Perception Of Subjective Contours Research Articles

Visual constraints for the perception of quantitative depth from temporal interocular unmatched features

Previous research ( Brooks & Gillam, 2006) has found that temporal interocular unmatched (IOUM) features generate a perception of subjective contours and can result in a perception of quantitative depth. In the present study we examine in detail the factors important for quantitative depth perception from IOUM features. In Experiments 1 and 2 observers were shown temporal IOUM features based on three dots that disappeared behind an implicit surface. Subjects reported a perception of a subjective surface and were able to perceive qualitative depth. In Experiments 3 and 4 metrical depth was perceived when binocular disparity features were added to the display. These results suggest that quantitative depth from IOUM information is perceived when binocular matched information is present in regions adjacent to the surface. In addition, the perceived depth of the subjective surface decreased with an increase in the width of the subjective surface suggesting a limitation in the propagation of quantitative depth to surface regions where qualitative depth information is available.

Details of the construction of perception: a closer look at illusory contours

Are we aware of neural activity in primary visual cortex? This question was asked in a classic paper written by Nobel laureate Francis Crick and Christof Koch more than a decade ago (Crick and Koch, 1995). By now, several studies have addressed the question, but so far consensus has not been reached (Rees, 2007; Tong, 2003). A recent study by Maertens et al. (2008) presents new relevant data on this issue. They demonstrated that activity in primary visual cortex (V1) is correlated with the perception of subjective contours. Maertens et al. used Kanizsa figures to induce a percept of illusory contours (IC, Figure ​Figure1),1), which are subjective in the sense that there is no actual change in luminance at the location of the perceived contour. Figure 1 Kanizsa stimuli used by Maertens et al. (2008). Illusory contours were generated by inducers (black “pack-men”) shown at different angles. The task was to judge whether the illusory contour was convex or concave. As control condition, ... The (illusory) perception of the contours is dependent on coherent orientation of the inducers. Hence, by either aligning or misaligning the inducers, very similar (local) physical parameters can create two subjectively different visual experiences. IC can therefore be used to provide information on neural correlates of conscious perception. An important feature of Maertens et al.’s experiment was the localization of neural activity specific for contour perception. This was accomplished by using spatially specific localizer stimuli for the contours and inducers separately, as well as by using a spatially more precise imaging sequence than traditionally used ones. This enabled the conclusion that the IC were related to retinotopically specific regions within V1, thereby extending previous research on IC where IC-related activity at such a level of detail was not observed. The results also add to existing evidence regarding V1 involvement in conscious perception. In previous research on conscious perception, some but not other studies have observed correlated activity in V1. A factor that could account for the inconsistent results is the specific task requirements. Specifically, it has been suggested that there is a reverse hierarchical sequence for conscious perception such that global, holistic features are perceived first and the details are added if required (Hochstein and Ahissar, 2002). In the Maertens et al. paper, the angle of the induced contrasts was varied between trials and the participant was required to judge the angle on each trial (Figure ​(Figure1).1). Hence, the task mandated a fine-scaled decision which required detail to be extracted – detail at such a fine scale that it may only be present in V1 due to its small receptive fields – and might therefore have induced correlated V1 activity. A hypothesis put forth by Crick and Koch in their 1995 paper was that, while activity in V1 may be necessary for conscious perception, it may not be sufficient. According to Crick and Koch, sensory regions need to be connected to prefrontal cortex to enable conscious perception. Whether involvement of frontal regions is necessary remains controversial (Eriksson et al., 2008), but a role of prefrontal cortex is indicated by a large number of neuroimaging studies showing a correlation between activity in frontal regions and conscious perception (Naghavi and Nyberg, 2005; Rees et al., 2002). In the Maertens et al. study, a limited part of the brain was sampled to improve spatial resolution. While this enabled the retinotopic mapping of the IC and thus constituted a great strength of study, this level of detail comes with a cost as nothing could be concluded regarding frontal involvement in the creation of IC. Future work, building on the methods used in the Maertens et al. study along with whole-brain sampling could shed light on the involvement of higher-order cortical regions and necessary vs. sufficient conditions for conscious perception. In sum, the Maertens et al. study is an excellent demonstration of the constructive nature of perception (von Helmholtz, 1910). Moreover, it shows that this constructive process is not confined to higher levels of the visual hierarchy, but can take place even at the lowest level of cortical processing.

The perception of subjective contours and neon color spreading figures in young infants

The goal of the present habituation-dishabituation study was to explore sensitivity to subjective contours and neon color spreading patterns in infants. The first experiment was a replication of earlier investigations that showed evidence that even young infants are capable of perceiving subjective contours. Participants 4 months of age were habituated to a subjective Kanizsa square and were tested afterward for their ability to differentiate between the subjective square and a nonsubjective pattern that was constructed by rotating some of the inducing elements. Data analysis indicated a significant preference for the nonsubjective pattern. A control condition ensured that this result was not generated by the difference in figural symmetry or by the local differences between the test displays. In the second experiment, infant perception of a neon color spreading display was analyzed. Again, 4-month-old infants could discriminate between the illusory figure and a nonillusory pattern. Furthermore, infants in a control group did not respond to the difference in symmetry and the local differences between two nonillusory targets. Overall, the results show that young infants respond to illusory figures that are generated by either implicit T-junctions (Experiment 1) or implicit X-junctions (Experiment 2). The findings are interpreted against the background of the neurophysiological model proposed by Grossberg and Mingolla (1985).

Perception of Subjective Contours in Fish

The ability of fish to perceive subjective (or illusory) contours, ie contours that lack a physical counterpart in terms of luminance contrast gradients, was investigated. In the first experiment, redtail splitfins (Xenotoca eiseni), family Goodeidae, were trained to discriminate between a geometric figure (a triangle or a square) on various backgrounds and a background without any figure. Thereafter, the fish performed test trials in which illusory squares or triangles were obtained by (i) interruptions of a background of diagonal lines, (ii) phase-shifting of a background of diagonal lines, and (iii) pacmen spatially arranged to induce perception of Kanizsa subjective surfaces. In all three conditions, fish seemed to generalise their responses to stimuli perceived as subjective contours by humans. Fish chose, correctly, squares or triangles made of interrupted or phase-shifted diagonal lines from uniform backgrounds of diagonal lines, as well as illusory square or triangle Kanizsa figures from figures in which the inducing pacmen were scrambled. In the second experiment, fish were trained to discriminate between a vertical and a horizontal bar with luminance contrast gradients, and then tested with vertically and horizontally oriented illusory bars, created either through interruption or spatial phase-shift of inducing diagonal lines. Fish appeared to be able to generalise the orientation discrimination to illusory contours. These results demonstrate that redtail splitfins are capable of perceiving illusory contours.

Infants’ perception of subjective contours from apparent motion

We examined infants’ perception of subjective contours in Subjective-Contour-from-Apparent-Motion (SCAM) stimuli [e.g., Cicerone, C. M., Hoffman, D. D., Gowdy, P. D., & Kim, J. S. (1995). The perception of color from motion. Perception & Psychophysics, 57, 761–777] using the preferential looking technique. The SCAM stimulus is composed of random dots which are assigned two different colors. Circular region assigned one color moved apparently, keeping all dots’ location unchanged. In the SCAM stimulus, adults can perceive subjective color spreading and subjective contours in apparent motion ( http://c-faculty.chuo-u.ac.jp/∼ymasa/okamura/ibd_demo.html). In the present study, we conducted two experiments by using this type of SCAM stimulus. A total of thirty-six 3–8-month-olds participated. In experiment 1, we presented two stimuli to the infants side by side: a SCAM stimulus consisting of different luminance, and a non-SCAM stimulus consisting of isoluminance dots. The results indicated that the 5–8-month-olds showed preference for the SCAM stimuli. In experiments 2 and 3, we confirmed that the infants’ preference for the SCAM stimulus was not generated by the local difference and local change made by luminance of dots but by the subjective contours. These results suggest that 5–8-month-olds were able to perceive subjective contours in the SCAM stimuli.

The Perception of Moving Subjective Contours by 4-Month-Old Infants

We investigated whether 4-month-old infants are capable of perceiving illusory contours produced by the Kanizsa-square display, first introduced by Prazdny (1983, Perception & Psychophysics 34 403-404), which tests whether a viewer perceives the illusory contour in the absence of brightness contrast (illusory brightness). Because the illusory square appears to move across the computer screen and infants are attracted to motion, this display holds their interest. In experiment 1, 4-month-old infants were tested for their ability to distinguish between a continuously moving illusory square and a continuously moving control display in which the pacman elements were rotated so that the perception of subjective contours did not occur. Data analysis revealed a significant preference for the subjective contour display. In experiment 2, habituation-dishabituation was used with 4-month-old infants. They were tested for their ability to discriminate between the illusory Kanizsa square that continuously moved back and forth and an illusory square which changed positions randomly. Although the infants did not show differences in dishabituation as a function of the habituation display, they looked significantly longer at the continuously moving display.

Subjective contour illusion: sex-related effect of unilateral brain damage.

The aim of the study was to investigate the effect of unilateral brain lesions on the perception of subjective contours. Brain damage resulted in a reduced ability to perceive the illusion. The effect, however, was hemisphere and sex dependent. In women an illusion decrement due to either the left or right hemisphere damage was observed, while in men only right hemisphere damage caused a decrement. No specific effect of intrahemispheric localisation of lesion was found. We conclude that mechanisms contributing to subjective contour perception are right hemisphere dependent in males whereas in females they are bilaterally represented.

Retention of local information in generation of subjective contours.

Temporal integration characteristics of subjective contour perception was investigated, using the sequential presentation of two pairs of disks with a sector removed. In the first experiment, by matching the contrast of a "real" stimulus, the perceived contrast of the subjective contours was measured as a function of the stimulus onset asynchrony (SOA) between the two pairs of the disks. With increasing SOA, the perceived contrast decreased gradually and levelled off at the SOA of ca 372 msec (1 SD = 119 msec). In the second experiment, the perceived contrast of the inducing disks was measured as a function of SOA using the matching method. The time limit of the additive effect for the contrast perception of the inducing disks was much shorter than that for subjective contours; the critical SOA was ca 65 msec (1 SD = 36 msec). The remarkable difference of the integration time was explained by a hierarchical process; the local spatial filtering, the retention of local information, and the completion of gaps by multiplicative or AND operation.

Brain Activity Related to the Perception of Illusory Contours

We have addressed the question of whether the brain's capacity to resolve an ambiguous retinal image depends upon the activity of early visual areas or whether it involves the investment of the received image with higher order cognitive hypotheses. To resolve the issue, we have used the technique of positron emission tomography to detect increases in regional cerebral blood flow (rCBF) in the brains of humans while they perceive the simple figures described by Schumann (1900) and by Kanizsa (1979). These figures produce striking percepts of surfaces or contours variously described as illusory, subjective, cognitive, or anomalous because they depend upon the brain's ability to complete the figures. If such completion is due to higher order cognitive processes or a combination of higher order and early areas, then, one might expect areas of increased rCBF outside the occipital lobe when subjects perceive these figures. However, if completion is mediated entirely by early visual areas, then the increases in rCBF will be restricted to these regions. Our results show that the perception of subjective contours is associated with significant activity in early visual areas only, particularly in area V2, leading us to conclude that the occipital cortex can contribute to the perception of these stimuli without higher order cognitive influence specific to the completion task.

Mechanisms of purely subjective contour tilt aftereffects

Neurones tuned for second-order stimuli—those which have edges defined by properties other than luminance and colour—have been frequently observed in prestriate cortex and in area V2 there are neurones which explicitly and unambiguously signal the orientation of purely subjective contours, i.e. contours with no Fourier components at the orientation of the perceived edge [von der Heydt, R. & Peterhans, E. (1989) Journal of Neuroscience, 9, 1731–1748]. No neurones in area V1 showed similar tuning characteristics. In addition, it has been demonstrated that like real contours, purely subjective test contours are subject to tilt aftereffects following prolonged viewing of an adapting stimulus, whether that stimulus is real or subjective [Paradiso, M. A., Shimojo, S. & Nakayama, K. (1989) Vision Research, 29, 1205–1213]. This result supports the assertion that the cortical processes responsible for real contour perception are also those giving rise to subjective contour perception. The data reported here further examined this hypothesis. Four experiments show that purely subjective contours exhibit both direct and indirect tilt aftereffects and tilt illusions like those observed with real contours. Further, they provide evidence that direct and indirect subjective contour effects, like direct and indirect real contour effects, arise via the operation of two mechanisms: a low level process, possibly lateral inhibition between orientation channels, and a second “higher-order” process. The data suggest that processing of orientation information beyond the striate cortex is similar to that which occurs in area V1 and the data are consistent with models of contour processing which assume that all perceived contours, both real and subjective, arise from a common mechanism.

“ノイズ入り”誘導パターンにおける主観的輪郭知覚の微小生成過程

“ノイズ入り”誘導パターンにおける主観的輪郭知覚の微小生成過程

Retinal Mechanisms in the Perception of Subjective Contours: The Contribution of Lateral Inhibition

One mechanism frequently proposed for the creation of subjective contours and their related brightness effects involves lateral neural interactions on the retina, such as the lateral inhibitory effects that underlie brightness contrast. Subjective contour stimuli were displayed under an intermittent light source, with rapid onset and slow offset as has been shown to increase lateral inhibitory interactions by allowing summation of neural onset transients. A sample of forty subjects, using magnitude estimates, reported increased subjective contour clarity and brightness effects under these exposure conditions. The effects were larger for relative brightness differences than for contour visibility. It appears that this technique may have applications in exploring retinal contributions to other aspects of the perception of subjective contours.

主観的輪郭の微小生成過程における図形手がかりの作用の検討

The purpose of this study was to investigate how the two figural cues, “alignment” and “incompletion”, would affect the microgenetic process of subjective contour (SC) perception. To change the effectiveness of each figural cue, four patterns of inducing figures (IFs) were used; “AI” pattern with both figural cues intact, “A” pattern in which the “incompletion” cue was weakened (with “alignment” as a main cue), “I” pattern in which the “alignment” cue was weakened (with “incompletion” as a main cue), and “N” pattern where both cues were obliterated. These IFs were exposed to four subjects tachistoscopically for 10-70 ms, followed by a masking pattern (dur. 500 ms) exposed without interval. All the subjects were asked to judge the clarity of the SC. The results showed that it took longer processing time to achieve the SC perception at the same clarity level as in the free observation with “I” and “N” patterns than with “AI” and “A” patterns. These results support the previous finding that “alignment” cue could generate the SC perception when each IF element was not clearly perceived and “incompletion” was not available as a main cue (Takahashi, 1990).

Evidence for perceptual organization in infants: Perception of subjective contours by young infants

In a series of experiments with 3- and 4-month-old infants, a familiarization/novelty preference procedure assessed the ability to perceive subjective contours. A pattern whose arrangement of elements induces the subjective contours that are perceived as a square by adult observers was used. By varying the orientation of elements, similar patterns without subjective contours were produced. With one exception, infants discriminated a pattern with subjective contours from patterns without subjective contours, but they did not discriminate among different patterns without subjective contours (Experiment 1). After being familiarized with real contours that formed a square (Experiment 2) and with three different patterns that produced the subjective contours of a square (Experiment 3), infants preferred patterns without subjective contours to a pattern that produced the subjective contours of a square. These three experiments together strongly suggest that infants as young as 3 months of age are able to perceive subjective contours. In Experiment 4 and its replication, infants perceived a pattern producing a subjective square or diamond as a set of components forming a square or diamond, namely, the lines and angles, and quite possibly as the complete form as well. Moreover, the percept of the subjective pattern may be used as the basis for discrimination among forms with real contours, indicative of a functional equivalence between subjective and real contours.

Segments of event-related potential map series reveal landscape changes with visual attention and subjective contours

Changes of the event-related potential (ERP) map landscape with time and condition were used to identify qualitative changes in the ERP generating process which are indicative of a change in the functional microstate. Twelve subjects attended or ignored unilaterally presented visual stimuli with and without subjective contours. ERP map series from 16 electrodes were adaptively segmented to identify periods of stable map landscape, using topographic descriptions (map maxima and minima). Attention as well as the subjective contours changed the map topography and increased the map amplitude. From 170 to 380 msec, they had similar effects on the antero-posterior map topography. Topographic differences between the effects of attention and subjective contours were also present, but affected mainly the left-right topography. The results are in line with the notion of attentional involvement in subjective contour perception and show that global modulation of exogenous brain activity cannot account for topographic changes with attention or with subjective contours. They further establish space-oriented data reduction as a powerful tool to identify components and to distinguish among hypotheses about the underlying generator processes.

Subjective contours, tilt aftereffects, and visual cortical organization

The tilt aftereffect (TAE) was used to study interactions between real and subjective contours. Subjects adapted to either real or illusory lines and were then shown test stimuli containing real or illusory lines. In our first experiment, we found that there is a marked asymmetry in the interactions between real and subjective stimuli. Adaptation to real lines produces comparable TAEs with real and subjective test lines. With either type of test stimulus the maximum effect occurs with a 10–20 deg difference between the orientations of the adaptation and test stimuli. Also, there is a strong TAE when the adaptation and test stimuli contain only subjective lines. However, there is a significantly weaker TAE when the adaptation stimulus is subjective and the test stimulus is real. In a second experiment we find that interocular transfer of tilt aftereffects is greater when the test stimulus is subjective than when it is real. These results are consistent with physiological reports that a subset of orientation selective cells in visual cortex is responsive to subjective contours and that these cells are more binocular, on average, than those responsive only to real contours. Our findings also suggest that the perception of subjective contours is based on the activation of neurons with properties, such as orientation selectivity, which are characteristic of early visual cortical areas.

The effects of perceptual set on the shape and apparent depth of subjective contours.

Two experiments showed the influence of perceptual set on the perception of subjective contours. In the first, the perceived shape of a subjective-contour figure (a minimal version of the Ehrenstein configuration) was varied by altering the observer’s viewing set. The second experiment showed that apparent depth emerged in subjective-contour figures when observers were set to perceive the illusory contours.

Attentional processing and the subjective contour illusion.

Leading explanations of the subjective contour illusion can be classified as being either "bottom-up" or "top-down." Bottom-up explanations assert that peripheral, physiological mechanisms often associated with the perception of real contours also account for subjective contour (SC) perception. In contrast, top-down explanations posit a more central locus of SC perception and are formulated on a molar, psychological level. A major aspect of bottom-up perceptual processing is that it is largely automatic. On the other hand, top-down processing implies a greater role for selective attention. In an effort to distinguish between bottom-up and top-down accounts of SC perception, the present investigation used a dual-task paradigm to test the relative attentional demands of real contour perception versus SC perception. In the primary task, subjects made speeded same-different discriminations of either paired SC forms or their real contour analogues. Half the subjects performed this primary task in conjunction with a six-digit short-term memory load secondary task. If subjective forms indeed impose a greater limited-capacity processing load than real forms, then the need to share processing capacity with a secondary task was expected to produce a greater increment in reaction time (RT) for subjective relative to real forms. The results indicated that the expected enhanced RT increment for subjective relative to real forms with the addition of a concurrent memory load was limited to same trials. This result implies that the nature of response indicators must be considered in assessing capacity requirements with the sort of dual-task paradigm used in the present investigation. Nevertheless, the fact that the increment in same RT with the addition of a concurrent memory load was greater for subjective relative to real forms accords with expectations derived from the notion that the perception of SCs is more attention demanding than that of real contours. If the interpretation of the present study is correct, then a comprehensive theory of SC perception will most likely be formulated within the top-down perspective of conceptually driven visual information processing.

Perceiving subjective contours during early childhood

The perception of subjective contours in visual displays that characteristically produce them among adults was studied for children between 3 and 6 years. Evidence for visually creating subjective contours was derived in two ways: (a) by means of direct perception, and (b) by means of recognition of the typically completed shape from among members of a matching series. With both procedures, there were significant age-related contours. A majority of 3-year-olds showed at least one instance of figural completion based on direct perception, and their average recognition performance was 60%. By 5 years, all children provided a clear indication that they had completed at least one figure by means of subjective contours, and their recognition accuracy was 100%. The overall findings provide age-related information that must be accounted for by theories of perceptual development; in particular, those dealing with pictorial perception.

Development of Visual Organization: The Perception of Subjective Contours

This study examines the development of infants' sensitivity to the organization of a subjective-contour stimulus array. 5- and 7-month-olds were sequentially shown 3 stimulus arrays of elements, only 1 of which was capable of producing subjective contours. Only the orientational relations among elements was varied. An infant habituation control procedure was used to test infants' abilities to discriminate these arrays. The results indicated that (1) only 7-month-olds showed consistent differential responsiveness to changes from an illusory array to a nonillusory array or vice versa, (2) 5-month-olds showed a weaker tendency to respond similarly and only when they had prolonged experience with the illusory array, and (3) neither age group showed much response recovery to a change from one nonillusory array to another. These findings are interpreted as indicating that infants can perceive subjective contours. However, the age of this accomplishment probably varies with both the characteristics of the array and the abilities of the observer.