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The gain and speed of smooth pursuit eye movements quickly drop whenever a moving tracked target disappears behind an occluder. The present study tests to what extent pursuit maintenance after target disappearance depends on the occluder's characteristics. In all experiments, a target moving for 2500 ms, (or 1250 ms) at 13.3°/s (or 26.6°/s), disappears behind an occluder for 700 ms (or 350 ms). Participants are asked to maintain their pursuit eye movements as long as possible after target disappearance. Experiment 1 compares smooth pursuit with four types of occluders and shows that a texture of flickering disks allows maintaining pursuit for long durations. Experiment 2 investigates the capability to maintain pursuit with occluders of varying flickering frequencies (3, 5, 10, 20, and 30 Hz). It is found that after target disappearance, smooth pursuit is maintained for longer durations with flicker at 10 and 20 Hz, relative to other flickering frequencies (3, 5, and 30 Hz). Experiment 3 tests whether disk size and disk density of a flickering occluding texture influence smooth pursuit maintenance. Finally, Experiment 4 tests the influence of the contrast distribution of the flickering disks on pursuit maintenance. Altogether, the results show that individuals can maintain smooth pursuit for long durations after target disappearance behind an occluding texture of disks flickering at temporal frequency above 5 Hz with balanced contrast. It is suggested that eye-induced reverse-phi motion responses in MT/MST neurons provide a positive visual feedback to the pursuit system, allowing generating smooth pursuit in the absence of explicit stimulus motion.

Visual shape and motion information, processed in distinct brain regions, should be combined to elicit a unitary coherent percept of an object in motion. In an fMRI study, we identified brain regions underlying the perceptual binding of motion and shape independently of the features-contrast, motion, and shape-used to design the moving displays. These displays alternately elicited a bound (moving diamond) or an unbound (disconnected moving segments) percept, and were either physically unchanging yet perceptually bistable or physically changing over time. The joint analysis of the blood-oxygen-level-dependent (BOLD) signals recorded during bound or unbound perception with these different stimuli revealed a network comprising the occipital lobe and ventral and dorsal visual regions. Bound percepts correlated with in-phase BOLD increases within the occipital lobe and a ventral area and decreased activity in a dorsal area, while unbound percepts elicited moderate BOLD modulations in these regions. This network was similarly activated by bistable unchanging displays and by displays periodically changing over time. The uncovered interplay between the two regions is proposed to reflect a generic binding process that dynamically weights the perceptual evidence supporting the different shape and motion interpretations according to the reliability of the neural activity in these regions.

This study investigates neural magneto-encephalographic (MEG) correlates of visual form and motion binding. Steady-state visual evoked fields (SSVEF) were recorded in MEG while observers reported their bound or unbound perception of moving bars arranged in a square shape. By using pairs of oscillating vertical and horizontal bars, "frequency-tagged" at f1 and f2, we identified a region with enhanced sustained power at 2f1+2f2 intermodulation frequency correlated with perceptual reports. Intermodulation power is more important during perceptual form/motion integration than during the perceptual segmentation of the stimulus into individual component motions, indicating that intermodulation frequency power is a neuromarker of form/motion integration. Source reconstruction of cortical activities at the relevant frequencies further reveals well segregated activity in the occipital lobe at the fundamental of the stimulation, f1 and f2, widely spread activity at 2f1 and 2f2 and a focal activity in the medial part of the right precentral sulcus region at the intermodulation component, 2f1+2f2. The present findings indicate that motion tagging provides a powerful way of investigating the processes underlying visual form/motion binding non-invasively in humans., (Copyright © 2011. Published by Elsevier B.V.)

This study explores the extent to which a display changing periodically in perceptual interpretation through smooth periodic physical changes-an inducer-is able to elicit perceptual switches in an intrinsically bistable distant probe display. Four experiments are designed to examine the coupling strength and bistable dynamics with displays of varying degree of ambiguity, similarity, and symmetry-in motion characteristics-as a function of their locations in visual space. The results show that periodic fluctuations of a remote inducer influence a bistable probe and regulate its dynamics through coupling. Coupling strength mainly depends on the relative locations of the probe display and the contextual inducer in the visual field, with stronger coupling when both displays are symmetrical around the vertical meridian and weaker coupling otherwise. Smaller effects of common fate and symmetry are also found. Altogether, the results suggest that long-range interhemispheric connections, presumably involving the corpus callosum, are able to synchronize perceptual transitions across the vertical meridian. If true, bistable dynamics may provide a behavioral method to probe interhemispheric connectivity in behaving human. Consequences of these findings for studies using stimuli symmetrical around the vertical meridian are evaluated.

Pupil size not only varies to changes in illumination but is also modulated by several cognitive factors, making it a potentially versatile physiological marker of cortical states. We recorded pupil dynamics while subjects continuously reported their bistable perception of ambiguous moving stimuli, plaids, and partially occluded rotating diamonds. We observed small (about 5% of surface change on average) but reliable pupil dilation around (-300 ms to 1.5 s) the button presses indicating the changes of percepts. We found that 70% of pupil dilation could be accounted for by the motor response. The remaining perceptual component was similar for spontaneously occurring transitions and transitions triggered by physical stimulus manipulations. Moreover, the amplitude of pupil modulation in the spontaneous condition was unrelated to the duration of each perceptual state. It is therefore unlikely that the mechanisms of endogenous perceptual bistability reflect in the pupil. In addition, we measured a clear constriction of the pupil after blinks (about 8% of surface change on average). As pupil changes have the potential to entail retino-cortical activity, their monitoring in studies of visual processing could prove worthwhile.

This study uses a rapid-serial-visual-presentation (RSVP) paradigm to test the extent to which shape and motion direction can be independently accessed and processed during the perception of structure-from-motion (SFM) stimuli. Subjects reported the number of occurrences of shape or motion direction during RSVP sequences of 3D-SFM stimuli. Overall, performance was better for motion than shape. In the motion task, observers were less accurate when the motion direction was repeated revealing a repetition blindness (RB) effect. In addition, the repetition of shape, although irrelevant to the motion task, resulted in increased performance, without change in RB rate. In contrast, there was no RB at the group level in the shape task and the repetition of the irrelevant motion direction had no effect on the performance. A closer look at the data showed that observers fall in two statistically distinct groups for the shape task. Some observers (N = 6) show a repetition advantage (RA) while the others (N = 5) show a repetition blindness (RB) effect. No behavioral differences between groups could be found for the motion task. The implications of these results for models of SFM processing are discussed in the light of the type/token theory (N. Kanwisher, 2001).

Numerous studies indicate that perceiving global object motion results from the integration of local component motions across space and time. Less attention has been paid to the issue of motion selection, necessary to avoid spurious associations of component motions belonging to different objects and to solve the so-called "superposition catastrophe problem" (F. Rosenblatt, 1961). We address this issue using outlines of geometrical shapes moving behind apertures that concealed their vertices such that recovering their global motion requires the selection and integration of some, but not all, component motions. Depending on which local motions are selected for motion integration, these stimuli yield the perception of either expansion/contraction, of global translation, or of segments moving independently. We show that the selection process depends on local and global stimulus parameters, including the local direction of figure's line-endings or the spatial configuration of component motions. In contrast, motion selection depends less on the width-i.e., spatial frequency content-or polarity of the edges. Finally, synchronous temporal modulation of component motions in the gamma range has little effect on motion selection. These results indicate that selecting component motions for motion integration is primarily determined by form constraints. As a consequence, current models assuming that mutually consistent component motions are bounded in a velocity space-lacking spatial organization should be revised to account for the present data. Alternately, interactions between visual areas selectively processing form and motion could be introduced in order to account for the perceptual binding of moving objects.

We report the results of psychophysical experiments with the so-called barber pole stimulus providing new insights on the neuronal processes underlying the analysis of moving features such as terminators or line-endings. In experiment 1, we show that the perceived direction of a barber pole stimulus, induced by line-ending motion, is highly dependent on the spatial frequency and contrast of the grating stimulus: perceived direction is shifted away from the barber pole illusion at high spatial frequency in a contrast dependent way, suggesting that line-ends are not processed at high spatial scales. In subsequent experiments, we use a contrast adaptation paradigm and a masking paradigm in an attempt to assess the spatial structure and location of the receptive fields that process line-endings. We show that the adapting stimulus that weakens most the barber pole illusion is localized within the barber pole stimulus and not at line-endings' locations. Current models of line-endings' motion processing are discussed in the light of these psychophysical results.

Line-ends, corners and junctions are important singularities for form analysis, object recognition, depth ordering or motion processing. In this study, we investigate the extent to which processing the motion of line ends depends on the spatial configuration of their immediate surround. To that aim, we used two vertical collinear line segments, translating clockwise or anti-clockwise along a circular path, together with a direction discrimination task. Direction discrimination was measured independently for outer line-ends--at both segments extremities--and inner line-ends--in between collinear segments--using line segments partially occluded by invisible masks such that the direction of either inner or outer line-ends' motion was restricted to a sinusoidal translation along a horizontal axis, and thus irrelevant for the motion task. Under these conditions, access to the direction of inner line-ends is longer and more difficult than it is for outer line-ends. Subsequent experiments show that these effects depend on the degree of collinearity between line segments. Similar experiments were performed after volunteers took a dose of Lorazepam, a benzodiazepine that facilitates the fixation of GABA on GABAA receptors. The results show that the differences between the processing of inner and outer line-ends is reduced, suggesting that the effect of the surround is modulated by inhibitory mechanisms. Using a simple model, we propose that this effect can be explained by a competition between a segmentation process based on surround suppression and contour integration through long-range horizontal connections, at or prior to motion processing stages.

Psychophysical and physiological studies suggest that long-range horizontal connections in primary visual cortex participate in spatial integration and contour processing. Until recently, little attention has been paid to their intrinsic temporal properties. Recent physiological studies indicate, however, that the propagation of activity through long-range horizontal connections is slow, with time scales comparable to the perceptual scales involved in motion processing. Using a simple model of V1 connectivity, we explore some of the implications of this slow dynamics. The model predicts that V1 responses to a stimulus in the receptive field can be modulated by a previous stimulation, a few milliseconds to a few tens of milliseconds before, in the surround. We analyze this phenomenon and its possible consequences on speed perception, as a function of the spatio-temporal configuration of the visual inputs (relative orientation, spatial separation, temporal interval between the elements, sequence speed). We show that the dynamical interactions between feed-forward and horizontal signals in V1 can explain why the perceived speed of fast apparent motion sequences strongly depends on the orientation of their elements relative to the motion axis and can account for the range of speed for which this perceptual effect occurs (Georges, Seriès, Frégnac and Lorenceau, this issue).

We report several experiments showing that a Gabor patch moving in apparent motion sequences appears much faster when its orientation is aligned with the motion path than when it is at an angle to it. This effect is very large and peaks at high speeds (64 degrees /s), decreases for higher and lower speeds and disappears at low speeds (4 degrees /s). This speed bias decreases as the angle between the motion axis and the orientation of the Gabor patch increases, but remains high for curvilinear paths, provided that element orientation is kept tangential to the motion trajectory. It is not accounted for by decision strategies relying on the overall length and duration of the motion sequence or the gap size (or spatial jump) between successive frames. We propose a simple explanation, thoroughly developed as a computational model in a companion paper (Seriès, Georges, Lorenceau & Frégnac: "Orientation dependent modulation of apparent speed: a model based on the dynamics of feedforward and horizontal connectivity in V1 cortex", this issue), according to which long-range horizontal connections in V1 elicit differential latency modulations in response to apparent motion sequences, whose read-out at an MT stage results in a perceptual speed bias. The consequences of these findings are discussed.

We present psychophysical experiments designed to reveal the role of facilitative contour interactions (the so-called 'association field') in apparent motion. We use the Ternus display (a trio of horizontally aligned elements oscillating in apparent motion). This display is perceived in 'element' motion when interframe intervals (IFIs) are short, and in 'group' motion when IFIs are long. Using Gabor elements arranged collinearly or in parallel, IFI is varied to find group motion thresholds. Consistent with a role for collinearity in perceptual grouping, thresholds are lower for collinear displays. The collinear vs. parallel comparison is made while manipulating contrast, spatial frequency, eccentricity, phase, orientation jitter and element separation. Results show a clear effect of contrast not observed in lateral masking paradigms or in 'pathfinder' stimuli, with higher contrast promoting within-frame grouping, and evidence of facilitatory interactions among parallel elements (although over a smaller scale). The tendency for collinear displays to group more than parallel displays declined with eccentricity with no clear difference evident at 12 deg. These changes in group motion thresholds indicate changing association strengths among the elements and is accounted for in terms of an association field. Alternative accounts in terms of second-order collector units or visible persistence are considered but are not supported by the data.

Visual analyses of form and motion proceed along parallel streams. Unified perception of moving forms requires interactions between these streams, although whether the interactions occur early or late in cortical processing remains unresolved. Using rotating outlined shapes sampled through apertures, we showed that binding local motions into global object motion depends strongly on spatial configuration. Identical local motion components are perceived coherently when they define closed configurations, but usually not when they define open configurations. Our experiments show this influence arises in early cortical levels and operates as a form-based veto of motion integration in the absence of closure.

To examine the relationship between visual motion processing for perception and pursuit, we measured the pursuit eye-movement and perceptual responses to the same complex-motion stimuli. We show that humans can both perceive and pursue the motion of line-figure objects, even when partial occlusion makes the resulting image motion vastly different from the underlying object motion. Our results show that both perception and pursuit can perform largely accurate motion integration, i.e. the selective combination of local motion signals across the visual field to derive global object motion. Furthermore, because we manipulated perceived motion while keeping image motion identical, the observed parallel changes in perception and pursuit show that the motion signals driving steady-state pursuit and perception are linked. These findings disprove current pursuit models whose control strategy is to minimize retinal image motion, and suggest a new framework for the interplay between visual cortex and cerebellum in visuomotor control.

Recovering the velocity of objects moving in the visual field requires both the integration and segmentation of local neuronal responses elicited by moving stimuli in primary visual cortex. Herein, we investigate the effects of the contrast, density, spatial proximity, spatial frequency, and spatial configuration of component motions on these complementary processes. Measuring the ability of human observers to discriminate the global direction of motion displays composed of spatially distributed patches of drifting gratings whose motion is locally ambiguous, we provide psychophysical evidence that linking component motion across space is facilitated at low contrast and high patch density. Furthermore, direction discrimination depends on the spatial frequency of component gratings and is more accurate for spatial configurations that contain "virtual" L junctions as compared to configurations composed of "virtual" T junctions. We suggest that the conditions yielding global motion coherence can be accounted for by the existence of anisotropic cooperative/competitive, contrast-dependent, long-range interactions among oriented direction-selective units. In addition, we bring evidence that motion segmentation processes rely upon the processing of moving local spatial discontinuities. The results are discussed in the light of recent psychophysical and physiological evidence that long-range excitatory and inhibitory interactions within primary visual cortex modulate perceptual linking.

Previous studies have shown that the perceptual integration of component motions distributed across space is inhibited whenever segmentation cues, such as line-ends, are salient. Herein, we investigate to what extent enhanced inhibition induced by lorazepam, a benzodiazepine facilitating the fixation of GABA on GABAA receptors, modifies the balance between motion integration and motion segmentation at the behavioural level. Motion integration was tested in 16 healthy volunteers taking a single and oral dose of either placebo or lorazepam (0.038 mg kg-1). The stimulus consisted of an outlined diamond presented behind four, otherwise invisible, apertures and translating along a circular trajectory (Lorenceau & Shiffrar (1992). Vision Research, 32, 263-273). Under these conditions, recovering the global diamond direction requires the integration of the component motions available within each aperture. The observers were asked to discriminate the global, clockwise or counter-clockwise, diamond direction under difficult--at high luminance contrasts--or easy--at low luminance contrasts--conditions. Overall, reaction times and error rates increased in the lorazepam group as compared to the placebo group, suggesting strong non-specific effects. However, the changes in performance in the lorazepam group are not homogeneous across conditions, suggesting that lorazepam also induces specific effects that modulate the integration/segmentation balance. Additional experiments performed with visible apertures or visible diamond vertices indicate that the effects of lorazepam are unlikely to reflect a deficit of motion processing or motion integration mechanisms since performance is only slightly impaired in the lorazepam as compared to the placebo group under these conditions. These results suggest that lorazepam might specifically modulate the saliency of line-ends, presumably because processing these features involves inhibitory mechanisms using GABA as a neuromediator, and in turn modify the balance between motion integration and segmentation.

Although it is in principle possible to determine the direction of motion of an object by combining the motion of its one-dimensional oriented contours (Fennema CL, Thompson WB. Comput. Graph. Image Processing 1979;9:301-315) there is still much debate on whether human observers can do so. The Intersection Of Constraint (IOC) rule proposed by Adelson and Movshon (Adelson EH, Movshon JA. Nature 1982;300:523-525), although compatible with the veridical object's motion, was challenged by recent psychophysical data obtained with type II plaids or lines moving behind apertures: perceived direction of motion is biased toward the vectorial average of the component motions, rather than in the direction predicted by the IOC rule. Since the velocity predicted by the vectorial rule is inconsistent with the physical velocity, its use leads to the puzzling prediction that the perceived position of a moving object becomes inconsistent with its actual position. In the present paper, the perceived path of a figure defined by its one-dimensional contours and moving behind apertures along a circular trajectory is compared with the discrepant predictions of the IOC and of the Vectorial model. The results show that the perceived path is close to veridical with these stimuli, therefore challenging the idea that the visual system uses a vector averaging rule.

To better understand how local motion detectors merge their responses so as to permit the global determination of objects' movements in the visual field, direction discrimination of performance was measured using a flexible class of moving dots--two sets of dots translating sinusoidally 90 deg out of phase along orthogonal axes. When dots' velocities are combined, a global motion along a circular trajectory emerges, clockwise or counter-clockwise depending on the sign of the phase lag. However, the results of the present experiments indicate that dot patterns are segregated into distinct, but interacting, streams when each dot motion can be accurately determined. In contrast, perceptual coherence of the global motion occurs when each local motion signal is "blurred" by a "motion noise". Direction discrimination performance then increases regularly with both noise amplitude and noise frequency, i.e., noise speed. Performance also increases when relative motion between dots is added. Testing different dot configurations indicates that performance is better for spatial arrangements that display structural properties (a square shape), as compared to overlapping random distributions. Interestingly, when the delay between stimulus onset and motion onset increases up to 300 msec, performance improves when dot patterns convey come form of structural organization but not when the dots are distributed at random. Relations of these results to existing models of motion integration are considered.

Accurate interpretations of image require the segmentation of motion signals produced by different objects with the simultaneous integration of motion signals produced by the same object. We investigated a motion integration paradigm in which the direction of an object's motion could only be determined from an integration of motion signals across the disconnected object edges. In a series of experiments we show that observers' ability to determine object motion depends significantly upon stimulus duration, luminance contrast and edge width. These effects suggest that the visual system, after some delay, relies upon relatively thick, luminance defined contour discontinuities to segment moving images.

To interpret the projected image of a moving object, the visual system must integrate motion signals across different image regions. Traditionally, researchers have examined this process by focusing on the integration of equally ambiguous motion signals. However, when the motions of complex, multi-featured images are measured through spatially limited receptive fields, the resulting motion measurements have varying degrees of ambiguity. In a series of experiments, we examine how human observers interpret images containing motion signals of differing degrees of ambiguity. Subjects judged the perceived coherence of images consisting of an ambiguously translating grating and an unambiguously translating random dot pattern. Perceived coherence of the dotted grating depended upon the degree of concurrence between the velocities of the grating terminators and dots. Depth relationships also played a critical role in the motion integration process. When terminators were suppressed with occlusion cues, coherence increased. When dots and gratings were presented at different depth planes, coherence decreased. We use these results to outline the conditions under which the visual system uses unambiguous motion signals to interpret object motion.

We studied how the visual system integrates locally ambiguous velocities into global unambiguous coherent motion in the presence or absence of a textured background. Line drawings of complex figures were presented through invisible (i.e. same luminance and hue as the background) circular apertures such that only straight line segments were visible. These figures were either presented against a uniform background or embedded in static textures made of similar line segments in such a way that figures cannot be detected if they remain static. Under our experimental conditions, the figures translated clockwise or counterclockwise along a circular path and observers were required to discriminate the global direction of motion. Because of the aperture problem, a single moving segment cannot disambiguate the global direction of the figures and integration across multiple line segments is therefore necessary to perform the task. We found that with figures at high contrast, the presence of a texture enhanced direction discrimination, while direction discrimination of figures at low contrast was impaired by the presence of the texture. These paradoxical effects of a static texture were further tested by manipulating the relative contrast between figures and texture, the motion onset asynchrony (the delay between stimulus onset and motion onset or MOA), the density, the orientation and the distribution of texture elements. The effects of the texture, either facilitation or suppression, increase with texture contrast. Accuracy improves with MOA and decreases with texture density. In general, at high figure contrast, accuracy is better whenever referents are present in the image. We suggest that facilitation by the texture at high figure contrast is accounted for by reduced salience of segmentation cues such as line terminators and increased accuracy of local velocity measurements. On the other hand, decreased performance at low figure contrast may reflect lateral suppression of the responses to motion signals by the texture.

In this study, the perceived speed of a tilted line translating horizontally (for a duration of 167 msec) is evaluated with respect to a vertical line undergoing the same translation. Perceived speed of the oblique line is shown to be underestimated when compared to the vertical line. This bias increases: (1) when the line is further tilted, (2) with greater line lengths, (3) with lower contrasts, and finally (4) with a speed of 2.1 deg/sec as compared to a higher speed of 4.2 deg/sec. These results may be accounted for by considering that two velocity signals are used by the visual system to estimate the speed of the line: the translation of this line (this signal does not depend on the line's orientation) and the motion component normal to the line (this signal depends on orientation). We suggest that these two signals are encoded by different types of units and that the translation signal is specifically extracted at the line endings. We further suggest that these signals are integrated by a weighted average process according to their perceptual salience. Other interpretations are considered at the light of current models dealing with the two-dimensional integration of different velocity signals.

The inverse relationship between the visible persistence of a briefly presented stimulus and its intensity is well established for static displays. However, with non-static displays, this relationship is only partially reported by previous studies. In order to clarify this topic, we investigated the effect of luminance on the visible persistence of a stimulus in apparent motion. Assuming that persistence duration is a normally distributed random variable, we studied whether the mean persistence of a stimulus could be systematically varied by varying its luminance. Our paradigm permits evaluation of this effect without changing the temporal interval between two successive presentations of the stimulus, thus avoiding the potential influence of this latter factor on persistence. Our results show that the inverse intensity effect still occurs at each of the successive locations of a stimulus in apparent motion. In addition, we provide evidence that increasing the spatial separation between the successive presentations, and decreasing the background luminance, result both in longer persistence duration. Altogether, these findings favour the hypothesis that persistence is actively suppressed by inhibitory interactions between adjacent neural zones.

We studied direction discrimination for lines moving obliquely relative to their orientation. Manipulating contrast, length and duration of motion, we found systematic errors in direction discrimination at low contrast, long length and/or short durations. These errors can be accounted for by a competition between ambiguous velocity signals originating from contour motion processing units and signals from line terminator processing units. The dynamic of this competition can be described by a simple model involving two different classes of processing units with different contrast thresholds, different integration time constants and different levels of response saturation.

Individual motion measurements are inherently ambiguous since the component of motion parallel to a homogeneous translating edge cannot be measured. Numerous models have proposed that the visual system solves this ambiguity through the integration of motion measurements across disparate contours. To examine this proposal, subjects observed a translating diamond through four stationary apertures. Since the diamond's motion could not be determined from any single contour, motion integration across contours was required to determine the diamond's direction of motion. We demonstrate that observers have difficulty accurately integrating motion information across space. Performance improved when the diamond stimulus was presented at 7 degrees eccentricity, through jagged apertures, or at low contrast. Taken together, these results imply that integration across space is more likely when the motion of contour terminators is less salient or reliable.

This study presents two distinct effects produced by manipulation of the background illumination on the directional sensitivity to colour- and orientation-carried motion. The two motion percepts were produced with two of a class of stimuli extensively used by the first and last authors in apparent-motion studies. The stimuli were designed to produce motion perception by virtue of spatiotemporal matching of (a) colour with orientation systematically mismatched (Colour across Orientation, CxO) and of (b) orientation with colour systematically mismatched (OxC). An increase in background illumination from dark to the equiluminance point (relative to the luminance of the discrete stimulus microelements) entails a significant increase and decrease of directional performances with CxO and OxC stimuli, respectively. It is proposed that these anti-symmetrical background effects have distinct neurophysiological origins. For CxO stimuli, improvement of directional performances at the equiluminant point is presumably due to the inactivation of the inhibitory effect of the luminance-motion pathway on the chromatic-motion pathway. The opposite effect obtained with OxC stimuli, previously referred to as the veto effect (Gorea and Papathomas, 1988 Invest. Ophthal. Vis. Sci. Suppl., 29, 265), is supposed to be entailed by the inactivation of the luminance-oriented mechanism, the only motion sensitive mechanism activated by this stimulus configuration.

A moving grating oriented +/- 45 degrees to the vertical can be perceived at choice as drifting along a left-right or up-down directional axis. When the drifting stimulus is presented alone, direction discrimination thresholds are independent of the specified response-axis. However, they strongly depend on it when the moving stimulus is superimposed on a vertical or horizontal stationary grating. Facilitation is always obtained when the drift direction of the intersections of the two gratings ('blobs') is collinear with the response-axis (i.e. when the orientations of the stationary grating and of the response-axis coincide), while inhibition is observed in the 'noncollinear' cases (i.e. when the orientations of the stationary grating and of the response-axis are orthogonal). These results are generalized in a series of reaction time (RT) experiments where the stimulus configuration described above was set at suprathreshold contrasts and where the orientation/direction of the drifting grating was variable. RT increased when the angle between the response-axis and the direction of the drifting grating increased (uncertainty effect), whether the test stimulus was presented alone, or superimposed on the stationary grating. The uncertainty effect was, however, significantly decreased under 'collinearity' conditions. The attenuation of the uncertainty effect was proportional with the velocity of the blobs and about equal in amount to the RT decrease obtained through the manipulation of the velocity of the drifting grating when presented alone (velocity effect). This observation strongly suggests that both component- and blob/plaid-related information contribute to the directional perception of a compound stimulus and that they sum algebraically.

Suprathreshold counterphase modulated gratings induce a bistable percept of drift or flicker. It is argued that these perceptual alternations might provide a new means for the investigation of directional selective mechanisms. The prevalance of either of the two perceptions has been studied as a function of the spatio-temporal characteristics of the stimulus and compared with: (1) the spatio-temporal contrast sensitivity surface for counterphase modulated gratings; (2) the motion/counterphase sensitivity ratio. Drift perception elicited by suprathreshold counterphase gratings attains a maximum for 8 c/deg, 12 Hz stimuli and decreases for any other experimental condition. For spatial frequencies below 1 c/deg, or temporal frequencies below 2 Hz, only flicker perception is reported. These phenomenal experiences do not show any systematic dependence on the involuntary eye movements of the observer. Comparison with the threshold measurements does not support their explanation in terms of the transient-sustained dichotomy, nor does it allow for a straightforward equivalance between the spatio-temporal characteristics of direction-selective mechanisms at threshold and at suprathreshold levels. It is suggested that the balance between flicker and motion is the perceptual outcome of the competition between lower and higher order motion detectors.