in Experiment 1 (Table 1). Each object was presented twice in random order. During testing, subjects sat on the aircraft floor, and oscillated the objects side to side between two markers 40 cm apart, while looking at them, for five seconds. After each oscillation period, the subjects were asked to estimate the mass of the object using a scale from 1 to 10 (1 being light and 10 being heavy) according to the procedure described by Grandy & Westwood[5]. A three-axis accelerometer (Gulf Coast Data Concept, LLC, Waveland, MS, USA) mounted on a wristband continuously recorded the subjects arm movement, for off-line calculation of the peak acceleration of subject's arm motion at each of the gravity level.