Global-motion aftereffect does not depend on awareness of the adapting motion direction

Asymmetrical thoracic-lumbar coordination during trunk rotation between adolescents with and without thoracic idiopathic scoliosis

STUDY DESIGN

Cross-sectional comparative study.

PURPOSE

To compare thoracic-lumbar kinematic changes and coordination based on coupling angles (CAs) in two different directions of trunk rotation between adolescents with idiopathic scoliosis (AIS) and control subjects. Altered three-dimensional (3D) deviations are often apparent in AIS groups during functional activities, such as gait. However, there is a lack of consistent evidence on coordinated motions during different directions of trunk rotation.

METHODS

This study included 14 AIS and 17 age-matched control subjects who were all right limb dominant. A motion capture system was utilized to analyze the spinal segment motions. The outcome measures included range of motion (ROM) at the first thoracic (T1), seventh thoracic (T7), and first lumbar (L1) spinous processes as well as the sacral tubercle (S1). The CAs compared in-phase (rotation from right to left) and anti-phase (rotation from left to right) trunk rotations.

RESULTS

Although there was no significant association with the spinal segments in the control group, the Cobb angle demonstrated significant positive correlations with anti-phase at T7 and L1 as well as in-phase at L1. Regarding the CAs, the groups demonstrated a significant interaction with both phases (F = 4.7, p = 0.04). The AIS group demonstrated positive correlations with ROM during in-phase at L1 and anti-phase at T7 and L1.

CONCLUSION

The coordination based on the CAs of the lumbar spine relative to the thoracic spine significantly decreased during left to right trunk rotation in the AIS group. These results indicated that the AIS group demonstrated directional dissociation toward the dominant side of lumbar rotation.

LEVEL OF EVIDENCE

III.

Adaptation to one perceived motion direction can generate multiple velocity aftereffects

Sensory adaptation is a useful tool to identify the links between perceptual effects and neural mechanisms. Even though motion adaptation is one of the earliest and most documented aftereffects, few studies have investigated the perception of direction and speed of the aftereffect at the same time, that is the perceived velocity. Using a novel experimental paradigm, we simultaneously recorded the perceived direction and speed of leftward or rightward moving random dots before and after adaptation. For the adapting stimulus, we chose a horizontally-oriented broadband grating moving upward behind a circular aperture. Because of the aperture problem, the interpretation of this stimulus is ambiguous, being consistent with multiple velocities, and yet it is systematically perceived as moving at a single direction and speed. Here we ask whether the visual system adapts to the multiple velocities of the adaptor or to just the single perceived velocity. Our results show a strong repulsion aftereffect, away from the adapting velocity (downward and slower), that increases gradually for faster test stimuli as long as these stimuli include some velocities that match some of the ambiguous ones of the adaptor. In summary, the visual system seems to adapt to the multiple velocities of an ambiguous stimulus even though a single velocity is perceived. Our findings can be well described by a computational model that assumes a joint encoding of direction and speed and that includes an extended adaptation component that can represent all the possible velocities of the ambiguous stimulus.

Invisible Adaptation: The Effect of Awareness on the Strength of the Motion Aftereffect

The ability to process information despite the lack of perceptual awareness is one of the most fascinating aspects of the visual system. Such unconscious processing is often investigated using adaptation, where any presence of the former can be traced by its footprint on aftereffects following the latter. We have investigated the mechanisms of the motion aftereffect (MAE) using random dot displays of varying motion coherence as well as crowding to modulate both the physical as well as the perceptual strength of the adaptation stimulus. Perceptual strength was quantitatively measured as the performance in a forced-choice direction-discrimination task. A motion-nulling technique was used to quantitatively measure the strength of the MAE. We show that the strength of the dynamic MAE is independently influenced by both the physical stimulus strength as well as the subjective perceptual strength, with the effect of the former being more prominent than that of the latter. We further show that the MAE still persists under conditions of subthreshold perception. Our results suggest that perceptual awareness can influence the strength of visual processing, but the latter is not fully dependent on the former and can still take place at its partial or even total absence.

Stimulus dependencies of an illusory motion: Investigations of the Motion Bridging Effect

The Motion Bridging Effect (MBE) is an illusion in which a motion that is not consciously visible generates a visible motion aftereffect that is predominantly in the same direction as the adapter motion. In the initial study of the MBE (Mattler & Fendrich, 2010), a ring of 16 points was rotated at angular velocities as high as 2250°/s so that observers saw only an unbroken outline circle and performed at chance when asked to report the ring's rotation direction. However, when the rotating ring was replaced by a veridically stationary ring of 16 points, the stationary ring appeared to visibly spin to a halt, principally in the same direction as the initial ring's rotation. Here we continue to investigate the stimulus dependencies of the MBE. We find the MBE, measured by the correspondence between the direction of the invisible rotation of the spinning ring and perceived rotation of the stationary ring, increases as the number of points used to construct the rings decreases and grows stronger as the diameter of the rings get larger. We consider the potential contributions of temporal frequency, retinal eccentricity, luminance levels, and the separation between the points forming the rings as mediators of these effects. Data is discussed with regard to the detection of real movement and apparent motion. We conclude that the detection of the rapid rotation of the spinning ring is likely to be modulated by temporal frequency of luminance changes along the ring perimeter while the point-distance may modulate an apparent motion produced by the transition from the perceptually unbroken spinning ring to the point-defined stationary ring.

The contribution of local and global motion adaptation in the repulsive direction aftereffect

After adapting to a certain motion direction, our perception of a similar direction will be repelled away from the adapting direction, a phenomenon known as the direction aftereffect (DAE). As the motion system consists of local and global processing stages, it remains unclear how the adaptation of the two stages contributes in producing the DAE. The present study addresses this question by independently inducing adaptation at local and global motion-processing levels. Local adaptation was manipulated by presenting test stimuli at either adapted or nonadapted locations. Global adaptation was manipulated by embedding one or five global motion directions in the adapting motion. Repulsive DAE, when measured using a multiple-element test pattern, was stronger when it was produced by global adaptation than when produced by local adaptation. Specifically, the DAE resulting from local adaptation (a) decreased when test orientations differed from adapting orientation, (b) decreased when local directions were disambiguated using plaid stimuli, (c) remained the same even when attention was focused at specific test locations during adaptation, and (d) increased when tested with a single element. Overall, these findings suggest that the strength of repulsive DAE depends on both the motion-processing level at which adaptation occurs and the level at which the DAE was tested. Furthermore, the repulsive DAE arising from local adaptation alone can be explained by the propagation of local speed repulsion instead of local direction repulsion. Findings are discussed in the context of how motion aftereffects arise from the adaptation of a hierarchical motion system.

Motion Perception: From Detection to Interpretation

Visual motion processing can be conceptually divided into two levels. In the lower level, local motion signals are detected by spatiotemporal-frequency-selective sensors and then integrated into a motion vector flow. Although the model based on V1-MT physiology provides a good computational framework for this level of processing, it needs to be updated to fully explain psychophysical findings about motion perception, including complex motion signal interactions in the spatiotemporal-frequency and space domains. In the higher level, the velocity map is interpreted. Although there are many motion interpretation processes, we highlight the recent progress in research on the perception of material (e.g., specular reflection, liquid viscosity) and on animacy perception. We then consider possible linking mechanisms of the two levels and propose intrinsic flow decomposition as the key problem. To provide insights into computational mechanisms of motion perception, in addition to psychophysics and neurosciences, we review machine vision studies seeking to solve similar problems.

Unbiased Measures of Interocular Transfer of Motion Adaptation

Numerous studies have measured the extent to which motion aftereffects transfer interocularly. However, many have done so using bias-prone methods, and studies rarely compare different types of motion directly. Here, we use a technique designed to reduce bias (Morgan, 2013, Journal of Vision, 13(8):26, 1–11) to estimate interocular transfer (IOT) for five types of motion: simple translational motion, expansion/contraction, rotation, spiral, and complex translational motion. We used both static and dynamic targets with subjects making binary judgments of perceived speed. Overall, the average IOT was 65%, consistent with previous studies (mean over 17 studies of 67% transfer). There was a main effect of motion type, with translational motion producing stronger IOT (mean: 86%) overall than any of the more complex varieties of motion (mean: 51%). This is inconsistent with the notion that IOT should be strongest for motion processed in extrastriate regions that are fully binocular. We conclude that adaptation is a complex phenomenon too poorly understood to make firm inferences about the binocular structure of motion systems.