The effects of eccentricity and vergence angle upon the relative tilt of corresponding vertical and horizontal meridia revealed using the minimum motion paradigm

When the corresponding horizontal meridia of the two eyes are aligned, the corresponding vertical meridia are tilted outwards in a temporal direction, a phenomenon first described by Helmholtz. However, it is not known if this effect is confined to the principal meridia or whether the same relationship exists between corresponding horizontal and corresponding vertical meridia at eccentric retinal locations. We sought to address this issue by exploiting the technique of Nakayama (1977 Proceedings of the Society of Photo-Optical Instrument Engineers 120 2-9) in which the positions of alternating dichoptic images that produce minimal apparent motion were used to measure the relative tilt of corresponding meridia at a range of eccentricities up to +/- 16 deg away from the fovea. Stimuli were composed of dichoptic images, one containing a blank field and the other a pair of dots, which alternated at a rate of 0.63 Hz and the relative tilt (binocular orientation difference) between the pairs of dots presented to the two eyes was varied between +/- 11 degrees. Nonius lines were used to maintain vergence angle, which was varied between 28 cm and infinity. Subjects judged which pair of alternating images produced the smallest amount of apparent motion (position change). It was found that at all eccentricities examined the corresponding horizontal meridia were generally aligned but the corresponding vertical meridia were consistently offset (extorted) by about +/- 2 degrees. The tilts of corresponding principal meridia were typically unaffected when vergence angle was varied, indicating that little or no cyclovergence accompanied changes in horizontal vergence. The results suggest that the binocular correspondence system appears to be mapped by a horizontal shear distortion that extends to retinal locations at least as far as 16 deg away from the foveae. The invariant extortion of corresponding principal vertical meridia with vergence state is consistent with previous suggestions that the empirical vertical horopter becomes progressively inclined with respect to the vertical as viewing distance increases.     

The effect of structure and degree of tilt on the tilted room illusion

The effect of structure and angle of tilt on the magnitude of the tilted-room illusion and its relationship to the illusion produced by a tilted-line field were examined in two experiments. In Experiment L, nine groups of 13 Ss were tested under three conditions of room tilt in the frontal plane (22.5, 45, and 67.5 deg) and under three conditions of structure (empty room, room with a back wall of stripes, and room with furniture). The results indicate that, while magnitude and direction of the illusion vary with degree of room tilt, structure increases the magnitude of the illusion only at a 45-deg room tilt. In Experiment 2, a field of lines was presented through a circular reduction tube to three groups of 13 Ss under three conditions of line tilt (22.5, 45, and 67.5 deg). An illusion occurred that was much smaller in magnitude and functionally different for the three tilt conditions when compared with the tilted-room illusion.     

Cognitive suppression of tilt sensations during linear horizontal self-motion in the dark

On the basis of models of otolith functioning, one would expect that, during sinusoidal linear self-motion in darkness, percepts of body tilt are experienced. However, this is normally not the case, which suggests that the otoliths are not responsive to small deviations from the vertical of the gravito-inertial force vector acting on them. Here we show that this is incorrect. Subjects usually know on what kind of linear motion device they are (going to be) moved, having seen it prior to experimentation. This may result in a cognitive suppression of such otolith responses. In the present study, subjects were kept completely unaware of how they were moved and were asked to report on how they thought they moved. About 50% of the reports included tilt percepts almost immediately. It is concluded that this reveals the presence of otolith responsiveness to even small and short-lived deviations of the gravito-inertial force vector from verticality, a responsiveness which is suppressed when (prior) cognitions exist that the motion path is purely in the horizontal plane.     

The relationship between external thoracopelvic angle and lumbar segmental axial twist angle using an ultrasound imaging technique

The relationship between externally measured and internal spine axial twist motion (rotation about a vertical axis) is not well understood. Ultrasound is a validated technique for measurement of vertebral axial twist motion and has the potential for measuring segmental vertebral axial twist in vivo. The objective of this study was to evaluate lumbar segmental axial twist in relation to external thoracopelvic twist from optical motion capture using an ultrasound imaging technique. Sixteen participants were tested in a custom-built axial twist jig, which isolated motion to the lumbar spine. Participants were moved from neutral to 75% of maximum axial twist range of motion in an upright kneeling posture. Thoracopelvic motion was recorded with a motion capture system and L1 to S1 vertebral axial twist was recorded using ultrasound. From motion capture, maximum thoracopelvic axial twist motion was 41.1 degrees. From ultrasound, the majority of axial twist motion occurred at the L2-L3 (46.8% of lumbar axial twist motion) and L5-S1 (33.5%) intervertebral joints. Linear regression linking axial twist at each vertebral level to thoracopelvic axial twist ranged from 0.43 to 0.79. These findings demonstrate a mathematical relationship between internal and external axial twist motion and the distribution of motion across the lumbar spine suggests that classic use of L4-L5 to represent lumbar spine motion may not be appropriate for axial twist modelling approaches.     

Visual angle and the word superiority effect

Two experiments are reported in which a word superiority effect is obtained under conditions where a fixed set of alternatives are employed with positional certainty as to the critical letter, trial type (word or nonword) is mixed, and the subject is told to fixate the position of the critical letter. A third experiment employed the same methodology except for the fact that the stimuli subtended a larger retinal angle. No word superiority effect was observed in the third experiment. It is suggested that the visual angle of the stimulus display is a crucial factor in experiments on the word superiority effect.     

The relationship superiority effect is moderated by the relationship context

Previous research has identified the perceived superiority effect, defined as the tendency to regard one's own relationship as better than other people's relationships, as a cognitive maintenance mechanism that supports commitment to close relationships. The present research was designed to test the hypothesis that the perceived superiority effect is moderated by the relationship context — that is, to whom one's relationship is being compared. Two different measures based on a spontaneous thought-listing procedure demonstrated that the perceived superiority effect is significantly stronger when comparing one's own relationship to the relationships of people in general than to the relationships of close friends. Correlational analyses suggested that this difference may reflect people's tendency to experience close friends' outcomes as personal outcomes.