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.     

Shifts in perceived position following adaptation to visual motion

Where do we perceive an object to be when it is moving? Nijhawan [1] has reported that if a stationary test pattern is briefly flashed in spatial alignment with a moving one, the moving element actually appears displaced in the direction in which it is moving. Nijhawan postulates that this may be the result of a mechanism that predicts the future position of the moving element so as to compensate for the fact that the element will have moved position from the time at which the light left it to the time at which the observer becomes aware of it (as a result of the finite time taken for neural transmission). There is an alternative explanation of this effect, however. Changes in the stimulus presentation could affect perceptual latency [2], and therefore the perceived position if in motion (as suggested for the Pulfrich pendulum effect [3] [4]). In other words, if the flashed probe of the Nijhawan demonstration takes longer to reach perceptual awareness than the moving stimulus, the latter will appear to be ahead of the probe. Here, I demonstrate an alternative way of testing this hypothesis. When an illusory movement is induced (via the motion aftereffect) within a stationary pattern, it can be shown that this also produces a change in its perceived spatial position. As the pattern is stationary, one cannot account for this result via the notion of perceptual lags.     

Conformational analysis of amphotericin B molecule

We examined the effects of adaptation and test contrasts on the duration of two types of motion aftereffect (MAE) that presumably reveal different levels of motion processing: MAE with a static test stimulus (static MAE), and that with a counterphasing test stimulus (flicker MAE). MAE duration increased with increasing adaptation contrast. When the test contrast was low, it increased rapidly, and saturated at a low adaptation contrast. When the test contrast was high, however, it gradually increased over a wide range of adaptation contrasts. These complex effects of stimulus contrasts could be well described by a dependency on adaptation contrast normalized by test contrast on a logarithmic axis. Little difference was found between the results for two types of MAE. The interaction between adaptation and test contrasts leads us to reject the idea that the shape of adaptation contrast dependency of MAE duration reflects that of the sensitivity function of motion detecting mechanisms. The results also suggest a functional similarity between the processes underlying static and flicker MAEs with regard to their responses to contrasts.     

Illusory concomitant motion in ambiguous stereograms: Evidence for nonstimulus contributions to perceptual organization.

Performed 3 experiments to test whether perceptual organization is cognitively or motivationally penetrable. In Exp I, 8 undergraduate and graduate students viewed a reversible stereogram while instructed to hold 1 depth organization. Responses about depth were recorded indirectly by recording responses about direction of the illusory concomitant motion that is perceptually coupled to depth in a stereogram. It is contended that, inasmuch as perceptually coupled variables covary without necessary stimulus covariation, a postperceptual locus for any intention effects they exhibit is unlikely. Results show that instruction influenced perceived depth to a degree influenced by stimulus bias. Exps II and III examined the possibility that instructed intention might influence perception indirectly by influencing eye movements. Eight graduate student viewers' vergence position was measured directly through responses about alignment of a vernier nonius fixation. Findings from Exp II indicate an interaction between hold instruction and stimulus bias. However, unlike Exp I, instructed responses were larger with the depth response than with the motion response. Results from Exp III reveal that differential instructions produced different responses about perceptual organization, but that they were not reliably accompanied by different vergence movements. Overall findings suggest that instructed intention may influence perceptual organization by influencing internal nonstimulus components to the perceptual process. (46 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Inhibition of return is not detected using illusory line motion

Inhibition of return (IOR) is the name that has been assigned to a response time (RT) delay to a stimulus presented at a recently stimulated spatial location. A commonly held explanation for the origins of IOR is that perceptual processing in inhibited and that this inhibition translates into slower RT. Three experiments with 10 subjects were used to directly test this perceptual explanation. The first two experiments assessed the level of perceptual facilitation present in the IOR paradigm using the frequency and latency of illusory line motion judgments. Contrary to the predictions of the perceptual view, the line motion and RT measures revealed only speeded processing at previously stimulated spatial locations. Experiment 3 required a simple detection response and used the same stimulus and timing parameters as those in Experiments 1 and 2. IOR was present, replicating the recent finding that judgments based on perceptual qualities of the stimulus do not demonstrate a RT delay, whereas simple detection tasks do show RT inhibition at previously stimulated locations. These findings are discussed in relation to a number of hypotheses about the origin of the RT delay.     

Motion capture changes to induced motion at higher luminance contrasts, smaller eccentricities, and larger inducer sizes

In the stimulus configuration for "motion capture" phenomenon, we varied luminance contrast of the center disk (target), eccentricity and stimulus size. The subjects had to judge the direction of perceived target motion. We found that motion capture changed to induced motion (the direction of illusory motion was reversed) at smaller eccentricities and larger stimulus sizes. At intermediate eccentricities, motion capture changed to induced motion with increasing luminance contrast of the target. By using magnitude estimation, we also found that even a luminance-defined target was captured ("homochromatic motion capture") and that a moving target was captured by a stationary inducer ("position capture"). Both motion and position capture effects were commonly observed at lower luminance contrasts of the target, larger eccentricities and smaller sizes. From these results, we propose a model of center-surround antagonistic motion contrast detectors in motion processing.     

Representation of motion boundaries in retinotopic human visual cortical areas

Edges are important in the interpretation of the retinal image. Although luminance edges have been studied extensively, much less is known about how or where the primate visual system detects boundaries defined by differences in surface properties such as texture, motion or binocular disparity. Here we use functional magnetic resonance imaging (fMRI) to localize human visual cortical activity related to the processing of one such higher-order edge type: motion boundaries. We describe a robust fMRI signal that is selective for motion segmentation. This boundary-specific signal is present, and retinotopically organized, within early visual areas, beginning in the primary visual cortex (area V1). Surprisingly, it is largely absent from the motion-selective area MT/V5 and far extrastriate visual areas. Changes in the surface velocity defining the motion boundaries affect the strength of the fMRI signal. In parallel psychophysical experiments, the perceptual salience of the boundaries shows a similar dependence on surface velocity. These results demonstrate that information for segmenting scenes by relative motion is represented as early as V1.     

Directions of motion after-effects induced by gratings and plaids

In three experiments the direction of motion after-effect (MAE) is measured following adaptation to two gratings moving in different directions presented in alternation (component-induced MAEs: CMAEs), and to moving plaid patterns composed of superimposed pairs of these gratings (plaid-induced MAEs; PMAEs). These MAEs are compared to: (i) the vector sum direction of the component gratings; (ii) the IOC-predicted direction of the plaids; and (iii) the perceived direction of the plaids as reported by observers. Contrary to previous findings (Burke D, Wenderoth P. Vis Res 1993;33:351-9), directions of PMAEs are shown to approximate the vector sum direction of the components, whereas directions of CMAEs are shown to approximate the mean (unweighted) direction of the components. This difference is attributed to the activity, and adaptation, of an additional population of neurones whose stimulus), or a counterphase moving plaid (a combined Fourier and non-Fourier stimulus), rules out the possibility that the discrepancy between PMAE direction and actual plaid direction is due to the use of test stimuli that do not adequately reflect adaptation by the Fourier and non-Fourier components of the adapting plaids (HR, Ferrera VP, Yo C. Vis Neurosci 1992;9:79-97). Various explanations of this paradoxical result are discussed, including: (i) that MAEs produced by Fourier components out-weigh (and possibly even mask) MAEs produced by non-Fourier plaid components; (ii) PMAEs are influenced by adaptation of a population of component-selective neurones that do not contribute to plaid perception; and, (iii) PMAEs are influenced by component-specific adaptation effects that are weighted according to relative component sensitivity, rather than relative component speed (Pantle A. Vis Res 14;1974:1229-36). We review psychophysical and neurophysiological evidence consistent with these explanations.     

A note on formal learning theory

Previous investigations have challenged the generality of the claim that perceived motion in an effective stimulus for smooth pursuit eye movements. The experiments extend the scope of these investigations. Three experiments test the hypothesis that perceived motion can serve as the stimulus for pursuit when the eye movement does not generate constraining retinal error information. Observers viewed retinally stabilized displays that elicited the perception that a stationary target was moving or that a moving target was moving faster than it was actually moving. The results failed to confirm the hypothesis. Relevant literature is reviewed. We conclude that perceived movement can act as a stimulus for pursuit only when the "perceptual target" has no retinal counterpart.     

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.     

Contribution of bottom-up and top-down motion processes to perceived position.

Perceived position depends on many factors, including motion present in a visual scene. Convincing evidence shows that high-level motion perception-which is driven by top-down processes such as attentional tracking or inferred motion-can influence the perceived position of an object. Is high-level motion sufficient to influence perceived position, and is attention to or awareness of motion direction necessary to displace objects' perceived positions? Consistent with previous reports, the first experiment revealed that the perception of motion, even when no physical motion was present, was sufficient to shift perceived position. A second experiment showed that when subjects were unable to identify the direction of a physically present motion stimulus, the apparent locations of other objects were still influenced. Thus, motion influences perceived position by at least two distinct processes. The first involves a passive, preattentive mechanism that does not depend on perceptual awareness; the second, a top-down process that depends on the perceptual awareness of motion direction. Each contributes to perceived position, but independently of the other. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The constructive nature of vision: direct evidence from functional magnetic resonance imaging studies of apparent motion and motion imagery

Echoplanar functional magnetic resonance imaging was used to monitor activation changes of brain areas while subjects viewed apparent motion stimuli and while they were engaged in motion imagery. Human cortical areas MT (V5) and MST were the first areas of the 'dorsal' processing stream which responded with a clear increase in signal intensity to apparent motion stimuli as compared with flickering control conditions. Apparent motion of figures defined by illusory contours evoked greater activation in V2 and MT/MST than appropriate control conditions. Several areas of the dorsal pathway (V3A, MT/MST, areas in the inferior and superior parietal lobule) as well as prefrontal areas including FEF and BA 9/46 responded strongly when subjects merely imagined moving stimuli which they had seen several seconds before. The activation during motion imagery increased with the synaptic distance of an area from V1 along the dorsal processing stream. Area MT/MST was selectively activated during motion imagery but not during a static imagery control condition. The comparison between the results obtained with objective motion, apparent motion and imagined motion provides further insights into a complex cortical network of motion-sensitive areas driven by bottom-up and top-down neural processes.     

Posture, locomotion, spatial orientation, and motion sickness as a function of space flight

This article summarizes a variety of newly published findings obtained by the Neuroscience Laboratory, Johnson Space Center, and attempts to place this work within a historical framework of previous results on posture, locomotion, motion sickness, and perceptual responses that have been observed in conjunction with space flight. In this context, we have taken the view that correct transduction and integration of signals from all sensory systems is essential to maintaining stable vision, postural and locomotor control, and eye-hand coordination as components of spatial orientation. The plasticity of the human central nervous system allows individuals to adapt to altered stimulus conditions encountered in a microgravity environment. However, until some level of adaptation is achieved, astronauts and cosmonauts often experience space motion sickness, disturbances in motion control and eye-hand coordination, unstable vision, and illusory motion of the self, the visual scene, or both. Many of the same types of disturbances encountered in space flight reappear immediately after crew members return to earth. The magnitude of these neurosensory, sensory-motor and perceptual disturbances, and the time needed to recover from them, tend to vary as a function of mission duration and the space travelers prior experience with the stimulus rearrangement of space flight. To adequately chart the development of neurosensory changes associated with space flight, we recommend development of enhanced eye movement systems and body position measurement. We also advocate the use of a human small radius centrifuge as both a research tool and as a means of providing on-orbit countermeasures that will lessen the impact of living for long periods of time with out exposure to altering gravito-inertial forces.     

Attention during adaptation weakens negative afterimages of perceptually colour-spread surfaces.

The visual system can complete coloured surfaces from stimulus fragments, inducing the subjective perception of a colour-spread figure. Negative afterimages of these induced colours were first reported by S. Shimojo, Y. Kamitani, and S. Nishida (2001). Two experiments were conducted to examine the effect of attention on the duration of these afterimages. The results showed that shifting attention to the colour-spread figure during the adaptation phase weakened the subsequent afterimage. On the basis of previous findings that the duration of these afterimages is correlated with the strength of perceptual filling-in (grouping) among local inducers during the adaptation phase, it is proposed that attention weakens perceptual filling-in during the adaptation phase and thereby prevents the stimulus from being segmented into an illusory figure. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The effects of motion adaptation and disparity in motion integration.

Two experiments, with 10 observers, examined the effects of pattern vs component adaptation on motion integration in stimuli with or without disparity. In Exp 1, Ss adapted to either downward pattern motion, downward component motion, or a grey screen and were then tested with plaids containing either crossed, uncrossed, or zero binocular disparity, moving downward. In Exp 2, the same test conditions were employed following adaptation to upward pattern motion. The total amount of time that coherence or transparent sliding was perceived was measured. Adaptation to component motion increased the amount of perceived coherent motion, whereas adaptation to pattern motion decreased it. Adaptation to the upward-moving pattern had no effect on perceived coherence. Results demonstrate the complex nature of the interaction between depth and motion mechanisms in motion integration. (French abstract) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

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 motion analogue of the Café Wall illusion

Detecting visual motion is computationally equivalent to detecting spatiotemporally oriented contours. The question addressed in this study is whether the illusory oriented contour in the space-space domain induces corresponding illusory motion perception. Two experiments were conducted. In experiment 1, the Café Wall pattern, which elicits a strong illusion of orientation (Café Wall illusion), was found to induce an illusion of motion when this pattern was converted to the space-time domain. The strength of the motion illusion depends on the mortar luminance and width, as for the Café Wall illusion. In experiment 2, the adaptation to this illusion of motion was found to induce a motion aftereffect in a static test, which indicates that a first-order-motion system contributes to the induction of the motion illusion. In fact, the motion-energy model was able to predict the strength of this motion aftereffect.     

Encoding of three-dimensional structure-from-motion by primate area MT neurons

We see the world as three-dimensional, but because the retinal image is flat, we must derive the third dimension, depth, from two-dimensional cues. Image movement provides one of the most potent cues for depth. For example, the shadow of a contorted wire appears flat when the wire is stationary, but rotating the wire causes motion in the shadow, which suddenly appears three-dimensional. The neural mechanism of this effect, known as 'structure-from-motion', has not been discovered. Here we study cortical area MT, a primate region that is involved in visual motion perception. Two rhesus monkeys were trained to fixate their gaze while viewing two-dimensional projections of transparent, revolving cylinders. These stimuli appear to be three-dimensional, but the surface order perceived (front as opposed to back) tends to reverse spontaneously. These reversals occur because the stimulus does not specify which surface is in front or at the back. Monkeys reported which surface order they perceived after viewing the stimulus. In many of the neurons tested, there was a reproducible change in activity that coincided with reversals of the perceived surface order, even though the stimulus remained identical. This suggests that area MT has a basic role in structure-from-motion perception.     

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)     

Contextual modulation of the motion aftereffect.

The authors examined center-surround effects for motion perception in human observers. The magnitude of the motion aftereffect (MAE) elicited by a drifting grating was measured with a nulling task and with a threshold elevation procedure. A surround grating of the same spatial frequency, temporal frequency, and orientation significantly reduced the magnitude of the MAE elicited by adaptation to the center grating. This effect was bandpass tuned for spatial frequency, orientation, and temporal frequency. Plaid surrounds but not contrast-modulated surrounds that moved in the same direction also reduced the MAE. These results provide psychophysical evidence for center-surround interactions analogous to those previously observed in electrophysiological studies of motion processing in primates. Collectively, these results suggest that motion processing, similar to texture processing, is organized for the purpose of highlighting regions of directional discontinuity in retinal images. (PsycINFO Database Record (c) 2010 APA, all rights reserved)