The brain may know more than cognitive theory can tell us: a reply to Ted Parks.

In reply to Parks' interpretation of Rock's cognitive theory of illusory figures, we maintain our point of view that such a theory has limited heuristic and explanatory power because it fails to predict subjects' responses in psychophysical tasks. As a result, the theoretical framework defended by Parks is not appropriate for suggesting candidate mechanisms of brain-behaviour function that could underly the phenomenal emergence of such figures.

Asymmetrical contrast effects induced by luminance and color configurations.

In the two experiments, the use of a psychophysical procedure of brightness/darkness cancellation shed light on interactions between spatial arrangement and figure-ground contrast in the perceptual filling in of achromatic and colored surfaces. Achromatic and chromatic Kanizsa squares with varying contrast, contrast polarity, and inducer spacing were used to test how these factors interact in the perceptual filling in of surface brightness or darkness. The results suggest that the neuronal processing of surfaces with apparent contrast, leading to figure-ground segregation (i.e., perceptual organization), is governed by mechanisms that integrate both luminance contrast and spatial information carried by the inducing stimuli, while discarding information on contrast polarity or color. The findings are discussed in relation to earlier observations on brightness assimilation and contrast. They support theories of nonantagonistic neural mechanisms suppressing local contrast or color signs in brightness-based figure-ground percepts. Such mechanisms might be necessary to cancel potentially conflicting polarities in geometrically complex visual stimuli so that perceptual filling in resulting in the most plausible representation of figure and ground can be achieved.

Do alignment thresholds define a critical boundary in long-range detection facilitation with co-linear lines?

Thresholds for line contrast detection (experiment 2) were measured with a two-alternative temporal forced-choice procedure as a function of the spatial position of a vertical target line with regard to two co-linear context lines. The different spatial positions of the target line corresponded to values near the position discrimination threshold (experiment 1) reflecting the just detectable lateral offset, or non-co-linearity, between the context lines which were vertically separated by about 100 minutes of visual arc. Target and context lines were vertically separated by about 30 minutes of arc, had equal contrast polarity in one case, and opposite contrast polarity in the other. Strong line contrast detection facilitation is found at perceptually co-linear target locations. This facilitation decreases noticeably at a horizontal target offset that corresponds to the alignment threshold measured with the context lines. The effects are independent of the relative contrast polarity of target and context and, as shown in a third experiment, also independent of both the relative length or number of lines, and the magnitude of their absolute co-axial separation. This independence seems to hold, provided individual line length and co-axial distance between lines are larger than what appears to be the lower limit of the long-range spatial domain for orientation or contour integration (i.e. 20 minutes of arc), as determined by previous studies. The findings reported here suggest that alignment thresholds are likely to define a critical lateral boundary in long-range detection facilitation with co-linear lines. They support models of contour integration based on interactions between neural mechanisms that integrate local signals of contrast, orientation, and relative position or end-to-end alignment. Such mechanisms may help to explain the formation of representations of virtual contours and object contours in human perception.

Dynamic characteristics of spatial mechanisms coding contour structures.

Psychophysical thresholds for the detection of luminance targets improve significantly when the targets are presented in a specific context of spatially separated, collinear inducing stimuli defining visual contours. This phenomenon is generally referred to as a special case of detection facilitation called spatial facilitation. Spatial facilitation has been observed with luminance-defined. achromatic stimuli on achromatic backgrounds as well as with targets and inducers defined by colour contrast. This paper reviews psychophysical results from detection experiments with human observers showing the conditions under which spatially separated contour inducers facilitate the detection of simultaneously presented target stimuli. The findings point towards two types of spatial mechanisms: (i) Short-range mechanisms that are sensitive to narrowly spaced stimuli of small size and, at distinct target locations, selective to the contrast polarity of targets and inducers. (ii) Long-range mechanisms that are triggered by longer stimuli, generate facilitation across wider spatial gaps between targets and inducers, and are insensitive to their contrast polarity. Spatial facilitation with chromatic stimuli requires a longer inducer exposure than spatial facilitation with achromatic stimuli, which is already fully effective at inducer exposures of 30 ms. This difference in temporal dynamics indicates some functional segregation between mechanisms for colour and luminance contrast in spatial coding. In general, spatially induced detection facilitation can to a large extent be explained by mechanisms involving from-short-to-long-range interactions between cortical detectors.

Response times to Ehrenstein illusions of varying subjective magnitude: complementarity of psychophysical measures.

The following experiments investigate the effects of contrast polarity, inducer spacing, and inducer type on three dependent variables measuring the perception of an illusory surface in Ehrenstein figures: subjective magnitude, response time, and frequency of perception. It was found that response time generally decreased when the other two behavioral indicators increased. However, it was also shown that subjective magnitude provided more discriminating measures of relatively strong illusory percepts, whereas frequency of perception and response time provided more discriminating measures of relatively weak illusory percepts. The findings generally confirm earlier work on the effect of inducer spacing and contrast polarity on the perceived strength of brightness illusions, and in particular reveal the complementarity of subjective magnitude, response time, and frequency of perception as critical measures of configurational effects in the perceptual processing of these phenomena.

Spatial facilitation by color and luminance edges: boundary, surface, and attentional factors.

The thresholds of human observers detecting line targets improve significantly when the targets are presented in a spatial context of collinear inducing stimuli. This phenomenon is referred to as spatial facilitation, and may reflect the output of long-range interactions between cortical feature detectors. Spatial facilitation has thus far been observed with luminance-defined, achromatic stimuli on achromatic backgrounds. This study compares spatial facilitation with line targets and collinear, edge-like inducers defined by luminance contrast to spatial facilitation with targets and inducers defined by color contrast. The results of a first experiment show that achromatic inducers facilitate the detection of achromatic targets on gray and colored backgrounds, but tend to suppress the detection of chromatic targets. Chromatic inducers facilitate the detection of chromatic targets on gray and colored backgrounds, but tend to suppress the detection of achromatic targets. Chromatic spatial facilitation appears to be strongest when inducers and background are isoluminant. The results of a second experiment show that spatial facilitation with chromatic targets and inducers requires a longer exposure duration of the inducers than spatial facilitation with achromatic targets and inducers, which is already fully effective at an inducer exposure of 30 ms only. The findings point towards two separate mechanisms for spatial facilitation with collinear form stimuli: one that operates in the domain of luminance, and one that operates in the domain of color contrast. These results are consistent with neural models of boundary and surface formation which suggest that achromatic and chromatic visual cues are represented on different cortical surface representations that are capable of selectively attracting attention. Multiple copies of these achromatic and chromatic surface representations exist corresponding to different ranges of perceived depth from an observer, and each can attract attention to itself. Color and contrast differences between inducing and test stimuli, and transient responses to inducing stimuli, can cause attention to shift across these surface representations in ways that sometimes enhance and sometimes interfere with target detection.

The effect of practice on the visual detection of near-threshold lines.

Two observers practised to detect small target lines of varying luminance presented either within a context of collinear inducing stimuli, or without the context in separate blocks. A two-alternative spatial-forced-choice procedure using the method of constant stimuli was employed. For blocks of 500 trials, reflecting individual performance on five successive days of training, the percentage of correct responses, and the response times were analyzed. After several thousands of trials, i.e. several weeks of practice, both observers managed to detect targets presented at their strongest luminance within the context condition. Without the context, these targets remained undetected. Response times (RT) vary nonsystematically during training. Once detection is observed at the highest target luminance, the accuracy of the individual responses (percentage of correct responses) systematically increases, and processing speed (RT) systematically decreases with increasing target intensity. These results show that, within the appropriate perceptual context, practice can lead to a better detectability of visual stimuli presented at luminance levels near detection threshold. This improvement is reflected by an optimization of the visual integration of the different luminance levels of the target (sensory coding), and processing speed (response routine) during training.

Detection facilitation by collinear stimuli in humans: dependence on strength and sign of contrast.

We measured detection of a thin vertical line (target) in the presence of a slightly thicker collinear, adjacent line (inducer). Sign and strength of contrast of the inducer were varied. Test lines could be either bright or dark. Detection thresholds were obtained through a temporal two-alternative forced-choice (2AFC) procedure with the method of constant stimuli. When target and inducer had equal contrast polarity, low thresholds of target lines were observed for low inducer contrasts and increased with increasing inducer contrast. With opposite contrast polarity of target and inducer, thresholds were high for low inducer contrasts and decreased for increasing contrast thereof. Our results support the hypothesis that cortical mechanisms with different sensitivity to the sign and strength of contrast participate in the detection facilitation of line contours.

Contour integration across polarities and spatial gaps: from local contrast filtering to global grouping.

This article introduces an experimental paradigm to selectively probe the multiple levels of visual processing that influence the formation of object contours, perceptual boundaries, and illusory contours. The experiments test the assumption that, to integrate contour information across space and contrast sign, a spatially short-range filtering process that is sensitive to contrast polarity inputs to a spatially long-range grouping process that pools signals from opposite contrast polarities. The stimuli consisted of thin subthreshold lines, flashed upon gaps between collinear inducers which potentially enable the formation of illusory contours. The subthreshold lines were composed of one or more segments with opposite contrast polarities. The polarity nearest to the inducers was varied to differentially excite the short-range filtering process. The experimental results are consistent with neurophysiological evidence for cortical mechanisms of contour processing and with the Boundary Contour System model, which identifies the short-range filtering process with cortical simple cells, and the long-range grouping process with cortical bipole cells.

Illusory form with inducers of opposite contrast polarity: evidence for multistage integration.

The perception of brightness differences in Ehrenstein figures and of illusory contours in phase-shifted line gratings was investigated as a function of the contrast polarity of the inducing elements. We presented either continuous lines or line-like arrangements composed of aligned dashes or dots whose spacing was varied. A yes/no procedure was used in which naive observers had to decide whether or not they perceived a brightness difference in a given Ehrenstein figure or an illusory contour in a phase-shifted line grating. The results show that brightness differences are perceived to some extent in Ehrenstein figures with inducers of opposite polarity of contrast; however, the percentage of yes response was systematically lower and response times were longer than for figures with inducers of the same polarity. Phase-shifted line gratings with lines of opposite polarity of contrast yielded stronger illusory contours and shorter response times than those with lines of the same polarity. When the sign of contrast was not the same within a given line of induction, neither differences in brightness nor illusory contours were perceived. The results suggest that the mechanisms that lead to apparent differences in brightness are more sensitive to input of the same contrast polarity, the mechanisms generating illusory contours more sensitive to input of opposite polarity. The data are discussed in the light of a multistage approach to illusory form perception and some implications for cortical models of illusory contour integration are discussed.

Contrast detection facilitation by spatially separated targets and inducers.

We measured contrast detection thresholds for a small (3.6 x 3.6 arc min) square target in the presence and absence of spatially identical pedestal stimuli, and of a spatially non-overlapping inducing line (3.6 x 23 arc min). Results for the pedestal stimuli replicated the classical "dipper function", thresholds being reduced by near-threshold pedestals and increased at higher pedestal contrasts. An inducer without a pedestal also decreased detection thresholds. When the inducer and pedestal were combined, their effects were additive. Thus the inducer facilitated target detection when the pedestal was absent but raised detection thresholds when the pedestal contrast was sufficient by itself to lower threshold. Inducers of opposite polarity to the target did not consistently decrease target thresholds, even when they were clearly visible, arguing against spatial uncertainty as the explanation of the inducer effect. The inducer effect was independent of the length of the inducer except with small (< 3.6 arc min) stimuli, and was abolished by increasing target-inducer separation beyond about 10 arc min.

Subthreshold summation with illusory contours.

Results from three experiments using spatial forced-choice techniques show that an illusory contour improves the detectability of a spatially superimposed, thin subthreshold line of either contrast polarity. Furthermore, the subthreshold line is found to enhance the visibility of the illusory contour. Stimuli which do not induce illusory contours, but reduce uncertainty about the spatial position of the line, give rise to a slight detection facilitation, but the threshold of 75% correct responses is not attained. The data indicate that superimposing illusory contours and subthreshold lines produces interactions which are similar to classic subthreshold summation. They thus provide psychophysical evidence for the functional equivalence of illusory contours and real lines suggested by recent neurophysiological findings.

Phenomena of illusory form: can we bridge the gap between levels of explanation?

The study of illusory brightness and contour phenomena has become an important tool in modern brain research. Gestalt, cognitive, neural, and computational approaches are reviewed and their explanatory powers are discussed in the light of empirical data. Two well-known phenomena of illusory form are dealt with, the Ehrenstein illusion and the Kanizsa triangle. It is argued that the gap between the different levels of explanation, bottom-up versus top-down, creates scientific barriers which have all too often engendered unnecessary debate about who is right and who is wrong. In this review of the literature we favour an integrative approach to the question of how illusory form is derived from stimulus configuration which provide the visual system with seemingly incomplete information. The processes that can explain the emergence of these phenomena range from local feature detection to global strategies of perceptual organisation. These processes may be similar to those that help us restore partially occluded objects in everyday vision. To understand better the Ehrenstein and Kanizsa illusions, it is proposed that different levels of analysis and explanation are not mutually exclusive, but complementary. Theories of illusory contour and form perception must, therefore, take into account the underlying neurophysiological mechanisms and their possible interactions with cognitive and attentional processes.

Psychophysical measures of illusory form perception: further evidence for local mechanisms.

Detection thresholds for a small light spot were measured at various distances from configurations (Kanizsa squares and other) consisting of white inducing elements on a dark background. Threshold distributions as a function of target position, number, size and spacing of contrast inducing elements were established. The data show that thresholds are elevated when the target is located close to one or more inducing element(s). Furthermore, threshold elevations diminish with increasing distance of the target from the configurations, increasing spacing and decreasing size of their inducing elements. When the target is flashed upon an illusory contour, no threshold elevation is observed in any of the conditions tested. Within incomplete illusory figures (only half of the square visible), the threshold gradients show the same tendencies. The present observations add further empirical support to the idea that illusory figures are built up by way of local mechanisms at early stages of processing.

Bright lines and edges facilitate the detection of small light targets.

Thresholds for the detection of a small light target (increment thresholds), measured at the ends of white lines and small luminance edges, are lower than when the target is presented on a plain field. This facilitation effect disappears when: (1) the line-end is 'stopped' by another line with perpendicular orientation; (2) the inducing line is black instead of white; and (3) when the inducer does not carry information about orientation (e.g., a small dot). These observations suggest that polarity specific and orientation selective neural activation, extending collinearly from the inducing lines and edges, produces a local increase in visual sensitivity. The possible role of such a mechanism in contour completion and the formation of illusory contours is discussed.

Local brightness mechanisms sketch out surfaces but do not fill them in: psychophysical evidence in the Kanizsa square.

In two experiments, brightness enhancement of the illusory surface in the Kanizsa square was investigated by means of a brightness matching procedure. The results show that specific properties of the inducing elements such as size, spacing, and luminance have effects on the matching threshold that are similar to those previously obtained in experiments on simultaneous contrast. The data from a third experiment demonstrate that increment thresholds measured within the Kanizsa square are elevated when the target is flashed on a position close to the inducing elements. The thresholds decrease considerably in the center of both test and control figures (representing or not representing an illusory square). These observations suggest that low-level mechanisms are likely to explain local brightness differences within the configurations but not global figure brightness. In other words, local contrast seems to generate brightness information that "sketches out" surfaces at their surrounds but does not "fill" them "in."

Psychophysical evidence for low-level processing of illusory contours and surfaces in the Kanizsa square.

Increment thresholds were measured on either side of one of the illusory contours of a white-on-black Kanizsa square and on the illusory contour itself. The data show that thresholds are elevated when measured on either side of the illusory border. These elevations diminish with increasing distance of the target spot from the white elements which induce the illusory figure. The most striking result, however, is that threshold elevations are considerably lower or even absent when the target is located on the illusory contour itself. At an equivalent position in a control figure where no illusory contour is visible, such a threshold decrease does not occur. The present observations add empirical support to low-level explanations of illusory contour perception.

Apparent brightness enhancement in the Kanizsa square with and without illusory contour formation.

The perceived strength of darkness enhancement in the centre of surfaces surrounded or not surrounded by illusory contours was investigated as a function of proximity of the constituent elements of the display and their angular size. Magnitude estimation was used to measure the perception of the darkness phenomenon in white-on-grey stimuli. Darkness enhancement was perceived in both types of the stimuli used, but more strongly in the presence of illusory contours. In both cases, perceived darkness enhancement increased with increasing proximity of the constituent parts of the display and with their angular size. These results suggest that the occurrence of darkness (or brightness) enhancement phenomena in the centre of the displays is not directly related to illusory contour formation.