On the perceived location of global motion

Motion-induced position mis-localization predicts the severity of Alzheimer's disease

Patients with Alzheimer's disease (AD) often exhibit motion processing deficits. It is unclear whether the localization of moving objects - a perceptual process tightly linked to motion - is impaired or intact in AD. In this study, we used the phenomenon of illusory shift of position induced by motion as a behavioural paradigm to probe how the spatial representation differs between AD patients and healthy elderly controls. We measured the magnitudes of motion-induced position shift in a group of AD participants (N = 24) and age-matched elderly observers (N = 24). We found that AD patients showed weakened position mis-localization, but only for motion stimuli of slow speeds. For fast motion, the position mis-localization did not differ significantly between groups. Furthermore, we showed that the magnitudes of position mis-localization can predict the severity of AD; that is, patients with more severe symptoms had less preserved position mis-localization. Our results suggest that AD pathology impacts not only motion processing per se, but also the perceptual process related to motion such as the localization of moving objects.

Errors in interception can be predicted from errors in perception

It has been hypothesised that our actions are less susceptible to visual illusions than our perceptual judgements because similar information is processed for perception and action in separate pathways. We test this hypothesis for subjects intercepting a moving object that appears to move at a different speed than its true speed due to an illusion. The object was a moving Gabor patch: a sinusoidal grating of which the luminance contrast is modulated by a two-dimensional Gaussian. We manipulated the patch's apparent speed by moving the grating relative to the Gaussian. We used separate two-interval forced choice discrimination tasks to determine how moving the grating influenced ten people's judgements of the object's position and velocity while they were fixating. Based on their perceptual judgements, and knowing that our ability to correct for errors that arise from relying on incorrect judgements are limited by a sensorimotor delay of about 100 msec, we predicted the extent to which subjects would tap ahead of or behind similar targets when trying to intercept them at the fixation location. The predicted errors closely matched the actual errors that subjects made when trying to intercept the targets. This finding does not support the two visual streams hypothesis. The results are consistent with the idea that the extent to which an illusion influences an action tells us something about the extent to which the action relies on the percept in question.

Motion-Induced Position Shifts Activate Early Visual Cortex

The ability to correctly determine the position of objects in space is a fundamental task of the visual system. The perceived position of briefly presented static objects can be influenced by nearby moving contours, as demonstrated by various illusions collectively known as motion-induced position shifts. Here we use a stimulus that produces a particularly strong effect of motion on perceived position. We test whether several regions-of-interest (ROIs), at different stages of visual processing, encode the perceived rather than retinotopically veridical position. Specifically, we collect functional MRI data while participants experience motion-induced position shifts and use a multivariate pattern analysis approach to compare the activation patterns evoked by illusory position shifts with those evoked by matched physical shifts. We find that the illusory perceived position is represented at the earliest stages of the visual processing stream, including primary visual cortex. Surprisingly, we found no evidence of percept-based encoding of position in visual areas beyond area V3. This result suggests that while it is likely that higher-level visual areas are involved in position encoding, early visual cortex also plays an important role.

Illusory position shift induced by plaid motion

In the motion-induced position shift (MIPS), the position of a moving pattern tapered by a stationary envelope is perceived to shift in the direction of the motion. It was found that plaid motion also elicited a MIPS in the direction of global motion and this global MIPS could not be predicted by the average of the local MIPSs due to component motions. We also used a pseudo plaid pattern and again observed a global MIPS that could not be predicted by the local MIPSs due to the components of the pseudo plaid pattern. We suggest the possibility that the receptive-field positions of global motion detectors shift in the direction opposite to global motion, resulting in a positional displacement in activation via population coding.

Motion-induced position shifts occur after motion integration

Low-level motion processing in the primate visual system involves two stages. The first stage (in V1) contains specialised motion sensors which respond to local retinal motion, and the second stage (in MT) pools local signals to encode rigid surface motion. Recent psychophysical research shows that motion signals influence the perceived position of an object (motion-induced position shift, MIPS). In the present paper we investigate the role played by the two processing stages in generating MIPS. We compared MIPS induced by single grating components (Gabor patches) to MIPS induced by plaids created by combining pairs of components. If motion signals at the lowest level of motion analysis (V1) influence position assignment, MIPS from plaids should reflect the position shift induced by each component when presented separately. On the other hand, if signals generated in MT (or later) influence perceived position, then MIPS from plaids should be consistent with a motion integration computation on the components. Results showed that MIPS from plaids is larger than the MIPS obtained from individual components, and can be explained by the output of an integration process that combines intersection-of-constraints and vector-sum computations.

Distinct position assignment mechanisms revealed by cross-order motion

Motion perception influences perceived position. It has been shown that first-order (luminance defined) motion shifts perceived position across a wide range of spatial and temporal frequencies. On the other hand, second-order (contrast defined) motion shifts perceived position over a narrow range of temporal frequencies, regardless of spatial frequency [Bressler, D. W., & Whitney, D. (2006). Second-order motion shifts perceived position. Vision Research, 46(6-7), 1120-1128]. These results suggest the presence of distinct position assignment mechanisms for first- and second-order motion. We investigated whether the first- and second-order systems independently encode and assign the position of a moving stimulus. To measure motion induced position shift we presented two horizontally offset Gabors placed above and below a central fixation point, with sine wave carriers drifting in opposite directions. Subjects judged the position of the top Gabor relative to the bottom one. We used both first-order Gabors (sinusoidal luminance modulation of a dynamic noise carrier enveloped by a static Gaussian) and second-order Gabors (sinusoidal contrast modulation of a dynamic noise carrier enveloped by a static Gaussian). Results showed a strong position shift in the direction of the carrier motion when both Gabors were first-order, a weak position shift when both Gabors were second-order, and no appreciable position shift when one Gabor was first-order and the other was second-order (cross-order motion). The absence of a position shift using cross-order motion supports the hypothesis that the two motion systems independently encode and assign the position of a moving object. These results are consistent with those of experiments investigating global spatial interactions between static first-order and second-order Gabor patches, indicating a commonality in the underlying spatial integration processes.

Spatial and temporal properties of the illusory motion-induced position shift for drifting stimuli

The perceived position of a stationary Gaussian window of a Gabor target shifts in the direction of motion of the Gabor's carrier stimulus, implying the presence of interactions between the specialized visual areas that encode form, position, and motion. The purpose of this study was to examine the temporal and spatial properties of this illusory motion-induced position shift (MIPS). We measured the magnitude of the MIPS for a pair of horizontally separated (2 or 8deg) truncated-Gabor stimuli (carrier=1 or 4cpd sinusoidal grating, Gaussian envelope SD=18arc min, 50% contrast) or a pair of Gaussian-windowed random-texture patterns that drifted vertically in opposite directions. The magnitude of the MIPS was measured for drift speeds up to 16deg/s and for stimulus durations up to 453ms. The temporal properties of the MIPS depended on the drift speed. At low velocities, the magnitude of the MIPS increased monotonically with the stimulus duration. At higher velocities, the magnitude of the MIPS increased with duration initially, then decreased between approximately 45 and 75ms before rising to reach a steady-state value at longer durations. In general, the magnitude of the MIPS was larger when the truncated-Gabor or random-texture stimuli were more spatially separated, but was similar for the different types of carrier stimuli. Our results are consistent with a framework that suggests that perceived form is modulated dynamically during stimulus motion.

Apparent position in depth of stationary moving three-dimensional objects

Motion signals contained within a stationary object projected on the fronto-parallel plane shift the object's apparent spatial position in the direction of the motion [see De Valois, R. L., & De Valois, K. K. (1991). Vernier acuity with stationary moving Gabors. Vision Research, 31(9), 1619-1626]. We report an analogous apparent position shift of three-dimensional objects that contain local elements that move in depth. Our stimulus was a transparent three-dimensional cylinder defined by 150 limited-lifetime dots, oriented such that it was end on and its tangent plane was circular. Dots moved in depth by changes in their binocular disparities. In the first experiment, observers judged the positions of the near and far ends of the cylinder, by moving marker lines in depth, for different dot speeds. The results showed that when dots moved towards the observer, the perceived location of the two ends of the cylinder appeared closer in depth. When dots moved away from the observer, the opposite effect was produced. Additionally, the amount of apparent position shift produced was dependent on dot speed, with faster speeds producing larger positional offsets. However, we found in the second experiment that when the cylinder contained randomly moving dots, or when the cylinder contained equal amounts of dots moving towards and away from the observer, positional shifts were very much reduced, or abolished. Our findings suggest that motion signals can induce a misperception of position in depth that is similar manner to that produced by motion within an object in the two-dimensional image plane.

Second-order motion shifts perceived position

Many studies have documented that first-order motion influences perceived position. Here, we show that second-order (contrast defined) motion influences the perceived positions of stationary objects as well. We used a Gabor pattern as our second-order stimulus, which consisted of a drifting sinusoidal contrast modulation of a dynamic random-dot background; this second-order carrier was enveloped by a static Gaussian contrast modulation. Two vertically aligned Gabors had carrier motion in opposite directions. Subjects judged the relative positions of the Gabors' static envelopes. The positions of the Gabors appeared shifted in the direction of the carrier motion, but the effect was narrowly tuned to low temporal frequencies across all tested spatial frequencies. In contrast, first-order (luminance defined) motion shifted perceived positions across a wide range of temporal frequencies, and this differential tuning could not be explained by differences in the visibility of the patterns. The results show that second-order motion detection mechanisms contribute to perceived position. Further, the differential spatial and temporal tuning of the illusion supports the idea that there are distinct position assignment mechanisms for first and second-order motion.

Visual motion due to eye movements helps guide the hand

Movement of the body, head, or eyes with respect to the world creates one of the most common yet complex situations in which the visuomotor system must localize objects. In this situation, vestibular, proprioceptive, and extra-retinal information contribute to accurate visuomotor control. The utility of retinal motion information, on the other hand, is questionable, since a single pattern of retinal motion can be produced by any number of head or eye movements. Here we investigated whether retinal motion during a smooth pursuit eye movement contributes to visuomotor control. When subjects pursued a moving object with their eyes and reached to the remembered location of a separate stationary target, the presence of a moving background significantly altered the endpoints of their reaching movements. A background that moved with the pursuit, creating a retinally stationary image (no retinal slip), caused the endpoints of the reaching movements to deviate in the direction of pursuit, overshooting the target. A physically stationary background pattern, however, producing retinal image motion opposite to the direction of pursuit, caused reaching movements to become more accurate. The results indicate that background retinal motion is used by the visuomotor system in the control of visually guided action.

Motion-Sensitive Neurones in V5/MT Modulate Perceived Spatial Position

Until recently, it was widely believed that object position and object motion were represented independently in the visual cortex. However, several studies have shown that adaptation to motion produces substantial shifts in the perceived position of subsequently viewed stationary objects. Two stages of motion adaptation have been proposed: an initial stage at the level of V1 and a secondary stage thought to be located in V5/MT. Indeed, selective adaptation can be demonstrated at each of these levels of motion analysis. What remains unknown is which of these cortical sites are involved in modulating the positional representation of subsequently viewed objects. To answer this question directly, we disrupted cortical activity by using transcranial magnetic stimulation (TMS) immediately after motion adaptation. When TMS was delivered to V5/MT after motion adaptation, the perceived offset of the test stimulus was greatly reduced. In marked contrast, TMS of V1 had no effect on the changes that normally occur in perceived position after motion adaptation. This result demonstrates that the anatomical locus at which motion and positional information interact is area V5/MT rather than V1/V2.

Motion distorts perceived depth

Two important tasks that the visual system has to perform are determining the direction of motion and the spatial location of objects. It has recently been shown that the perceived location of an object moving in the frontal-plane is displaced along the direction of motion (e.g. Nature 397 (1999) 610; Vision Research 31 (1991) 1619). The aim of the present study is to examine the extent of this interaction between motion and perceived location. The observers' task was to indicate which of two vertically separated moving stimuli was closer. The two stimuli were presented at various relative disparity offsets. The stimuli consisted of moving dot patterns (optic-flow) that simulated either fronto-parallel motion (all the dots moved one direction) or motion in depth. Motion of the dots towards the centre of the stimulus simulated object motion away from the observer and motion of the dots away from the centre of the stimulus simulated object motion towards the observer. Results indicate that motion-in-depth information can bias perceived stereoscopic-based depth. Simulated motion towards the observer made the object appear closer to the observer than the depth signalled by the disparity information and simulated motion away from the observer made it seem further away. The results of this study, when combined with those of previous studies, show that motion can distort our entire three dimensional representation of space.