Modal and amodal completion generate different shapes

Mechanisms of contour completion are critical for computing visual surface structure in the face of occlusion. Theories of visual completion posit that mechanisms of contour interpolation operate independently of whether the completion is modal or amodal--thereby generating identical shapes in the two cases. This identity hypothesis was tested in two experiments using a configuration of two overlapping objects and a modified Kanizsa configuration. Participants adjusted the shape of a comparison display in order to match the shape of perceived interpolated contours in a standard completion display. Results revealed large and systematic shape differences between modal and amodal contours in both configurations. Participants perceived amodal (i.e., partly occluded) contours to be systematically more angular--that is, closer to a corner--than corresponding modal (i.e., illusory) contours. The results falsify the identity hypothesis in its current form: Corresponding modal and amodal contours can have different shapes, and, therefore, mechanisms of contour interpolation cannot be independent of completion type.     

Curved apparent motion induced by amodal completion

We investigated whether amodal completion can bias apparent motion (AM) to deviate from its default straight path toward a longer curved path, which would violate the well-established principle that AM follows the shortest possible path. Observers viewed motion sequences of two alternating rectangular tokens positioned at the ends of a semicircular occluder, with varying interstimulus intervals (ISIs; 100-500?ms). At short ISIs, observers tended to report simple straight-path motion-that is, outside the occluder. But at long ISIs, they became increasingly likely to report a curved-path motion behind the occluder. This tendency toward reporting curved-path motion was influenced by the shape of tokens, display orientation, the gap between tokens and the occluder, and binocular depth cues. Our results suggest that the visual system tends to minimize unexplained absence of a moving object, as well as its path length, such that AM deviates from the shortest path when amodal integration of motion trajectory behind the curved occluder can account for the objective invisibility of the object during the ISI.     

Modal completion in the Poggendorff illusion: support for the depth-processing theory

The Poggendorff illusion is one of the most prominent geometrical-optical illusions and has attracted enduring interest for more than a hundred years. Most modern theories explain the illusion by postulating various kinds of distortion of the "test" component of the figure by the context or the inducing component. They make no reference to the importance of processes involved in three-dimensional scene perception for understanding the illusion. We measured the strength of the Poggendorff illusion in configurations containing solid inducing surfaces as opposed to the usual parallel lines. The surface, oblique-line, and background luminances were manipulated separately to create configurations consistent with modal completion of the obliques in front of the surface. The marked decrease in the size of the illusion in conditions favoring modal completion is consistent with claims that perceived spatial layout is a major determinant of the Poggendorff illusion.     

Shrinkage and expansion by amodal completion: a critical review

The whole literature on the so-called dimensional effects of amodal completion (shrinkage of a partially occluded figure and expansion of modally visible parts of the same figure) is critically reviewed. It is claimed that phenomenal shrinkage is the sum of at least two illusions independent of figure ground distinction: (1) shrinkage of spaces divided into a few parts, and (2) shrinkage of empty spaces. It is also shown that a partially occluded figure can widen, rather than shrink. It is concluded that the stratification in two phenomenally superimposed figures accompanies the shrinkage effect, but is irrelevant to its generation. Further experiments are needed in order to draw a well-grounded conclusion on the expansion effect.     

Flank transparency: the effects of gaps,line spacing,and apparent motion

We analyze the properties of a dynamic color-spreading display created by adding narrow colored flanks to rigidly moving black lines where these lines fall in the interior of a stationary virtual disk. This recently introduced display (Wollschl?ger et al, 2001 Perception 30 1423-1426) induces the perception of a colored transparent disk bounded by strong illusory contours. It provides a link between the classical neon-color-spreading effect and edge-induced color spreading as discussed by Pinna et al (2001 Vision Research 41 2669-2676). We performed three experiments to quantitatively study (i) the enhancing influence of apparent motion; (ii) the degrading effect of small spatial discontinuities (gaps) between lines and flanks; and (iii) the spatial extent of the color spreading. We interpret the results as due to varying degrees of objecthood of the dynamically specified disk: increased objecthood leads to increased surface visibility in both contour and color.     

What variables control (long-range) apparent motion?

A solid (ie filled-in) two-dimensional form is seen in motion toward another solid object of different shape and not towards an outline object of the same shape. The interpretation of these findings in terms of luminance-domain spatial frequency filtering is challenged by experiments in which the same results were obtained with kinetic and cyclopean forms.     

An electroluminescent lamp device for critical flicker fusion and apparent motion

A circuit that uses electroluminescent lamps as light sources for the study of critical flicker fusion (CFF) and apparent motion phenomena is described. The technique involves the modulation of a 2- KH z exciting current with a multivibrator-formed pulse adjustable over a suitable range of frequencies. Components are readily available from commercial sources.     

An airplane illusion: apparent velocity determined by apparent distance

When a small drone plane appears to be a normal-sized airplane, it appears to be very far away and moving too fast. This is the airplane illusion. In the illusory situation, familiar size determines the apparent size and distance of the plane. It sets the depth for the frontal-plane component of the perceived motion and the relative depth difference for the motion-in-depth component. Because these perceived distances are very large, the perceived velocities are very large in the respective directions. Cognition can override familiarity and produce a veridical perception of the drone.     

Evans on transparency: a rationalist account

Gareth Evans famously observed that he can answer the question ‘Do you think there is going to be a third world war?’ by attending to “precisely the same outward phenomena as I would attend to if I were answering the question ‘Will there be a third world war?’” (The varieties of reference, Oxford University Press, Oxford, p 225, 1982). I argue that this observation follows from two independently plausible ideas in philosophy of mind. The first is about rationality and consciousness: it is that to be rational is in part to be required to believe that you are in a conscious state if you are in one, at least if various background conditions are met. The second is about consciousness and attention: it is that consciousness in a belief state consists in its subject engaging, to a sufficient extent, in a certain sort of world-directed attention. I also argue that this suggestion is superior to others that have been made in the literature regarding Evan’s observation. 

     

Amodal completion in bonobos

We trained two bonobos to discriminate among occluded, complete, and incomplete stimuli. The occluded stimulus comprised a pair of colored shapes, one of which appeared to occlude the other. The complete and incomplete stimuli involved the single shape that appeared to have been partially covered in the occluded stimulus; the complete stimulus showed the full shape, whereas the incomplete stimulus showed the same portion of the shape that was visible in the occluded stimulus. The correct response was to select the two-part occluded stimulus. Consistent with amodal completion, the bonobos committed a higher percentage of errors to the complete stimuli than to the incomplete stimuli. As well, the percentage of errors to the complete stimuli rose after repeated training with several different shapes. Extensive experience with two-dimensional images enhances amodal completion of partially occluded stimuli in bonobos.     

Semantic aspects of morphological processing: transparency effects in Serbian

We examined the contribution of semantics to morphological facilitation in the visual lexical decision task at two stimulus onset asynchronies (SOAs) with Serbian materials. Primes appeared in Roman or Cyrillic characters. Targets always were printed in Roman. When primes were presented at an SOA of 250 msec, decision latencies to verbal targets (e.g., VOLIM) showed greatest facilitation after inflectionally (e.g., VOLE) related primes, significantly less after semantically transparent derived primes (e.g., ZAVOLE), and less again after semantically opaque derived primes (e.g., PREVOLE). Latencies after semantically transparent and opaque derived target words did not differ at an SOA of 48 msec. Both were slower than after inflectionally related primes. Stated generally, effects of semantic transparency among derivationally related verb forms were evident at long SOAs, but not at short ones. Under alphabet-alternating conditions, magnitudes of facilitation were greater overall, but the pattern was similar. The outcome suggests that restricted processing time for the prime limits the contribution of semantics to morphological processing and calls into question accounts that posit a task-invariant semantic criterion for morphological decomposition within the lexicon.     

Flank transparency: transparent filters seen in dynamic two-color displays

Flank transparency is the perception of a colored transparent filter evoked by apparent-motion displays containing as few as two colors. Displays of flank transparency contain a random array of line segments placed on a uniform background. Small flanks are added to the line segments if the segments fall in the interior of a moving virtual shape, such as a virtual disk. This leads to the perception of a colored transparent disk with well-defined boundaries moving over the array of lines. Current qualitative and quantitative models of luminance and color conditions for perceptual transparency do not account for flank transparency as they require displays containing at least three different colors.     

Musical stem completion: humming that note

This study looked at how people store and retrieve tonal music explicitly and implicitly using a production task. Participants completed an implicit task (tune stem completion) followed by an explicit task (cued recall). The tasks were identical except for the instructions at test time. They listened to tunes and were then presented with tune stems from previously heard tunes and novel tunes. For the implicit task, they were asked to sing a note they thought would come next musically. For the explicit task, they were asked to sing the note they remembered as coming next. Experiment 1 found that people correctly completed significantly more old stems than new stems. Experiment 2 investigated the characteristics of music that fuel retrieval by varying a surface feature of the tune (same timbre or different timbre) from study to test and the encoding task (semantic or nonsemantic). Although we did not find that implicit and explicit memory for music were significantly dissociated for levels of processing, we did find that surface features of music affect semanticjudgments and subsequent explicit retrieval.     

Amodal completion in infants: Straight continuity versus symmetry1

A visual pattern's orientation influences how adults amodally complete that pattern. We examined whether this is also the case with infants using Markovich's pattern, consisting of a black polygon partially occluded by a gray disc, which is completed as either a vertically symmetrical hexagon or as a pentagon, depending on the pattern's orientation. Our experiment consisted of two prefamiliarization test trials, six familiarization trials, and two postfamiliarization test trials. In the familiarization trials, two identical Markovich patterns, upright or slanted, were presented to 7–8‐month‐old infants (N = 28) side by side on a CRT monitor. In the pre‐ and postfamiliarization test trials, a pentagon and a hexagon were presented side by side. Infants looked longer at the hexagon than at the pentagon in the postfamiliarization test trials. This suggests that infants completed the pattern as a pentagon regardless of the pattern's orientation. This lack of an orientation effect suggests that infants do not use the figure's symmetry as a guiding principle for amodal completion.     

Phenomenal transparency in achromatic checkerboards.

The study explored the luminance relations that determine the occurrence of achromatic transparency in phenomenal surfaces on complex backgrounds. Let the luminances of the left and right parts of a transparent surface on a bipartite background and those of the left and right parts of the bipartite background be p and q and m and n, respectively. Metelli proposed that this surface looks transparent when the rule p < q if m < n (or p > q if m > n) is satisfied, and Masin and Fukuda that it looks transparent when the inclusion rule is satisfied, that is, when p epsilon (m, q) or q epsilon (p, n). These rules also apply to achromatic checkerboards formed by one checkerboard enclosed in another checkerboard. This study shows that only the inclusion rule correctly predicted the occurrence of transparency in these checkerboards.