Effect of field of view on the levitation illusion.

Supine subjects inside a furnished room in which both they and the room are pitched 90 degrees backwards may experience themselves and the room as upright relative to gravity. This effect is known as the levitation illusion because observers report that their arms feel weightless when extended, and objects hanging in the room seem to "levitate". This illusion is an extreme example of a visually induced illusion of static tilt. Visually induced tilt illusions are commonly experienced in wide-screen movie theatres, flight simulators, and immersive virtual reality systems. For technical reasons an observer's field of view is often constrained in these environments. No studies have documented the effect of field-of-view (FOV) restriction on the incidence of the levitation illusion. Preliminary findings suggest that when concurrently manipulating the FOV and observer position within an environment, the incidence of levitation illusions depends not only on the field of view but also on the visible scene content.

Gravity and perceptual stability during translational head movement on earth and in microgravity.

We measured the amount of visual movement judged consistent with translational head movement under normal and microgravity conditions. Subjects wore a virtual reality helmet in which the ratio of the movement of the world to the movement of the head (visual gain) was variable. Using the method of adjustment under normal gravity 10 subjects adjusted the visual gain until the visual world appeared stable during head movements that were either parallel or orthogonal to gravity. Using the method of constant stimuli under normal gravity, seven subjects moved their heads and judged whether the virtual world appeared to move "with" or "against" their movement for several visual gains. One subject repeated the constant stimuli judgements in microgravity during parabolic flight. The accuracy of judgements appeared unaffected by the direction or absence of gravity. Only the variability appeared affected by the absence of gravity. These results are discussed in relation to discomfort during head movements in microgravity.

The relative role of visual and non-visual cues in determining the perceived direction of "up": experiments in parabolic flight.

In order to measure the perceived direction of "up", subjects judged the three-dimensional shape of disks shaded to be compatible with illumination from particular directions. By finding which shaded disk appeared most convex, we were able to infer the perceived direction of illumination. This provides an indirect measure of the subject's perception of the direction of "up". The different cues contributing to this percept were separated by varying the orientation of the subject and the orientation of the visual background relative to gravity. We also measured the effect of decreasing or increasing gravity by making these shape judgements throughout all the phases of parabolic flight (0 g, 2 g and 1 g during level flight). The perceived up direction was modeled by a simple vector sum of "up" defined by vision, the body and gravity. In this model, the weighting of the visual cue became negligible under microgravity and hypergravity conditions.

Post-rotatory nystagmus and turning sensations after active and passive turning.

We measured post-rotatory nystagmus and sensations of body rotation in standing subjects brought to rest in the dark after 3 minutes of each of the following conditions: 1) passive turning about the mid-body axis, involving only vestibular stimulation, 2) active turning about the mid-body axis, involving both vestibular stimulation and motor-proprioceptive activity in the legs, and 3) stepping round while remaining facing in the same direction on the center of a rotating platform with the head held in a stationary holder (apparent turning), involving only motor-proprioceptive activity. The same acceleration-velocity profile was used in all conditions. Post-rotatory nystagmus (slow phase) occurred in the same direction to passive body turning and was reduced in velocity after active body turning. After apparent turning, nystagmus was in the opposite direction as attempted body turning. Our theoretical analysis suggests that nystagmus after active turning should conform to the mean of the responses after passive and apparent turning rather than to their sum. The results conform more closely to the mean than to the sum, but with greater weight given to vestibular inputs than to motor-proprioceptive inputs. Post-rotatory sensations of self-rotation were in the expected opposite direction after passive turning and were lower in magnitude after active turning. After apparent turning, sensations of self-rotation were in the same direction as those after attempted turning--an effect known as the antisomatogyral illusion.

Human cyclovergence as a function of stimulus frequency and amplitude.

By the use of scleral search coils a continuous record of human cyclovergence was obtained while two identical 80 degrees textured patterns, presented dichoptically, oscillated in the frontal plane in counterphase through 1, 3 and 6 degrees of cyclorotation at frequencies between 0.05 and 2 Hz. The amplitude and gain of the response decreased exponentially with increasing stimulus frequency. As stimulus amplitude increased, response amplitude also increased but gain was highest for low-amplitude cyclorotations. For an amplitude of 1 degrees and a frequency of 0.05 Hz the gain reached 0.87 for two subjects. The phase lag increased from a few degrees at a frequency of 0.05 Hz to over 100 degrees at a frequency of 2 Hz. These results suggest that cyclovergence is designed to correct for small, slow drifts in the stereoscopic alignment of the images in the two eyes. Although the disparity in the textured display was not interpreted as slant, it provided a strong stimulus for cyclovergence. The cyclovergence caused a transfer of cyclodisparity into a superimposed vertical line, which was then perceived as slanting in depth.

The dynamics of vertical vergence.

We measured the gain and phase of vertical vergence in response to disjunctive vertical oscillations of dichoptic textured displays. The texture elements were m-scaled to equate visibility over the area of the display and were aperiodic and varied in shape so as to avoid spurious binocular matches. The display subtended 65 degrees and oscillated through peak-to-peak amplitudes from 18 arc min to 4 degrees at frequencies from 0.05 to 2 Hz - larger ranges than used in previous investigations. The gain of vergence was near 1 when the stimulus oscillated at 18 arc min at a frequency of 0.1 Hz or less. As the amplitude of stimulus oscillation increased from 18 arc min to 4 degrees, vergence gain decreased at all frequencies, which is evidence of a nonlinearity. Gain declined with increasing stimulus frequency but was still about 0.5 at 2 Hz for an amplitude of 18 arc min. Phase lag increased from less than 10 degrees at a stimulus frequency of 0.05 Hz to between 100 degrees and 145 degrees at 2 Hz. Overall, the dynamics of vertical vergence resemble the dynamics of horizontal vergence and cyclovergence.

Effect of field size, head motion, and rotational velocity on roll vection and illusory self-tilt in a tumbling room.

The effect of field size, velocity, and visual fixation upon the perception of self-body rotation and tilt was examined in a rotating furnished room. Subjects sat in a stationary chair in the furnished room which could be rotated about the body roll axis. For full-field conditions, complete 360 degrees body rotation (tumbling) was the most common sensation (felt by 80% of subjects). Constant tilt or partial tumbling (less than 360 degrees rotation) occurred more frequently with a small field of view (20 deg). The number of subjects who experienced complete tumbling increased with increases in field of view and room velocity (for velocities between 15 and 30 degrees s-1). The speed of perceived self-rotation relative to room rotation also increased with increasing field of view.

The Play Activity Questionnaire: a parent report measure of children's play preferences.

Three studies describe the development and validation of the Play Activity Questionnaire (PQ), a parent report measure of children's play preferences. In Study 1, the 15-item PQ was completed by parents of 239 6- to 8-year-old children, and exploratory factor analysis revealed four play factors: Active and Adventurous, Athletic, Rough-and-Tumble, and Quiet. In Study 2, the factor structure was replicated with PQ data on 203 children of a broader age range (4 to 12 years). The play factors accounted for 65.7% of the variance. Boys' scores exceeded girls' scores on the Athletic and Rough-and-Tumble play factors. Girls' scores exceeded boys' scores on the Quiet play factor. Sex differences were not observed on the Active and Adventurous play factor. Interparent agreement was moderately high, and PQ factor scores were unrelated to social class or nationality. Individual factor scores showed moderate to high stability over 5 months. Validity of the play factors was examined in Study 3 by relating factor scores to measures of activity level, aggression, and sex-typed play. Play factor scores were associated with several of these measures, suggesting that the PQ is a valid measure of children's play behavior with potential applications in the fields of developmental psychopathology and psychoneuroendocrinology, and in studies of normal play and sex-role development.

Effects of stimulus size and eccentricity on horizontal and vertical vergence.

We measured the gain and phase of horizontal and vertical vergences of five subjects as a function of stimulus area and position. Vergence eye movements were recorded by the scleral search coil method as subjects observed dichoptic displays oscillating in antiphase either from side to side or up and down with a peak-to-peak magnitude of 0.5 degree at either 0.1 Hz or 1.0 Hz. The stimulus was a central textured disc with diameter ranging from 0.75 degree to 65 degrees, or a peripheral annulus with outer diameter 65 degrees and inner diameter ranging from 5 degrees to 45 degrees. The remaining field was black. For horizontal vergence at both stimulus frequencies, gain and the phase lag were about the same for a 0.75 degree stimulus as for a 65 degrees central stimulus. For vertical vergence, mean gain increased and mean phase lag decreased with increasing diameter of the central stimulus up to approximately 20 degrees. Thus, the stimulus integration area is much smaller for horizontal vergence than for vertical vergence. The integration area for vertical vergence is similar to that for cyclovergence, as revealed in a previous study. For both types of vergence, response gains were higher and phase lags smaller at 0.1 Hz than at 1.0 Hz. Also, gain decreased and phase lag increased with increasing occlusion of the central region of the stimulus. Vergence gain was significantly higher for a 45 degrees central disc than for a peripheral annulus with the same area. Thus, the central retina has more power to evoke horizontal or vertical vergence than the same area in the periphery. We compare the results with similar data for cyclovergence and discuss their ecological implications.

Child outcome following mid-trimester amniocentesis: development, behaviour, and physical status at age 4 years.

Four-year-old children whose mothers had mid-trimester amniocentesis (n = 88) were compared with children whose mothers chose not to have the procedure (n = 46). Intelligence, visual-motor-perceptual skills, language, behaviour, social competence, temperament, physical growth, hearing, and middle-ear function were measured using standardized procedures. Health histories were obtained by maternal report. The results suggest that the wide range of developmental and behavioural variables studied is not influenced by removal of amniotic fluid in the mid-trimester. However, mothers who had amniocentesis were more likely to report a history of ear infections in their child (P = 0.04). In support of this finding were the results of audiological assessment which demonstrated a trend toward a higher rate of bilateral middle-ear impedance abnormalities in children whose mothers had amniocentesis (P = 0.06). Further study of the upper respiratory system is recommended to explore potential long-term sequelae of mid-trimester amniocentesis.