Endogenous attention and illusory line motion reexamined

P. Downing and A. Treisman's (1997) failure to replicate an effect of endogenous attention on the direction of illusory line motion (ILM) was reexamined. Four experiments with slightly modified stimulus presentation methods based on gradient theories of ILM found that endogenous attention directed to 1 of 2 similar priming objects is capable of influencing experienced motion direction within a subsequently presented line. The endogenous effect on ILM was consistent with a concomitant response-time discrimination task, was robust across naive and informed participants, occurred whether eye fixation was monitored or not, and occurred under conditions where multiple motion response categories were available to participants. The endogenous effect disappeared when participants moved their eyes to the attended item, when there was no motivation to endogenously attend, and when the presentation methods of P. Downing and A. Treisman (1997) were used.     

Displacement of location in illusory line motion

Six experiments examined displacement in memory for the location of the line in illusory line motion (ILM; appearance or disappearance of a stationary cue is followed by appearance of a stationary line that is presented all at once, but the stationary line is perceived to “unfold” or “be drawn” from the end closest to the cue to the end most distant from the cue). If ILM was induced by having a single cue appear, then memory for the location of the line was displaced toward the cue, and displacement was larger if the line was closer to the cue. If ILM was induced by having one of two previously visible cues vanish, then memory for the location of the line was displaced away from the cue that vanished. In general, the magnitude of displacement increased and then decreased as retention interval increased from 50 to 250 ms and from 250 to 450 ms, respectively. Displacement of the line (a) is consistent with a combination of a spatial averaging of the locations of the cue and the line with a relatively weaker dynamic in the direction of illusory motion, (b) might be implemented in a spreading activation network similar to networks previously suggested to implement displacement resulting from implied or apparent motion, and (c) provides constraints and challenges for theories of ILM.     

The effect of timing and spatial separation on the velocity of auditory apparent motion

Previously, it was shown that the minimum conditions for the illusion of auditory apparent motion (AAM) depend on stimulus timing but not spatial separation. In the present experiment, the effects of stimulus timing and source separation on the perceived velocity of AAM were examined. Eight listeners estimated the velocity, duration, and distance traveled of AAM, using a no-modulus, magnitude estimation procedure. Four burst durations (25, 50, 100, and 300 msec), 10 stimulus onset asynchronies (SOAs; 30, 40, 50, 60, 70, 80, 90, 100, 110, and 120 msec) and two separations (10° and 40°) were tested. Perceived velocity estimates were related to the total duration (burst duration + SOA) of the stimulus sequence. The effect of separation on velocity was extremely small but statistically significant. These results are similar to those obtained previously on the minimum conditions for AAM. Duration estimates were related only to total duration, but separation estimates were related to both separation and total duration. These results suggest that velocity is possibly a primary dimension of AAM that is independent of source separation.     

Flash-induced forward and reverse illusory line motion in offset bars

Illusory line motion (ILM) refers to perception of motion in a bar that onsets or offsets all at once. When the bar onsets or offsets between two boxes after one of the boxes flashes, the bar appears to shoot out of the flashed box (_flashILM). If the bar offsets during the flash, it appears to contract into the flashed box (reverse ILM; rILM). Onset bars do not show rILM. Moreover, rILM and _flashILM are not correlated, indicating they are two different illusions. To date, rILM has only been studied using a 50-ms flash where the bar offsets 16.7 ms after flash onset. It is not clear if rILM is due to the 16.7-ms flash-bar-removal stimulus onset asynchrony (SOA) or due to the flash offsetting after the bar. The current studies explore these parameters to better understand the conditions that lead to rILM. The results suggest that _flashILM is sensitive to the temporal interval between flash onset and bar offset, while rILM appears to arise when the flash offsets after the bar has been removed regardless of the temporal interval between flash onset and bar removal. These results are consistent with _flashILM reflecting visual exogenous attention while rILM may reflect the low-level spreading of subthreshold activation radiating from the flashed box. The findings are incorporated into the recent work that suggests that the literature concerning ILM is possibly conflating a number of different illusions of line motion, including polarized gamma motion (PGM), transformational apparent motion (TAM), and exogenous attention induced motion (_flashILM).     

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 is 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.     

A comparison of colour,shape, and flash induced illusory line motion

When a bar suddenly appears between two boxes, the bar will appear to shoot away from the box that matches it in colour or in shape—a phenomenon referred to as attribute priming of illusory line motion (ILM; _colourILM and _shapeILM, respectively). If the two boxes are identical, ILM will still occur away from a box if it changes luminance shortly before the presentation of the bar (_flashILM). This flash condition has been suggested to produce the illusory motion due to the formation of an attentional gradient surrounding the flashed location. However, _colourILM and _shapeILM cannot be explained by an attentional gradient as there is no way for attention to select the matching box prior to the presentation of the bar. These findings challenge the attentional gradient explanation for ILM, but only if it is assumed that ILM arises for the same underlying reason. Two experiments are presented that address the question of whether or not _flashILM is the same as _colourILM or _shapeILM. The results suggest that while _colourILM and _shapeILM reflect a common illusion, _flashILM arises for a different reason. Therefore, the attentional gradient explanation for _flashILM is not refuted by the occurrence of _colourILM or _shapeILM, which may reflect transformational apparent motion (TAM).     

The spatial and temporal conditions for perceiving velocity as constant

Perceived velocities during a brief period of exposure (< 1.2 s) were measured to examine how much time is necessary to perceive velocity as constant. Moving sinusoidal gratings were used as stimuli at relatively low velocities. At the beginning of each stimulus presentation, a moving pattern was perceived as stationary until a critical time had passed. After that, perceived velocity was positively correlated with moving distance, irrespective of physical velocity below a critical moving distance. Beyond the critical moving distance, velocity was perceived as constant. A simple model is presented to explain these results.     

Primed and unprimed rebounding illusory apparent motion

Although sequences of uncorrelated random dots can yield a wide range of illusorily coherent motion percepts (including translation, rotation, contraction, expansion, shear, and rebounding motion), past priming studies have relied on two-alternative forced choice tasks that only measure unidirectional (positive or negative) priming effects. In Experiment 1 we showed that when participants are primed with unidirectional motion and given an additional option to report bidirectional (rebounding) motion, they do so frequently, suggesting that unidirectional motion can “default” to a rebounding percept. Furthermore, rebounding percepts are more prevalent during trials with long frame durations, suggesting a role for attention in forming and maintaining these illusory percepts. In Experiment 2 we compared rebounding percepts that followed unidirectional, drifting primes with rebounding percepts that followed bidirectional, rebounding primes, and found that these two types of illusory rebounding motion percepts differ systematically in their temporal structures. We argue that rebounding percepts following drifting primes can be understood as a breakdown of positive priming into an underlying oscillatory state, whereas rebounding percepts following rebounding primes may be understood either as (1) the initialization of the same oscillatory process, or (2) the entrainment of a two-step motion pattern by a higher-order mechanism.     

Exploring the modulation of attentional capture by attentional control settings using performance and illusory line motion

Modulation of attentional capture by attentional control settings was explored using performance and phenomenology. Trials began with four figure-8s presented above, below, left, and right of fixation. Any figure-8 (or none) brightened uninformatively (cue) before presentation of either a digit target (2 or 5 made from a figure-8 by deletion) calling for a speeded identification, or a line connecting adjacent figure-8s calling for a motion judgement. Such lines are seen drawn away from an adjacent cue. Digit targets appeared only on the horizontal or vertical axes, encouraging voluntary attention to two (target-relevant) of the four figure-8s. Whereas voluntary attention reduced attentional capture from cues at taskirrelevant locations (when lines were unlikely to be presented near the task-irrelevant locations), it had no effect on motion judgements, suggesting that cue-elicited exogenous attention affects perceptual arrival times while voluntary attention prioritizes locations for further processing without affecting arrival times.     

The perceived strength of illusory contours

Illusory contours are not well understood, partially because a lack of physical substance complicates their specification via physical standards. One solution is to gauge illusory contours with respect to luminance-defined contours, which are easily quantified physically. Accordingly, we chose a metric (perceived contrast) that expresses illusory contour strength in terms of the physical contrast of luminance-defined contours. Using this metric, adult observers adjusted the contrast of a luminance-defined contour until it matched the perceived contrast of an illusory contour. Illusory contour length, inducer size, and inducer contrast all influenced illusory contour strength.. The results are adequately explained via low-level visual processes. It appears that matching paradigms can be beneficial in quantitative studies of illusory contours.     

Effect of field size, head motion, and rotational velocity on roll vection and illusory self-tilt in a tumbling room

The effect of field size, velocity, and visual fixation upon the perception of self-body rotation and tilt was examined in a rotating furnished room. Subjects sat in a stationary chair in the furnished room which could be rotated about the body roll axis. For full-field conditions, complete 360 degrees body rotation (tumbling) was the most common sensation (felt by 80% of subjects). Constant tilt or partial tumbling (less than 360 degrees rotation) occurred more frequently with a small field of view (20 deg). The number of subjects who experienced complete tumbling increased with increases in field of view and room velocity (for velocities between 15 and 30 degrees s-1). The speed of perceived self-rotation relative to room rotation also increased with increasing field of view.     

The spatial and temporal characteristics of perceiving 3-D structure from motion

In four experiments, a scalar judgment of perceived depth was used to examine the spatial and temporal characteristics of the perceptual buildup of three-dimensional (3-D) structure from optical motion as a function of the depth in the simulated object, the speed of motion, the number of elements defining the object, the smoothness of the optic flow field, and the type of motion. In most of the experiments, the objects were polar projections of simulated half-ellipsoids under-going a curvilinear translation about the screen center. It was found that the buildup of 3-D structure was: (1) jointly dependent on the speed at which an object moved and on the range through which the object moved; (2) more rapid for deep simulated objects than for shallow objects; (3) unaffected by the number of points defining the object, including the maximum apparent depth within each simulated object-depth condition; (4) not disrupted by nonsmooth optic flow fields; and (5) more rapid for rotating objects than for curvilinearly translating objects.     

The perceived strength of illusory contours.

Illusory contours are not well understood, partially because a lack of physical substance complicates their specification via physical standards. One solution is to gauge illusory contours with respect to luminance-defined contours, which are easily quantified physically. Accordingly, we chose a metric (perceived contrast) that expresses illusory contour strength in terms of the physical contrast of luminance-defined contours. Using this metric, adult observers adjusted the contrast of a luminance-defined contour until it matched the perceived contrast of an illusory contour. Illusory contour length, inducer size, and inducer contrast all influenced illusory contour strength. The results are adequately explained via low-level visual processes. It appears that matching paradigms can be beneficial in quantitative studies of illusory contours.     

Dependence of illusory motion on directional consistency in oblique components

Pinna and Brelstaff (2000 Vision Research 40 2091-2096) reported a motion illusion on viewing two concentric circles consisting of quadrangular components with black and white sides on a grey background. Our results suggest that the illusion is based on the integration of motion signals derived from oblique components, and on the consistency in the direction among those components. Furthermore, arrays of these oblique components can elicit the perception of motion not only for the oblique components themselves, but also for other objects in the picture. We propose that the motion illusion depends not only upon detection of the illusory motion signal at each local oblique component, but also upon the accumulation of the signal all over the stimulus configuration.     

The involvement of illusory contours in stroboscopic motion

It was tested whether illusory contours, as they appear in the Ehrenstein figure, are able to contribute to stroboscopic motion in addition to the contribution of their inducing (real) contours. It was found that form information associated with illusory contours can be used by the stroboscopic motion mechanism. This is evidence that illusory contours can have functional effects that resemble those of real contours.     

Using illusory line motion to differentiate misrepresentation (Stalinesque) and misremembering (Orwellian) accounts of consciousness

It has been suggested that the difference between misremembering (Orwellian) and misrepresentation (Stalinesque) models of consciousness cannot be differentiated (Dennett, 1991). According to an Orwellian account a briefly presented stimulus is seen and then forgotten, whereas by a Stalinesque account it is never seen. At the same time, Dennett suggested a method for assessing whether an individual is conscious of something. An experiment was conducted which used the suggested method for assessing consciousness to look at Stalinesque and Orwellian distinctions. A visual illusion, illusory line motion, was presented and participants were requested to make judgments that reflected what they were aware of. The participants were able to make responses indicating that they were aware of the actual stimulus in some conditions, but only of the illusion in others. This finding supports a claim that the difference between the Orwellian and Stalinesque accounts may be empirically observable and that both types of events may occur depending on task and stimulus parameters.     

The spatial and temporal characteristics of perceiving 3-D structure from motion.

In four experiments, a scalar judgment of perceived depth was used to examine the spatial and temporal characteristics of the perceptual buildup of three-dimensional (3-D) structure from optical motion as a function of the depth in the simulated object, the speed of motion, the number of elements defining the object, the smoothness of the optic flow field, and the type of motion. In most of the experiments, the objects were polar projections of simulated half-ellipsoids undergoing a curvilinear translation about the screen center. It was found that the buildup of 3-D structure was: (1) jointly dependent on the speed at which an object moved and on the range through which the object moved; (2) more rapid for deep simulated objects than for shallow objects; (3) unaffected by the number of points defining the object, including the maximum apparent depth within each simulated object-depth condition; (4) not disrupted by nonsmooth optic flow fields; and (5) more rapid for rotating objects than for curvilinearly translating objects.     

Discrimination of spatial and temporal intervals defined by three light flashes: Effects of spacing on temporal judgments and of timing on spatial judgments

Observers were presented stimulus patterns consisting of a sequence of three laterally displaced light flashes, which defined two spatial intervals and two temporal intervals. The position and time of the second flash were varied factorially, and observers were asked to make relative judgments of either the two spatial intervals or the two temporal intervals. “Induction” effects of stimulus timing on spatial judgments and of stimulus spacing on temporal judgments were both found; however, the directionality of these effects differed between subjects. The results are inconsistent with the hypothesis, derived from previous findings, that such effects are determined primarily by a tendency toward perceiving constant velocity of apparent motion; it is proposed that the directionality of the induction effects is determined largely by the strategy adopted by the observer for combining spatial and temporal stimulus information.