Contour suppression during stroboscopic motion and metacontrast

Metacontrast masking reduces the estimated duration of visible persistence

A brief visual display can give rise to a sensation that outlasts the duration of the physical stimulus. The duration of this visible persistence has been estimated with paradigms that require the temporal integration of two brief sequential displays (frames) separated by a blank temporal gap. Temporal integration is said to occur when the visible persistence generated by the first frame is sufficiently long to bridge the inter-frame temporal gap. The longest gap at which integration still occurs is taken as an estimate of the duration of visible persistence. In the present work, we show that the duration of visible persistence has been underestimated in at least some of the experiments involving the temporal integration of successive displays. This is because the trailing frame can act as a metacontrast mask that foreshortens the visibility of the leading frame. Specifically, we show that operations that reduce the strength of metacontrast masking yield longer estimates of visible persistence. The relationship between metacontrast masking and visible persistence had been mentioned in some individual studies, but a comprehensive examination of that relationship is currently unavailable. Finally, we show that estimates based on single displays (e.g., the Sperling paradigm) also fail to provide untainted estimates because, in single displays, visible persistence is confounded with informational persistence.

Are the inverse-duration and inverse-proximity effects in temporal integration subserved by independent mechanisms?

Displays shorter than about 100 ms are normally seen as lasting longer than their physical duration. This visible persistence can bridge a temporal gap between two sequential stimuli causing them to be temporally integrated into a single percept. We investigated two findings in the temporal-integration literature: the inverse duration effect (temporal integration is progressively impaired as the duration of the first stimulus is increased) and the inverse proximity effect (temporal integration is progressively impaired as the spatial proximity between the stimuli is increased). In two experiments we asked whether the two effects are separable (i.e., whether they are subserved by independent mechanisms) or interact with one another. To estimate the duration of visible persistence we used the missing element paradigm in Experiment 1 and directional stroboscopic motion between two lines in Experiment 2. In both experiments we manipulated the duration of the leading stimulus and the spatial gap between the elements of the two sequential displays. Additive-factors logic was employed to examine the separability of the effects of duration and proximity. Independence (separability) of the two factors would be evidenced in a graph in which the functions of duration over proximity are parallel. The results pointed uniformly to separability. A plausible mechanism for the inverse duration effect is the burst of processing activity time-locked to stimulus onset. A plausible mechanism for the inverse proximity effect is lateral inhibition that acts to reduce the visible persistence of the leading stimulus.

Predictions of U-Shaped Backward Pattern Masking from Considerations of the Spatio-Temporal Frequency Response

The threshold detectability of a briefly presented target stimulus consisting of a vertical sinusoidal grating was affected not only by the spatial frequency content of an equally briefly presented, two-octave-wide masking noise, but also by the time interval separating the onsets of the target and its mask. Over a range of stimulus onset asynchronies, in which the mask onset either preceded, coincided with, or followed the target onset, a mask with a low spatial frequency content had its greatest masking effect on a high spatial frequency target grating when the mask followed the target by 120–180 ms. When the mask had a high spatial frequency content and the target was of low spatial frequency, or when the target was entered on the mask frequency band, optimal masking effects occurred when the onsets of the mask and target coincided. The results are discussed in relation to previous masking studies, particuarly those in which U-shaped backward pattern masking functions are obtained.

The effect of spatial competition between object-level representations of target and mask on object substitution masking

One of the processes determining object substitution masking (OSM) is thought to be the spatial competition between independent object file representations of the target and mask (e.g., Kahan & Lichtman, 2006). In a series of experiments, we further examined how OSM is influenced by this spatial competition by manipulating the overlap between the surfaces created by the modal completion of the target (an outline square with a gap in one of its sides) and the mask (a four-dot mask). The results of these experiments demonstrate that increasing the spatial overlap between the surfaces of the target and mask increases OSM. Importantly, this effect is not caused by the mask interfering with the processing of the target features it overlaps. Overall, the data indicate, consistent with Kahan and Lichtman, that OSM can arise through competition between independent target and mask representations.

Lateral masking in cycling displays: the relative importance of separation, flanker duration, and interstimulus interval for object-mediated updating

A central bar repeatedly presented in alternation with two flanking bars can lead to the disappearance of the central bar. Recently it has been suggested that this masking effect could be explained by object-mediated updating: the information from the central bar is integrated into the representation of the flankers, leading not only to the disappearance of the central bar as a separate object, but also to the perception of the flankers in apparent motion between their real position and the position of the central bar. This account suggests that the visibility of the central bar should depend on the same factors as those that influence the construction and maintenance of object representations. Therefore separation between central bar and flankers should not influence visibility as long as the time interval between them is adequate to make an interpretation of the scene in terms of one object moving from one location to the other possible location. We found that if the time interval between the central bar and the flankers is neither too short nor too long, the central bar becomes invisible even at large separations. These findings are inconsistent with traditional accounts of the cycling lateral masking displays in terms of local inhibitory mechanisms.

A spatio-temporal interaction on the apparent motion trace

During the perception of apparent motion, activity along the apparent motion trace has been found in the primary visual cortex. It has been hypothesized that this activity interferes with stimuli presented on the apparent motion trace ("motion masking"). We investigated whether this perceptual interference varies with regard to the trajectory of a moving object token in a detection task. We found a general decrease of detectability of targets presented on the trace. Surprisingly, targets presented in time with the trajectory were detected significantly more often than targets which appeared out of time. We relate this finding to a spatio-temporally specific prediction of visual events along the apparent motion trace.

Dynamics of shape interaction in human vision

Spatial context can alter perceived shape, and temporal context can influence the perception of a stimulus. We sought to determine the time course of shape interactions by using a paradigm in which closed shape contours are laterally displaced over space and time. Target and masks are separated by various stimulus onset asynchrony (SOA) values, yielding forward, backward, and simultaneous masking conditions. Results indicate that spatial lateral interactions of shape are amplified by temporal asynchrony, reaching a peak at SOAs of 80-110 ms. Mask amplitude scales all effects and masking is shape specific. When a single mask follows the target, both spatial configuration and mask onset transient are critical in determining depth of masking. When the target is followed by two sequential masks, the possibility of apparent motion determines whether one or both masks drive masking. These findings suggest that temporal interactions of shape are dependent on an interactive combination of shape specificity and transients, that apparent motion plays a modulatory role, and that target shape is determined after a temporal window, not at its onset.

Linking dynamical perceptual decisions at different levels of description in motion pattern formation: computational simulations

A two-level dynamical model of motion pattern formation is developed in which local motion/ nonmotion perceptual decisions are based on inhibitory competition between area V1 detectors responsive to motion-specifying versus motion-independent stimulus information, and pattern-level perceptual decisions are based on inhibitory competition between area MT motion detectors with orthogonal directional selectivity. The model accounts for the effects of luminance perturbations on the relative size of the pattern-level hysteresis effects reported by Hock and Ploeger (2006) and also accounts for related experimental results reported by Hock, Kelso, and Schöner (1993). Single-trial simulations demonstrated the crucial role of local motion/nonmotion bistability and activation-dependent future-shaping interactions in stabilizing perceived global motion patterns. Such interactions maintain currently perceived motion patterns by inhibiting the soon-to-be-stimulated motion detectors that otherwise would be the basis for the perception of an alternative pattern.

Visual attention and the mechanism of metacontrast

The U-shaped metacontrast function may result from the superimposition of two monotonic components which reflect the effects of mechanisms similar to the peripheral and central processes suggested for backward pattern masking by Turvey (Psychol Rev 80:1-52, 1973). In an experiment using the disc-ring paradigm, it was demonstrated that the decreasing and increasing branches of the metacontrast function are differently affected by the exposure duration of the mask and a task-irrelevant stimulus (distractor) appearing in the contralateral visual hemifield. The phenomenal representation of masking is different for the two parts of the curve. It is suggested that masking in the second part of the masking function, but not in the first, is related to the control of visual attention.

Meta- and paracontrast reveal differences between contour- and brightness-processing mechanisms

We investigated meta- and paracontrast masking using tasks requiring observers to judge the surface brightness or else the contours of target stimuli. The contour task revealed strongest metacontrast at SOAs shorter than those obtained for the brightness task. Paracontrast revealed related temporal differences between the tasks. Additionally, the paracontrast results support the existence not only of prolonged inhibitory effects but also of facilitatory effects. The combined results comport with the existence of cortical mechanisms for: (i) fast contour processing, (ii) slow surface-brightness processing, (iii) prolonged inhibition, and (iv) facilitation.

Recent models and findings in visual backward masking: a comparison, review, and update

Visual backward masking not only is an empirically rich and theoretically interesting phenomenon but also has found increasing application as a powerful methodological tool in studies of visual information processing and as a useful instrument for investigating visual function in a variety of specific subject populations. Since the dual-channel, sustained-transient approach to visual masking was introduced about two decades ago, several new models of backward masking and metacontrast have been proposed as alternative approaches to visual masking. In this article, we outline, review, and evaluate three such approaches: an extension of the dual-channel approach as realized in the neural network model of retino-cortical dynamics (Ogmen, 1993), the perceptual retouch theory (Bachmann, 1984, 1994), and the boundary contour system (Francis, 1997; Grossberg & Mingolla, 1985b). Recent psychophysical and electrophysiological findings relevant to backward masking are reviewed and, whenever possible, are related to the aforementioned models. Besides noting the positive aspects of these models, we also list their problems and suggest changes that may improve them and experiments that can empirically test them.

The roles of location specificity and masking mechanisms in the attentional blink

In a series of four experiments using rapid serial visual presentations of two target letters embedded in numeral distractors, with different numbers of display positions and with or without masking, we show that (1) the nonmonotonic, U-shaped attentional blink (AB) function, which occurs when all items are presented at the same display location, is eliminated in favor of a monotonic function when targets and distractors are presented randomly dispersed over four or nine adjacent positions; (2) the AB monotonicity is maintained with the spatially distributed presentation even when backward masks are used in all possible stimulus positions and when the location of the next item in sequence is predictable; and (3) the U-shaped AB is not due to position-specific forward or backward masking effects occurring at early levels of visual processing. We tentatively conclude that the U-shaped AB is primarily a function of the interruption of late visual processing produced when the item following the first target occurs at the same location. In order for the AB to severely disrupt performance, the item following the first target must be presented at the same location as the target so that it can serve both as a distractor and as a mask interrupting of interfering with subsequent visual processing.

Visual interactions in the path of apparent motion

When two stationary visual objects appear in alternating sequence, they evoke the perception of a single object moving back and forth between them. This is known as stroboscopic or apparent motion and forms the basis of perceived continuity in, for example, motion pictures. When the spatiotemporal separation between the inducing objects is optimal, the subjective appearance of apparent motion is nearly indistinguishable from that of real motion. Here we report that the detection and identification of a simple visual form in the path of apparent motion is impaired by the illusory perception of an object moving through the empty space between the locations at which the inducing objects are presented. This observation may be a manifestation of perceptual completion or 'filling in' during apparent motion perception. We propose that feedback from higher to lower visual cortical areas activates an explicit neural representation of a moving object, which can then disrupt the representation of visual stimuli in the path of the movement.

Motion and metacontrast with simultaneous onset of stimuli

Coherent directional motion can be seen if an image is displayed in two sequential frames (F1 and F2), where F2 is a translated version of F1. A similar two-frame sequence can produce metacontrast masking: the visibility of a leading target (F1) is reduced by a trailing, spatially nonoverlapping mask (F2). Strict temporal succession of the stimuli has been considered essential for both motion and masking. This requirement for a minimum stimulus-onset asynchrony (SOA) is known as the SOA law. Contrary to the SOA law, we found that motion and masking can be obtained with simultaneous onsets of the stimuli, provided that F2 outlasts F1. We compared motion and metacontrast with simultaneous onsets of the stimuli (SIM paradigm) with the traditional paradigm in which an interstimulus interval (ISI) is inserted between the leading and the trailing stimuli (ISI paradigm). We studied the effects in light-adapted and in dark-adapted viewing, each over a wide range of stimulus intensities. Homologous results were obtained with the two paradigms, thus disconfirming the SOA law. Models of motion sensors, such as that proposed by Reichardt [in Sensory Communication, W. A. Rosenblith, ed. (MIT Press, Cambridge, Mass., 1961), p. 303], are inherently capable of explaining the motion results obtained with both paradigms. The masking results with the SIM paradigm disconfirm theories based on onset-locked slow excitatory and fast inhibitory responses but can be explained in terms of Bridgeman's network model [Bull. Math. Biol. 40, 605 (1978)]. In light of the results obtained with the two paradigms, we discuss, and tentatively support, the suggestion that motion and metacontrast may be complementary parts of a unitary perceptual system.

Transposition in backward masking. The case of the travelling gap

The transfer of attributes from a test stimulus (TS) to a masking stimulus (MS) during backward masking was investigated. Results indicated that a gap inserted on a TS transferred to a MS and that the degree of transfer varied as an inverted U-shaped function of ISI. Two explanations of the findings were offered. One was based on preservation of distinctive features and the second was based on "filling in" theory and the decrease in masking which occurs with increased spatial separation. The suggestion was made that an explanation of the transposition effect should be consistent with theories of backward masking and apparent motion.

Stationary patterns suppress the perception of stroboscopic motion

Two spatially separated vertical bar stimuli briefly flashed in temporal sequence produced strong sensations of stroboscopic apparent motion; particularly at intermediate onset asynchronies. The sustained presence of two additional stationary vertical bars flanking the two movement-inducing bars during their presentation significantly decreased the rated magnitude of the sensation of stroboscopic motion. Control experiments rule out contrast reduction of the movement-inducing bars by the stationary flanking bars as a source of the decrease of the rated magnitude of stroboscopic motion. These results are related to similar effects observed in metacontrast and suggest that sustained channels responding to stationary patterns inhibit transient channels responding to brief or rapid image displacements giving rise to perception of stroboscopic motion.

Spatial/temporal interactions: backward and forward metacontrast masking with sine-wave gratings

A metacontrast masking paradigm is presented in which the detectability of a sine-wave target is measured in the presence of a spatially-flanking sine-wave mask. The onset of the mask either precedes (forward masking) or follows (backward masking) the onset of the target. Target detectability is measured as a function of stimulus onset asynchrony (SOA) for stimuli varying in spatial frequency and contrast. For low spatial-frequency targets, target detectability varies as a U-shaped function of SOA both in forward and backward masking. For high spatial frequency targets, U-shaped masking is observed only in backward masking. The magnitude of the masking effect at each SOA of maximal masking (SOAmax) depends on the spatial-frequency similarity of target and mask. SOAmax does not vary with contrast, but does vary with spatial frequency. These data are considered within the context of a model positing inhibitory interactions between the responses of fast- and slow-responding spatial-frequency selective channels, where the latency to channel response increases with spatial frequency.