Global induced motion and visual stability in an optic flow illusion

When an expansion flow field of moving dots is overlapped by planar motion, observers perceive an illusory displacement of the focus of expansion (FOE) in the direction of the planar motion (Duffy and Wurtz, Vision Research, 1993;33:1481-1490). The illusion may be a consequence of induced motion, wherein an induced component of motion relative to planar dots is added to the motions of expansion dots to produce the FOE shift. While such a process could be mediated by local 'center-surround' receptive fields, the effect could also be due to a higher level process which detects and subtracts large-field planar motion from the flow field. We probed the mechanisms underlying this illusion by adding varying amounts of rotation to the expansion stimulus, and by varying the speed and size of the planar motion field. The introduction of rotation into the stimulus produces an illusory shift in a direction perpendicular to the planar motion. Larger FOE shifts were perceived for greater speeds and sizes of planar motion fields, although the speed effect saturated at high speeds. While the illusion appears to share a common mechanism with center-surround induced motion, our results also point to involvement of a more global mechanism that subtracts coherent planar motion from the flow field. Such a process might help to maintain visual stability during eye movements.     

Illusory line motion in visual search: attentional facilitation or apparent motion?

A line, presented instantaneously, is perceived to be drawn from one end when a dot is flashed at that end prior to the presentation of the line. Although this phenomenon, called illusory line motion, has been attributed to accelerated processing at the locus of attention, preattentive (stimulus-driven) motion mechanisms might also contribute to the line-motion sensation. We tested this possibility in an odd-target-search task. The stimulus display consisted of two, four, or eight pairs of dots and lines. All lines were presented on the same side of the dots (eg right), except for the target line, which was presented on the opposite side (left). Subjects were asked to report the presence or absence of the target, which was presented in half of the trials. Low error rates for target detection (about 10%) even when the display consisted of eight dot-line pairs (ie display size was eight) indicated that illusory line motion could be perceived simultaneously at many locations. The interstimulus interval (ISI) between the dots and lines (0-2176 ms) and the contrast polarity (both dots and lines were brighter than the background, or dots were darker and lines were brighter) were also manipulated. When an ISI of a few hundred milliseconds was inserted, target detection was nearly impossible with larger display sizes. When the contrast polarity was changed, the target-detection performance was impaired significantly, even with no ISI. Moreover, it was found that the effects of display size, ISI, and contrast polarity were comparable in searches for a two-dot apparent-motion target. These results support the idea that preattentive, apparent-motion mechanisms, as well as attentional mechanisms, contribute to illusory line motion.     

The perception of color from motion

We introduce and explore a color phenomenon which requires the prior perception of motion to produce a spread of color over a region defined by motion. We call this motion-induced spread of color dynamic color spreading. The perception of dynamic color spreading is yoked to the perception of apparent motion: As the ratings of perceived motion increase, the ratings of color spreading increase. The effect is most pronounced if the region defined by motion is near 1 degree of visual angle. As the luminance contrast between the region defined by motion and the surround changes, perceived saturation of color spreading changes while perceived hue remains roughly constant. Dynamic color spreading is sometimes, but not always, bounded by a subjective contour. We discuss these findings in terms of interactions between color and motion pathways.     

Perception of induced visual motion: Effects of relative position, shape, and size of the surround.

In the 1st of 3 experiments with 64 undergraduates, the induced motion perceived in a stationary central point of light was primarily determined by the movement of the outermost of 2 oppositely moving surrounds, regardless of surround shape. Exp II found that moving square surrounds were more effective than moving circular surrounds in generating induced motion. In Exp III, perceived motion of the stationary light was directly related to the size of the moving square surround. These results, which indicate that induced motion is a function of the relative position, shape, and size of the moving surround(s), may be due to changes in the observer's egocentric orientation and perception of straight ahead. (French abstract) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The shape of self-motion perception--I. Equivalence classification for sustained motions

Two completely different motions of a subject relative to the earth can induce exactly the same stimuli to the vestibular, somatosensory and visual systems. When this happens, the subject may experience disorientation and misperception of self-motion. We have identified large classes of motions that are perceptually equivalent, i.e. indistinguishable by the subject, under three sets of conditions: no vision, with vision and earth-fixed visual surround, and with vision during possible movement of the visual surround. For each of these sets of conditions, we have developed a classification of all sustained motions according to their perceptual equivalences. The result is a complete list of the possible misperceptions of sustained motion due to equivalence of the forces and other direct stimuli to the sensors under the given conditions. This research expands the range of possible experiments by including all components of linear and angular velocity and acceleration. Many of the predictions in this paper can be tested experimentally. In addition, the equivalence classes developed here predict perceptual phenomena in unusual motion environments that are difficult or impossible to investigate in the laboratory.     

Vector analysis and process combination in motion perception.

Conducted 3 experiments with 102 undergraduates that support an altered explanation of the vector analysis that occurs in certain motion displays discovered by G. Johansson (1950). What seemed the result of a perceptual vector analysis is ascribed to the outcome of 2 independent stimulus conditions—configural change and S-relative—to which such displays can give rise because of external vector analysis. In 2 of Johansson's displays, conditions for configurational change were altered by adding stationary reference points in the surround of the displays. Veridical perception of the displays resulted in a majority of instances. It was also found that the different motions that resulted from configurational change and from S-relative stimulation could combine to form unitary perceived motions and that this happened frequently under some conditions. (13 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Influence of stimulus speed upon the antagonistic surrounds of area MT/V5 neurons

We investigated the effect of stimulus speed upon surround antagonism in macaque MT/V5 neurons, using probe stimuli placed at different positions in the surround. Their speed was varied, while the stimulation of the excitatory receptive field (RF) was held at optimal speed. Most Surrounds proved asymmetric, arising from a single region on one side of RF, although bilaterally and circularly symmetric surrounds were occasionally observed. Surround organization was generally retained at faster or slower surround speeds. Speed-dependent changes usually entailed diminished position dependence of surround influence, consequent to reduced surround effect at the position producing maximum inhibition. The effect of a stimulus covering the entire surround was much less dependent upon motion speed. Results show that surround non-uniformity is a robust finding in MT/V5 and endows neurons with multiple mechanisms for extracting surface orientation in depth.     

Implied velocity and acceleration induce transformations of visual memory.

Investigated the phenomenon of representational momentum as reported by the 1st 2 authors (see record 1984-16934-001) in cases where visual memories are distorted by implied motions of the elements of a pattern, conducting 3 experiments with 48 undergraduates. It was predicted that these memory distortions should be sensitive not only to the direction of the implied motions but also to changes in the implied velocity. Ss observed a sequence of dot-pattern displays that implied that the dots were moving at either a constant velocity or constant acceleration, but in separate directions. Discrimination functions for recognizing the final pattern in the sequence revealed that Ss' memories had shifted forward, corresponding to small continuations of the implied motions. The induced memory shifts increased in size as the implied velocity and acceleration of the dots increased but were eliminated when the display sequence implied a deceleration of the dots to a final velocity of zero. It is suggested that mentally extrapolated motion may have some of the same inertial properties as actual physical motion. (30 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Visual motion aftereffects: critical adaptation and test conditions

The visual motion aftereffect (MAE) typically occurs when stationary contours are presented to a retinal region that has previously been exposed to motion. It can also be generated following observation of a stationary grating when two gratings (above and below it) move laterally: the surrounding gratings induce motion in the opposite direction in the central one. Following adaptation, the centre appears to move in the direction opposite to the previously induced motion, but little or no MAE is visible in the surround gratings [Swanston & Wade (1992) Perception, 21, 569-582]. The stimulus conditions that generate the MAE from induced motion were examined in five experiments. It was found that: the central MAE occurs when tested with stationary centre and surround gratings following adaptation to surround motion alone (Expt 1); no MAEs in either the centre or surround can be measured when the test stimulus is the centre alone or the surround alone (Expt 2); the maximum MAE in the central grating occurs when the same surround region is adapted and tested (Expt 3); the duration of the MAE is dependent upon the spatial frequency of the surround but not the centre (Expt 4); MAEs can be observed in the surround gratings when they are themselves surrounded by stationary gratings during test (Expt 5). It is concluded that the linear MAE occurs as a consequence of adapting restricted retinal regions to motion but it can only be expressed when nonadapted regions are also tested.     

Eye movement as a cue to figure motion in anorthoscopic perception.

When a figure moves behind a narrow aperture in an opaque surface, if it is perceived as a figure, its shape will often appear distorted. Under such anorthoscopic conditions, the speed or direction of the object's motion is ambiguous. However, when the observer simultaneously tracks a moving target, a figure is always perceived, and its precise shape is a function of the speed or direction of tracking. The figure is seen as moving with the speed or in the direction of the target. Thus, it is argued that eye movement serves as a cue to the figure's motion, which, in turn, determines its perceived length or orientation. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Speed discrimination of stereoscopic (cyclopean) motion

This study investigated the degree to which speed of stereoscopic translational motion (i.e. moving binocular disparity information) can be discriminated in a display that minimizes position information. Observers viewed dynamic random-element stereograms depicting arrays of randomly positioned stereoscopic dots that moved bidirectionally. Two tasks were performed: a speed discrimination task and a displacement discrimination task. Across a range of conditions, speed could be discriminated under conditions in which displacement could not. Thus, speed of stereoscopic motion can be discriminated when position information is minimal. This result indicates that stereoscopic motion is sensed in a way that cannot be explained by feature tracking or by inferring the motion from memory of position and time.     

Brightness induction from uniform and complex surrounds: a general model

We studied the brightness induced from complex non-figural achromatic surrounds. A spatially uniform test field was surrounded by a random texture composed of two sets of dots. The luminance of each set of dots was modulated sinusoidally at 0.5 Hz. The mean luminance, phase and amplitude of modulation of each set were controlled independently so as to modulate the luminance and/or the contrast of the surround. Brightness induction was measured by a modulation nulling technique. The results were fit by a model in which the total brightness induced by a surround is equal to a weighted spatial summation of the induced effects from each point in the surround. The model incorporates local luminance gain controls in the test and surround fields and assumes that the magnitude of induction from each surround element is gain controlled by the difference between the mean luminance of the test and the individual surround elements.     

Deficits and recovery of first- and second-order motion perception in patients with unilateral cortical lesions

Unilateral lesions in the posterior parietal cortex can degrade motion perception in the contralesional visual hemifield. Our aim was to investigate whether deficits caused by cortical lesions may be different for first- and second-order motion perception, and to study the time scale of any potential recovery. In nine patients with circumscribed lesions mainly in the parietal and fronto-parietal cortex, thresholds for direction discrimination were measured for stimuli presented peripherally in their ipsi- and contralesional hemifield. Subjects had to identify the direction of a vertically moving object embedded in a background of dynamic random dot noise. The object consisted of various proportions of signal and noise dots. Signal dots were either (a) coherently moving in the same direction as the object (first-order), (b) stationary (second-order: drift-balanced), or (c) coherently moving in the opposite direction (second-order: theta). Noise dots were flickering. Two patients showed significant threshold elevations for all three types of motion stimuli presented in their contralesional hemifield, while thresholds for ipsilesional targets were unaffected. Neither showed any selective deficit of first- versus second-order motion perception, but second-order motion was more impaired. Their lesions probably included the motion area V5-MT, which was spared in the other seven patients. One of the patients, who was retested several times during a 27-month postlesional period, showed complete recovery for first- and second-order motion direction discrimination, as well as for the detection of speed differences.     

Velocity decomposition and surface decomposition--reciprocal interactions between motion and form processing

Physically unidirectional motion of short-lived random dot arrays was found to perceptually decompose into two motion components (velocity decomposition) in a configuration in which two squares appear to partially overlap transparently (surface decomposition). In the experiments in which the velocity of the short-lived random dots in the overlapping area was varied, both the velocity decomposition and the surface decomposition were found to be strongest when the velocity of the overlapping area was close to the vector sum of the velocities of random dots in adjacent non-overlapping areas. On the other hand, neither velocity decomposition nor surface decomposition was found either when random dot arrays were put in occlusion configurations or when continuous random dots were used. While previous studies have indicated a one-way influence either from motion to form processing, or from form to motion processing, the present study further suggests that there is a strong reciprocal interaction between motion and form processing. A possibility is that the reciprocal interaction is iterative so that the representations for velocity and surface decomposition are gradually formed.     

Color from motion: dichoptic activation and a possible role in breaking camouflage

'Color from motion' describes the perception of a spread of subjective color over achromatic regions seen as moving. The effect can be produced in a display of multiple frames shown in quick succession, each frame consisting of a fixed, random placement of colored dots on a high-luminance white background with color assignments of some dots, but not dot locations, changing from frame to frame. Evidence is presented that the perception of apparent motion and the spread of subjective color can be activated by binocular combination of disjoint signals to each eye. The dichoptic presentation of every odd-numbered frame of the full stimulus sequence presented to one eye and, out of phase, every even-numbered frame to the other eye produces a compelling perception of color from motion equal to that seen with the full sequence presented to each eye alone. This is consistent with the idea that color from motion is regulated in sites at or beyond the convergence of monocular pathways. When the background field in the stimulus display is of low luminance, an amodally complete object, fully colored and matching the dots defining the moving region in hue and saturation, is seen to move behind a partially occluding screen. Observers do not perceive such an object in still view. Hence, color from motion can be used by the visual system to produce amodal completion, which suggests that it may play a role in enhancing the visibility of camouflaged objects.     

Ventral intraparietal area of the macaque: anatomic location and visual response properties

1. The middle temporal area (MT) projects to the intraparietal sulcus in the macaque monkey. We describe here a discrete area in the depths of the intraparietal sulcus containing neurons with response properties similar to those reported for area MT. We call this area the physiologically defined ventral intraparietal area, or VIP. In the present study we recorded from single neurons in VIP of alert monkeys and studied their visual and oculomotor response properties. 2. Area VIP has a high degree of selectivity for the direction of a moving stimulus. In our sample 72/88 (80%) neurons responded at least twice as well to a stimulus moving in the preferred direction compared with a stimulus moving in the null direction. The average response to stimuli moving in the preferred direction was 9.5 times as strong as the response to stimuli moving in the opposite direction, as compared with 10.9 times as strong for neurons in area MT. 3. Many neurons were also selective for speed of stimulus motion. Quantitative data from 25 neurons indicated that the distribution of preferred speeds ranged from 10 to 320 degrees/s. The degree of speed tuning was on average twice as broad as that reported for area MT. 4. Some neurons (22/41) were selective for the distance at which a stimulus was presented, preferring a stimulus of equivalent visual angle and luminance presented near (within 20 cm) or very near (within 5 cm) the face. These neurons maintained their preference for near stimuli when tested monocularly, suggesting that visual cues other than disparity can support this response. These neurons typically could not be driven by small spots presented on the tangent screen (at 57 cm). 5. Some VIP neurons responded best to a stimulus moving toward the animal. The absolute direction of visual motion was not as important for these cells as the trajectory of the stimulus: the best stimulus was one moving toward a particular point on the face from any direction. 6. VIP neurons were not active in relation to saccadic eye movements. Some neurons (10/17) were active during smooth pursuit of a small target. 7. The predominance of direction and speed selectivity in area VIP suggests that it, like other visual areas in the dorsal stream, may be involved in the analysis of visual motion.     

Use of Exact Solutions of Wave Propagation Problems to Guide Implementation of Nonlinear Seismic Ground Response Analysis Procedures

One-dimensional nonlinear ground response analyses provide a more accurate characterization of the true nonlinear soil behavior than equivalent-linear procedures, but the application of nonlinear codes in practice has been limited, which results in part from poorly documented and unclear parameter selection and code usage protocols. In this article, exact (linear frequency-domain) solutions for body wave propagation through an elastic medium are used to establish guidelines for two issues that have long been a source of confusion for users of nonlinear codes. The first issue concerns the specification of input motion as “outcropping” (i.e., equivalent free-surface motions) versus “within” (i.e., motions occurring at depth within a site profile). When the input motion is recorded at the ground surface (e.g., at a rock site), the full outcropping (rock) motion should be used along with an elastic base having a stiffness appropriate for the underlying rock. The second issue concerns the specification of viscous damping (used in most nonlinear codes) or small-strain hysteretic damping (used by one code considered herein), either of which is needed for a stable solution at small strains. For a viscous damping formulation, critical issues include the target value of the viscous damping ratio and the frequencies for which the viscous damping produced by the model matches the target. For codes that allow the use of “full” Rayleigh damping (which has two target frequencies), the target damping ratio should be the small-strain material damping, and the target frequencies should be established through a process by which linear time domain and frequency domain solutions are matched. As a first approximation, the first-mode site frequency and five times that frequency can be used. For codes with different damping models, alternative recommendations are developed.     

Chromatic induction: border contrast or adaptation to surrounding light?

Chromatic induction from a surrounding light is measured with an additional remote field outside the surround. Chromatic induction from the surround into a central test field is found to be attenuated by a remote inhomogeneous 'checkerboard', composed of squares at two different chromaticities. A uniform remote field, on the other hand, either at the average or at the most extreme chromaticity of the 'checkerboard', has a weaker effect on chromatic induction than the inhomogeneous field, implying that chromatic contrast within the remote region is a critical factor. The complete set of experiments is accounted for by chromatic contrast gain control: chromatic induction, mediated by a neural signal for contrast at the edge of the test, is attenuated by contrast within the remote region. A contrast gain control set by variation in chromaticity over a broad area can contribute to the stable color appearance of surfaces embedded within complex scenes by minimizing chromatic induction from locally adjacent regions.     

A model for encoding multiple object motions and self-motion in area MST of primate visual cortex

Many cells in the dorsal part of the medial superior temporal (MST) region of visual cortex respond selectively to specific combinations of expansion/contraction, translation, and rotation motions. Previous investigators have suggested that these cells may respond selectively to the flow fields generated by self-motion of an observer. These patterns can also be generated by the relative motion between an observer and a particular object. We explored a neurally constrained model based on the hypothesis that neurons in MST partially segment the motion fields generated by several independently moving objects. Inputs to the model were generated from sequences of ray-traced images that simulated realistic motion situations, combining observer motion, eye movements, and independent object motions. The input representation was based on the response properties of neurons in the middle temporal area (MT), which provides the primary input to area MST. After applying an unsupervised optimization technique, the units became tuned to patterns signaling coherent motion, matching many of the known properties of MST cells. The results of this model are consistent with recent studies indicating that MST cells primarily encode information concerning the relative three-dimensional motion between objects and the observer.     

An objective criterion for apparent motion based on phase discrimination.

Studies of visual apparent motion have relied on observers' subjective self-reports of experienced motion, for which there is no objective criterion of right or wrong. A new method of phase discrimination is reported that may offer an objective indicator of apparent motion. Ss discriminated the direction of an objective 75-ms phase shift, away from strict temporal alternation of 2 stimulus dots. Accuracy increased from 50% to 100% correct as rate of alternation and distance between the dots was decreased, in conformity with Korte's 3rd law of apparent motion. This and additional evidence suggests that phase discrimination may be mediated by asymmetries between the experienced strengths of leftward and rightward motion. Phase discrimination may also be adaptable to the study of apparent motion and related phenomena in other sensory modalities and other animal species. (PsycINFO Database Record (c) 2010 APA, all rights reserved)